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

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.