Modulation of sarcoplasmic reticulum Ca2+ release by glycolysis in cat atrial myocytes

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Abstract
In cardiac myocytes, glycolysis and excitation–contraction (E–C) coupling are functionally coupled. We studied the effects of inhibitors (2‐deoxy‐d‐glucose (2‐DG), iodoacetate (IAA)), intermediates (glucose‐6‐phosphate (G6P), fructose‐6‐phosphate (F6P), fructose‐1,6‐bisphosphate (FBP), phosphoenolpyruvate (PEP)) and products (pyruvate, l‐lactate) of glycolysis on sarcoplasmic reticulum (SR) Ca2+ release and uptake in intact and permeabilized cat atrial myocytes. In field‐stimulated (0.5–0.7 Hz) intact myocytes, 2‐DG (10 mm) and IAA (1 mm) caused elevation of diastolic [Ca2+]i and [Ca2+]i transient alternans (Ca2+ alternans) followed by a decrease of the amplitude of the [Ca2+]i transient. Focal application of 2‐DG resulted in local Ca2+ alternans that was confined to the region of exposure. 2‐DG and IAA slowed the decay kinetics of the [Ca2+]i transient and delayed its recovery (positive staircase) after complete SR depletion, suggesting impaired activity of the SR Ca2+‐ATPase (SERCA). 2‐DG and IAA reduced the rate of reuptake of Ca2+ into the SR which was accompanied by a 15–20% decrease of SR Ca2+ load. Major changes of mitochondrial redox state (measured as FAD autofluorescence) were not observed after inhibition of glycolysis. Pyruvate (10 mm) and l‐lactate (10 mm) elicited similar changes of the [Ca2+]i transient. Pyruvate, l‐lactate and IAA – but not 2‐DG – induced intracellular acidosis. Recording of single channel activity of ryanodine receptors (RyRs) incorporated into lipid bilayers revealed complex modulation by glycolytic intermediates and products (1 mm each): some were without effect (G6P, PEP, l‐lactate) while others either increased (F6P, +40%; FBP, +265%) or decreased (pyruvate, −58%) the open probability of the RyR. Consistent with these findings, spontaneous SR Ca2+ release (Ca2+ sparks) in permeabilized myocytes was facilitated by FBP and inhibited by pyruvate. The results indicate that in atrial myocytes glycolysis regulates Ca2+ release from the SR by multiple mechanisms including direct modulation of RyR activity by intermediates and products of glycolysis and modulation of SERCA activity through local changes of glycolytically derived ATP.