1. Plasma noradrenaline concentrations and blood pressure were measured in 45 patients with essential hypertension and 34 matched normotensive subjects. Plasma noradrenaline was similar in both groups, but in the hypertensive patients plasma noradrenaline correlated with blood pressure. 2. The increase in forearm flow in response to an intra-arterial infusion of phentolamine was determined in 12 of the hypertensive and 14 of the normotensive subjects to assess the alpha-adrenoceptor-mediated component of vascular resistance. Although the dilator response to phentolamine was similar in both groups, in the hypertensive patients it was correlated with the control plasma noradrenaline (r = 0.83, P less than 0.01) as well as the height of mean blood pressure (r = 0.73, P less than 0.01). 3. These results suggest that in hypertensive patients plasma noradrenaline can be a marker for both sympathetic activity and the alpha-adrenoceptor-mediated component of vascular resistance.
Pithing of anaesthetized normotensive cats significantly lowered the arterial blood pressure and augmented plasma renin activity (PRA). Captopril dose-dependently diminished mean arterial blood pressure in both pithed and intact anaesthetized normotensive cats. The hypotensive effectiveness of captopril was most pronounced in pithed cats. Captopril inhibited the hypertensive response to intravenously administered noradrenaline in pithed cats, but did not attenuate the hypertensive response to noradrenaline in intact cats. The results do not exclude that the hypotensive activity of captopril in intact cats may be causally independent of the attenuating effect of converting enzyme inhibition on the postjunctional alpha-adrenoceptor mediated vasoconstriction as observed in pithed animals.
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