Effects of change in concentration of calcium on myocardial contractility depressed by thiamylal and halothane.

The effects of change in the concentration of Ca++ on myocardial contractility depressed by thiamylal and halothane were studied in isolated dog heart muscle. The concentration of Ca++ in the bathing medium was increased stepwise from 1.9 to 20.9 mM. An increase in Ca++ concentration resulted in an increase both in net-shortening (delta1) and in maximum velocity of shortening at 0.4 g preload (V'max) which reached the maximum values at 11.4mM of Ca++. A further increase in Ca++ concentration rather decreased delta1 and V'max. The concentration-response relation curves in the presence of thiamylal (3.3 mg%) shifted to the right almost parallel to the control curve, with the peaks at 15.2 mM of Ca++. The result indicates that the depression of myocardial contractility produced by thiamylal is counteracted competitively by Ca++. However, the curves in the presence of halothane (5.0 +/- 0.4 mg%) were kept lower than those of the control, with the peaks at 7.6 mM of Ca++. The result shows that the counteraction of Ca++ to the depression of myocardial contractility was limited in the presence of halothane. A similar tendency was observed in maximum force of isometric contraction (Fm). Moreover, in the presence of halothane, the administration of higher concentrations of Ca++ almost always accompanied "after-contraction". From the result obtained and the literature concerned it was suggested that the mechanism for producing the depression of myocardial contractility might be different between thiamylal and halothane in relation to Ca++ movements at the Ca++ storage sites within the myocardium.