Sensory neuropeptides and hypoxic pulmonary vasoconstriction in the rat

Endogenous vasodilators such as endothelially derived relaxant factor have been shown to modulate hypoxic pulmonary vasoconstriction. Sensory peptides such as substance P (SP) and calcitonin gene related peptide (CGRP) are also potent pulmonary vasodilators in both animals and humans. Their possible role in the modulation of the normal hypoxic pressor response has been examined in an isolated, ventilated, and blood perfused rat lung preparation.

Animals (n = 7) were pretreated with 50 mg/kg capsaicin administered subcutaneously to deplete nerve endings of sensory neuropeptides. A control group (n = 7) received a subcutaneous dose of capsaicin vehicle. One week later the rats were killed and the rise in pulmonary artery pressure was measured during four successive periods of hypoxic ventilation (FIO2 0.03), and after four injections of angiotensin II (1.0 microgram).

A 60% depletion of SP levels was measured in the sciatic nerves of animals treated with capsaicin. The hypoxic pressor response was not significantly altered in capsaicin treated animals compared with controls, except during the fourth hypoxic episode when it was augmented. The angiotensin II pressor response was the same in both groups during each of the injections.

The sensory neuropeptide SP (and possibly CGRP) does not have a major role in modulating the pulmonary vascular response to hypoxia.