Role of sensory neuropeptides in the responses of pulmonary and cerebral blood vessels to acute hypoxia in rats

OBJECTIVE: To study the effects of sensory neuropeptides on pulmonary circulation and cerebral blood flow during acute hypoxia in SD rats. METHODS: The changes in hypoxic pulmonary vasoconstriction (HPV) and hypoxic cerebral vasodilatation (HCVD) were observed in mature SD rats pretreated with capsaicin (50 mg/kg) when they were newborn. The hypoxic changes of plasma substance P in the pulmonary circulation and the peptidergic nerve fibers in the lungs were determined with radioimmunoassay and immunohistochemical staining respectively. The effects of exogenous SP on HPV and HCVD were studied as well. RESULTS: The mean pulmonary artery pressure (2.46 +/- 0.07kPa), pulmonary vascular resistance (3665.09 +/- 164.08kPa.s/L) and the percentage change of pulmonary vascular resistance (delta PVR%) during hypoxia (98.67% +/- 4.07%) in group capsaicin (Caps) were higher than those in group Veh (vehicle control) (2.22 +/- 0.04 kPa, 2945.14 +/- 119.23kPa.s/L and 41.21% +/- 2.08% respectively) (t = 2.89, 3.49 and 12.18, P < 0.01) and those in group NC (normal control) (2.22 +/- 0.03 kPa, 2908.19 +/- 130.85 kPa.s/L and 39.13% +/- 1.48%) (t = 3.09, 3.59 and 13.48, P < 0.01). The wave amplitude in rheoencephalogram (H) in group Caps (11.27 +/- 1.02 m omega) was less than those in group Veh (14.96 +/- 1.08 m omega) (t = 2.49, P < 0.05) and group NC (15.86 +/- 1.33 m omega) (t = 2.69, P < 0.05) during normoxia. It increased significantly in groups Veh (41.36% +/- 6.74%) (t = 9.44, P < 0.01) and NC (48.79 +/- 6.10%) (t = 11.21, P < 0.01) during acute hypoxia, however it dropped in group Caps (-23.55 +/- 2.38%). The plasma SP-like immunoreactivity level in the outflow blood of pulmonary circulation increased in hypoxic rats (SPop-SPip = 139.04 +/- 27.97 pmol/L) while it decreased in group Caps (SPop-SPip = -45.84 +/- 6.52 pmol/L) (t = 5.41, P < 0.01). The SP- and CGRP-immunoreactive fibers in lungs appeared to be sparser and thinner during hypoxic inspiration for 3-5 min. SP decreased delta PVR% from 47.42% +/- 4.70% to 20.58% +/- 2.66% (t = 6.84, P < 0.01) while it increased delta H% from 12.51% +/- 4.31% to 37.66% +/- 5.17% (t = 3.93, P < 0.01) during hypoxia. CONCLUSIONS: Sensory neuropeptides might contribute to the modulation of the basic tone of pulmonary and cerebral vessels. They could be released during acute hypoxia modulating HPV and mediating HCVD.