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<0.001) and DCT (78%,p<0.01). Reciprocally, a significant (p<0.001) increase in enzyme activity is induced 3 h after a single PRL injection (140 g/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<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.