Effects of tyramine on blood pressure and plasma catecholamines in normal and hypertensive subjects

Responses of blood pressure and plasma catecholamines to intravenous injection of tyramine at increasing dosage (30, 45, and 60 microgram/kg, respectively) were evaluated in 25 normal subjects and 20 patients with mild essential hypertension. Basal plasma norepinephrine and epinephrine concentrations before tyramine injections were similar in the two groups. Following tyramine injection, plasma epinephrine was unchanged. Responses of plasma catecholamines and blood pressure to tyramine were similar in the two groups. Plasma norepinephrine increased significantly 2 min after a dose of 30 microgram/kg, but higher tyramine doses failed to produce a further increase in plasma norepinephrine. In contrast, pressor responses to tyramine were dose-dependent. Maximal pressor responses were observed within 23 min after injection. These findings reveal a dissociation between changes in blood pressure and plasma norepinephrine following injection of tyramine. Lack of steady state may limit the value of tyramine bolus injections as a tool for the quantitation of pressor responsiveness to variations in endogenous sympathetic output. Alternatively, it is possible that the pressor effect of tyramine may be mediated at least in part by a norepinephrine independent mechanism.