[Na] and [K] dependence of the Na/K pump current-voltage relationship in guinea pig ventricular myocytes.

Abstract

Na/K pump current was determined between -140 and +60 mV as steady-state, strophanthidin-sensitive, whole-cell current in guinea pit ventricular myocytes, voltage-clamped and internally dialyzed via wide-tipped pipettes. Solutions were designed to minimize all other components of membrane current. A device for exchanging the solution inside the pipette permited investigation of Na/K pump current-voltage (I-V) relationships at several levels of pipette [Na] ([Na]pip) in a single cell; the effects of changes in external [Na] ([NA]o) or external [K] ([K]O) were also studied. At 50 mM [Na]pip, 5,4, mM [K]o and .apprx. 150 mM [Na]o, Na/K pump current was steeply voltage dependent at negative potentials but was approximately consistant at positive potentials. Under those conditions, reduction of [Na]o enhanced pump current at negative potentials but had little effect at positive potentials: at zero [Na]o, pump current was only weakly voltage dependent. At 5,4 mM [K]o and .apprx. 150 mM [Na]o, reduction of [Na]pip from 50 mM scaled down the sigmoid pump I-V relationship and shifted it slightly to the right (toward more positive potentials). Pump current at 0 mV was activated by [Na]pip according to the Hill equation with best-fit K0.5 .simeq. 11 mM and Hill coefficient nH .simeq. 1.4. At zero [Na]o, reduction of [Na]pip seemed to simply scale down the relatively flat pump I-V relationship: Hill fit parameters for pump activation by [Na]pip at 0 mV were K0.5 .simeq. 10 mM, NH .simeq. 1.4. At 50 mM [Na]pip and high [Na]o, reduction of [K]o from 5.4 mM scaled down the sigmoid I-V relationship and shifted it slightly to the right: at 0 mV, K0.5 .simeq. 1.5 mM and nH .simeq. 1.0. At zero [Na]o, lowering [K]o simply scaled down the flat pump I-V relationships yielding, at 0 mV, K0.5 .simeq. 0.2 mM, nH .simeq. 1.1. The voltage-independent activation of Na/K pump current by both intracellular Na ions and extracellular K ions, at zero [N]o, suggesting that neither ion binds within the membrane field. Extracellular Na ions, however, seem to have both a voltage-dependent and a voltage-independent influence on the Na/K pump: they inhibit outward Na/K pump current in a strongly voltage-dependent fashion, with higher apparent affinity at more negative potentials (K0 1.15 .simeq. 90 mM at -120 mV, and .apprx. 170 nM at -80mV), and they compete with extracellular K ions in a seemingly voltage-independent manner. Possibly, Na ions are released from the Na/K pump to the exterior in two stages, one involving charge translocation and the second involving a subsequent voltage-insensitive competition with external k ions.