Abstract
In order to clarify the interaction of volatile anesthetics and extracellular calcium ion on the myocardial contraction, effects of both isoflurane (1.0%) and halothane (0.5%) on the extracellular calcium ion concentration ([Ca(2+)](O))-tension curve were studied. Increasing [Ca(2+)](O) enhanced the myocardial contraction response, and the maximal response was obtained at [Ca(2+)](O) of 3.0 mM. Halothane depressed the maximal value of the tension development in response to increasing [Ca(2+)](O), while isoflurane did not ( P < 0.01). The probit response of the developed tension to the changes in [Ca(2+)](O) indicated that isoflurane increased the median effective concentration (EC(50)) of [Ca(2+)](O) significantly from 0.484 +/- 0.051 (mean +/- SEM) to 0.870 +/- 0.056 mM ( P = 0.001), but halothane did not ( P = 0.018). Therefore, 1.0% isoflurane was concluded to move the [Ca(2+)](O)-tension curve to the right, while a downwards shift occurred with 0.5% halothane.
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