Effects of prolactin on Na-K-ATPase activity along the rat nephron

To test prolactin (PRL) action on osmoregulation in mammals, we evaluated in the rat the effect of this hormone on a major enzyme in renal regulation of water and electrolyte: renal Na-K-ATPase. Enzyme activity was determined by cytochemistry in medullary ascending limb (MAL) and distal convoluted tubule (DCT) from rats treated either by bromocriptine, or by PRL. Three hours after a bromocriptine injection (0.1 mg/100 g IP) a significant decrease of Na-K-ATPase activity is observed in both MAL (80% of control values, p less than 0.001) and DCT (78% p less than 0.01). Reciprocally, a significant (p less than 0.001) increase in enzyme activity is induced 3 h after a single PRL injection (140 micrograms/100 g IM), in both segments (MAL: 165%, DCT: 172% of control activities) and persists 6 h after the injection (MAL: 130%, DCT: 118%). Na-K-ATPase activity was correlated to plasma PRL levels (r = 0.78 in DCT, r = 0.89 in MAL). A direct effect of PRL on the tubule is suggested by results from experiments in which PRL, at various concentrations, is added in vitro on renal slices before Na-K-ATPase activity measurements. The increase in Na-K-ATPase activity exhibits a log-dose dependency with PRL concentration (p less than 0.01) and is still observed when AVP antagonist is added before PRL incubation, ruling out the possible role of AVP contamination of PRL. These results suggest a direct effect of PRL on renal Na-K-ATPase in MAL and DCT.