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Objectives: To determine the disposition of orally administered cefpodoxime proxetil in foals and adult horses and measure the minimum inhibitory concentrations (MICs) of the drug against common bacterial pathogens of horses., Animals: 6 healthy adult horses and 6 healthy foals at 7 to 14 days of age and again at 3 to 4 months of age., Procedure: A single dose of cefpodoxime proxetil oral suspension was administered (10 mg/kg) to each horse by use of a nasogastric tube. In 7- to 14-day-old foals, 5 additional doses were administered intragastrically at 12-hour intervals. The MIC of cefpodoxime for each of 173 bacterial isolates was determined by use of a commercially available test., Results: In 7- to 14-day-old foals, mean +/- SD time to peak serum concentration (Tmax) was 1.7 +/- 0.7 hours, maximum serum concentration (Cmax) was 0.81 +/- 0.22 microg/mL, and elimination half-life (harmonic mean) was 7.2 hours. Disposition of cefpodoxime in 3- to 4-month-old foals was not significantly different from that of neonates. Adult horses had significantly higher Cmax and significantly lower Tmax, compared with values for foals. The MIC of cefpodoxime required to inhibit growth of 90% of isolates for Salmonella enterica, Escherichia coli, Pasteurella spp, Klebsiella spp, and beta-hemolytic streptococci was 0.38, 1.00, 0.16, 0.19, and 0.09 microg/mL, respectively., Conclusions and Clinical Relevance: Oral administration at a dosage of 10 mg/kg every 6 to 12 hours would appear appropriate for the treatment of equine neonates with bacterial infections.

Six healthy adult horses (5 mares and 1 stallion) were given a single dose of acetylsalicylic acid (ASA), 20 mg/kg of body weight, by intravenous (IV), rectal, and intragastric (IG) routes. Serial blood samples were collected via jugular venipuncture over a 36-h period, and plasma ASA and salicylic acid (SA) concentrations were determined by high-performance liquid chromatography. After IV administration, the mean elimination rate constant of ASA (+/- the standard error of the mean) was 1.32 +/- 0.09 h(-1), the mean elimination half-life was 0.53 +/- 0.04 h, the area under the plasma concentration-versus-time curve (AUC) was 2555 +/- 98 microg x min/mL, the plasma clearance was 472 +/- 18.9 mL/h/kg, and the volume of distribution at steady state was 0.22 +/- 0.01 L/kg. After rectal administration, the plasma concentration of ASA peaked at 5.05 +/- 0.80 microg/mL at 0.33 h, then decreased to undetectable levels by 4 h; the plasma concentration of SA peaked at 17.39 +/- 5.46 microg/mL at 2 h, then decreased to 1.92 +/- 0.25 microg/mL by 36 h. After rectal administration, the AUC for ASA was 439.4 +/- 94.55 microg x min/mL and the bioavailability was 0.17 +/- 0.037. After IG administration, the plasma concentration of ASA peaked at 1.26 +/- 0.10 microg/mL at 0.67 h, then declined to 0.37 +/- 0.37 microg/mL by 36 h; the plasma concentration of SA peaked at 23.90 +/- 4.94 microg/mL at 4 h and decreased to 0.85 +/- 0.31 microg/mL by 36 h. After IG administration, the AUC for ASA was 146.70 +/- 24.90 microg x min/mL and the bioavailability was 0.059 +/- 0.013. Administration of a single rectal dose of ASA of 20 mg/kg to horses results in higher peak plasma ASA concentrations and greater bioavailability than the same dose given IG. Plasma ASA concentrations after rectal administration should be sufficient to inhibit platelet thromboxane production, and doses lower than those suggested for IG administration may be adequate.

Clarithromycin offers numerous advantages over erythromycin and thus, is an attractive alternative for the treatment of Rhodococcus equi infections in foals. The disposition of clarithromycin was investigated in 6 foals after intragastric administration at a dose of 10 mg/kg body weight. Detectable serum concentrations of clarithromycin were found in 3 of 6 foals at 10 minutes and in all foals by 20 minutes post-administration. Time to peak serum concentration (Tmax) was 1.5 hours and peak serum concentration (Cmax) was 0.92+/-0.17 microg/ml. Mean serum concentrations decreased to 0.03 microg/ml at 24 h. No adverse reactions were noted during or after IG administration in any of the foals. Based on the pharmacokinetic parameters, the MIC90 of R. equi isolates, and predicted steady state concentrations, an oral dose of 7.5 mg/kg given every 12 hours would appear appropriate for the treatment of R. equi infections in foals.

