Influence of KATP channel openers on the permeability of cardiac mitochondrial membranes for potassium ions, protons and ADP during mild acidosis

Abstract

The inhibition of ATP-ase or the impairment of ADP/ATP transport is suggested to be responsible for the cardioprotective effects of KATP channel openers. In this study, we investigated the ability of KATP channel openers to affect K+ and H+ flux to the matrix of rat heart mitochondria as well as permeability of mitochondrial membranes for ADP under mildly acidic (pH 6.8) conditions. K+ and H+ flux to the mitochondrial matrix was registered spectrophotometrically as the swelling of non-respiring mitochondria at 540 nm in the KNO3 and NH4NO3 medium. The apparent KM for ADP of mitochondria was estimated from the least-squares fit to the Michaelis–Menten equation. The results showed that the KATP channel openers diazoxide and pinacidil could activate potassium ion and proton flux to the mitochondrial matrix both under normal (pH 7.4) and mildly acidic (pH 6.8) conditions. Mild acidosis increased the apparent KM for ADP of mitochondria two times (up to 60.6 ± 3.4 µM) as compared to control (33.0 ± 3.2 µM). The KATP channel opener diazoxide (100 µM) increased the apparent KM for ADP by 40% under control and by 30% under mildly acidic conditions. Our results suggest that KATP channel openers could suppress the ADP/ATP exchange during ischemija and thus promote preservation of ATP in cardiomyocytes. Keywords: rat heart mitochondria, KATP channel, ADP/ATP exchange, diazoxide, pinacidil, acidosis