Effects of insulin, ADH, and cyclic AMP on sodium transport in the toad bladder

These studies further define the mechanisms by which insulin stimulates Na transport in the toad bladder. Serosal but not mucosal addition of insulin, 100-1,000 mU/ml, stimulated short-circuit current (SCC) by 25-50%. The initial rise in SCC occurred at 5 min and the peak response at 15-25 min. Doses of insulin greater than 250 mU/ml increased SCC values for up to 3 h. Actinomycin D did not block the early rise in SCC produced by insulin, but it blocked the delayed effects. Insulin increased SCC in substrate-depleted bladders, although the increase in SCC was less (P less than 0.01) than in nonsubstrate-depleted bladders. Pyruvate addition to substrate-depleted bladders restored to normal the rise in SCC observed after insulin. Simultaneous addition of ADH and insulin led to an increase in SCC that was greater than the sum of the responses observed when each hormone was added independently. Synergistic effects on SCC were also obtained with cyclic AMP and insulin. Insulin did not increase cyclic AMP levels in toad bladder epithelial cells. It is suggested that insulin stimulates active Na transport by two mechanisms: 1) a rapid phase, which may involve unmasking of pump sites within the membrane, and 2) a delayed effect which seems to require protein synthesis. The synergism of which seems to require protein synthesis. The synergism of insulin with ADH or cyclic AMP may reflect a facilitative effect of insulin on ADH or cyclic AMP-sensitive pump sites or, alternatively, the uncovering of latent pump sites that then may be available to stimulation by ADH or cyclic AMP.