Abstract

Extracellular calcium (Ca²⁺_e)-induced relaxation of isolated, phenylephrine (PE)-contracted mesenteric arteries is dependent on an intact perivascular sensory nerve network that expresses the Ca²⁺-sensing receptor (CaSR). Activation of the receptor stimulates an endocannabinoid vasodilator pathway, which is dependent on cytochrome P450 and phospholipase A₂ but largely independent of the endothelium. In the present study, we determined the role of nitric oxide (NO) in perivascular nerve CaSR-mediated relaxation of PE-contracted mesenteric resistance arteries isolated from mice. Using automated wire myography, we studied the effects of NO synthase (NOS) gene knockout (NOS^−/−) and pharmacologic inhibition of NOS on Ca²⁺_e-induced relaxation of PE-contracted arteries. Endothelial NOS knockout (eNOS^−/−) upregulates but neuronal NOS knockout (nNOS^−/−) downregulates CaSR expression. NOS^−/− reduced maximum Ca²⁺_e-induced relaxation with no change in EC₅₀ values, with eNOS^−/− having the largest effect. The responses of vessels to calindol and Calhex 231 indicate that the CaSR mediates relaxation. l-N⁵-(1-iminoethyl)-ornithine reduced Ca²⁺_e-induced relaxation of PE-contracted arteries from C57BL/6 control mice by ≈38% but had a smaller effect in vessels from eNOS^−/− mice. 7-Nitroindazole had no significant effect on relaxation of arteries from NOS^−/− mice, but both N^G-nitro-l-arginine methylester and N^G-monomethyl-l-arginine significantly reduced the relaxation maxima in all groups. Interestingly, the nNOS-selective inhibitor S-methyl-l-thiocitrulline significantly increased the EC₅₀ value by ≈60% in tissues from C57BL/6 mice but reduced the maximum response by ≈80% in those from nNOS^−/− mice. Ca²⁺-activated big potassium channels play a major role in the process, as demonstrated by the effect of iberiotoxin. We conclude that CaSR signaling in mesenteric arteries stimulates eNOS and NO production that regulates Ca²⁺_e-induced relaxation.