The effect of halothane on sarcoplasmic reticulum (SR) Ca²⁺ regulation was investigated in saponin-permeabilized rat atrial myocytes. Rats (250-300 g) were killed humanely (Schedule 1) and atrial myocytes were isolated by enzymatic digestion. Cells were perfused with weakly Ca²⁺-buffered solutions approximating to the intracellular milieu, and Ca²⁺ release from the SR was detected using fluo-3. Spontaneous Ca²⁺ release from the SR occurred when the bathing [Ca²⁺] was increased to ~250 nM. Spontaneous release occurs when the SR Ca²⁺ content reaches a ‘threshold’ level. As the Ca²⁺ content increases, the frequency of localised Ca²⁺ release events (Ca²⁺ sparks) rises until a propagated Ca²⁺ wave is initiated (Cheng et al. 1996). The increase in spark frequency may involve Ca²⁺ binding to a regulatory site within the SR lumen, which leads to an increase in the open probability of the ryanodine receptor (RyR).

In the presence of 5 mM ATP, 1 mM halothane had no apparent effect on the amplitude or frequency of spontaneous Ca²⁺ release (Fig. 1). As reported in ventricular myocytes (Yang & Steele, 2000), decreasing the cytosolic [ATP] to 0.05 mM markedly increased the amplitude of the spontaneous Ca²⁺ transients and reduced the frequency of release. These changes are believed to reflect a reduction in the open probability of the RyR, due to reduced occupancy of the adenine nucleotide-binding site. This allows the [Ca²⁺] within the SR to reach a higher level before propagated Ca²⁺ release occurs. In the continued presence of low [ATP], introduction of 1 mM halothane induced a marked decrease in the amplitude of the spontaneous Ca²⁺ transient (36.5 ± 4.2 %, n = 6, mean ± S.E.M.) and an increase in the release frequency (61 ± 7.6 %, n = 6, mean ± S.E.M.). This effect was reversible on removal of halothane. Further experiments showed that at 5 mM ATP, > 2 mM halothane was required to influence spontaneous Ca²⁺ release. However, in the presence of 0.05 mM ATP, halothane had a significant effect at < 0.5 mM. This is within the range (0.05-0.7 mM) recorded from arterialized blood during anaesthesia (Davies et al. 1972).

These effects of halothane are consistent with an increase in the open probability of the RyR. The apparent ATP dependence suggests that halothane may have little direct effect on SR Ca²⁺ regulation under normal conditions. However, if cells become metabolically impaired, halothane may serve to maintain the sensitivity of the RyR as the [ATP] decreases, thereby limiting the rise in SR Ca²⁺ content. This may reduce the severity of spontaneous Ca²⁺ release during ischaemia and reperfusion.

The financial support of the British Heart Foundation is acknowledged.

All procedures accord with current UK legislation.