Decreased vascular permeability response to substance P in airways of genetically hypertensive rats.

1. The inbred genetically hypertensive strain (GH) of the Otago Wistar rat possesses more sensory neurons containing the neuropeptide substance P (SP) than does its genetically related control normotensive strain. 2. As SP contributes to airway inflammation by increasing microvascular permeability, we assessed the extravasation of Evans Blue dye in trachea and main bronchus of anaesthetized GH and control rats, in the presence of endogenous (capsaicin-liberated) or exogenous SP. 3. Following intravenous administration of either capsaicin (75 microg kg(-1)) or SP (3.3 nmol kg(-1)), extravasation of Evans Blue in airways from GH rats was only about 60% of that in airways of control rats. This difference was not gender-specific and responses to capsaicin were abolished by pretreatment with a selective NK1 receptor antagonist SR 140333 (360 nmol kg(-1)). 4. By contrast, the extravasation of dye caused by intravenous 5-hydroxytryptamine (0.5 micromol kg(-1)) was similar in magnitude in both GH and control strains. 5. Falls in systemic arterial blood pressure in response to exogenous SP (0.1-3 nmol kg(-1)) or acetylcholine (0.2-2 nmol kg(-1)) were also very similar between strains, but those in response to capsaicin (75 microg kg(-1)) in the GH rats were about double those in control rats. The hypotensive response to SP was abolished by SR 140333, but that to capsaicin was unaffected. 6. Our results indicate that the increased peripheral innervation density by SP-nerves in GH rats is accompanied by reduced inflammatory responses to SP. This does not involve decreased vasodilator potency of SP and is therefore probably related to altered endothelial responsiveness.