Effects of diazepam on the isolated chick embryo heart.

Diazepam decreased the rate and amplitude of contraction in isolated embryonic chick hearts in a dose-dependent manner in both the noninnervated hearts obtained from 4-day-old embryos and the innervated hearts from 7-day-old embryos. The concentration of diazepam necessary to reduce the heart rate and contractile amplitude to 50% of the control values was about 1 X 10(-4) M. Concentrations less than 1.0 X 10(-5) M had no detectable depressant effects. Prior administration of atropine did not alter the depression induced by diazepam. Norepinephrine was able to stimulate the amplitude of contraction in the diazepam-depressed heart while atropine was without effect. The vehicle used in the clinical injectable preparation of diazepam had no depressant effects. The mechanism of action of the diazepam-induced depression on the isolated embryonic chick heart may be a direct depression of the myocardium.