Concentrations of caffeine (CA) and two metabolites were measured simultaneously in venous blood and splenius muscle of adult horses using a semi-automated in vivo microdialysis sampling technique. Dialysates from muscle and jugular vein were collected continuously for 48 h and drug levels were determined by high performance liquid chromatography (HPLC). Following i.v. injection, CA (3 mg/kg) attained a peak blood level of nearly 5400 +/- 600 ng/mL and decreased with a half-life of 15.3 +/- 0.7 h. Pharmacokinetic and statistical comparisons between CA concentrations in jugular dialysates and plasma samples revealed no significant differences between these sampling techniques. However, measurements in muscle and blood revealed unexpected pharmacokinetic differences, including significantly elevated concentrations of CA in muscle for 4 h following drug administration. In contrast, the CA metabolites theophylline (TP) and theobromine (TB) exhibited delayed appearances in muscle and blood with peak concentrations of 300 +/- 60 ng/mL (TP) and 150 +/- 50 ng/mL (TB) detected in both tissues 1 day following CA administration. This study demonstrates that our novel semi-automated microdialysis procedure for continuous monitoring of drug and metabolite levels may be useful for related studies in other domesticated large animal species.

Pharmacokinetics and distribution of orbifloxacin into body fluids and endometrium was studied in 6 mares after intragastric (IG) administration at a single dose rate of 7.5 mg/kg body weight. Orbifloxacin concentrations were serially measured in serum, synovial fluid, peritoneal fluid, urine, cerebrospinal fluid, and endometrial tissues over 24 hours. Minimum inhibitory concentrations of orbifloxacin were determined for 120 equine pathogens over an 11-month period. The mean peak serum concentration (Cmax) was 2.41+/-0.30 microg/mL at 1.5 hours after administration and decreased to 0.17+/-0.01 microg/mL (Cmin) at 24 hours. The mean elimination half-life (t1/2) was 9.06+/-1.33 hours and area under the serum concentration vs time curve (AUC) was 20.54+/-1.70 mg h/L. Highest mean peritoneal fluid concentration was 2.15+/-0.49 microg/mL at 2 hours. Highest mean synovial fluid concentration was 1.17+/-0.28 microg/mL at 4 hours. Highest mean urine concentration was 536.67+/-244.79 microg/mL at 2 hours. Highest mean endometrial concentration was 0.72+/-0.23 microg/g at 1.5 hours. Mean CSF concentration was 0.46+/-0.55 microg/mL at 3 hours. The minimum inhibitory concentration of orbifloxacin required to inhibit 90% of isolates (MIC90) ranged from < or = 0.12 to > 8.0 microg/mL, with gram-negative organisms being more sensitive than gram-positive organisms. Orbifloxacin was uniformly absorbed in the 6 mares and was well distributed into body fluids and endometrial tissue. At a dosage of 7.5 mg/kg once a day, many gram-negative pathogens, such as Actinobacillus equuli, Escherichia coli, Pasteurella spp., and Salmonella spp. would be expected to be susceptible to orbifloxacin.

Serum concentrations and pharmacokinetics of enrofloxacin were studied in 6 mares after intravenous (IV) and intragastric (IG) administration at a single dose rate of 7.5 mg/kg body weight. In experiment 1, an injectable formulation of enrofloxacin (100 mg/mL) was given IV. At 5 min after injection, mean serum concentration was 9.04 microg/mL and decreased to 0.09 microg/mL by 24 h. Elimination half-life was 5.33 +/- 1.05 h and the area under the serum concentration vs time curve (AUC) was 21.03 +/- 5.19 mg x h/L. In experiment 2, the same injectable formulation was given IG. The mean peak serum concentration was 0.94 +/- 0.97 microg/mL at 4 h after administration and declined to 0.29 +/- 0.12 microg/mL by 24 h. Absorption of this enrofloxacin preparation after IG administration was highly variable, and for this reason, pharmacokinetic values for each mare could not be determined. In experiment 3, a poultry formulation (32.3 mg/mL) was given IG. The mean peak serum concentration was 1.85 +/- 1.47 microg/mL at 45 min after administration and declined to 0.19 +/- 0.06 microg/mL by 24 h. Elimination half-life was 10.62 +/- 5.33 h and AUC was 16.30 +/- 4.69 mg x h/L. Bioavailability was calculated at 78.29 +/- 16.55%. Minimum inhibitory concentrations of enrofloxacin were determined for equine bacterial culture specimens submitted to the microbiology laboratory over an 11-month period. The minimum inhibitory concentration of enrofloxacin required to inhibit 90% of isolates (MIC90) was 0.25 microg/mL for Staphylococcus aureus, Escherichia coli, Salmonella spp., Klebsiella spp., and Pasteurella spp. The poultry formulation was well tolerated and could be potentially useful in the treatment of susceptible bacterial infections in adult horses. The injectable enrofloxacin solution should not be used orally.

