Modification by antihypertensive drugs of reflex circulatory responses induced by vertical tilting or bilateral carotid occlusion

Bilateral carotid artery occlusion (BCO) caused a significant increase in blood pressure and heart rate in Dialurethane anesthetized dogs, while vertical tilting caused an increase in heart rate with no significant change in blood pressure. Mecamylamine, bretylium, bethanidine and dibenamine (each at 10 mg/kg, i.v.) markedly reduced the positive chronotropic effects of both BCO and vertical tilting, reduced the pressor response to BCO and resulted in an inability to maintain blood pressure when the dogs were tilted to a vertical position. Hydralazine decreased blood pressure at 0.5 and 1.0 mg/kg, i.v., but only the higher dose caused severe inhibition of the tilt reflex. Bethanidine or guanethidine given chronically produced a biphasic response to tilting, first a decrease in blood pressure then an overshoot above control levels, while responses to BCO were minimally depressed. Clonidine given either acutely or chronically had no effect on the responses to tilting but inhibited those to BCO. Reserpine in small doses given chronically also selectively depressed the carotid occlusion reflex. Oral doses of drug given chronically were selected to cause essentially equal reductions on blood pressure of renal hypertensive dogs. Thus, a high incidence of clinical orthostatic hypotension appears associated with those antihypertensive agents which depress reflex changes induced by both procedures, whereas unimportant postural circulatory changes may be anticipated for drugs which selectively depress the carotid occlusion reflex.