Plasma guanosine 3',5'-monophosphate responses to methacholine and epinephrine in humans

The im injection of methacholine into healthy volunteers caused a dose-dependent increase in plasma cGMP levels; this increase was antagonized by atropine, while it was not affected by phentolamine or propranolol. The im injection of epinephrine caused a small but significant rise in plasma cGMP concentrations; this rise was completely blocked by the simultaneous injection of phentolamine, while it was not affected by atropine or propranolol. These data show that changes in the plasma concentration of cGMP may reflect not only cholinergic but also alpha-adrenergic functions in humans.