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Liver biopsies from 9 out of every 10 obese individuals exhibit pathological changes of unknown aetiology and 3 out of every 10 reflect severe injury in the form of periportal fibrosis. To examine the hypothesis that excessive fibrosis in obesity arises in part from a predisposition to injury of the liver by drugs and xenobiotics, we administered 5, 10 and 25 mg/kg doses of the model periportal hepatotoxin, allyl alcohol, to obese Sprague-Dawley rats and age-matched non-obese controls. Alanine aminotransferase activity (ALT) in plasma was ten-fold more elevated in obese animals than in non-obese animals given the 25 mg/kg dose (P < 0.05). On fitting the ALT results to a non-linear, parametric model by iterative non-linear least squares regression, we found that the slope of the log dose ALT curve was similar for obese and non-obese rats. However, the minimum dose required to produce elevated ALT (DMIN) was 50% lower for obese animals (DMIN 6.47 +/- 2.75 vs. 13.3 +/- 0.96 mg allyl alcohol; P < 0.05). In a subsequent experiment, allyl alcohol was administered to obese rats based on ideal body weight, which is defined as the mean total body weight of an age-matched non-obese animal. With this dosing normalization, the 25 mg/kg ideal body weight doses translated to administration of a fixed dose of 13.5 mg allyl alcohol to obese rats. Obese rats treated in this fashion exhibited more severe necrosis in the periportal zone (median necrosis score 2 versus 0-1, P < 0.05) and increased mortality over controls (44% versus 0%; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Previous literature indicates possible interrelationships between the endogenous opioids or endorphins, pain response, and obesity or eating behaviour. The pain response was, therefore, examined in a rat model of obesity induced by palatable food high in unsaturated fats. Pellet-fed control and energy-dense obese and nonobese rats were tested for latency of response to a thermal stimulus using the tail flick test. Obese rats exhibited a statistically significant increase in tail flick latency compared to controls. In addition, the observed latencies were significantly correlated to the body weight of the rats (r = 0.52, p < 0.01). These data suggest that dietary-induced obese rats are similar to obese humans in being less sensitive to painful stimuli, consistent with an increase in endogenous opioids in obesity.

Obese human patients and obese overfed rats treated chronically with gentamicin suffer greater renal injury than nonobese patients and control animals. To understand the mechanism of this heightened susceptibility to the nephrotoxic effects of gentamicin, this study examines the plasma-time course and renal uptake of gentamicin in control and obese overfed rats following a single bolus dose. Gentamicin was administered ip to control rats at 30 mg/kg total body mass and to obese rats at 30 mg/kg ideal body mass plus 40% of excess body mass. Following gentamicin dosing, only 1 of 11 concentration-time points taken over 6/hr postdosing was different between control and obese groups. In addition, the area under the plasma concentration curve extrapolated to infinite time was not different between obese and control rats (mean +/- SD of 4.47 +/- 0.85 vs. 4.13 +/- 0.35 mg.min.ml-1, p greater than 0.5). The gentamicin plasma concentrations after 6 hr were less than 1 microgram/ml and not different between the groups; however, the concentration of gentamicin in the kidneys was 33% greater in obese than control rats at this time (324 +/- 66.9 vs. 244 +/- 34.7 micrograms/g, p less than 0.05). The fraction of dose and the total amount of drug that accumulated in the kidneys were also greater in the obese rats (42 and 72% increases). Considered with the results of previous studies, it appears that obese overfed rats sustain more severe nephrotoxicity following comparable plasma gentamicin exposure because of increased renal uptake and/or retention of drug.

