The effect of isoproterenol on cardiovascular function in the stage 24 chick embryo

The developing cardiovascular system of the chick embryo is susceptible to teratogenic effects of catecholamines. Yet the mechanism for the teratogenetic action is unclear. Since catecholamines affect cardiovascular physiology, we studied the acute effect of the beta-agonist isoproterenol on mean atrial pressure, heart rate, mean dorsal aortic blood flow, mean arterial pressure and vascular resistance in stage 24 chick embryos. Dorsal aortic blood velocity was measured with a 20-MHz pulsed-Doppler velocity meter and intravascular pressure was measured with a servo-null pressure system. Isoproterenol in doses of 2 X 10(-4) micrograms (2.5 micrograms/kg), 8 X 10(-4) micrograms (10 micrograms/kg), and 1.2 X 10(-3) micrograms (15 micrograms/kg) was injected intravenously in 5-microliters aliquots of chick Ringer's solution. Additional groups of embryos were treated with the beta-antagonist propranolol, and isoproterenol plus propranolol. Control embryos received 5 microliters chick Ringer's solution to assess the hemodynamic effects of a volume injection. We found that isoproterenol caused no change in mean atrial pressure, heart rate, or mean arterial pressure. However, isoproterenol caused a dose-related decrease in dorsal aortic blood flow and a 2.5-fold increase in vascular resistance. The effects of isoproterenol were blocked by propranolol, which suggested that the increase in vascular resistance was mediated by beta-receptor stimulation.