The effect of fractions of plasma from essential hypertensive (n = 27) and normotensive subjects (n = 26) on the tension of aortic strips (n = 27) from normotensive rats was examined. The fractions obtained from hypertensive patients had been shown to increase blood pressure, when injected intravenously in a normotensive rat. In aortic strips the hypertensive fractions elicited a transient relaxation of variable amplitude and subsequently a sustained contraction. The normotensive fractions did not alter tension of the strips significantly. In Ca2+ free medium and after addition of nifedipine hypertensive plasma fractions did not induce a contraction, whereas in Na+ free medium the contraction was not abolished. It is concluded that in the fraction of hypertensive plasma containing substances with a molecular weight in the range of 1000-1500 Da a vasopressor agent with direct actions on arterial smooth muscle is present. The substance probably acts by increasing Ca2+ influx in vascular smooth muscle.
