Effects of Guanidine Derivatives on Mitochondrial Function: III. THE MECHANISM OF PHENETHYLBIGUANIDE ACCUMULATION AND ITS RELATIONSHIP TO IN VITRO RESPIRATORY INHIBITION

Abstract

Steady-state concentrations of citric acid cycle compounds accumulating during state 3 oxidation of pyruvate by guinea pig heart mitochondria have been measured using isotopic and fluorometric techniques; incubations partially inhibited with several guanidine derivatives and other inhibitors were compared with controls. The changes in levels of intermediates which occurred with guanidine derivatives were quantitatively and qualitatively identical to those with amytal; this pattern of intermediates was therefore not limited to those compounds which possess hypoglycemic propertiesin vitro. With antimycin, rotenone and nigericin the pattern of intermediates was specific for each agent, and differed from that with guanidine derivatives and amytal. Reduced pyridine nucleotides were also estimated at progressively increasing degrees of respiratory inhibition by these same agents. Lower concentrations of phenethylbiguanide and amytal produced identical increases in state 3 level of reduced pyridine nucleotide, while higher phenethylbiguanide concentrations were associated with a phosphatedependent decrease in reduced pyridine nucleotide level in both state 3 and state 4 which was not observed with amytal, and not accompanied by a stimulation of respiratory rate. These changes resemble the metabolic condition termed state 6, and are consistent with a calciumlike activity of guanidine derivatives. Changes in level of reduced pyridine nucleotide were observed which were specific to rotenone and nigericin; these changes, combined with the patterns of citric acid cycle intermediates observed with these inhibitors, are useful in interpreting the effects of these agents on the functional state of intact mitochondria.