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KCI등재 학술저널

Relatoinship between Sarcoplasmic Reticular Calcium Release and Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> Exchange in the Rat Myocardial Contraction

Relatoinship between Sarcoplasmic Reticular Calcium Release and Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> Exchange in the Rat Myocardial Contraction

<P> Suppressive role of Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> 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 Ca<SUP>2&#8290;</SUP> 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<SUP>&#8290;</SUP>-reduced, Ca<SUP>2&#8290;</SUP>-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<SUP>&#8290;</SUP> current inhibition. 0 Ca application suppressed the inward current by 39&#8273;4% while the current was further suppressed after 0 Na-0 Ca application by 53&#8273;3%. Caffeine increased this inward current by 44&#8273;3% in spite of 14 mM EGTA. Finally, the Na<SUP>&#8290;</SUP> current-dependent fraction of the inward current was increased in a stimulation frequency-dependent manner. From these results, it is concluded that the Ca<SUP>2&#8290;</SUP> exit-mode (forward-mode) Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> exchange suppresses the LA tension by extruding Ca<SUP>2&#8290;</SUP> 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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