Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

Abstract PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na + from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na + channel (ENaC), Na + /K + -pump, and basolateral K + channels, which regulate Na + transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 μg/ml, PRL had no effect on the SCC. PRL (1 μg/ml) was sufficient to stimulate the SCC since 1 and 10 μg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 μg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na + /K + -pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers’ solution on each side. PRL (10 μg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na + /K + -pump [although PRL (1 μg/ml) had no effect on this SCC]. The effect of PRL on basolateral K + channels was investigated using apically nystatin-permeabilized skin with high-K Ringer’s solution on the apical side. PRL (10 μg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K + channels. Thus, although PRL stimulates the Na + /K + -pump, this effect probably contributes less than that on ENaC to the regulation of Na + transport across adult bullfrog skin.