Comparison of Ca2+, Sr2+, and Mn2+ fluxes in mitochondria of the perfused rat heart.

The amount of readily exchangeable Ca2+ in mitochondria of an isolated working rat heart is less than 10 ng-ions/g heart. We therefore conclude that either no Ca2+ enters mitochondria or that the Ca+ which does enter is removed continuously. Using Sr2+ and Mn2+, we obtained evidence that the mitochondrial Na+-Ca2+ exchanger was indeed operational in releasing metal from mitochondria of the heart. When Ca2+ in the perfusate was replaced by Sr2+, we found that a significant amount of Sr2+ (approximately 100 ng-ions/g heart) entered mitochondria. When the heart then was returned to a Ca2+-containing perfusate, over 80% of the Sr2+ was washed out of mitochondria within 30 seconds. When low levels of Mn2+ were added to the perfusate, we found that Mn2+ accumulated in mitochondria irreversibly. This is evidence for the operation of the Na+-Ca2+ exchanger because Na+ was found to release Ca2+ and Sr2+ but not Mn2+ from isolated rat heart mitochondria. Our estimates indicate that when the Na+-Ca2+ exchanger is maximally operative, as in the Sr2+-perfused heart, the flux of Sr2+ through mitochondria is at most 10% of the total flux needed for the activation of contraction. The low level of Ca2+ in the mitochondria of Ca2+-perfused hearts suggests a much smaller flux of Ca2+ through the mitochondria in this case. We therefore conclude that mitochondria play little if any role in the beat-to-beat regulation of normal Ca2+ fluxes in the rat heart.