Adrenergic innervation and sensitivity to vasoconstrictor hormones of inner muscle of sheep pulmonary artery.

Adrenergic nerves penetrated further from the adventitia into the media of medium-sized pulmonary arteries (first branch of main artery), than into carotid arteries, of sheep. They innervated more than 80% of the media in the pulmonary compared to 50-75% in the carotid artery, so that no smooth muscle cells in even the innermost part of the pulmonary arteries were far from a nerve fibre. Inner muscle of pulmonary arteries, unlike that of carotids and other systemic arteries, was little more sensitive than outer muscle to the constrictor action of norepinephrine, with or without desipramine present to block uptake of the norepinephrine by nerves. Nor was it significantly more sensitive than outer muscle to constrictor actions of histamine or 5-hydroxytryptamine; both inner and outer muscle gave only small responses to 5-hydroxytryptamine and to other agents released from blood clot; this is likely to be important in an artery highly prone to embolisation. The results suggest that the lower pressure in pulmonary arteries allows a higher degree of penetration of the vessel wall by nerves, which in turn induces low sensitivity to constrictor hormones in inner as well as outer muscle.