Cardiac Subsarcolemmal and Interfibrillar Mitochondria Display Distinct Responsiveness to Protection by Diazoxide

Objective: Cardiac subsarcolemmal (SSM) and interfibrillar (IFM) mitochondrial subpopulations possess distinct biochemical properties and differ with respect to their protein and lipid compositions, capacities for respiration and protein synthesis, and sensitivity to metabolic challenge, yet their responsiveness to mitochondrially active cardioprotective therapeutics has not been characterized. This study assessed the differential responsiveness of the two mitochondrial subpopulations to diazoxide, a cardioprotective agent targeting mitochondria. Methods: Mitochondrial subpopulations were freshly isolated from rat ventricles and their morphologies assessed by electron microscopy and enzymatic activities determined using standard biochemical protocols with a plate reader. Oxidative phosphorylation was assessed from State 3 respiration using succinate as a substrate. Calcium dynamics and the status of Ca2+-dependent mitochondrial permeability transition (MPT) pore and mitochondrial membrane potential were assessed using standard Ca2+ and TPP+ ion-selective electrodes. Results: Compared to IFM, isolated SSM exhibited a higher sensitivity to Ca2+ overload-mediated inhibition of adenosine triphosphate (ATP) synthesis with decreased ATP production (from 375 +/- 25 to 83 +/- 15 nmol ATP/min/mg protein in SSM, and from 875 +/- 39 to 583 +/- 45 nmol ATP/min/mg protein in IFM). In addition, SSM exhibited reduced Ca2+-accumulating capacity as compared to IFM (230 +/- 13 vs. 450 +/- 46 nmol Ca2+/mg protein in SSM and IFM, respectively), suggestive of increased Ca2+ sensitivity of MPT pore opening. Despite enhanced susceptibility to stress, SSM were more responsive to the protective effect of diazoxide (100 mM) against Ca2+ overload-mediated inhibition of ATP synthesis (67% vs. 2% in SSM and IFM, respectively). Conclusion: These results provide evidence for the distinct sensitivity of cardiac SSM and IFM toward Ca2+-dependent metabolic stress and the protective effect of diazoxide on mitochondrial energetics.