Objective: To determine whether plasma concentrations of benzodiazepines (BDZ) in dogs following intranasal (IN) administration of diazepam are comparable to concentrations following IV administration., Animals: 6 (4 male, 2 female) healthy adult Greyhounds., Procedure: Dogs were randomly assigned to 2 groups of 3 dogs in a crossover design. Diazepam (0.5 mg/kg of body weight) was administered intravenously to dogs in group 1 and intranasally to dogs in group 2. Blood was collected from the jugular vein of each dog into tubes containing lithium heparin before and 3, 6, 9, 12, 15, 20, 30, 60, 120, 240, and 480 minutes following diazepam administration. After a 4-day washout period, dogs in group 1 received diazepam intranasally, dogs in group 2 received diazepam intravenously, and blood was again collected. Plasma concentration of BDZ was determined by use of a fluorescence polarization immunoassay., Results: Mean (+/- SD) peak plasma concentration of BDZ following IV administration (1,316 +/- 216 microg/L) was greater than that following IN administration (448 +/- 41 microg/L). Time to peak concentration was < or = 3 minutes following IV administration and 4.5 +/- 1.5 minutes following IN administration. Mean bioavailability of BDZ following IN administration was 80 +/- 9%., Conclusions and Clinical Relevance: Diazepam is rapidly and efficiently absorbed following IN administration of the parenteral formulation. Plasma concentrations match or exceed the suggested therapeutic concentration (300 microg/L). Intranasal administration of diazepam may be useful for treatment of seizures in dogs by owners or when intravenous access is not readily available.

The objectives of this study were to determine the pharmacokinetics and tissue concentrations of doxycycline after repeated intragastric administration, and to determine the minimum inhibitory concentrations (MIC) for equine pathogenic bacteria. In experiment 1, 2 mares received a single intragastric dose of doxycycline hyclate (3 mg/kg bwt). Mean peak serum concentration was 0.22 microg/ml 1 h postadministration. In experiment 2, 5 doses of doxycycline hyclate (10 mg/kg bwt), dissolved in water, were administered to each of 6 mares via nasogastric tube at 12 h intervals. The mean +/- s.e. peak serum doxycycline concentration was 0.32+/-0.16 microg/ml 1 h after the first dose and 0.42+/-0.05 microg/ml 2 h after the fifth dose. The mean trough serum concentrations were > 0.16 microg/ml. Highest mean synovial concentration was 0.46+/-0.13 microg/ml and highest mean peritoneal concentration was 0.43+/-0.07 microg/ml, both 2 h after the fifth dose. Highest urine concentration was mean +/- s.e. 145+/-25.4 microg/ml 2 h after the last dose. Highest endometrial concentration was mean +/- s.e. 1.30+/-0.36 microg/ml 3 h after the fifth dose. Doxycycline was not detected in any of the CSF samples. Mean +/- s.e. Vd(area) was 25.3+/-5.0 l/kg and mean t1/2 was 8.7+/-1.6 h. In experiment 3, minimum inhibitory concentrations of doxycycline were determined for 168 equine bacterial culture specimens. The MIC90 was < or = 1.0 microg/ml for Streptococcus zooepidemicus and 0.25 microg/ml for Staphylococcus aureus. Based on drug concentrations achieved in the serum, synovial and peritoneal fluids and endometrial tissues and MIC values determined in the present study, doxycycline at a dose of 10 mg/kg bwt per os every 12 h may be appropriate for the treatment of infections caused by susceptible (MIC < 0.25 microg/ml) gram-positive organisms in horses.

A radioimmunoassay was developed for prednisolone using IgG purified from rabbit antiserum. The assay was employed to determine the pharmacokinetics of prednisolone following intravenous administration of 450 mg of prednisolone sodium succinate (Solu Delta Cortef) to five adult Thoroughbred horses. The RIA had a sensitivity of 2 ng/ml and was relatively specific. It had cross-reactivity with 21-deoxycortisol (83.3%) cortisol (27.8%), 11-beta-hydroxyprogesterone (39.2%) and 17-hydroxyprogesterone (50%). However, it did not cross-react with naturally occurring steroids (cholesterol, testosterone, estradiol or progesterone) or synthetic steroids (betamethasone, methylprednisolone, prednisone or triamcinolone). Radioimmunoassay of the horse serum samples detected the presence of prednisolone for 5 to 7 hours post administration. The pharmacokinetic parameters tested and their means were a half-life of 1.150 +/- 0.233 (+/- SEM) hours, an excretion constant of 0.686 +/- 0.018 Ke/hr, a volume of distribution of 607 +/- 109 ml/kg, and a clearance rate of 374 +/- 47 ml/hr/kg. RIA also detected the presence of prednisolone in the urine beginning one hour post administration. The prednisolone in the urine increased significantly at 2 hours and reached a peak at 4 hours post administration. The urinary levels decreased at 5, 6, and 7 hours and peaked again at 8 hours. The level then gradually decreased and reached the minimal detectable levels in 48 hours. These results showed that the RIA was sensitive and relatively specific for the determination of prednisolone.(ABSTRACT TRUNCATED AT 250 WORDS)

