The role of Na(+)-Ca2+ exchange in activation of excitation-contraction coupling in rat ventricular myocytes.
1. The purpose of this study was to determine whether mechanisms other than Ca2+ influx via L-type Ca2+ 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(+)-Ca2+ exchange since it appears to be sensitive to both voltage and Ni2+. We conclude that a contribution of reverse Na(+)-Ca2+ 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.
Wasserstrom
J A
JA
Department of Medicine, Northwestern University Medical School, Chicago, IL 60611, USA. J. A. Wasserstrom: ja-wasserstrom@nwu.edu
Vites
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D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D004594:Electrophysiology; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology / Q000378:metabolism; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism / Q000502:physiology*; D013696:Temperature; D016276:Ventricular Function
D002121:Calcium Channel Blockers; D002586:Cesium; D012964:Sodium; D011188:Potassium; D002118:Calcium