Halothane alters control of intracellular Ca²⁺ mobilization in single rat ventricular myocytes

In an attempt to understand the cellular mechanisms underlying volatile anesthetic-induced myocardial depression, halothane-induced negative inotropy was investigated in an animal model through continuous monitoring of intracellular Ca²⁺ concentration ([Ca²⁺](i)) in rat ventricular myocytes loaded with fura-2. Single cells were stimulated with 15 mM caffeine or 50 mM extracellular K⁺ (K⁺(O)) or were paced by extracellular glass suction pipette electrode. With each stimulus modality, halothane (0.6-1.5%) caused a significant (P < 0.05) and dose-dependent depression of the Ca²⁺ transient. Caffeine and electrically stimulated Ca²⁺ transients were reduced, in 1.5% halothane, to 35 ± 14 and 42 ± 8% of control, respectively. Resting or basal [Ca²⁺](i) was unaffected by halothane. Halothane did not elicit spontaneous Ca²⁺ transients in these cells. Single cells stimulated by trains of electrical stimuli at 1.0, 1.5, and 2.0 Hz showed a change in [Ca²⁺](i) from prestimulus levels to a stimulated baseline steady state that appeared to increase with stimulus frequency. Halothane at 0.7% increased the change in resting to stimulated baseline [Ca²⁺](i) and depressed net transients (P < 0.05) at 1.0 and 1.5 Hz. In contrast, 0.1 μM ryanodine depressed the Ca²⁺ transients in myocytes stimulated by trains of stimuli, but did not potentiate the change in stimulated baseline [Ca²⁺](i) at any pacing rate. The results are consistent with the hypothesis that halothane reduces Ca²⁺(i) availability by causing a net loss of Ca²⁺ from the sarcoplasmic reticulum. The results from experiments using onset of pacing to induce a sudden increase in Ca²⁺(i) load in previously quiescent myocytes suggest that halothane may act to limit sarcoplasmic reticulum and/or sarcolemmal uptake/extrusion mechanisms, as compared to ryanodine, which depletes sarcoplasmic reticulum Ca²⁺ stores without affecting reuptake and extrusion.