Five healthy adult mares and 1 gelding were given a single dose (15 mg/kg of body weight) of metronidazole per rectum. After manual evacuation of feces from the rectum, a suspension of crushed tablets and water (40 ml) was administered via a 28-F catheter advanced 30 cm into the rectum. Blood samples were obtained by jugular venipuncture, and metronidazole concentration was measured serially for the 14 hours after drug administration. Mean serum concentration of metronidazole peaked at 4.5 micrograms/ml, 0.83 hour after administration, and decreased to 0.38 microgram/ml, 14 hours after administration. Mean elimination rate constant was 0.23/h, and the harmonic mean elimination half-life was 3.04 hours. Further study is necessary to determine a therapeutic dose regimen for metronidazole administered per rectum.

Serum concentrations of metronidazole were determined in 6 healthy adult mares after a single IV injection of metronidazole (15 mg/kg of body weight). The mean elimination rate (K) was 0.23 h-1, and the mean elimination half-life (t1/2) was 3.1 hours. The apparent volume of distribution at steady state was 0.69 L/kg, and the clearance was 168 ml/h/kg. Each mare was then given a loading dose (15 mg/kg) of metronidazole at time 0, followed by 4 maintenance doses (7.5 mg/kg, q 6 h) by nasogastric tube. Metronidazole concentrations were measured in serial samples of serum, synovia, peritoneal fluid, and urine. Metronidazole concentrations in CSF and endometrial tissues were measured after the fourth maintenance dose. The highest mean concentration in serum was 13.9 +/- 2.18 micrograms/ml at 40 minutes after the loading dose (time 0). The highest mean synovial and peritoneal fluid concentrations were 8.9 +/- 1.31 micrograms/ml and 12.8 +/- 3.21 micrograms/ml, respectively, 2 hours after the loading dose. The lowest mean trough concentration in urine was 32 micrograms/ml. Mean concentration of metronidazole in CSF was 4.3 +/- 2.51 micrograms/ml and the mean concentration in endometrial tissues was 0.9 +/- 0.48 micrograms/g at 3 hours after the fourth maintenance dose. Two mares hospitalized for treatment of bacterial pleuropneumonia were given metronidazole (15.0 mg/kg, PO, initially then 7.5 mg/kg, PO, q 6 h), while concurrently receiving gentamicin, potassium penicillin, and flunixin meglumine IV. Metronidazole pharmacokinetics and serum concentrations in the sick mares were similar to those obtained in the healthy mares.

Three sets of paired circular and square full-thickness skin wounds were made on the dorsum of the metacarpus (n = 48) of 8 horses. Each wound was 6.25 cm2 in area. The wounds were treated topically with an ointment, nonadherent dressing, and bandaged with a snug elastic wrap. Wounds were photographed every other day until healing was complete. Wound areas were measured and exponential and linear wound healing models were applied to the wound healing data generated. Wound healing variables measured for each wound were: number of days to healing, maximal size attained, rate of wound contraction (calculated by use of first-order and linear models), final wound size, and percentage of wound that healed by contraction. The exponential model fit the data significantly better than the linear model. The maximal size attained by circular wounds was significantly smaller than the maximal size attained by square wounds. Wound shape did not influence the rate of wound healing. On the basis of our findings, conversion of circular defects to square defects would not speed wound healing.

