Effects of vasopressin, theophylline and cyclic adenosine monophosphate on short-circuit current across isolated rabbit ileal mucosa

Localization of the action of cholera toxin on adenyl cyclase in mucosal epithelial cells of rabbit intestine

Brush borders and plasma membranes have been purified from mucosal epithelial cells of rabbit ileum under control conditions and after treatment for 3 hr with cholera toxin in vivo. The activity of several enzymes in these preparations was measured. It was concluded that adenyl cyclase, like NaK-ATPase, seems not to be a normal constituent of brush borders. Both these enzymes are present in plasma membrane preparations derived largely from the basal and lateral margins of the epithelial cells, both may be phospholipid dependent enzymes and both are affected by cholera toxin. Adenyl cyclase activity is increased while NaK-ATPase is decreased. The activities of alkaline phosphatase, leucineaminopeptidase, 5'-nucleotidase, glucose-6-phosphatase, and Mg-ATPase were not found to be affected by the toxin. Cholera toxin, which makes contact with the luminal side of the epithelial cells, in the natural disease and in the experimental model, would appear to exert its pathologic effect on adenyl cyclase at the opposite (basal and lateral) side of the cells.

Effect of cholera enterotoxin on ion transport across isolated ileal mucosa

The effects of cholera enterotoxin on intestinal ion transport were examined in vitro. Addition of dialyzed filtrate of Vibrio cholerae (crude toxin) to the luminal side of isolated rabbit ileal mucosa caused a delayed and gradually progressive increase in transmural electric potential difference (PD) and shortcircuit current (SCC). A similar pattern was observed upon addition of a highly purified preparation of cholera toxin, although the changes in PD and SCC were smaller. Na and Cl fluxes across the short-circuited mucosa were determined with radioisotopes 3-4 hr after addition of crude toxin or at a comparable time in control tissues. The toxin caused a net secretory flux of Cl and reduced to zero the net absorptive flux of Na. Similar flux changes were observed when either crude or purified toxin was added in vivo and tissues were mounted in vitro 3-4 hr later. Additon of D-glucose to the luminal side of toxin-treated mucosa produced a large net absorptive flux of Na without altering the net Cl and residual ion fluxes. Adenosine 3',5'-cyclic phosphate (cyclic AMP) and theophylline had previously been shown to cause a rapid increase in SCC and ion flux changes similar to those induced by cholera toxin. Pretreatment of ileal mucosa with either crude or purified cholera toxin greatly reduced the SCC response to theophylline and dibutyryl cyclic AMP, which, together with the flux data, suggest that both cyclic AMP and cholera toxin stimulate active secretion by a common pathway. Inhibition of the SCC response to theophylline was observed after luminal but not after serosal addition of toxin. In vitro effects of cholera toxin correlated closely with in vivo effects: heating toxin destroyed both; two V. cholerae filtrates which were inactive in vivo proved also to be inactive in vitro; PD and volume flow measurements in isolated, in vivo ileal loops of rabbit revealed that the PD pattern after addition of toxin is similar to that seen in vitro and also correlates closely with changes in fluid movement. The results suggest that stimulation by cholera toxin of a cyclic AMP-dependent active secretory process of the intestinal epithelial cells is a major cause of fluid loss in cholera.

Pharmacological properties of centrally administered ouabain and their modification by other drugs

1. Ouabain given by intracerebroventricular injection to mice in small doses (0.1-0.4 mug) produced a dose related depression of central nervous activity, characterized by a reduction in spontaneous locomotor activity, hypothermia, catalepsy and ptosis, lowered body posture and lack of response to external stimuli. Doses above 0.4 mug were excitatory, convulsant and lethal.2. The depressant effects could be antagonized by (+)-amphetamine, desmethylimipramine, dibutyryl cyclic 3'5'-adenosine monophosphate and caffeine.3. The MAO inhibitor nialamide produced only a small antagonism of ouabain, resulting in a greater rate of recovery from the depressant effects of ouabain.4. The depressant effects were associated with a marked elevation of whole-brain dopamine levels with little change in noradrenaline or 5-hydroxytryptamine.5. The dopamine-beta-hydroxylase inhibitor sodium diethyldithiocarbamate, administered by intracerebroventricular injection, produced effects qualitatively similar to those seen after ouabain.6. An interference with central transmitter function is postulated as a possible mode of action of intracerebroventricularly injected ouabain.

