Prolactin stimulates Na+-K+-ATPase activity located in the outer renal medulla of the rat

The effect of rat prolactin on rat renal Na+-K+-ATPase activity was investigated by a cytochemical technique. Rat prolactin caused stimulation of Na+-K+-ATPase activity only in the outer medulla of the kidney, and not in renal cortical structures. Peak enzyme activity in cultured rat renal segments occurred after tissue had been exposed to rat prolactin for 2 min, and the time of maximal stimulation did not vary with the concentration of prolactin. There was a curvilinear response in Na+-K+-ATPase activity over the rat prolactin concentration range, 0.04-40 ng/l, but higher prolactin concentrations caused inhibition of enzyme activity. Na+-K+-ATPase response was totally blocked by specific rat prolactin antiserum. Human prolactin had no consistent effect on rat medullary Na+-K+-ATPase activity. Addition of specific tri-iodothyronine and arginine vasopressin antisera to rat prolactin was without effect, confirming that the stimulatory action of rat prolactin on Na+-K+-ATPase was not due to contamination with these hormones which are known to stimulate this enzyme.