Serum concentrations of cefepime (BMY-28142) were determined for four dosing regimes, 10 mg/kg or 20 mg/kg, given as single subcutaneous (SC) or intramuscular injections (IM) to dogs. Serial serum samples were analyzed for the presence of cefepime by high-performance liquid chromatography. In experiment 1, the overall mean (+/- SEM) serum concentration (for a 12-hour period) after a dose of 20 mg/kg for SC and IM routes (4.9 +/- 0.74 micrograms/ml and 5.5 +/- 0.63 micrograms/ml, respectively) was twice that for the 10 mg/kg dose given either SC or IM (2.2 +/- 0.31 micrograms/ml and 2.8 +/- 0.47 micrograms/ml, respectively). There was no significant difference (p greater than 0.05) in mean serum concentrations for SC and IM routes of administration at the same dosage. In subsequent experiments, 5 doses of cefepime (20 mg/kg) were administered IM at 12-hour (experiment 2) or 24-hour (experiment 3) intervals. The mean (+/- SEM) peak serum concentration was 12.1 +/- 1.59 micrograms/ml, 2 hours after the 2nd injection in experiment 2. In experiment 3, the mean (+/- SEM) peak serum concentration was 10.9 +/- 1.34 micrograms/ml, 4 hours after the 1st injection. Mean trough concentrations in experiment 2 were greater than or equal to 0.5 microgram/ml and less than or equal to 0.5 in experiment 3. Multiple IM doses produced transient edema at the injection site and mild lameness in all dogs. Cefepime was highly active against single canine isolates of Staphylococcus intermedius, Pseudomonas aeruginosa and Escherichia coli, with minimum inhibitory concentrations of 0.125 microgram/ml, 1 microgram/ml and 0.3 microgram/ml, respectively.

Six calves with suppurative arthritis were given a single IM injection of sodium cephapirin at a dosage of 10 mg/kg of body weight. Cephapirin concentrations were serially measured in serum and in normal and suppurative synovial fluid over a 24-hour period. Mean peak serum concentration was 6.33 microliters/ml at 20 minutes after injection. The highest cephapirin concentrations in normal and suppurative synovial fluid were 1.68 and 1.96 micrograms/ml, respectively, 30 minutes after injection. Overall mean cephapirin concentration in normal synovial fluid for the first 4 hours (1.04 +/- 0.612 micrograms/ml) was not significantly different from that in suppurative synovial fluid (0.88 +/- 0.495 micrograms/ml; P greater than 0.05). Elimination half-life was 0.60 hours and clearance was 1,593 ml/h/kg.

Eight calves with suppurative arthritis were each given a single intramuscular injection of ampicillin trihydrate at a dose of 10 mg/kg. Ampicillin concentrations were measured serially in serum and in suppurative and normal synovial fluid over a 24-hour period. The mean peak serum concentration was 2.5 +/- 0.54 micrograms/ml 2 hours after injection. The highest concentration in normal synovial fluid was 3.5 +/- 0.40 micrograms/ml at 4 hours and the highest concentration in suppurative synovial fluid was 2.7 +/- 0.58 micrograms/ml at 2 hours. Overall mean ampicillin concentration in normal synovial fluid for the first 8 h (2.9 +/- 0.32 micrograms/ml) was significantly different from that in suppurative synovial fluid (2.1 +/- 0.33 micrograms/ml) and serum (1.9 +/- 0.30 micrograms/ml; p less than 0.05).

Each of seven mares was given an intravenous (IV) injection of 40% dimethyl sulfoxide (DMSO) at a dosage of 1 g/kg, over 35 min, immediately followed by a single IV injection of a trimethoprim (TMP) and sulfamethoxazole (SMZ) combination (SMZ 83%, TMP 17%) at a combined dosage of 44 mg/kg (7.48 mg/kg TMP; 36.52 mg/kg SMZ). Each horse served as its own control and was alternately treated with an identical dose of TMP-SMZ treatment alone at least seven days following or preceding the DMSO and TMP-SMZ treatment. Serum and cerebrospinal fluid (CSF) concentrations of TMP and SMZ were measured over a six hour period. Dimethyl sulfoxide treatment caused no significant difference in the mean serum concentration of SMZ or in the mean CSF concentrations of TMP or SMZ. The mean serum concentration of TMP was significantly (p less than 0.05) increased at the two, four and six hour sampling time in the mares receiving pretreatment with DMSO. The clearance of TMP was also significantly (p less than 0.05) decreased from 675 mL/h/kg to 327 mL/h/kg by DMSO administration. Concentrations of TMP and SMZ in the CSF in both treatment groups exceeded the minimum inhibitory concentrations for many common bacterial pathogens of equine origin. In addition, CSF concentration of TMP exceeded the serum concentrations required for 50% inhibition of dihydrofolate reductases of protozoan origin. Serum TMP and SMZ concentration were similar to those reported to be effective against Toxoplasma gondii in in vitro studies on the killing or inhibition of the organism.

