Reducing Ryanodine Receptor Open Probability as a Means to Abolish Spontaneous Ca 2+ Release and Increase Ca 2+ Transient Amplitude in Adult Ventricular Myocytes

Abstract

The aim of this work was to investigate whether it is possible to remove arrhythmogenic Ca 2+ release from the sarcoplasmic reticulum that occurs in calcium overload without compromising normal systolic release. Exposure of rat ventricular myocytes to isoproterenol (1 μmol/L) resulted in an increased amplitude of the systolic Ca 2+ transient and the appearance of waves of diastolic Ca 2+ release. Application of tetracaine (25 to 50 μmol/L) decreased the frequency or abolished the diastolic Ca 2+ release. This was accompanied by an increase in the amplitude of the systolic Ca 2+ transient. Cellular Ca 2+ flux balance was investigated by integrating Ca 2+ entry (on the L-type Ca 2+ current) and efflux (on Na–Ca 2+ exchange). Isoproterenol increased Ca 2+ influx but failed to increase Ca 2+ efflux during systole (because of the abbreviation of the duration of the Ca 2+ transient). To match this increased influx the bulk of Ca 2+ efflux occurred via Na–Ca 2+ exchange during a diastolic Ca 2+ wave. Subsequent application of tetracaine increased systolic Ca 2+ efflux and abolished the diastolic efflux. The increase of systolic efflux in tetracaine resulted from both increased amplitude and duration of the systolic Ca 2+ transient. In the presence of isoproterenol, those Ca 2+ transients preceded by diastolic release were smaller than those where no diastolic release had occurred. When tetracaine was added, the amplitude of the Ca 2+ transient was similar to those in isoproterenol with no diastolic release and larger than those preceded by diastolic release. We conclude that tetracaine increases the amplitude of the systolic Ca 2+ transient by removing the inhibitory effect of diastolic Ca 2+ release.