Disappearance of insulin response after enzymatic treatment of sodium-transporting amphibian epithelia

K+ -stimulated Na+ transport in frog-skin epithelia

Increasing [K+] from 2.5 mmol/l to 115 mmol/l on the serosal side of the frog skin produces a rapid decrease of short-circuit current (Isc) that is followed, within a few minutes, by a recovery of Isc to near or above its control value. After isolation of the epithelium by a procedure involving collagenase treatment and physical removal of the corium, increasing serosal [K+] still produced a depression of Isc but no significant recovery phase. By itself, collagenase treatment reduced but did not eliminate the recovery phase. The recovery phase was also markedly depressed by the beta-adrenergic blocker oxprenolol, but not by propranolol, atropine or indomethacin. Amiloride, given during the recovery phase, caused Isc to reverse to a small outward value. These results suggest that the recovery phase of Isc seen in the response to increased serosal [K+] represents an increase in Na+ influx through amiloride-sensitive channels which is triggered by the release of an intermediary agent, possibly a beta-adrenergic agonist, from some structure in the corium.

Vasopressin-like effects of psychotropic drugs in amphibian epithelia

Amphibian epithelia have been used as models for studying the effects of psychotropic drugs on membrane transport. Several of these agents added to the internal or to the external media, at concentrations greater than 10(-3) M, had inhibitory, "ouabain-like" effects on Na transport. In contrast, stimulatory, "vasopressin-like" effects were seen at lower concentrations. The stimulation was additive to that of oxytocin if the drug was present in the external solution but nonadditive if in the internal solution. On water transport, harmala alkaloids had a vasopressinomimetic action in toad skin, while inhibition was seen with Li and amitriptyline. To account for these multiple effects, it is hypothesized that psychotropic drugs act on the following cell targets: the Na pump, the cyclic nucleotide system, microtubules, and membrane calcium sites at the outer barrier of the epithelium. Direct, biochemical evidence is needed to substantiate this hypothesis.