Cytosolic Ca2+ concentration of rat ventricular cells was measured under varying experimental conditions by using a fluorescent Ca2+
indicator,
Fura-2. Resting [Ca2+]i of rat myocyte was 150 +/- 30 nM (n = 39), and this value was compatible with others. The Perfusion of
cardioplegic solution significantly increased [Ca2+]i, and this effect was further augmented by
hypothermia (p < 0.05). Application of
nifedipine (5 x 10(-7) M) to the perfusate or pretreatment of
caffeine (10 mM) had no apparent effect on this
cardioplegia-induced [Ca2+]i change. But Ni2+ (5 mM), an antagonist of Na+/Ca2+ exchange mechanism, prevented the [Ca2+]i change during
cardioplegia (p < 0.05). Magnitude of
cardioplegia-induced [Ca2+]i increase was also dependent on the Ca2+ concentration of
cardioplegic solution. These results suggest that Na+/Ca2+ exchange may play an important role in
cardioplegia-induced [Ca2+]i change. To rule out the possibility whether the protective effect of hypothermic
cardioplegia is due to the preservation of high-energy
phosphate store or decreasing the transmembrane ionic fluxes by phase transition, we exhausted a energy store of cardiac cell by application of
2,4 dinitrophenol to the bath and measured its effect on [Ca2+]i change during
cardioplegia. Hypothermic
cardioplegia delayed the onset of [Ca2+]i increase and decreased its amplitude compared to those of normothermic
cardioplegia. From the above results, hypothermic
cardioplegia may protect the cardiac cells from ischemic insult by preserving a high-energy
phosphate store. Application of Ni2+ to the
cardioplegic solution or reduction of external Ca2+ concentration also had some protective effect, since it prevented [Ca2+]i increase during
cardioplegia.