Renal Na-K-ATPase activity during saline infusion in the rabbit

During saline infusion, sodium reabsorption (RNa) in the diluting segment (thick ascending limb of Henle's loop) increases acutely. The mechanism for this higher pumping rate of outer medullary Na-K-ATPase is unknown. Following left-sided nephrectomy, immediate i.v. infusion of hypertonic saline increased RNa in the remaining whole right kidney by 28 +/- 14% (p less than 0.05). Na-K-ATPase activity in outer medulla was raised by (delta) 23 +/- 4% above the left kidney (p less than 0.05), whereas cortical activity was unchanged. The mechanism for this increase in Na-K-ATPase activity was explored. The catalytic rate per enzyme did not differ in the two kidneys and equalled 5 340 min-1. The increase was therefore due to higher tissue concentration of active enzyme. The response was fully developed during continuous infusion within 20 min, and of equal magnitude whether protein synthesis had been inhibited by cycloheximide (delta = 23 +/- 7%) or stimulated by unilateral nephrectomy 6 days earlier combined with saline infusion for 2 h (delta = 34 +/- 10%). Thus, during hypertonic saline infusion, the increased RNa in the outer medulla was partly accounted for by the activation of latent Na-K-ATPase. High delivery of sodium to the diluting segment for more than 20 min during hypertrophy caused no further activity change.