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We tested the hypothesis that changes in arterial blood flow modify the function of endothelial Ca2+-activated K+ channels [calcium-activated K+ channel (K(Ca)), small-conductance calcium-activated K+ channel (SK3), and intermediate calcium-activated K+ channel (IK1)] before arterial structural remodeling. In rats, mesenteric arteries were exposed to increased [+90%, high flow (HF)] or reduced blood flow [-90%, low flow (LF)] and analyzed 24 h later. There were no detectable changes in arterial structure or in expression level of endothelial nitric-oxide synthase, SK3, or IK1. Arterial relaxing responses to acetylcholine and 3-oxime-6,7-dichlore-1H-indole-2,3-dione (NS309; activator of SK3 and IK1) were measured in the absence and presence of endothelium, NO, and prostanoid blockers, and 6,12,19,20,25,26-hexahydro-5,27:13,18:21,24-trietheno-11,7-metheno-7H-dibenzo [b,n] [1,5,12,16]tetraazacyclotricosine-5,13-diium dibromide (UCL 1684; inhibitor of SK3) or 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; inhibitor of IK1). In LF arteries, endothelium-dependent relaxation was markedly reduced, due to a reduction in the endothelium-derived hyperpolarizing factor (EDHF) response. In HF arteries, the balance between the NO/prostanoid versus EDHF response was unaltered. However, the contribution of IK1 to the EDHF response was enhanced, as indicated by a larger effect of TRAM-34 and a larger residual NS309-induced relaxation in the presence of UCL 1684. Reduction of blood flow selectively blunts EDHF relaxation in resistance arteries through inhibition of the function of K(Ca) channels. An increase in blood flow leads to a more prominent role of IK1 channels in this relaxation.

To test the hypothesis that endogenous neuropeptide Y (NPY) counteracts the vasodilator effects of calcitonin gene-related peptide (CGRP), we used isolated mesenteric resistance arteries of rats and mice. With immunohistochemistry, we observed CGRP-containing fibers along and in the vicinity of a subset of NPY- or tyrosine hydroxylase-immunoreactive fibers. The CGRP1 receptor component calcitonin-related-like receptor was expressed by periarterial nerves and smooth muscle cells, whereas receptor activity-modifying protein 1 was observed primarily on the smooth muscle. In organ chambers, exogenous CGRP caused relaxations that were reversed by exogenous NPY. The effects were inhibited by 1-piperidinecarboxamide, N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]-carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)-methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl) (BIBN4096BS, a CGRP1 receptor antagonist; pK(B) = 8.54 +/- 0.52) and (R)-NZ-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]argininamide (BIBP3226, a Y1 antagonist; pK(B) = 7.00 +/- 0.49), respectively. Pretreatment with capsaicin (1 muM; 20 min) and the presence of BIBN4096BS (20 nM) increased contractile responses to K(+) (20-40 mM) and electrical field stimulation (EFS; 1-32 Hz). NPY increased contractile responses to K(+) and BIBP3226 (400 nM) reduced contractile responses to EFS. These effects were inhibited by capsaicin and BIBN4096BS, respectively. Furthermore, the relaxing effect of exogenous CGRP (10 nM) during phenylephrine-induced contraction (30 muM) was reversed by EFS, and this effect was reduced in the presence of BIBP3226. We confirmed that bioactive concentrations of endogenous CGRP and NPY can be released from periarterial sensory-motor and sympathetic nerves, respectively, and we demonstrate for the first time functional antagonism between endogenous NPY and CGRP at the level of the smooth muscle.