Rhythmic contractile activity of the pulmonary artery studied in vivo in cats.

The rhythmic contractions of the intact pulmonary artery branch were recorded in anaesthetized cats. In order to investigate the temporal relationship between the contractions and the cardiac events, the heart rate was varied by various manoeuvres. The rhythmic contractile activity synchronous with pulse pressure was found to be locked to the pulsatile activity of the right atrium. Administration of acetylcholine or electrical stimulation to the cut peripheral end of either vagus produced similar reduction in the rate of contractions and the heart rate, a 1:1 relationship between the two was always maintained. Prior treatment with atropine abolished the acetylcholine and vagal stimulation responses. Thus, a cholinergic mechanism which affects the activity of the cardiac pacemaker also influences the arterial smooth muscle contractility.