Metabolic Inhibition Alters Subcellular Calcium Release Patterns in Rat Ventricular Myocytes

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Abstract
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 2+ 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 2+ buffer EGTA (5 mmol/L) and the fluorescent Ca 2+ -indicator fluo-3 (1 mmol/L), depolarization from −40 to 0 mV elicited a striped pattern of Ca 2+ release. This pattern represents the simultaneous activation of multiple Ca 2+ 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 2+ current ( I Ca ), the spatially averaged Ca 2+ transient, and E-C coupling gain, but no reduction in sarcoplasmic reticulum Ca 2+ content. The striped pattern of subcellular Ca 2+ release became fractured, or disappeared altogether, corresponding to a marked decrease in the area of the cell exhibiting organized Ca 2+ release. There was no significant change in the intensity or kinetics of local Ca 2+ release. The mechanism is not fully explained by dephosphorylation of L-type Ca 2+ channels, because a similar degree of I Ca “rundown” in control cells did NOT result in fracturing of the Ca 2+ release pattern. We conclude that metabolic inhibition interferes with E-C coupling by (1) reducing trigger Ca 2+ , and (2) directly inhibiting sarcoplasmic reticulum Ca 2+ release site open probability.