A Study on the Excitation-Contraction Coupling of Myocardium ---Analysis on Isolated, perfused Hearts using Multi-Nuclear NMR Methods.

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This research program focused on the mechanism for the disruption of intracellular calcium homeostasis induced by ischemia and reperfusion. Intracellular ionic calcium concentration ([Ca^<2+>]_i) was measured with fluorine-19 NMR (F-NMR) coupled with a calcium indicator, 5F-BAPTA. The results obtained in this program are as follows.1)[Ca^<2+>]_i significantly increased at the end of ischemia and during the early phase of reperfusion when hearts were subjected into 30-min ischemia at30ﾟC. The increase of [Ca^<2+>]_i was attenuated by pretreatment ofCa^<2+> channel blocker, indicating that opening of Ca^<2+> channel is one of the mechanism of Ca^<2+> increase during ischemia. 2)Hearts reperfused after 15-min global ischemia at 37ﾟshowed myocardial stunning. In contrast, hearts reperfused with high-[Na]_o. solution showed significantly better recovery. Nevertheless, reperfusion with solutions in which the additional Na was substituted either by sucrose or by choline chloride did not impro … More ve functional recovery, indicating that the beneficial effects of high-[Na]_o. reperfusion are not due either to high ionic strength or to high osmolarity. These results indicate that high-[Na]_o. reperfusion protects against stunning, supporting the idea that the Na^+/Ca^+ exchange plays an important role in the mechanism of stunned myocardium. 3)Myocardial content of sugar phosphates ([SP]) and [Ca^<2+>]_i were measured to elucidate the contribution of glycolysis to intracellular Ca^<2+> homeostasis. After the depletion of glycogen, the perfusion without substrate of glycolysis showed no significant changes in end-diastolic LV pressure (EDP) and [SP]. Even after the blockage of glycolysis, omission of glucose from the perfusate increased neither [SP] nor EDP. Nevertheless, addition of glucose to perfusate consistently increased EDP with accumulation of SP. F-NMR revealed that EDP correlates significantly with [Ca^<2+>]_i. These results indicate that disruption of intracellular Ca^<2+> homeostasis is caused by the accumulation of SP, rather than the inhibition of glycolysis. Less