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KMID : 0811720000040030197
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Korean Journal of Physiology & Pharmacology
2000 Volume.4 No. 3 p.197 ~ p.210
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Relatoinship between Sarcoplasmic Reticular Calcium Release and Na -Ca2 Exchange in the Rat Myocardial Contraction
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Chang Mann Ko/Eun-Gi Kim
Soon Jin Kim/Chang Mann Ko
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Abstract
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Suppressive role of Na+?Ca2+ exchange in myocardial tension generation was examined in the negative frequency-force relationship (FFR) of electric field stimulated left atria (LA) from postnatal developing rat heart and in the whole-cell clamped adult rat ventricular myocytes with high concentration of intracellular Ca2+ buffer (14 mM EGTA). LA twitch amplitudes, which were suppressed by cyclopiazonic acid in a postnatal age-dependent manner, elicited frequency-dependent and postnatal age-dependent enhancements after Na+?reduced,Ca2+?depleted (26 Na-0 Ca) buffer application. These enhancements were blocked by caffeine pretreatment with postnatal age-dependent intensities. In the isolated rat ventricular myocytes, stimulation with the voltage protocol roughly mimicked action potential generated a large inward current which was partially blocked by nifedipine or Na+ current inhibition. 0 Ca application suppressed the inward current by 39±4 while the current was further suppressed after 0 Na-0 Ca application by 53±3 Caffeine increased this inward current by 44±3 in spite of 14 mM EGTA. Finally, the Na+ current-dependent fraction of the inward current was increased in a stimulation frequency-dependent manner. From these results, it is concluded that the Ca2+ exit-mode (forward-mode) Na+?Ca2+ exchange suppresses the LA tension by extruding Ca2+ out of the cell right after its release from sarcoplasmic reticulum (SR) in a frequency-dependent manner during contraction, resulting in the negative frequency-force relationship in the rat LA.
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