Effect of ketamine on contractile performance of isolated frog myocardium and comparison of ketamine, thiopental and droperidol.

The influence of ketamine on the inotropic and chronotropic responsiveness of heart muscle was examined in spontaneously beating frog ventricular preparations. Ketamine produced a slight positive inotropic effect in isolated strips from the ventricle of the frog at 10(-5) M concentration. It does, however, possess negative inotropic properties at the higher doses studied (> or = 10(-4) M). Ketamine also decreased the heart rate of the ventricle at doses greater than 10(-4) M. Prior muscarinic blockade with atropine (10(-6) M) and histaminergic- H2 blockade with cimetidine (10(-5) M), did not affect the negative inotropic action of ketamine. Thiopental and droperidol also depressed the contractile performance of the frog myocardium. pD2 values of ketamine, thiopental and droperidol were found to be 3.44 +/- 0.63, 3.36 +/- 0.74, 4.18 +/- 0.45 respectively. These results suggest that the negative inotropic effects of ketamine is evident only at high concentrations and appears to be non specific.