The role of Na$^+$-Ca$^2+$ exchange in activation of excitation-contraction coupling in rat ventricular myocytes.
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J. Physiol. (June 1996)

1. The purpose of this study was to determine whether mechanisms other than Ca$^2+$ influx via L-type Ca$^2+$ current (ICa) might contribute to activation of contraction in rat ventricular myocytes. The whole-cell voltage-clamp technique was used with normal transmembrane K$^+$ and Na$^+$ gradients at 34 degrees C. The sarcoplasmic reticulum (SR) was conditioned with one to three prepulses to +100 mV for 100 ms. 2. Cell shortening (delta L) increased with test voltage up to a plateau level at about +20 mV, beyond which cell shortening remained fairly constant, thus describing a sigmoidal voltage dependence. This relationship was obtained when holding potential (Vh) was either -40 or -70 mV; however, greater shortening was obtained at the more negative Vh. 3. The sigmoidal V-delta L relationship was converted to a bell shape following the magnitude of ICa when internal Cs+ was substituted for K$^+$ and when the temperature was reduced to 22 degrees C. 4. At 34 degrees C, block of ICa with nifedipine (10 microM) decreased shortening by about 50\% but did not alter the voltage dependence of delta L when Vh was either -40 or -70 mV. Addition of Ni2+ (4-5 mM) blocked all remaining contractions. 5. When cell shortening was triggered by an action potential voltage clamp, there was again about 50\% of the contraction that was insensitive to nifedipine but was blocked by Ni2+. 6. Our results demonstrate that there is a significant contribution of a nifedipine-insensitive mechanism to the activation of contraction. This mechanism is likely to be reverse mode Na$^+$-Ca$^2+$ exchange since it appears to be sensitive to both voltage and Ni2+. We conclude that a contribution of reverse Na$^+$-Ca$^2+$ exchange to activation of excitation-contraction coupling occurs in rat heart at near-physiological conditions which include warm temperatures, normal transmembrane Na$^+$ and K$^+$ gradients and activation in response to an action potential.
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