Mitochondrial release of Ca2+ during sustained nerve activity in the electric organ of Torpedo marmorata.

The calcium content of mitochondria isolated from the Torpedo electric organ was determined by atomic absorption spectroscopy. The mitochondrial fraction was found to contain 830 ng-atom Ca/mg protein. For comparison a fraction of isolated nerve endings contained 250 ng-atom Ca/mg protein. When tissue was continuously stimulated at a frequency of 5 Hz via the electromotor nerves, the Ca content of the mitochondrial fraction which is likely to contain both pre- and postsynaptic mitochondria was reduced proportionally to the decrease in transmitter content of the isolated nerve terminal fraction. After 5,000 pulses, the mitochondrial fraction had lost 80% of its Ca contents. When tissue was stimulated after previous perfusion with d-tubocurarine and blockade of electrical response the same number of pulses caused only a loss of 20--40% in the Ca content of the mitochondrial fraction. Release of transmitter was not affected by d-tubocurarine. The results suggest that repetitive stimulation in the absence of postsynaptic receptor blockade causes loss of Ca from both nerve terminal mitochondria and mitochondria of the myotube-derived electrocytes. In the presence of d-tubocurarine Ca is lost from presynaptic mitochondria only.