EFFECT OF NA-PIP ON K-O ACTIVATION OF THE NA-K PUMP IN ADULT-RAT CARDIAC MYOCYTES

To determine if environmental factors influence the external K (K-o) dependence of Na-K pump current (I-p), we systematically varied internal (pipette) Na (Na-pip) and Na-K pump activity while measuring the K-o dependence in adult rat cardiac myocytes. For each Na-pip, reactivation of I-p by K-o was dose dependent. The maximal I-p (I-pmax) and apparent affinity for K-0 binding to the Na-K pump (K-0.5) increased as Na-pip increased. The results of making an equimolar substitution of tetramethylammonium for K and Cs, and partial I-p inhibition with ouabain, also showed that I-pmax and K-0.5 increased as Na-pip increased. We simulated pump activity as a function of intracellular Na (Na-i) and K-o using a cyclic model of the Na-K pump and found that the model predicts K-0.5 for K-o binding increases as Na increases, even when the conditions are adjusted by removing pipette K and partial pump inhibition with ouabain.