The objectives of this study were to determine 1) the effect of Banamine on the seminal concentration of PGE2 and 2) the ability of sperm cells from treated rams to undergo acrosome reaction in vitro as an indirect measure of their fertilizing capacity. Seven rams, approximately 55 kg bodyweight and 2 to 5 yr of age, were divided into two groups: Group 1, treated (n = 4) and Group 2, controls (n = 3). Treatment consisted of administration of 75 mg i.m. Banamine twice daily, 6 to 8 h apart, for 45 d. On Day 0 (first day of treatment) and on Days 2, 9, 11, 16, 23, 25, 28, 30, 36, 39, 43 and 46 semen samples were collected from both groups using an electroejaculator. Blood samples were obtained for determination of serum levels of Banamine using high-performance liquid chromatography. Semen samples were examined for motility and morphology. Highly motile (>/=85%), normal-appearing semen samples were pooled on each day of collection and 25 ul of the pooled sample (1x10(6)/ml) of each group were induced to undergo acrosome reaction in vitro using ionophore A23187. Acrosome reaction was demonstrated using a staining technique designed for demonstrating the process in bull sperm cells. The percentage of acrosome-reacted and non-acrosome-reacted sperm was determined by random microscopic examination of 100 sperm cells using a double-blind approach. The supernatants of the remainder of the semen samples were assayed for levels of PGE2 using RIA. Values for acrosome-reacted sperm cells and PGE2 levels on the first day of treatment from both groups were compared with corresponding values from each day of sampling using Wilcoxon Rank Sums test (P<0.05). In Group 1, the mean serum level of Banamine was 3.02+/-0.58ug/ml. There was a significant increase in the ability of sperm cells from rams in Group 1 to undergo acrosome reaction as treatment progressed compared with the sperm cells from rams in Group 2. However, there was a significant decrease in concentration of PGE2 in semen from rams in Group 1 compared with those from Group 2. The results of this study suggest an inverse relationship between the capacity of sperm cells to undergo acrosome reaction and concentration of PGE2 in semen of rams treated with a nonsteroidal anti-inflammatory drug.

Six healthy adult mares were given a single IV dose (25 mg/kg of body weight) of chloramphenicol sodium succinate. Chloramphenicol concentrations in serum, synovial fluid, peritoneal fluid, and urine were measured serially over a 48-hour period. The highest measured serum chloramphenicol concentration was 6.21 micrograms/ml at 0.5 hour. Chloramphenicol was detected in synovial and peritoneal fluids, with mean peak concentrations of 3.89 micrograms/ml and 3.50 micrograms/ml, respectively, at 0.5 hour. Serum and synovial concentrations declined rapidly and were not measurable at 3 hours. Chloramphenicol could not be detected in peritoneal fluid at 6 hours. The serum half-life was 0.43 hour and the apparent volume of distribution was 2.83 L/kg. Urine concentrations of chloramphenicol peaked at 0.5 hour at 106.72 micrograms/ml and also declined rapidly. The drug could not be detected in the urine at 36 hours.

The effect of a subcutaneous injection of estradiol on the secretion of pituitary prolactin in the rat and the relationship between serum estradiol level and luteinizing hormone (LH) secretion in mare were reviewed. In addition, the effect of estradiol injection on LH secretion and the metabolism of [14C] estradiol in intact and bile duct fistulated pony mares were studied. Low (0.1 mug/day/rat) to moderate dose (5 mug/day/rat) of estradiol benzoate injected subcutaneously to mature or immature rats significantly increased pituitary content of prolactin and serum prolactin level five- to tenfold. On the other hand, high dose of estradiol (10 mug/day/rat or more) was less effective in stimulating prolactin secretion, and it appeared that progesterone injected concurrently with estradiol had some inhibitory action on the stimulatory effect of estradiol. Studies in pony mares showed that the physiologic level of serum estradiol during proestrus was important for the induction of the ovulatory surge of LH. Intramuscular injection of a low dose (2 or 4 mg/mare) of estradiol was stimulatory, whereas a high dose (8 mg/mare) was inhibitory for LH secretion in pony mares. Results of the estradiol metabolism studies indicated a relatively long half-life for estradiol in the mare. The majority of the [14C] estradiol metabolites appeared in the urine within 24 hr following intravenous injection. Enterohepatic circulation appeared to be important for estradiol metabolism in mare.

Twenty-one adult cats, allotted into 2 groups, were given gentamicin sulfate at dosages of either 5.0 mg/kg of body weight or 2.5 mg/kg as a single IM injection. During a 24-hour period, serum concentrations of gentamicin were measured serially, using a fluorescence immunoassay. The mean peak serum concentration of gentamicin in cats given 5.0 mg/kg was 23.1 micrograms/ml at postinjection hour (PIH) 0.5; thereafter, the mean serum concentration steadily decreased to 2.0 micrograms/ml at PIH 24. The mean peak serum concentration for cats administered 2.5 mg/kg was 9.1 micrograms/ml at PIH 0.5; thereafter, the mean serum concentration steadily decreased to 1.3 micrograms/ml at PIH 12. Serum therapeutic concentrations, without exceeding toxic concentrations, were attained at the 2.5 mg/kg dosage.

