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

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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)