Interaction of the Na+-K+ pump and Na+-Ca2+ exchange via [Na+](i) in a restricted space of guinea-pig ventricular cells

Abstract

1. The whole-cell Na+-K+ pump current (I(Na-K)) and Na+-Ca2+ exchange current (I(Na-Ca)) were recorded in guinea-pig ventricular myocytes to study the interaction between the two Na+ transport mechanisms. 2. I(Na-K) was isolated as an external K+-induced current, and I(Na-Ca) as an external Ca2+-induced or Ni2+-sensitive current. The experimental protocol used for one ion carrier did not affect the other. 3. The amplitude of I(Na-K) decreased to 54 ± 17% of the initial peak during continuous application of K+ with 20 mM Na+ in the pipette. The outward I(Na-Ca), which was intermittently activated by brief applications of Ca2+, decreased during activation of I(Na-K), and recovered after cessation of I(Na-Ca) via a depletion of Na+ under the sarcolemma. 4. To estimate changes in Na+ concentration ([Na+](i)) under the sarcolemma, the reversal potential (V(rev)) of I(Na-Ca) was measured. Unexpectedly, V(rev) hardly changed during activation of I(Na-K). However, when I(Na-Ca) was blocked by Ni2+ at the same time that I(Na-K) was activated, V(rev) changed markedly, maximally by +100 mV, immediately after the removal of Ni2+ and K+. 5. Subsarcolemmal [Na+](i) was calculated from the V(rev) of I(Na-Ca) on the assumption that the subsarcolemmal Ca2+ concentration ([Ca2+](i)) was fixed with EGTA, and mean [Na+](i) was calculated from both the time integral of I(Na-K) and the cell volume. The subsarcolemmal [Na+](i) was about seven times greater than the mean [Na+](i). 6. The interaction between the Na+-K+ pump and Na+-Ca2+ exchange was well simulated by a diffusion model, in which Na+ diffusion was restricted to one-seventh (14%) of the total cell volume.