Changes in blood pressure and plasma catecholamines caused by tyramine and cold exposure

Tyramine may be used to stimulate release of endogenous norepinephrine. We have compared the increases in blood pressure and plasma catecholamines in normal volunteers during (a) tyramine infusions, and (b) the more physiological sympathetic stimulus of cold exposure. In a second study, the cardiac component of the pressor effect of tyramine was assessed by measuring systolic time intervals, and by infusing tyramine in beta-blocked subjects. Tyramine, 15.0 micrograms/kg/min for 30 min, elevated systolic BP from 122 +/- 11 to 149 +/- 4 mm Hg, without increasing diastolic BP or heart rate. Plasma norepinephrine rose from 0.547 +/- 0.184 to 0.836 +/- 0.096 ng/ml; plasma epinephrine was unchanged. Thirty-min exposure to 4 degrees C elevated both systolic BP (from 105 +/- 8 to 116 +/- 9 mm Hg) and diastolic BP (from 72 +/- 4 to 81 +/- 6 mm Hg). This was associated with a greater rise in plasma norepinephrine, from 0.357 +/- 0.131 to 1.143 +/- 0.393 ng/ml; plasma epinephrine was again unchanged. A single oral dose of propranolol 160 mg caused approximately a two-fold right shift in the systolic BP dose response to tyramine, and blocked the tyramine-induced shortening of the presystolic ejection period. Tyramine appears to exert its pressor effect mainly by stimulation of cardiac beta-receptors. This may account for the relatively small rise in plasma norepinephrine (relative to cold exposure) since the heart does not contribute a high proportion of circulating norepinephrine.