Reflex Vasoconstrictor and Vasodilator Responses in Man

The application of ice to the forehead and the Valsalva maneuver are known to cause reflex vasoconstriction by stimulation of adrenergic pathways of the sympathetic nervous system. In the experiments reported here, the effects of these stimuli on blood flow to the forearm of man were studied after intra-arterial injection of adrenergic blocking drugs. Changes in flow were measured with water plethysmographs applied to both forearms. One forearm served as "control," and the other forearm, into which the drugs were injected, was the "experimental" one. The stimuli caused a consistent reduction of flow to the control forearm, but after the intra-arterial infusion of small doses of guanethidine there was increased flow to the experimental arm simultaneous with decreased flow to the control arm. The increased flow in response to the stimuli was accentuated after phentolamine and reduced after atropine. The results indicate that the application of ice to the forehead and the Valsalva maneuver stimulate adrenergic vasoconstrictor pathways and cholinergic vasodilator pathways simultaneously. The vasoconstrictor response predominates, and the vasodilator effect is unmasked only after adrenergic blockade. It appears, therefore, that mental stresses are not the only stimuli which can cause reflex cholinergic vasodilatation in the forearm of man. Work by others would suggest that leg exercise may also cause cholinergic dilatation after adrenolytic agents. It is possible that cholinergic vasodilatation may play a role in the severe orthostatic fall in blood pressure, as well as in the exercise hypotension, observed in patients treated with guanethidine.