MECHANISM OF ACTION OF CAPSAICIN ON SUBMUCOSAL ARTERIOLES IN THE GUINEA-PIG ILEUM

Vasomotor neurons in the enteric nervous system release acetylcholine to dilate submucosal arterioles, but it is not known whether sensory nerves that project to these vessels also can provide a vasodilator innervation. This possibility was examined by determining the mechanism of action of capsaicin on guinea pig ileal submucosal arterioles in vitro. Capsaicin dilated all vessels that had been preconstricted with prostaglandin F2alpha; mean effective concentration was 11 nM, and maximal dilation occurred at 60-200 nM. The vasodilation showed marked desensitization upon repeated applications of capsaicin. Tetrodotoxin blocked the capsaicin-induced vasodilation but not the desensitization observed upon repeated application. Muscarinic receptor antagonists did not affect the actions of capsaicin. Capsaicin did not dilate arterioles whose extrinsic sensory afferent fibers had been surgically removed. Substance P and human calcitonin gene-related peptide II dilated arterioles; these dilations were not inhibited after desensitization of the capsaicin-induced vasodilation. Thus capsaicin dilates submucosal arterioles by selectively activating extrinsic afferent fibers that release vasodilator transmitter substances onto these vessels.