ATP-dependent effects of halothane on SR Ca2+ regulation in permeabilized atrial myocytes

Previous work suggests that modification of sarcoplasmic reticulum (SR) function may contribute to the cardioprotective effect of halothane during ischaemia and reperfusion. The aim of this study was to investigate the effects of halothane on spontaneous Ca(2+) release from the sarcoplasmic reticulum (Ca(2+) sparks and waves).

Rat atrial myocytes were permeabilized with saponin and perfused with solutions approximating to the intracellular milieu and containing fluo-3. SR Ca(2+) release was detected using confocal microscopy.

In the presence of 5 mM ATP, halothane (0.25-2 mM) had no significant effect on the amplitude or frequency of spontaneous Ca(2+) waves. However, in the presence of 0.05 mM ATP, halothane (0.25-2 mM) induced a concentration-dependent decrease in the amplitude and an increase in the frequency of spontaneous Ca(2+) waves, e.g., 1 mM halothane decreased the amplitude by 34.7+/-3.5% (n=9) and increased the frequency by 67+/-19.9% (n=7). In the presence of 5 mM ATP, 1 mM halothane had no significant effect on the amplitude or frequency of Ca(2+) sparks. When [ATP] was reduced to 0.05 mM, Ca(2+) spark frequency decreased by 67.9+/-14% and the amplitude increased by 27.5+/-4.9% (n=13). Subsequent introduction of halothane (0.5-1 mM) induced a transient burst of Ca(2+) sparks, consistent with ryanodine receptor (RyR) activation. Further experiments showed that the decrease in Ca(2+) spark frequency following ATP depletion was associated with a progressive increase in the SR Ca(2+) content over 1-2 min. This rise in SR Ca(2+) content did not occur when 1 mM halothane was present during ATP depletion.

These data suggest that the sensitivity of the RyR to activation by halothane increases at low [ATP]. In metabolically impaired cells, halothane would be expected to lessen any rise in SR Ca(2+) content and to reduce the amplitude of spontaneous Ca(2+) release. These effects of halothane are considered in relation to the events that occur during ischaemia and reperfusion.