Changes in intracellular Ca²⁺ concentration of rabbit coronary artery smooth muscle cell during ischemic cardioplegic period

To elucidate the possibility whether an elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) in rabbit coronary artery myocytes during ischemic cardioplegic period may serve as one of the mechanisms of the "no-reflow' phenomenon or not, the changes in [Ca²⁺]_i were measured under ischemic cardioplegia conditions using a fluorescent Ca²⁺ indicator, fura 2/AM. When single cells were perfused with cardioplegic or ischemic cardioplegic solutions, [Ca²⁺]_i was significantly increased and the degree of [Ca²⁺] elevation was further augmented by the ischemic cardioplegic solution. Pretreatment of a sarcoplasmic reticulum emptying agent, 20 mM caffeine, had no effect on ischemic cardioplegia-induced [Ca²⁺]_i changes, but application of a Ca²⁺ channel blocker, 5 x 10^-1M nifedipine, or an antagonist of Na⁺/Ca²⁺ exchange, 5 mM Ni²⁺, significantly inhibited the [Ca²⁺]_i elevation, respectively. The magnitude of ischemic cardioplegia-induced [Ca²⁺]_i elevation was dependent on the Ca²⁺ concentration of perfusate in the range of 0 and 25 mM. When Ni²⁺ was added to the reperfusion solution, recovery of ischemic cardioplegia-induced [Ca²⁺]_i elevation was very rapid compared with the controls. It is concluded that ischemic cardioplegia-induced [Ca²⁺]_i elevation may serve as one of the mechanisms of the "no-reflow' phenomenon in rabbit coronary artery smooth muscle cells. We propose that Na⁺/Ca²⁺ exchange may serve as a key function in ischemic cardioplegia-induced [Ca²⁺]_i elevation.