Characterization of acetylcholinesterase in rabbit intrapulmonary arteries

Acetylcholine (ACh) acts on the pulmonary vasculature to evoke vasodilation and, in some species, vasoconstriction. The actions of ACh are terminated by its rapid hydrolysis by cholinesterases. Aside from histochemical localization studies, there is little information on cholinesterase enzymes in pulmonary blood vessels. The present study addresses the hypothesis that pulmonary blood vessels contain sufficient cholinesterase activity to regulate the action of ACh in these tissues. Accordingly, studies were undertaken to characterize and quantify cholinesterase activities in pulmonary arteries and veins, quantify inhibition of enzyme activity, and investigate functional physiological consequences of cholinesterase inhibition. Cholinesterase activities in aorta and trachea also were examined for comparison. Kinetic studies showed that the lobar pulmonary arterial enzyme has a Michaelis constant of 55.3 +/- 17.0 microM and a maximum velocity of 8.6 +/- 2.7 nmol/min/mg protein similar to cholinesterases found in other peripheral tissues. Studies with selective inhibitors revealed that > 98% of total enzyme activity was attributable to acetylcholinesterase. Similar levels of enzyme activity were found in homogenates of lobar branch intrapulmonary arteries, intrapulmonary veins, and aorta. The majority of enzyme activity was localized to the membrane fraction, although a moderate amount was found in the cytosol. Studies in intact intrapulmonary lobar arteries showed that these vessels had cholinesterase activity comparable with that found in intact trachealis muscle and that neostigmine (10 nM to 10 microM) caused concentration-dependent inhibition of enzyme activity. In isolated intrapulmonary lobar arteries, functional studies showed that 1 and 10 microM neostigmine significantly potentiated ACh-induced contractions.(ABSTRACT TRUNCATED AT 250 WORDS)