Cytochrome P450IIE1 (IIE1) is a microsomal xenobiotic-activating enzyme that is inducible not only by various chemical agents but also by fasting and diabetes. Using a rat model that mimics human obesity, we have found that hepatic IIE1 levels are also increased by this common clinical disorder. Liver microsomes from rats made obese by feeding with an energy-dense diet displayed elevated aggregate P450 content (+28%) and enhanced catalytic activities associated with IIE1, including low-Km N-nitrosodimethylamine demethylation (+66%), aniline hydroxylation (+52%), p-nitrophenol hydroxylation (+170%), and acetaminophen-cysteine conjugate formation (+28%). In contrast, obesity had no significant effect on cytochrome b5 content, P450 reductase activity, benzphetamine demethylation, or erythromycin demethylation, with the latter two reactions being linked with rat IIC11 and IIIA1, respectively. The enhancement of IIE1-dependent drug-metabolizing activities noted in liver microsomes from obese rats was paralleled by a similar increase (111%) in hepatic IIE1 protein content in these animals, as assessed on immunoblots developed with anti-hamster IIE1 IgG. Anti-IIE1-inhibitable rates of microsomal p-nitrophenol metabolism, a reaction highly correlated with IIE1 content (r = 0.88, p less than 0.01), were over 3-fold higher in obese rats than in nonobese controls, providing additional evidence for the obesity-related increase of hepatic IIE1. The induction of IIE1 by the pathophysiological condition of obesity may provide a biochemical basis for the increased incidence of occult liver disease and certain cancers noted in obese individuals.

Liver microsomes from obese and control Sprague-Dawley rats were compared for cytochrome P-450 content and the ability to metabolize various prototype substrates. Over a 40-week period, the obesity-producing energy-dense diet increased average total body mass by 50%, liver mass by 32%, and body fat mass by 292%. Spectrally detectable cytochrome P-450 per mg protein increased by 36% in hepatic microsomes from obese rats. The livers from obese rats also contained more cytochrome P-450 (87%), while microsomal protein, NADPH-cytochrome c reductase, aryl hydrocarbon hydroxylase, and UDP-glucuronosyl transferase per organ rose slightly (12-40%) but not significantly. No change in the specific activities of these enzymes occurred. Young and adult rats were transferred from pellet diet to energy-dense diet for 3 weeks to examine the influence of diet vs. obesity. This short-term dietary change increased microsomal protein per g liver as well as cytochrome P-450 per liver, per g liver, and per mg protein. Adult animals increased in body weight by 24%, making them overweight and borderline obese. However, young animals showed no increase in body or liver weight, suggesting a direct effect of the energy-dense diet on liver P-450. Dietary obesity thus increased both the relative and total amounts of liver cytochrome P-450 in rats, but not the specific activities of other enzymes. These changes in cytochrome P-450 are consistent with the increased clearance seen for several oxidized drugs in obese humans and suggest that the obese overfed rat represents a useful animal model.

This disposition of acetaminophen was examined in an overfed rat model of human obesity following iv administration of a subtoxic 303 +/- 5 mg/kg dose based upon ideal body weight. Weanling Sprague-Dawley rats were maintained on a nutritionally complete semisynthetic diet containing 60% vegetable shortening and were compared to animals given a standard laboratory diet over the same 22-week period. At the time of study obese rats outweighed controls by 42% (637 +/- 32 g vs. 450 +/- 7 g, respectively). The absolute clearance of acetaminophen from plasma increased by 27% in obese rats. Higher partial formation clearance to acetaminophen glucuronide and sulfate conjugates accounted for most of this increase. Clearance by sulfhydryl conjugation was also substantially increased (56%), indicating that metabolism via toxic oxidation also occurred at a greater rate in obese animals. Absolute renal clearance of the glucuronide and sulfate conjugates but not acetaminophen increased with obesity, whereas parent volume of distribution remained unchanged. Some rats raised on the energy-dense diet remained lean and were examined separately as a dietary control group. Acetaminophen disposition in these animals was indistinguishable from pellet-fed controls, suggesting that acetaminophen elimination changes in overweight animals resulted from obesity itself and not from the obesity inducing energy-dense diet. Although animals placed on the energy-dense diet ate fewer grams of food per day, their caloric intake was similar to that of pellet-fed animals when adjusted for differences in body weight and caloric content of the diets.(ABSTRACT TRUNCATED AT 250 WORDS)

