Sensory nerves, vascular endothelium and neurogenic relaxation of the guinea-pig isolated pulmonary artery

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Abstract
1. In the presence of atropine and guanethidine (3 μmol/l each), electrical field stimulation (1–20 Hz) produced frequency-dependent relaxations of the histamine-(3 μmol/l) induced vascular tone in isolated rings from the guinea-pig pulmonary artery. The electrically-evoked relaxations were abolished by tetrodotoxin (1 μmol/l). The amplitude of these nerve-mediated, nonadrenergic non-cholinergic (NANC) relaxations was unaffected by removal of the vascular endothelium produced through rubbing of the internal surface. 2. Capsaicin (1 μmol/l) produced a prompt and sustained relaxation of the histamine-induced tone which was unaffected by removal of the endothelium. A second application of capsaicin 60–120 min later had no further relaxant effect, indicating desensitization. After in vitro capsaicin desensitization, the electrically-evoked NANC relaxations were abolished, both in the presence or absence of the vascular endothelium. 3. Substance P evoked a prompt and transient relaxation in precontracted arterial rings with intact endothelium and a transient small contraction in rings in which the endothelium had been mechanically removed. The selective NK-1 receptor agonist, [Pro9]-substance P sulfone closely mimicked the relaxation produced by substance P while the selective NK-2 or NK-3 receptor agonists had no relaxant effect. Tachyphylaxis to substance P did not modify the amplitude of the capsaicin-induced relaxation. 4. Human alpha calcitonin gene-related peptide (CGRP) produced a prompt and sustained relaxation both in the presence and absence of the vascular endothelium. 5. Ruthenium red (10 μmol/l) blocked the relaxation to capsaicin while leaving unaffected the relaxation to electrical field stimulation or CGRP (0.1 μmol/l). 6. Both substance P (SP)-and CGRP-like immunoreactivities (LI) were detected in extracts of the guinea-pig pulmonary artery. Capsaicin (1 μmol/l) evoked a prompt and simultaneous outflow of both SP- and CGRP-LI. A second application of capsaicin 60 min later failed to increase SP- or CGRP-LI outflow, indicating complete desensitization. A small but clearly detectable release of both SP-LI and CGRP-LI was also evoked by electrical field stimulation. 7. These findings provide evidence that the neurogenic NANC vasodilation in the guinea-pig pulmonary artery is due to antidromic activation of peripheral endings of capsaicin-sensitive primary afferents. Endogenous CGRP is a likely mediator for this vasodilation. No evidence was found that endogenous SP might contribute to vasodilation by activating NK-1 receptors on endothelial cells.