Biliary excretion of dye was measured in 2 clinically normal and 2 heterozygote Corriedale sheep (the mutant Corriedale is characterized by depressed biliary transport of conjugated sulfobromophthalein (SBP) compounds) during infusion of the preformed glutathione conjugate of SBP. Maximal rates of excretion of conjugated SBP compounds in bile were comparable in heterozygote Corriedale and clinically normal sheep. These 2 heterozygote sheep do not express the biliary transport defect observed in mutant Corriedale sheep during SBP-glutathione infusion.

Biliary excretion of dye was evaluated in three normal and one mutant Corriedale sheep (characterized by depressed biliary transport of many organic anions) during infusion of unconjugated sulfobromophthalein (BSP) and its preformed glutathione conjugate (BSP-GSH). Maximal dye excretion rates in bile in normal sheep were higher when BSP-GSH rather than BSP was administered. In confirmation of earlier studies, dye excretion in bile was markedly depressed in the mutant animal. However, movement of unconjugated BSP from liver cells into bile remained relatively intact whereas transport of conjugated BSP compounds was virtually absent. Preservation of unconjugated dye entry into bile suggests the presence of a transport mechanism for unconjugated BSP which is preserved in mutant sheep and is distinct from that involved in BSP-GSH excretory transport. Renal clearance of conjugated BSP compounds was greater than of unconjugated BSP and clearance values were comparable in the normal and mutant sheep. Thus, no excretory defect comparable to that present in liver was identifiable in the kidney.

Six healthy adult horse mares were each given a single injection of sodium cephapirin (20 mg/kg of body weight, IV), and serum cephapirin concentrations were measured serially over a 6-hour period. The mean elimination rate constant was 0.78 hour-1 and the elimination half-life was 0.92 hours. The apparent volume of distribution (at steady state) and the clearance of the drug were estimated at 0.17 L/kg and 598 ml/hour/kg, respectively. Each mare was then given 4 consecutive IM injections of sodium cephapirin (400 mg/ml) at a dosage level of 20 mg/kg. Cephapirin concentrations in serum, synovial fluid, peritoneal fluid, CSF, urine, and endometrium were measured serially. After IM administration, the highest mean serum concentration was 14.8 micrograms/ml 25 minutes after the 4th injection. The highest mean synovial and peritoneal concentrations were 4.6 micrograms/ml and 5.0 micrograms/ml, respectively, 2 hours after the 4th injection. The highest mean endometrial concentration was 2.2 micrograms/g 4 hours after the 4th injection. Mean urine concentrations reached 7,421 micrograms/ml. Cephapirin did not readily penetrate the CSF. When cephapirin was given IM at the same dose, but in a less concentrated solution (250 mg/ml), serum concentrations peaked at 25.0 micrograms/ml 20 minutes after injection, but the area under the serum concentration-time curve was not significantly different (P greater than 0.05). The bioavailability of the drug was greater than or equal to 95% after IM injection.

Six healthy pony foals, from 2 to 11 days of age, were given a single IM injection of amikacin sulfate (250 mg/ml) at a dosage rate of 7 mg/kg of body weight. Serum amikacin concentrations were measured serially over a 24-hour period. The mean peak serum concentration was 14.7 micrograms/ml at 0.5 hour. The elimination rate constant for amikacin was 0.24/hour, the elimination half-life was 3.0 hours, and the apparent volume of distribution was 0.58 L/kg.

Five healthy adult dogs were given a single IV dose (40 mg/kg of body weight) of ticarcillin disodium. Serum concentrations were measured serially over a period of 12 hours. Five days later, the drug was administered IM to the dogs at the same dose rate, and serum concentrations were measured serially for 12 hours. The mean peak serum concentration after IM administration was 120.5 micrograms/ml at 1.5 hours. Pharmacokinetic values following IV administration were (i) elimination rate constant = 0.8/hour-1, (ii) half-life = 0.8 hour, (iii) serum clearance = 292 ml/hr/kg, and (iv) apparent volume of distribution = 347 ml/kg. Estimated values after IM administration were (i) elimination rate constant = 0.6/hour, (ii) half-life = 1.1 hours, (iii) serum clearance = 218 ml/hr/kg, and (iv) apparent volume of distribution = 345 ml/kg; only the elimination rate constants were significantly different between the 2 routes of administration.

