Sodium pump quantity and turnover in rabbit descending colon at different rates of sodium absorption

³H-Ouabain binding to isolated epithelia and basolateral membrane vesicles of Na⁺-transporting epithelial cells of rabbit descending colon was determined to quantify the number of operative Na⁺-pump sites at different rates of transcellular Na⁺ transport which was varied over a wide range by chronic dietary Na⁺ restriction or Na⁺ loading. Both in intact epithelia and in basolateral membrane vesicles the maximal number of specific ouabain binding sites was higher in preparations from animals transporting Na⁺ at high rates than in preparations from animals transporting Na⁺ at low rates. The affinity of ouabain to its binding site and the association and dissociation rate constants were not dependent on the rate of Na⁺ transport. In intact epithelia the Na⁺ turnover rate per pump unit was twice as high in tissues with high Na⁺ transport than in tissues with low Na⁺ transport. In basolateral membrane vesicles the Na⁺ turnover rate was considerably higher than in intact epithelia and there was no difference in turnover rate between vesicle preparations obtained from tissues transporting Na⁺ at high or low rates. Hence, factors within the intact cell appear to control the turnover rate of the Na⁺-pump.