Identification of sodium‐calcium exchange current in single ventricular cells of guinea‐pig.

Abstract

1. The Na-Ca exchange current was investigated in single ventricular cells from guinea-pig hearts by combining the techniques of whole-cell voltage clamp and intracellular perfusion. 2. The membrane conductance was minimized by blocking Ca and K channels as well as the Na-K pump. Under these conditions, when Ca2+ was loaded internally by a pipette solution containing 430 nM-Ca2+, changing the Li+-rich external solution to a Na+-rich one induced a significant inward current. Applying external Na+ in the absence of internal Ca2+ did not appreciably change the current. 3. In contrast, perfusing 1 mM-external Ca2+ in the presence of internal Na+ which was loaded by a 20 mM-Na+ pipette solution, induced a marked outward current. Ca2+ superfusion in the absence of internal Na+ caused only a small current change. 4. The current-voltage relation of external-Ca2+- and external-Na+-induced current showed almost exponential voltage dependence as given by the equation i = a exp (rEF/RT), where a is a scaling factor that determines the magnitude of the current and r is a partition parameter used in the rate theory and represents the position of the energy barrier in the electrical field, which indicates the steepness of the voltage dependence of the current. E, F, R and T have their usual meanings. The value of a was 1-2 microA/microF and r about 0.35 for the Ca2+-induced outward current. At very positive or negative potentials, the current magnitude became smaller than expected from an exponential relation. 5. The current was blocked by heavy metal cations, such as La3+, Cd2+, Mn2+ and Ni2+ and partially blocked by amiloride and D600. 6. The temperature coefficient (Q10) value of the Ca2+-induced outward current was 3.6 +/- 0.4 (n = 4) at 0 mV and 4.0 +/- 0.9 at 50 mV in the range between 21 and 36 degrees C. 7. The outward current magnitude showed a sigmoidal dependence upon the external Ca2+ concentration with a half-maximum concentration, K1/2 of 1.38 mM and a Hill coefficient of 0.9 +/- 0.2 (n = 5). 8. Sr2+ could replace Ca2+ with K1/2 of 7 mM. Mg2+ and Ba2+, however, did not replace Ca2+. 9. The inward current component also showed a sigmoidal external Na+ dependence with K1/2 of 87.5 +/- 10.7 mM and a Hill coefficient of 2.9 +/- 0.4 (n = 6). 10. The reversal potential of the current was obtained near the values expected for 3 Na+:1 Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in [Ca2+]i mediated by sodium‐calcium exchange in smooth muscle cells isolated from the guinea‐pig ureter.

“…With regard to P. I. AARONSON AND C. D. BENHAM Ca2+ extrusion, however, similar (although wide) ranges of temperature dependence have been reported for the Na+-Ca2+ exchanger, and for the ATP-driven Ca2+ pump (e.g. Baker et al 1969;Larsen, Hinds & Vincenzi, 1978;Kimura et al 1987), so that the relative contributions of these two processes may not have been markedly perturbed. In spite of these potential drawbacks, however, the technique has proved useful for characterizing the properties of Na+{Ca2+ exchange in cardiac cells both when [Ca2+], was buffered (Kimura et al 1987) and when it was allowed to vary (Barcenas-Ruiz et al 1988).…”

“…Baker et al 1969;Larsen, Hinds & Vincenzi, 1978;Kimura et al 1987), so that the relative contributions of these two processes may not have been markedly perturbed. In spite of these potential drawbacks, however, the technique has proved useful for characterizing the properties of Na+{Ca2+ exchange in cardiac cells both when [Ca2+], was buffered (Kimura et al 1987) and when it was allowed to vary (Barcenas-Ruiz et al 1988). Ca2+ buffering has been compared in perfused cells and in ester-loaded cells using Fura-2, which has similar Ca2+-buffering properties to Indo-1; these studies (Gray, 1988) suggested that Ca2+ buffering was not seriously compromised by perfusion.…”

“…It has previously been reported that reduction of [Ca2+]i to extremely low levels inhibits Na+-Ca2+ exchange in squid axon and cardiac cells (DiPolo & Beauge', 1983;Kimura et al 1987). This effect might be mediated through a Ca2+-dependent intracellular second messenger system (Caroni & Carafoli, 1983), or through an as yet undefined allosteric effect.…”

“…The advantage of this approach is that it is possible to measure in great detail the kinetics of the exchange-generated current, while controlling the electrochemical gradients of Na+ and Ca2+ with some degree of confidence (Kimura, Miyamae & Noma, 1987;Lipp & Pott, 1988). Recently, Barcenas-Ruiz, Beuckelmann & Weir (1988) have used Fura-2 and the whole-cell clamp to monitor both currents and changes in the intracellular Ca21 concentration ([Ca2+]i) mediated by Na+-Ca2+ exchange in heart cells.…”

Effects of Membrane Potential on Na+ -Dependent Mg2+ Extrusion from Rat Ventricular Myocytes.

“…Membrane depolarization would not affect a 2 : 1 exchange, but should decrease the driving force for Mg 2ϩ efflux through the exchange of either 3 : 1 or 4 : 1 stoichiometries with the reversal at positive potentials; it has been shown that the inward Na ϩ -Ca 2ϩ exchange current (influx of 3 Na ϩ and efflux of 1 Ca 2ϩ ) is inhibited by depolarization [29]. The observed acceleration of ⌬[Mg 2ϩ ] i by depolarization appears to be consistent with 1 : 1 (Na ϩ : Mg 2ϩ ) exchange.…”