Stimulation of mitochondrial pyruvate metabolism and citrulline synthesis by dexamethasone. Effect of isolation and incubation media.

Hepatic mitochondria isolated in 0.3 M-sucrose or 0.3 M-mannitol from rats treated for 3h with dexamethasone displayed stimulated rates of pyruvate carboxylation and decarboxylation and citrulline synthesis when compared with organelles from control animals. Mitochondria isolated in mannitol also displayed elevated rates of pyruvate carboxylation and decarboxylation when compared with those isolated in sucrose, and this stimulation was shown to be independent of the lengthy isolation procedure. Citrulline synthesis proceeded at similar rates in mitochondria isolated in either sugar. The concentration of exchangeable adenine nucleotides was identical in mitochondria isolated in sucrose or mannitol, suggesting that those prepared in the former sugar are not more permeable to metabolites than those prepared in the latter. The matrix volume of mitochondria isolated in mannitol was greater than that of mitochondria isolated in sucrose, and the effect of mannitol on pyruvate metabolism was mimicked by swelling the organelles in hypo-osmotic sucrose. Measurements of the extra-matrix volume by using [14C]sucrose or [14C]mannitol suggest that mannitol can permeate mitochondria to a greater extent than can sucrose. The possibility that mannitol elicits its effect by entering the mitochondrial matrix and so initiating swelling is discussed.