The overfed rat served as the animal model for examining the influence of obesity on the hepatotoxic and nephrotoxic potential of metabolically activated drugs, and acetaminophen served as the prototype drug. Weanling Sprague-Dawley rats were given a standard pellet diet or semisynthetic, energy-dense diet designed to produce obesity. After 24 weeks, when overfed rats outweighed controls by more than 50%, animals received 710 mg/kg of acetaminophen i.p., based on total body weight. Toxicity evaluation included biochemical signs of organ injury over the first 24 hr and histopathologic changes in tissue morphology at 48 hr. Both enzyme release (alanine aminotransferase into plasma, alkaline phosphatase into urine) and frank cellular necrosis in liver and kidney of obese rats greatly exceeded that in pellet-fed controls. Contributing to the potentiation of injury were higher peak plasma concentrations of acetaminophen in obese animals resulting from total body weight dosing. However, liver and kidney injury and mortality remained elevated when peak plasma concentrations were matched by fat-free mass dosing, indicating that increased toxicity also was related to obesity. Incomplete recovery of acetaminophen and metabolites from obese animals (45 vs. 71% in control rats) caused by a functional renal impairment made it impossible to determine the metabolic fate of acetaminophen in overfed animals from the analysis of urine collections. Drug products measured in urine were summed with amounts remaining in carcass at sacrifice, computed as terminal plasma concentrations times respective distribution volumes. These results showed obese rats to form more glucuronide and less sulfate conjugate than did pellet-fed controls, coinciding with clinical evidence for enhanced glucuronidation in obese humans.(ABSTRACT TRUNCATED AT 250 WORDS)

The obese overfed rat effectively models many of the pharmacological changes in human obesity. Recent data show that the obese rat is unusually susceptible to liver damage by several metabolically activated drugs that may be more toxic in obese humans. Results of the present study suggest a specific molecular locus for this interaction. In obese rats, P450 content of liver and the microsomal concentration of P450 were elevated 88% and 31%, respectively, over nonobese controls. Increases in microsomal ethanol oxidation were of identical magnitude. The ethanol-inducible form of P450 that is responsible for microsomal ethanol oxidation, P450IIE1, bioactivates several drugs that are shown to cause increased injury in obese rats. Collectively, these findings indicate that specific forms of P450 may become up-regulated in obesity, increasing the risk of a biochemically defined spectrum of drug-induced organ injuries.

The current studies examine acetaminophen pharmacokinetics and biotransformation in obese animals for possible shifts in metabolic conjugation reactions. Obesity was produced in Sprague-Dawley rats with an energy-dense cafeteria feeding regimen. Acetaminophen half-life remained unchanged and apparent volume of distribution increased slightly in obese versus pellet-fed control rats following an ip dose of 287 mg/kg. However, obese animals exhibited lower plasma concentrations of acetaminophen sulfate and excreted less sulfate conjugate but more glucuronide conjugate in urine. Absolute clearance of acetaminophen from plasma was similar for both groups of rats but formation clearance of acetaminophen sulfate was lower and formation clearance of acetaminophen glucuronide and was higher than control in obese rats. Renal clearance of unchanged drug and both conjugated metabolites appeared to rise with the degree of obesity. The many parallels in acetaminophen disposition shared with the obese human show the overfed rat to be a promising model for metabolic and physiologic changes associated with human obesity.

Existing methods for predicting creatinine clearance provide accurate estimates for normal-weight patients but not for patients who are obese. Studies into this problem began with an animal model of obesity, the obese overfed rat. Mean creatinine clearance was found to vary in direct proportion to fat-free body mass, determined in both obese and normal animals. The relevance of this observation to renal function in humans was evaluated by analyzing published studies reporting creatinine clearance and creatinine excretion rates in obese and normal persons. Measured creatinine clearance correlated well with estimated fat-free body mass (r = 0.772, p less than 0.02), and urinary excretion of creatinine normalized to fat-free mass correlated impressively with age (r = 0.960). Formulas derived from these observations allow for the prediction of creatinine clearance at steady state: (formula; see text) In initial tests of these formulas, their predictions appeared to be as accurate as existing methods for the normal-weight population and far superior to these methods when applied to the obese population. Therefore, when creatinine clearance is not measured in obese patients, the estimation of this parameter with the proposed formulas should improve the ability to select the appropriate dose for drugs that are cleared principally by renal filtration.

