Early kinetics of glucagon action in isolated hepatocytes at the mitochondrial level

The temporal relationship between the effect of glucagon on respiratory functions and the changes in metabolites related to gluconeogenesis has been studied. Mitochondria prepared from hepatocytes after incubation with glucagon for 1 min already displayed a maximal stimulation of state-3 respiration. The increase in succinate dehydrogenase activity was almost fully expressed 3 min after glucagon. With respect to the utilization of pyruvate, 2-oxoglutarate and glutamate, glucagon produced a significant effect within 1 min. The rate of this decrease was linear for about 3 min slowing down thereafter. The stimulation of glucose production from lactate became significant within 1 min and remained constant up to 15 min. The influence of glucagon on the mitochondrial redox state also was an early event. It was maximally shifted to the more reduced state within 2 min and declined within 15 min. Under the conditions employed no effect of glucagon on urea synthesis or branched-chain amino acid release up to 15 min incubation time was discernible. Glucagon influenced the respiratory parameters virtually independent of Ca2+, in contrast to its action on intermediary metabolism. As to the hormone specificity, no enhancement of state-3 respiration and succinate dehydrogenase activity was caused by phenylephrine or isoproterenol. From the time course studies presented, it appears that the mitochondrial effects of glucagon might be causally interrelated with the regulation of gluconeogenesis. Moreover, our results indicate that the stimulation of state-3 respiration represents the earliest, specific action of glucagon at the mitochondrial level.