Sensitivity of the developing chick myocardium to the positive inotropic effects of calcium and isoproterenol.

The present results show that the sensitivity of the chick myocardium to the positive inotropic effect of Ca++ decreases during development and that the Ca++ concentration of the physiological solution used must be lowered below 'normal' to study the effects of positive inotropic agents in preparations from younger embryos. Isoproterenol elicits positive inotropic responses in 7-9 day embryonic ventricle and in newborn chick atria; however, the 4-day embryonic myocardium is unresponsive to isoproterenol.