On the inter-relationship between glucagon action, the oxidation-reduction state of pyridine nucleotides, and calcium retention by rat liver mitochondria.

Experiments were performed to investigate the proposition (Lehninger, A. L., Vercesi, A., and Bababunmi, E. A. (1978) Proc. Natl. Acad Sci. U. S. A. 75,1690-1694) that the oxidation-reduction state of mitochondrial pyridine nucleotides is a determinant in the ability of mitochondria to retain ca’+. Use was made of the fact that glucagon administration to rats in vivo induces a stable enhancement in the ability of the mitochondria subsequently isolated from the liver to retain accumulated CaZ+ (Prpic, V., Spencer, T. L., and Bygrave, F. L. (1978) Biochen J. 176,705-714). Addition of oxalacetate to saline-challenged (control) mitochondria that already had accumulated Ca’+, led within 15 s to the complete oxidation of the mitochondrial NADH. In the same time period, only about 20% of the NADPH had become oxidized and Ca2+ was still retained by the mitochondria. Approximately 30 s after the addition of oxalacetate, the rate of NADPH oxidation slowed about 10-fold at which time the release of Ca’+ from the mitochondria began to ccur. The rate of NADH oxidation was similar in mitochondria isolated from glucagon-challenged and from saline-challenged rat liver. That of NADPH oxidation was slower in the glucagon-challenged mitochondria especially during the above-mentioned latter phase. Under identical experimental conditions, Ca’+ was retained by the mitochondria from glucagon-challenged liver but not by those from saline-challenged liver. Other experiments involving variations to either the pH of the incubation medium, or the dose of glucagon administered, provided further evidence of a correlation between the ability of mitochondria to retain Ca2+ and their resistance to oxidize NADPH. A correlation could not be detected between the ability of mitochondria to retain Ca‘+ and their ability to oxidize NADH. The data do not permit conclusions t s be drawn about the nature of the mechanisms linking the oxidationreduction state of NADP+-NADPH with that of Ca’+ retention, but an argument is put that the energylinked transhydrogenase reaction that leads to the generation of NADPH and which is stimulated in mitochondria from glucagon-challenged liver, may be involved.