Effects of atropine on adrenergic responsiveness and reflex vasoconstriction in humans

To determine whether cholinergic mechanisms contribute to blood pressure regulation, we examined the effects of atropine on pressor responses to phenylephrine and vasoconstrictor responses to the cold pressor test (CPT) and lower body negative pressure (LBNP) in 16 healthy subjects. Heart rate, blood pressure and cardiac index were significantly increased, and central venous pressure (CVP) was significantly decreased after atropine (0.04mg/kg). After atropine, pressor responses to phenylephrine at doses of 20, 40 and 80 micrograms were significantly increased by 101, 112 and 93% (p less than 0.001); however, atropine attenuated pressor responses to CPT (p less than 0.001) and forearm vasoconstrictor responses to LBNP. In 6 propranolol-treated subjects, atropine also augmented pressor responses to phenylephrine but attenuated pressor responses to CPT. These results suggest that atropine augments the pressor response to an alpha-agonist, which may be partly explained by inhibition of compensatory vascular responses mediated by cholinergic mechanisms.