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The role of Na(+)‐Ca2+ exchange in activation of excitation‐contraction coupling in rat ventricular myocytes.
Author(s) -
Wasserstrom J A,
Vites A M
Publication year - 1996
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1996.sp021401
Subject(s) - nifedipine , contraction (grammar) , chemistry , biophysics , voltage clamp , myocyte , membrane potential , ryanodine receptor , endoplasmic reticulum , electrophysiology , excitation–contraction coupling , coupling (piping) , medicine , calcium , biology , materials science , biochemistry , organic chemistry , metallurgy
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.