(9) A COMPARISON BETWEEN THE RAPID MODE OF CALCIUM UPTAKE IN HEART MITOCHONDRIA AND LIVER MITOCHONDRIA

L. Buntinas, K.K. Gunter, G.C. Sparagna and T.E. Gunter
Dept. of Biochem. & Biophys., Univ. of Rochester, Rochester, NY 14642

Mitochondrial Ca2+ transport has recently been shown to possibly be involved in: 1) sequestration of Ca2+ from cytosolic transients so as to increase NADH levels and activate metabolic rate in a manner dependent on transient frequency,2) modulation of cytosolic Ca2+ waves, and 3) induction of apoptosis. It is important to understand how the Ca2+ transport mechanisms present within mitochondria mediate and differentiate between these functions. A recently-discovered mechanism of Ca2+ entry into mitochondria, the RaM or "rapid mode" has been shown to be very effective in sequestering Ca2+ from cytosolic Ca2+ transients. Here we compare the properties of the RaM in heart mitochondria with those previously observed in liver mitochondria.

As with liver mitochondria, heart mitochondria also show a period of rapid uptake at the beginning of a Ca2+ pulse, which is not due to binding or exchange. As was found with liver mitochondria, this uptake is inhibited by binding of Ca2+ to an external inhibition site. Unlike the RaM in liver mitochondria, release of Ca2+ from this external inhibition site does not occur within a fraction of a second but requires over a minute at low [Ca2+] for completion. Furthermore, uptake via the RaM in heart is not effected by ATP and GTP as in liver, and unlike liver is inhibited by ADP and particularly by AMP. Spermine, present endogenously in both heart and liver, is a potent activator of the RaM in liver, but is an inhibitor in heart mitochondria at a physiological concentration. Ruthenium red, which was found to completely inhibit the RaM in liver mitochondria when used at about 10 times higher concentration than necessary to inhibit the uniporter, had a differential effect on the RaM in heart mitochondria. It seemed to inhibit the RaM in some cases, but not in others. Evidence for a mode of rapid Ca2+ release following Ca2+ uptake via RaM has been observed in both liver and heart mitochondria.

Supported by AHA NY State Affiliate grant 960166.