Regulation of Mitochondrial Matrix PH and Adenosine 5'-triphosphatase Activity During Ischemia in Slow Heart-rate Hearts. Role of Pi/H+ Symport

During ischemia in so-called slow heart-rate hearts, there is a marked inhibition of the mitochondrial ATPase mediated by inhibitor protein binding to the enzyme (Rouslin, W., and Pullman, M. E. (1987) J. Mol. Cell. Cardiol. 19, 661-668). This ischemia-induced ATPase inhibition is triggered by a drop in mitochondrial matrix pH (Rouslin, W. (1987) J. Biol. Chem. 262, 3472-3476) which occurs as a result of the cell acidification which develops rapidly during the ischemic process. One effect of the ATPase inhibition is a marked slowing of the net rate of tissue ATP hydrolysis and, thus, a prolongation of cell viability during ischemia. In the present study, we demonstrate that matrix acidification in intact mitochondria from slow heart-rate hearts appears to be mediated by the Pi transporter. Pi/H+ symport appears to be the primary process which mediates matrix acidification and thus ATPase inhibition in intact slow heart-rate heart mitochondria made acidotic in vitro and, presumably, also in mitochondria in situ during the ischemic process. In contrast, intact mitochondria from a so-called fast heart-rate species, which exhibited only a low level of ischemia-induced ATPase inhibition in situ (Rouslin, W. (1987) Am. J. Physiol. 252, H622-H627), failed to exhibit a Pi- and pH-dependent mitochondrial ATPase inhibition mechanism in vitro. The Pi-dependent mitochondrial ATPase inhibition mechanism reported here for slow heart-rate hearts is consistent with a role for Pi as a coordinating signal promoting the conservation of cell ATP during myocardial ischemia.