Reducing ryanodine receptor open probability as a means to abolish spontaneous Ca2+ release and increase Ca2+ transient amplitude in adult ventricular myocytes

The aim of this work was to investigate whether it is possible to remove arrhythmogenic Ca2+ release from the sarcoplasmic reticulum that occurs in calcium overload without compromising normal systolic release. Exposure of rat ventricular myocytes to isoproterenol (1 mu mol/L) resulted in an increased amplitude of the systolic Ca2+ transient and the appearance of waves of diastolic Ca2+ release. Application of tetracaine ( 25 to 50 mu mol/L) decreased the frequency or abolished the diastolic Ca2+ release. This was accompanied by an increase in the amplitude of the systolic Ca2+ transient. Cellular Ca2+ flux balance was investigated by integrating Ca2+ entry ( on the L-type Ca2+ current) and efflux ( on Na-Ca2+ exchange). Isoproterenol increased Ca2+ influx but failed to increase Ca2+ efflux during systole ( because of the abbreviation of the duration of the Ca2+ transient). To match this increased influx the bulk of Ca2+ efflux occurred via Na - Ca2+ exchange during a diastolic Ca2+ wave. Subsequent application of tetracaine increased systolic Ca2+ efflux and abolished the diastolic efflux. The increase of systolic efflux in tetracaine resulted from both increased amplitude and duration of the systolic Ca2+ transient. In the presence of isoproterenol, those Ca2+ transients preceded by diastolic release were smaller than those where no diastolic release had occurred. When tetracaine was added, the amplitude of the Ca2+ 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 Ca2+ transient by removing the inhibitory effect of diastolic Ca2+ release.