CALCIUM-ANTAGONISTS AND BAY K8644 PROMOTE DEPOLARIZATION OF THE RAT-HEART MITOCHONDRIAL-MEMBRANE POTENTIAL - FURTHER EVIDENCE FOR A ROLE IN ALTERATION OF OXIDATIVE-METABOLISM

Studies were carried out using a tetraphenylphosphonium (TPP+)-selective electrode to monitor the effect of selected calcium (Ca2+) antagonists and the dihydropyridine Ca2+ agonist Bay K8644 on membrane potential (PSI) associated with isolated rat heart mitochondria. Verapamil and diltiazem (10-500 muM), standard Ca2+ antagonists, produced a depolarization of both liver and heart mitochondria at concentrations >150 muM. In contrast, nitrendipine (10-200 muM), a dihydropyridine compound, produced a concentration-related inhibition of PSI in mitochondria from both sources, effects which were statistically significant at concentrations >50 muM. Cinnarizine (10-100 muM) and bepridil (10-100 muM) also produced inhibition of heart PSI, these effects being particularly noted in the presence of bepridil, where depolarization of the membrane was statistically significant with only 10 muM drug. The results indicate the complexity of action of these drugs at the mitochondrial level. In general, drug actions on PSI appear to be correlated with previously reported effects on Ca2+ transportation rather than oxidative phosphorylation associated with rat heart mitochondria. The findings also illustrate that the mitochondrial actions of cardiovascular compounds may be of relevance in situ, particularly during ischaemia/reperfusion when mitochondria become loaded with Ca2+.