Adrenergic regulation of calcium-deficient hypertension in chick embryos

Chick embryos rendered calcium deficient by long-term culture outside the eggshell develop hypertension and tachycardia (R. Tuan and H. Q. Nguyen, J. Exp. Med. 165: 1418-1423, 1987). To characterize the hypertension of cultured shell-less (SL) chick embryo, we have compared their cardiovascular response to adrenergic drugs with normal (NL) embryos at day 14 of incubation. Blood pressure and pulse rate were measured before and after administration of norepinephrine (NE), phentolamine (PA), isoproterenol (IP), and propranolol (PP). Base-line blood pressures (systolic and diastolic) were uniformly higher in SL than NL embryos. In both embryos, blood pressure was elevated by NE and PP but lowered by IP and PA; pulse rates were decreased by PP and PA and increased by NE and IP. Pharmacological sensitivity to NE and PA, based on their effective dosages and magnitude of response in blood pressure and pulse rate, was higher in SL embryos. On the other hand, the embryos did not differ significantly in their sensitivity to IP and PP. The plasma concentration of catecholamines was considerably higher in SL embryos. Although serum calcium was lower in SL embryos, myocardial calcium content was not significantly different. Thus the cardiovascular function of SL embryos is likely to be under more sensitive and potentiated alpha-adrenergic regulation, perhaps a result of different calcium handling by several tissues (e.g., myocardium). These properties of the SL chick embryo indicate that it is a novel and useful experimental system to study the relationship between calcium homeostasis and development of hypertension.