Depressant effects of volatile anesthetics upon rat and amphibian ventricular myocardium: insights into anesthetic mechanisms of action.

To clarify the mechanisms by which volatile anesthetics may depress myocardial contractility, the depressant effects of equivalent concentrations of isoflurane, enflurane and halothane were compared in rat and frog ventricular myocardium, preparations which differ markedly in excitation-contraction coupling. In Tyrode solution, right ventricular papillary muscles from rat exhibited very large, rapidly developing contractions after rest, with a subsequent negative force-frequency relation as the stimulation rate was increased to 0.1, 0.25, 0.5, 1, 2, and 3 Hz. The large contractions after rest and at 0.1 Hz were depressed by 0.75% halothane and 1.7% enflurane to about 60% of control, but less so by 1.3% isoflurane (approximately 0.8 MAC). Halothane at 1.5% was more depressant than 2.5% isoflurane at all stimulation rates, while 3.5% enflurane caused intermediate depression (approximately 1.6 MAC). Contractions in frog ventricular strips were studied in Ringer solution following rest and at stimulation rates of 0.1, 0.25, 0.5, and 1 Hz, in the absence and presence of equivalent anesthetic concentrations. At 0.1 to 1 Hz, isoflurane was less depressant than equivalent concentrations of halothane. Enflurane (1.7%) was less depressant than 0.75% halothane at 0.1 and 0.25 Hz; 3.5% enflurane was more depressant than 2.5% isoflurane at 1 Hz. Anesthetic effects on sustained contractures were also studied in frog ventricular strips that were superfused for 4-5 min with 40, 60, 80, and 100 mM K Ringer solution. Contractures induced by 80 and 100 mM K solution were depressed more by halothane (to 60% of control) than by isoflurane or enflurane (approximately 85% of control). However, only enflurane depressed the contractions at 1 Hz more than the sustained contractures in 100 mM K Ringer. The Ca2+ for activating contractions in rat ventricle is derived largely from the sarcoplasmic reticulum, the intracellular Ca2+ accumulation and release organelle. In contrast, Ca2+ for activating contractions in the frog ventricle originates primarily from the external medium. These results suggest that halothane is more potent than isoflurane in reducing the amount of Ca2+ rapidly released from the sarcoplasmic reticulum (as observed in rat), as well as in depressing entry of extracellular Ca2+ to activate myofibrils (as in frog). Enflurane appears to have intermediate potency with actions distinct from halothane and isoflurane. The greater potency of halothane may also be due in part to greater direct depression of actin-myosin ATPase.