Relatoinship between Sarcoplasmic Reticular Calcium Release and Na<SUP>⁢</SUP>-Ca<SUP>2⁢</SUP> Exchange in the Rat Myocardial Contraction
Relatoinship between Sarcoplasmic Reticular Calcium Release and Na<SUP>⁢</SUP>-Ca<SUP>2⁢</SUP> Exchange in the Rat Myocardial Contraction
- 대한생리학회-대한약리학회
- The Korean Journal of Physiology & Pharmacology
- 제4권 제3호
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2000.01197 - 210 (14 pages)
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<P> Suppressive role of Na<SUP>⁢</SUP>-Ca<SUP>2⁢</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⁢</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>⁢</SUP>-reduced, Ca<SUP>2⁢</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>⁢</SUP> 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<SUP>⁢</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⁢</SUP> exit-mode (forward-mode) Na<SUP>⁢</SUP>-Ca<SUP>2⁢</SUP> exchange suppresses the LA tension by extruding Ca<SUP>2⁢</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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