Do modulators of the mitochondrial K(ATP) channel change the function of mitochondria in situ?

Pharmacological opening of mitochondrial cardiac ATP-sensitive potassium (K(ATP)) channels has the chance to be a promising but still controversial cardioprotective mechanism. Physiological roles of mitochondrial K(ATP) channels in the myocardium remain unclear. We studied the effects of diazoxide, a specific opener of these channels, on the function of rat mitochondria in situ in saponin-permeabilized fibers using an ionic medium that mimics the cytosol. In the presence of NADH-producing substrates (malate + glutamate), neither 100 microm diazoxide nor 100 microm glibenclamide (a K(ATP) channel blocker) changed the mitochondrial respiration in the absence or presence of ADP. Because the K(ATP) channel function could be modified by changes in adenine nucleotide concentrations near the mitochondria, we studied the effects of diazoxide and glibenclamide on the functional activity of mitochondrial kinases. Both diazoxide and glibenclamide did not change the in situ ADP sensitivity in the presence or absence of creatine (apparent K(m) values for ADP were, respectively, 59 +/- 9 and 379 +/- 45 microm). Similarly, stimulation of the mitochondrial respiration with AMP in the presence of ATP due to adenylate kinase activity was not affected by the modulators of K(ATP) channels. However, when succinate was used as substrate, diazoxide significantly inhibited basal respiration by 22% and maximal respiration by 24%. Thus, at a cardioprotective dose, the main functional effect of diazoxide depends on respiratory substrates and seems not to be related to K(ATP) channel activity.