Six mares were given 5 IM injections (at 12-hour intervals between doses) of amikacin sulfate at a dosage of 7 mg/kg of body weight. Serum amikacin concentrations were measured serially throughout the study; synovial, peritoneal, endometrial, and urine concentrations were determined after the last injection. Amikacin concentrations of the CSF were measured serially in 3 of the 6 mares; 1 of the 3 mares had septic meningitis. Mean serum amikacin concentrations peaked at 1 to 2 hours after IM injection. The highest mean serum concentration was 19.2 micrograms/ml (1.5 hours after the 5th injection). The highest mean synovial concentration was 10.8 micrograms/ml at 2 hours after the 5th injection; the highest mean peritoneal concentration was 16.2 micrograms/ml at 3 hours after the 5th injection. The mean endometrial amikacin concentration was 2.5 micrograms/g (1.5 hours after the 5th injection). Amikacin reached a CSF concentration of 0.97 micrograms/ml in the mare with meningitis, but amikacin was not detected in CSF of healthy mares. Urine concentrations reached 1,458 micrograms/ml. Pharmacokinetic values were estimated after the 1st injection (elimination rate constant = 0.31/hour; half-life = 2.3 hours; apparent volume of distribution = 0.26 L/kg), and after the 5th injection (elimination rate constant = 0.28/hour; half-life = 2.6 hours; apparent volume of distribution = 0.30 L/kg); significant differences were not observed.

The use of animal models in the experimental production of liver diseases similar to those of man is still in its infancy. There is a need to discover new models more closely related to counterpart syndromes in man in the fields of hepatorenal syndrome, neonatal jaundice, Wilson's disease, cholelithiasis, viral hepatitis, biliary atresia, and cirrhosis, to mention only a few. With the continued indiscriminate inbreeding of companion animals as well as the planned inbreeding of laboratory animals, there is little doubt that many more will soon be available. The current availability of mutant rats and sheep with bilirubin transport defects has allowed for a better understanding of how organic anions are transported by the liver. Many other currently available experimental animal models herein briefly reviewed have been only superficially studied. It is the intent of this chapter to provide for post-doctoral students an appreciation for the many animal model systems available for experimental hepatic research.

Three stallions, 3 to 5 yr old and approximately 550 kg bodyweight, were used in a switchback experimental design to study the effect of daily, oral administration of 3g PBZ on the concentrations of PBZ and PGFM in blood (plasma) and seminal plasma (SP). Control and treatment periods were each 24 days' duration. Blood and semen samples were simultaneously collected every three days during these periods. Each stallion served consecutively as a control, treated, control, and treated subject for 24 days in each of the four periods. Concentrations of PBZ were obtained using HPLC and PGFM by specific RIA. Concentrations (mean +/- SE) of PBZ averaged 9.2 +/- 0.12 ug/ml in plasma but were undetectable in SP following the treatments. There was no significant difference in the plasma levels of PGFM between pre- and post- treatment values. However, there was a significant difference (P<0.001) in PGFM concentrations of seminal plasma before and after treatment. Results of this study suggest that daily, oral adminisration of 3g PBZ for 24 days to mature stallions can significantly decrease seminal plasma concentrations of PGFM. The physiological significance of this observation remains speculative.

Six healthy adult mares were each given an oral loading dose of ormetoprim(OMP)-sulfadimethoxine (SDM) at a dosage of 9.2 mg of OMP/kg and 45.8 mg of SDM/kg, followed by four maintenance doses of 4.6 mg of OMP/kg and 22.9 mg of SDM/kg, at 24 h intervals. Ormetoprim and SDM concentrations were measured in serum, synovial fluid, peritoneal fluid, cerebrospinal fluid, urine and endometrium. The highest mean serum OMP concentration was 0.92 micrograms/mL 0.5 h after the first dose; the highest mean SDM concentration was 80.9 micrograms/mL 8 h after the first dose. The highest mean synovial fluid concentrations were 0.14 microgram of OMP/mL and 28.5 micrograms of SDM/mL 12 h after the first dose. The highest mean peritoneal fluid concentrations were 0.19 micrograms of OMP/mL 6 h after the first dose and 25.5 micrograms of SDM/mL 8 h after the fifth dose. The highest mean endometrial concentrations were 0.56 micrograms of OMP/g and 28.5 micrograms of SDM/g 4 h after the fifth dose. The mean cerebrospinal fluid concentrations were 0.08 micrograms of OMP/mL and 2.1 micrograms of SDM/mL 5 h after the fifth dose. Mean trough urine drug concentrations were greater than or equal to 0.4 micrograms of OMP/mL and greater than or equal to 172 micrograms of SDM/mL. Two of the mares were each given a single intravenous (IV) injection of OMP and SDM at a dosage of 9.2 mg of OMP/kg and 45.8 mg of SDM/kg. Excitation and muscle fasciculations were observed in both mares after IV administration and all scheduled blood samples could be collected from only one of the two mares.(ABSTRACT TRUNCATED AT 250 WORDS)