INSULIN ENHANCES SODIUM SENSITIVITY OF NA-K-ATPASE IN ISOLATED RAT PROXIMAL CONVOLUTED TUBULE

Insulin has been shown to stimulate the rate of ouabain-sensitive Rb-86 influx in the isolated rat proximal convoluted tubule (PCT). To study the mechanism of this activation of Na-K-adenosinetriphosphatase (Na-K-ATPase), we determined the actions of insulin on 1) the maximal activity (V-max) of Na-K-ATPase hydrolytic activity; 2) the maximal rate of ouabain-sensitive Rb-86 influx (after intracellular Na loading); 3) the rate of ouabain-sensitive Rb-86 influx under conditions where intracellular Na concentration is rate limiting, either in the presence or in the absence of 5 x 10(-4) M amiloride and/or low extracellular Na concentration (3 mM); and 4) the Na sensitivity of the Na-K-ATPase hydrolytic activity. The maximal rates of Na-K-ATPase hydrolytic activity and of ouabain-sensitive Rb-86 uptake were unchanged by insulin. In contrast, we confirmed that insulin enhanced Rb-86 uptake (in peq.mm(-1).min(-1)) in the absence of inhibitor of the Na/H exchanger [18.2 +/- 1.7 to 24.1 +/- 1.3 (SE), P < 0.03] and, in addition, demonstrated a similar stimulation in the presence of either 5 x 10(-4) M amiloride (7.2 +/- 0.6 to 10.7 +/- 0.9, P < 0.01), 3 mM extracellular Na (4.1 +/- 0.4 to 5.6 +/- 0.2, P < 0.05), and both amiloride and 3 mM extracellular Na (2.1 +/- 0.7 to 4.5 +/- 0.4, P < 0.03). Finally, insulin increased the sensitivity of Na-K-ATPase to Na as the apparent dissociation constant decreased from 46.5 +/- 5.3 to 27.6 +/- 3.0 mM (P < 0.03). These results suggest that, in isolated rat PCT, insulin stimulates the Na-K-ATPase without changing its V-max by enhancing the Na sensitivity of the enzyme and thereby by increasing the pumping efficiency of each pump unit.