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

1. The whole-cell Na+-K+ pump current (INa-K) and Na+-Ca2+ exchange current (INa-Ca) were recorded in guinea-pig ventricular myocytes to study the interaction between the two Na+ transport mechanisms. 2. INa-K was isolated as an external K+-induced current, and INa-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 INa-K decreased to 54 +/- 17% of the initial peak during continuous application of K+ with 20 mM Na+ in the pipette. The outward INa-Ca, which was intermittently activated by brief applications of Ca2+, decreased during activation of INa-K, and recovered after cessation of INa-K, activation. These findings revealed a dynamic interaction between INa-K and INa-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 INa-Ca was measured. Unexpectedly, V-rev hardly changed during activation of INa-K. However, when INa-Ca was blocked by Ni2+ at the same time that INa-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 INa-Ca, on the assumption that the subosarcolemmal Ca2+ concentration ([Ca(])(2+)i) was fixed with EGTA, and mean [Na+](i) was calculated from both the time integral of INa-K and the cell volume. The subsarcolemmal [Na+](i) was about seven times greater than the mean [Na+](i). 6.The interaction between 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.