Metabolic Inhibition Alters Subcellular Calcium Release Patterns in Rat Ventricular Myocytes: Implications for Defective Excitation-Contraction Coupling During Cardiac Ischemia and Failure

Metabolic inhibition (MI) contributes to contractile failure during cardiac ischemia and systolic heart failure, in part due to decreased excitation-contraction (E-C) coupling gain. To investigate the underlying mechanism, we studied subcellular Ca release patterns in whole cell patch clamped rat ventricular myocytes using two-dimensional high-speed laser scanning confocal microscopy. In cells loaded with the Ca buffer EGTA (5 mmol/L) and the fluorescent Ca-indicator fluo-3 (1 mmol/L), depolarization from −40 to 0 mV elicited a striped pattern of Ca release. This pattern represents the simultaneous activation of multiple Ca release sites along transverse-tubules. During inhibition of both oxidative and glycolytic metabolism using carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP, 50 nmol/L) and 2-deoxyglucose (2-DG, 10 mmol/L), there was a decrease in inward Ca current (ICa), the spatially averaged Ca transient, and E-C coupling gain, but no reduction in sarcoplasmic reticulum Ca content. The striped pattern of subcellular Ca release became fractured, or disappeared altogether, corresponding to a marked decrease in the area of the cell exhibiting organized Ca release. There was no significant change in the intensity or kinetics of local Ca release. The mechanism is not fully explained by dephosphorylation of L-type Ca channels, because a similar degree of ICa“rundown” in control cells did NOT result in fracturing of the Ca release pattern. We conclude that metabolic inhibition interferes with E-C coupling by (1) reducing trigger Ca, and (2) directly inhibiting sarcoplasmic reticulum Ca release site open probability.