Sarcolemmal calcium transporters in myocardial ischemia

To determine the potential pathophysiologic role of alterations in sarcolemmal Ca2+ transport mechanisms, we investigated the effects of up to 120 min of global ischemia in the rabbit heart on the three major Ca2+ transport proteins in the sarcolemma: the Na+-Ca2+ exchanger, the ATP-dependent Ca2+ pump, and the L-type Ca2+ channel. We purified sarcolemmal vesicles from control rabbit hearts and rabbit hearts made ischaemic for 20, 30, 60, 90, and 120 min. Purification of K+-p-nitrophenylphosphatase activity was about 30-fold compared to the initial homogenate, and was the same for control and ischemic hearts. We measured the initial velocity of Na+-Ca2+ exchange and found no inhibition after 20 min of ischemia, a 22% reduction in Vmax after 30 min of ischemia, and approximately a 50% reduction in Vmax after 60, 90, and 120 min of ischemia. At no time was there any change in the Ca2+ affinity of the Na+-Ca2+ exchanger. Solubilization and reconstitution of the Na+-Ca2+ exchanger into asolectin vesicles restored the velocity to the same level as control reconstituted vesicles after 60 min of ischemia, but not after 90 or 120 min of ischemia. In contrast to Na+-Ca2+ exchange, the initial velocity of the sarcolemmal ATP-dependent Ca2+ pump was unaffected by up to 2 h of ischemia. The number of L-type Ca2+ channels, measured by nitrendipine binding, was reduced by 21% after 60 min of ischemia. Decreased Ca2+ efflux due to direct inhibition of the Na+-Ca2+ exchanger, as well as inhibition by low pH and increased intracellular Na2+ in ischemia, probably contribute to Ca2+ overload and irreversible myocyte injury. Conversely, decreased Ca2+ influx due to decreased availability of L-type Ca2+ channels, as well as decreased capacity for reversed Na+-Ca2+ exchange, could contribute to contractile dysfunction during ischemia and myocardial stunning following reperfusion.