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

Abstract

The effects of changes in Ca2+ concentration on myocardial contractility depressed by thiamylal and halothane were studied in isolated dog heart muscle. The concentration of Ca2+ in the bathing medium was increased stepwise from 1.9-20.9 mM. An increase in Ca2+ concentration increased both net-shortening (.DELTA.1) and maximum velocity of shortening at 0.4 g preload (V''max) which reached maximum values at 11.4 mM of Ca2+. A further increase in Ca2+ concentration decreased .DELTA.1 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 Ca2+. The depression of myocardial contractility produced by thiamylal is probably counteracted competitively by Ca2+. The curves in the presence of halothane (5.0 .+-. 0.4 mg%) were kept lower than those of the control, with peaks at 7.6 mM of Ca2+. The counteraction of the depression of myocardial contractility by Ca2+ was limited in the presence of halothane. A similar tendency was observed in maximum force of isometric contraction (Fm). In the presence of halothane, the administration of higher concentrations of Ca2+ almost always accompanied after-contraction. The mechanism for producing the depression of myocardial contractility might be different between thiamylal and halothane in relation to Ca2+ movements at the Ca2+ storage sites within the myocardium.