Stimulation of intestinal mucosal adenyl cyclase by cholera enterotoxin and prostaglandins

The effects of several prostaglandins (PG) and a highly purified preparation of cholera enterotoxin (CT) on intestinal mucosal adenyl cyclase activity and the effect of CT on intestinal mucosal cyclic 3',5'-adenosine monophosphate concentration were determined in guinea pig and rabbit small intestine and were correlated with the effects of the same agents on ion transport. Adenyl cyclase activity, measured in a crude membrane fraction of the mucosa, was found at all levels of the small intestine with the highest activity per milligram protein in the duodenum. The prostaglandins, when added directly to the assay, increased adenyl cyclase activity; the greatest effect (2-fold increase) was obtained with PGE(1) (maximal effect at 0.03 mM) and PGE(2). The prostaglandins also increased short-circuit current (SCC) in isolated guinea pig ileal mucosa, with PGE(1) and PGE(2) again giving the greatest effects. The prior addition of theophylline (10 mM) reduced the subsequent SCC response to PGE(1) and vice versa. It was concluded, therefore, that the SCC response to PGE(1), like the response to theophylline, represented active Cl secretion. CT increased adenyl cyclase activity in guinea pig and rabbit ileal mucosa when preincubated with the mucosa from 1 to 2.5 hr in vitro or for 2.5 hr in vivo but not when added directly to the assay. The increments in activity caused by PGE(1) and NaF were the same in CT-treated and control mucosa. Cyclic 3',5'-AMP concentration in rabbit ileal mucosa was increased 3.5-fold after a 2 hr preincubation with CT in vitro. Phosphodiesterase activity in the crude membrane fraction of the mucosa was unaffected by either CT or PGE(1). A variety of other agents including insulin, glucagon, parathormone, thyroid-stimulating hormone, L-thyroxine, thyrocalcitonin, vasopressin, and epinephrine all failed to change adenyl cyclase activity. It is concluded that CT and certain prostaglandins produce small intestinal fluid secretion by increasing mucosal adenyl cyclase activity, thereby stimulating an active secretory process.

Elevated concentration of adenosine 3':5'-cyclic monophosphate in intestinal mucosa after treatment with cholera toxin

A heat-labile factor in cell-free filtrate of a Vibrio cholerae culture induces a marked rise in the wet-weight concentration of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in the intestinal mucosa of the dog. The increase becomes appreciable 1-1.5 hr after intraluminal administration of the filtrate, about the same time as the onset of intestinal secretion in response to a heat-labile enterotoxin in the filtrate. These results are consistent with the hypothesis that cyclic AMP may be an intermediary in the intestinal secretory response to cholera toxin.

The effect of adenosine triphosphate on the transmural potential in rat small intestine

1. ATP either in the mucosal or serosal fluid caused a transient increase in the potential difference and short-circuit current across the wall of rat jejunum in vitro, the serosal solution becoming more positive.2. Similar responses were observed in the ileum and colon, and in in vivo preparations of small intestine.3. The response is relatively specific for ATP.4. The transient nature of the response is not due to rapid hydrolysis of extracellular ATP.5. High concentrations of K(+) in the mucosal medium, serosal ouabain or mucosal 2,4-dinitrophenol all inhibit the response without altering the time course.6. Significant inhibition was not observed in the presence of ADP or in Mg(2+)-free salines.7. The results are consistent with an intracellular action of ATP in the epithelium to stimulate net ion transport. The results do not demonstrate whether or not extracellular ATP can act as an energy donor for an electrogenic ion pump.8. Theophylline prolongs the time course of the response, and the involvement of the adenyl cyclase system cannot be excluded as an explanation for the findings.