Stimulation of pyruvate transport in metabolizing mitochondria through changes in the transmembrane pH gradient induced by glucagon treatment of rats

Abstract

Glucagon treatment of rats allowed the isolation of liver mitochondria with enhanced rates of pyruvate metabolism measured in either sucrose or KCl media. No change in the activity of the pyruvate carrier itself was apparent, but under metabolizing conditions, use of the inhibitor of pyruvate transport, alpha-cyano-4-hydroxycinnamate, demonstrated that pyruvate transport limited the rate of pyruvate metabolism. The maximum rate of transport under metabolizing conditions was enhanced by glucagon treatment. Problems involved in measuring the transmembrane pH gradient under metabolizing conditions are discussed and a variety of techniques are used to estimate the matrix pH. From the distribution of methylamine, ammonia and D-lactate and the Ki for inhibition by alpha-cyano-4-hydroxycinnamate it is concluded that the matrix is more acid than the medium and that the pH of the matrix rises after glucagon treatment. The increase in matrix pH stimulates pyruvate transport. The membrane potential, ATP concentration and O2 uptake were also increased under metabolizing conditions in glucagon-treated mitochondria. These changes were correlated with a stimulation of the respiratory chain which can be observed in uncoupled mitochondria [Yamazaki (1975) J. Biol. Chem. 250, 7924--7930]. The mitochondrial Mg2+ content (mean +/- S.E.M.) was increased from 38.8 +/- 1.2 (n = 26) to 47.5 +/- 2.0 (n = 26) ng-atoms/mg by glucagon and the K+ content from 126.7 +/- 10.3 (n = 19) ng-atoms/mg. This may represent a change in membrane potential induced by glucagon in vivo. The physiological significance of these results in the control of gluconeogenesis is discussed.