Functional coupling between glycolysis and excitation-contraction couplingunderlies alternans in cat heart cells

1. Electromechanical alternans was characterized in single cat atrial and v entricular myocytes by simultaneous measurements of action potentials, memb rane current, cell shortening and changes in intracellular Ca2+ concentrati on ([Ca2+](1)). 2. Using laser scanning confocal fluorescence microscopy, alternans of elec trically evoked [Ca2+](1) transients revealed marked differences between at rial and ventricular myocytes. In ventricular myocytes, electrically evoked [Ca2+](1) transients during alternans were spatially homogeneous. In atria l cells Ca2+ release started at subsarcolemmal peripheral regions and subse quently spread toward the centre of the myocyte, In contrast to ventricular myocytes, in atrial cells propagation of Ca2+ release from the sarcoplasmi c reticulum (SR) during the small-amplitude [Ca2+](1) transient was incompl ete, leading to failures of excitation-contraction (EC) coupling in central regions of the cell. 3. The mechanism underlying alternans was explored by evaluating the trigge r signal for SR Ca2+ release (voltage-gated L-type Ca2+ current, I-Ca,I-L) and SR Ca2+ load during alternans. Voltage-clamp experiments revealed that peak I-Ca,I-L was not affected during alternans when measured simultaneousl y with changes of eel shortening. The SR Ca2+ content, evaluated by applica tion of caffeine pulses, was identical following the small-amplitude and th e large amplitude [Ca2+](1) transient. These results suggest that the prima ry mechanism responsible for cardiac alternans does not reside in the trigg er signal for Ca2+ release and SR Ca2+ load. 4. beta-Adrenergic stimulation with isoproterenol (isoprenaline) reversed e lectromechanical alternans, suggesting that under conditions of positive ca rdiac inotropy and enhanced efficiency of EC coupling alternans is less lik ely to occur. The occurrence of electromechanical alternans could be elicited by impairme nt of glycolysis. Inhibition of glycolytic flux by application of pyruvate, iodoacetate or beta-hydroxybutyrate induced electromechanical and [Ca2+](1 ) transient alternans in both atrial and ventricular myocytes. 6. The data support the conclusion that in cardiac myocytes alternans is th e result of periodic alterations in the gain of EC coupling, i.e. the effic acy of a given trigger signal to release Ca2+ from the SR. It is suggested that the efficiency of EC coupling is locally controlled in the microenviro nment of the SR Ca2+ release sites by mechanisms utilizing ATP, produced by glycolytic enzymes closely associated with the release channel.