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

Na/K pump current was determined between -140 and +60 mV as steady-state, strophanthidin-sensitive, whole-cell current in guinea pig 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 permitted investigation of Na/K pump current-voltage (I-V) relationships at several levels of pipette Na ( Napip) in a single cell; the effects of changes in external Na ( Nao) or external K ( Ko) were also studied. At 50 mM Napip, 5.4 mM Ko, and approximately 150 mM Nao, Na/K pump current was steeply voltage dependent at negative potentials but was approximately constant at positive potentials. Under those conditions, reduction of Nao enhanced pump current at negative potentials but had little effect at positive potentials: at zero Nao, pump current was only weakly voltage dependent. At 5.4 mM Ko and approximately 150 mM Nao, reduction of Napip 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 Napip according to the Hill equation with best-fit K0.5 approximately equal to 11 mM and Hill coefficient nH approximately equal to 1.4. At zero Nao, reduction of Napip seemed to simply scale down the relatively flat pump I-V relationship: Hill fit parameters for pump activation by Napip at 0 mV were K0.5 approximately equal to 10 mM, nH approximately equal to 1.4. At 50 mM Napip and high Nao, reduction of Ko from 5.4 mM scaled down the sigmoid I-V relationship and shifted it slightly to the right: at 0 mV, K0.5 approximately equal to 1.5 mM and nH approximately equal to 1.0. At zero Nao, lowering Ko simply scaled down the flat pump I-V relationships yielding, at 0 mV, K0.5 approximately equal to 0.2 mM, nH approximately equal to 1.1. The voltage-independent activation of Na/K pump current by both intracellular Na ions and extracellular K ions, at zero Nao, suggests 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 (K0.5 approximately equal to 90 mM at -120 mV, and approximately 170 mM at -80 mV), and they compete with extracellular K ions in a seemingly voltage-independent manner.(ABSTRACT TRUNCATED AT 400 WORDS)