Increasing ryanodine receptor open probability alone does not produce arrhythmogenic calcium waves - Threshold sarcoplasmic reticulum calcium content is required

Diastolic waves of Ca2+ release have been shown to activate delayed afterdepolarizations as well as some cardiac arrhythmias. The aim of this study was to investigate whether increasing ryanodine receptor open probability alone or in the presence of beta-adrenergic stimulation produces diastolic Ca release from the sarcoplasmic reticulum (SR). When voltage-clamped rat ventricular myocytes were exposed to caffeine (0.5 to 1.0 mmol), diastolic Ca2+ release was seen to accompany the first few stimuli but was never observed in the steady state. We attribute the initial phase of diastolic Ca2+ release to a decrease in the threshold SR Ca2+ content required to activate Ca2+ waves and its subsequent disappearance to a decrease of SR content below this threshold. Application of isoproterenol (1 mu mol/L) increased the amplitude of the systolic Ca2+ transient and also the SR Ca2+ content but did not usually produce diastolic Ca2+ release. Subsequent addition of caffeine, however, resulted in diastolic Ca2+ release. We estimated the time course of recovery of SR Ca2+ content following recovery from emptying with a high (10 mmol/L) concentration of caffeine. Diastolic Ca2+ release recommenced only when SR content had increased back to its final level. We conclude that increasing ryanodine receptor open probability alone does not produce arrhythmogenic diastolic Ca2+ release because of the accompanying decrease of SR Ca2+ content. beta-Adrenergic stimulation increases SR content and thereby allows the increased ryanodine receptor open probability to produce diastolic Ca2+ release. The implications of these results for arrhythmias associated with abnormal ryanodine receptors are discussed.