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The effect of challenge by Escherichia coli STa enterotoxin on pig jejunal mucosal surface pH was investigated in vivo. Exposure to STa resulted in a rapid and reversible alkalinisation (P less than 0.001) of the jejunal mucosa from 6.27 +/- 0.11 (5) to 6.89 +/- 0.03 (5). This action of STa is probably mediated through cyclic 3'5'-guanosine monophosphate (cGMP) since the 8-bromo analogue of cGMP induced the same effect as that observed after STa challenge. The action of STa on mucosal pH was partially inhibited by pre-administration of an antisecretory factor (ASF) preparation. The action of 8-bromo cGMP was unchanged by the presence of ASF. This implies that ASF inhibition occurs during the early stages of STa action prior to stimulation of guanylate cyclase. This effect of STa on the pig jejunal mucosal surface pH, or acid microclimate, may explain why weak acid supplementation of oral rehydration solutions can be ineffective in certain types of diarrhoeal disease.

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions, to elucidate the influence of three levels of dietary NaCl and of luminal NaCl concentrations on coprodeal and colonic transport of NaCl. Net Na+ and Cl- absorption rates were increased in response to low dietary Na+ levels and higher luminal NaCl concentrations. Net Na+ absorption was always against the prevailing electrochemical gradient, saturably dependent on luminal Na+ concentrations in low Na+ diet birds, and linearly dependent on luminal concentrations in medium and high Na+ diet birds. Net Cl- absorption was usually down its electrochemical gradient. The Cl- flux rates were strongly linearly dependent on Na+ flux rates, and statistically this relationship could account for all significant variation of Cl- fluxes associated with dietary and perfusate treatments. Transmural electrical potential differences were invariably serosa positive, and partly correlated with Na+ fluxes.

1. Sheets of isolated mucosa from the colon of chickens on either a high or a low NaCl diet were mounted in Ussing chambers. They were bathed in Krebs-phosphate medium and exposed to sodium concentrations ranging from 3.0 to 140 mmol/l (choline-chloride replacement). 2. In the short-circuited state the net fluxes of sodium in the mucosa (m)-serosa (s) direction, followed saturation kinetics with Km = 25 mmol/l in the low-NaCl chickens and 124 mmol/l in the high-NaCl chickens. 3. The unidirectional fluxes of sodium in the s-m direction were linearly related to the electro-chemical driving force. The apparent permeabilities were 8.6 +/- 0.3 x 10(-6) cm/s in the low-NaCl and 11.6 +/- 0.3 x 10(-6) cm/s in the high-NaCl birds. 4. The chloride fluxes were in the short-circuited state of equal magnitude in the m-s and the s-m direction, and in the two dietary states. Changes in the electrical driving force resulted in changes in chloride fluxes compatible with passive flow.

The kinetics of amiloride action on the isolated epithelium of the hen coprodaeum are reported. Tissues were taken from birds fed on low salt diets for 9-10 days, conditions which induce a high resting short circuit current due to sodium and sensitive to amiloride. The relation between the inhibition of amiloride sensitive short circuit current and blocker concentration obeyed simple mass laws with an apparent stoichiometry of 1:1 between amiloride and the sodium entry sites. The concentration of amiloride producing its half maximal effect (Ki) was 1.77 +/- 0.20 microM at a sodium concentration of 130 mM. There was a shallow dependence of Ki on sodium concentration, the value of Ki falling to 0.78 +/- 0.1 microM at 1.3 mM Na. The relation of Ki to Na concentration was linear indicating competitive antagonism. The sodium concentration which half saturates the amiloride site (KNa) was 80 mM. This value is very different from the concentration of sodium which half saturates SCC (Kscc = 5-7 mM) suggesting there are at least two sites at which sodium can modify the transporting characteristics. These data are compared to those for other epithelia where Kscc and KNa are rather similar. The benzyl derivative of amiloride (benzamil) was found to be 11.6 times more potent than amiloride on this tissue. The potency ration is similar to that for other sodium transporting epithelia suggesting that the structure of the ion translocation mechanism is partly conserved between species although the Ki for amiloride may vary by an order of magnitude.

