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

Abstract

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. 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. 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‐K activation. These findings revealed a dynamic interaction between I_Na‐K and I_Na‐Ca via a depletion of Na⁺ under the sarcolemma. 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⁺. 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. 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.