Failure of the baroreceptor reflex mechanisms has been proposed as a basis for the early hypotension seen in monkeys after high-dose, whole-body irradiation. The present work involved the testing of baroreflex sensitivity by carotid occlusion and phenylephrine injection before and after 1000 rads60 Co in nine unanesthetized, restrained monkeys. During the early postradiation minutes, at the time of deepest hypotension, both of the baroreflex tests revealed depressed baroreflex sensitivity (diminished blood pressure and heart rate responses). After 8-15 min postirradiation, the phenylephrine, but not the occlusion, test demonstrated a reversal to significant baroreflex hypersensitivity which persisted 24 hr or more. Early shifts in the set level relation between blood pressure and heart rate also occurred after irradiation. No failure of the baroreflex mechanisms was evident, in contradiction to a previous finding.

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