The effects of pre-treatment in vivo with aldosterone and dexamethasone were investigated on rabbit distal colon. Apical Na-permeability and net sodium transport were measured in vitro. In this epithelium, Na-transport is entirely electrogenic. It can therefore be measured electrically as the fraction of short circuit current which is blockable by amiloride. The epithelia were studied in an Ussing chamber and the electrical values recorded by a computerized digital voltage clamp. Transepithelial parameters, and the transapical membrane parameters (in preparations depolarized from the serosal side) were investigated after treatment with the two hormones. Under transepithelial conditions, aldosterone and dexamethasone stimulated the short circuit current (Isc) from control (17.4 microA/cm2) to a similar degree (86.6 and 93.8 microA/cm2). However, whereas aldosterone did not alter the transepithelial resistance (RT) significantly, dexamethasone reduced RT from 357 to 167 omega X cm2. The stimulation of the potential difference (VT) under control condition (6.6 mV) was therefore significantly different between aldosterone (28.7 mV) and dexamethasone (16 mV). Mucosal amiloride (0.1 mM) inhibited Isc and VT completely under all conditions. Steady state current-voltage relations were obtained by voltage clamping the tissues in "staircase" increments before and after mucosal treatment with amiloride. As measured by the difference between these two states, Na-currents were calculated both for the transepithelial and the transapical condition. Intracellular Na-activity and apical Na-permeability were then calculated by the Nernst and Goldman-Hodgkin-Katz equations. These values were found to be increased after treatment with both hormones. Dexamethasone was a more potent stimulator of both values.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-circuit current (SCC) and electrical potential difference (PD) were measured in two epithelia of the lower gut of the domestic fowl, coprodeum and colon, mounted in vitro in the Ussing-chamber. The birds were suddenly switched from a low-NaCl to a high-NaCl diet, and 24 h aldosterone treatment (3 X 42.7 micrograms/kg i.m.) given from zero to 5 days after dietary change. The effects on SCC and PD of addition of amino acids, and later amiloride, to the bathing media were measured, because a low-NaCl-diet which releases endogenous aldosterone, inhibits amino acid stimulation (in colon) and induces amiloride inhibition of SCC and PD (in both coprodeum and colon). Birds were slaughtered 4 h after the last injection as single injections of aldosterone showed maximal change of SCC and PD, and maximal amino acid and amiloride effects, at that time. Control intramuscular injections of aldosterone showed a mono-exponential decline of the plasma concentration with a half time of 47 min. In colon the aldosterone treatment induced the low-NaCl diet pattern of the transport parameters regardless of the delay after resalination. In coprodeum SCC declined gradually with time and reached 25% of the value at low-NaCl diet when aldosterone was given 5 days after resalination. The data suggest that colon maintains a high flush of sodium through the epithelial cells regardless of whether the entry is via the amiloride-blockable sodium channel or the non-electrolyte stimulated pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the antisecretory factor (ASF) on experimental porcine enterotoxin-induced jejunal secretion was tested. The heat-labile enterotoxin (LT) from Escherichia coli and cholera toxin (CT) was used for challenge in ligated intestinal loops. Less than 10 units of ASF inhibited the LT-induced secretion, while that due to CT required more than 10 units of ASF. ASF was effective only when administered prior to toxin challenge, and could be given either intravenously or intra-intestinally. Mixing of ASF with specific anti-ASF antibodies prior to injection abolished its antisecretory effect. LT- and CT-induced secreted fluid contained equal concentrations of Na+, K+ and Cl-, and the ionic concentration was not affected by ASF. Less than 0.1 units of ASF per pituitary gland was present in 3- and 5-week old pigs, while it increased to 4.5 units in 28-week old animals, and to 12.2 units in pigs older than two years. However, after intra-intestinal vaccination with 2.0 mg CT, the pituitary ASF content in the 5-week old animals increased to 2.0 units within 24 h.

Short circuit currents (Isc), resting electrical potential differences (p.d.) and resistances were measured in vitro for stripped rectal ("colonic") and coprodeal epithelia from fowls transferred to low from high NaCl diet for 1, 2, 4 or 8 days (NaCl depletion), and from birds adapted to low NaCl diet and sacrificed 8, 16 and 24 h after NaCl-loading (resalination). NaCl-depletion enhanced Isc and p.d. fully by 4 days in rectum but took 8 days in coprodeum (half times 1.2 days and 4 days respectively); rectal amino acid-sensitive Isc was abolished and amiloride-sensitive Isc was established by 4 days (half-time 1 day). Resistance changes were transient and relatively small in rectum, but coprodeal resistance decreased. Resalination reversed these changes very rapidly in both tissues (within 25-50 h; half times 10-20 h). It is concluded that responses to NaCl-depletion are mediated by aldosterone (and possibly by some other unknown agonist); responses to resalination may be due in part to suppressed aldosterone secretion and possibly to the evocation of some other controller (perhaps prolactin or corticosterone) antagonising aldosterone.

The binding of 3H-benzamil to homogenates of epithelium removed from the colon and coprodaeum of hens was studied. A low capacity high affinity binding component was detected. The binding constants for benzamil and amiloride to this component were similar to those obtained when these ligands were used to inhibit transport in intact epithelia. The mean potency ratio of benzamil to amiloride was 12.8 measured by binding compared with 11.6 from functional inhibition. Binding activity was present in tissues taken from animals fed on low sodium diets and those containing a normal sodium content. In the presence of sodium the affinity of benzamil was slightly reduced, but only in tissues taken form animals on a low salt diet. Only a small fraction of the total binding activity was present in the apical surface of low salt tissues indicating that in homogenates a small percentage of the total activity is associated with functional sodium entry sites. It is suggested that the major part of the binding activity detected in homogenates represents components of the sodium ion translocation mechanism which are en route for the apical membrane.

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions to elucidate the influence of three levels of dietary Na+ and of luminal fluid composition on transport of electrolytes and water. Net rates of secretion of K+ and absorption of NH4+ were increased in response to dietary Na+ depletion and increased luminal Na+ concentrations, but these fluxes were linked to Na+ transport and no dietary or perfusion treatment effects could be detected on them independently of effects on Na+ flux. Similar results were found for Na+-linked water absorption from isosmotic perfusates. Serosal-mucosal net osmotic flow with Na+-free perfusates was significantly reduced by Na+-depletion, but no significant dietary effect could be detected on the serosal-mucosal osmotic permeability coefficient. The reflexion coefficient was effectively unity. Mucosal-serosal inorganic phosphate flux was small. The flux was independent of Na+ flux and dietary Na+ levels, and apparently facilitated by serosalmucosal osmotic flow. The data allowed determination of the absorption/secretion in coprodeum and colon of the ions investigated, as compared to the renal excretion rate.