Neural regulation of the pulmonary vasculature in a semi-arboreal snake, Elaphe obsoleta

The innervation of the pulmonary vasculature of the semi-arboreal rat snake, Elaphe obsoleta, was examined with glyoxylic acid-induced catecholamine histochemistry, peptide immunohistochemistry, and in vitro perfusion of the pulmonary vasculature. An adrenergic innervation was present on the pulmonary artery, the smaller pulmonary arteries, the veins draining the lung, and the main pulmonary vein. Vasoactive intestinal polypeptide-like immunoreactive axons were observed on the pulmonary artery and vein, small arteries, and occasionally small veins within the lung parenchyma. A dense plexus of substance P-like immunoreactive (SP-LI) axons was observed on the distal extrinsic pulmonary artery. SP-LI axons were found on the more distal arteries within the lung parenchyma, but not on the veins. The distribution of calcitonin gene-related peptide- and SP-LI axons was similar suggesting that the axons are sensory nerves. In the perfused pulmonary vasculature, vagal stimulation caused a predominant vasoconstriction which was abolished by atropine indicating it was cholinergic in nature. A post-stimulus vasodilatation was abolished by bretylium and propranolol indicating it was adrenergic in nature. The responses to nerve stimulation were located in both the extrinsic and intrinsic pulmonary vasculature. No evidence for non-adrenergic, non-cholinergic transmission to the vascular smooth muscle was found. The extensive, functional innervation of the main pulmonary artery, as well as the more distal vasculature within the lung, may reflect adaptation to cardiovascular problems imposed by an elongated body and arboreal habits.