Effects of the diuretic drug furosemide were examined in obese animals to evaluate the hypothesis that organ damage by reactive drug metabolites may be potentiated by this disease. Obese overfed Sprague-Dawley rats that were treated ip with 450 mg/kg furosemide on the basis of total body mass suffered a 58% mortality rate over 24 h. This contrasted with 0% mortality in animals of normal body mass. On the basis of median histopathology scores, organ necrosis was judged to be greater in the liver (2+) and kidneys (1+) of obese rats than in the liver (1+) and kidneys (less than 1+) of normal controls (p less than 0.05). Obese animals demonstrated a fourfold rise in fat mass over controls. The low solubility of furosemide in lipid makes it probable that aggravated drug toxicity in obese rats dosed to total body mass resulted in part from elevated furosemide concentrations in lean body mass. In a subsequent study designed to minimize this possibility, furosemide was administered on the basis of fat-free body mass to equalize initial drug exposure in obese and control rats. Even with this downward dosage adjustment, obese animals suffered increased hepatic necrosis (median score of 2+ versus 0 in treated controls), greater impairment of renal function (plasma creatinine concentration of 2.41 mg/dl versus 0.96 mg/dl in treated controls), and more extensive enzymuria (enzyme excretion 175-300% more elevated than in treated controls). In conclusion, obese rats appear to be at increased risk of furosemide-induced liver and kidney injury due to at least two factors: (1) increased exposure of target organs in lean body mass to furosemide when the dosing of this poorly lipophilic drug was based on total body mass, and (2) increased susceptibility of target organs in lean body mass to furosemide injury when dosing was adjusted downward to reflect fat-free body mass and to equalize initial drug exposure.

The present study was undertaken to examine the appropriateness of the obese overfed rat and the obese Zucker rat as animal models for evaluating drug disposition changes in human obesity. It was found that 11 of 12 characteristics that control or influence drug clearance and volume of drug distribution in obese humans were qualitatively reproduced in the obese overfed rat. In contrast, existing literature shows that the obese Zucker rat resembles the obese human in only five of 12 characteristics, with meaningful discrepancies in fat-free mass, creatinine clearance, and thyroid function. Perhaps of greatest significance were changes in hepatic cytochrome P-450, which increased in proportion to total body mass in the obese overfed rat but remain unchanged in the Zucker rat. Although P-450 status in human obesity is unknown, the overfed rat model provides an opportunity for examining increased oxidative drug elimination that appears as an established feature of human obesity. In conclusion, the obese overfed rat appears to be superior to the obese Zucker rat as an animal model for evaluating the pharmacological consequences of human obesity, particularly those in which reproducing drug pharmacokinetics is an important consideration.

Obese humans suffer from excessive organ dysfunction, altered drug pharmacokinetics and may be at increased risk of various drug toxicities. A recent report shows that gentamicin nephrotoxicity in critically ill patients is more frequent and more severe than usual in individuals who are substantially overweight. The present study utilizes an overfed rat model to examine the influence of obesity on the nephrotoxic potential of gentamicin. After 52 weeks on an energy-dense diet, obese animals outweighed pellet controls by more than 80% (913 +/- 86 vs. 507 +/- 52 g; X +/- S.D., n = 7). When animals were treated twice daily for 6 days with 30 mg/kg of gentamicin i.p. based on total body mass, obese rats sustained more cortical necrosis than control (median score 3+ vs. 0), higher serum creatinine (4.36 +/- 2.72 vs. 0.71 +/- 0.17) and greater creatinine adjusted N-acetyl hexosaminidase excretion. The impact of obesity on intrinsic susceptibility to gentamicin nephrotoxicity was assessed by dosing animals for 5 days to ideal body mass plus 40% of excess body mass, the current clinical practice for achieving normal gentamicin concentrations in obese patients. Obese rats again sustained more frequent and severe cortical necrosis (2+ vs. 0) and excreted more N-acetyl hexosaminidase than control animals. Urine pH averaged 1.7 U below normal in obese animals, but restoration to normal values by 2 weeks on the pellet diet did not diminish the toxicity increase. Results from the overfed rat closely resemble the recent clinical observation that obese patients sustain more frequent and severe kidney damage from aminoglycoside antibiotics.(ABSTRACT TRUNCATED AT 250 WORDS)