Calcitonin gene-related peptide-induced relaxation in isolated small superior mesenteric arteries from adult stroke-prone spontaneously hypertensive rats

Influence of methanandamide and CGRP on potassium currents in smooth muscle cells of small mesenteric arteries

Cannabinoids have potent vasodilatory actions in a variety of vascular preparations. Their mechanism of action, however, is complex. Apart from acting on vascular smooth muscle or endothelial cannabinoid receptors, several studies point to the activation of type 1 vanilloid (TRPV1) receptors on primary afferent perivascular nerves, stimulating the release of calcitonin gene-related peptide (CGRP). In the present study, the direct influence of the cannabinoid methanandamide and the neuropeptide CGRP on the membrane potassium ion (K(+)) currents of rat mesenteric myocytes was explored. Methanandamide (10 μM) decreased outward K(+) currents, an effect similar to that observed in smooth muscle cells from the rat aorta. Conversely, CGRP (10 nM) significantly increased whole-cell K(+) currents and this effect was abolished by preexposure to tetraethylammonium chloride (1 mM) or iberiotoxin (100 nM), inhibitors of large-conductance calcium-dependent K (BK(Ca)) channels but not by glibenclamide (10 μM), an inhibitor of ATP-dependent K channels. In the presence of the CGRP receptor antagonist CGRP(8-37) (100 nM), the adenylyl cyclase inhibitor SQ22536 (100 μM), or the protein kinase A inhibitor Rp-cAMPS (10 μM), CGRP had no effect. These findings show that methanandamide does not increase membrane K(+) currents in smooth muscle cells of small mesenteric arteries, supporting an indirect mechanism for the reported hyperpolarizing influence in this vessel. Moreover, CGRP acts directly on these smooth muscle cells by increasing BK(Ca) channel activity in a CGRP receptor and cyclic adenosine monophosphate-dependent way. Collectively, these data indicate that methanandamide relaxes and hyperpolarizes intact mesenteric vessels by releasing CGRP from perivascular nerves.

Calcitonin gene-related peptide activates different signaling pathways in mesenteric lymphatics of guinea pigs

The effects of calcitonin gene-related peptide (CGRP) on constriction frequency, smooth muscle membrane potential (V(m)), and endothelial V(m) of guinea pig mesenteric lymphatics were examined in vitro. CGRP (1-100 nM) caused an endothelium-dependent decrease in the constriction frequency of perfused lymphatic vessels. The endothelium-dependent CGRP response was abolished by the CGRP-1 receptor antagonist CGRP-(8-37) (1 microM) and pertussis toxin (100 ng/ml). This action of CGRP was also blocked by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NNA; 10 microM), an action that was reversed by the addition of L-arginine (100 microM). cGMP, adenylate cyclase, cAMP-dependent protein kinase (PKA), and ATP-sensitive K+ (K+(ATP)) channels were all implicated in the endothelium-dependent CGRP response because it was abolished by methylene blue (20 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), dideoxyadenosine (10 microM), N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89; 1 microM) and glibenclamide (10 microM). CGRP (100 nM), unlike acetylcholine, did not alter endothelial intracellular Ca2+ concentration or V(m). CGRP (100 nM) hyperpolarized the smooth muscle V(m), an effect inhibited by L-NNA, H89, or glibenclamide. CGRP (500 nM) also caused a decrease in constriction frequency. However, this was no longer blocked by CGRP-(8-37). CGRP (500 nM) also caused smooth muscle hyperpolarization, an action that was now not blocked by L-NNA (100 microM). It was most likely mediated by the activation of the cAMP/PKA pathway and the opening of K+(ATP) channels because it was abolished by H89 or glibenclamide. We conclude that CGRP, at low to moderate concentrations (i.e., 1-100 nM), decreases lymphatic constriction frequency primarily by the stimulation of CGRP-1 receptors coupled to pertussis toxin-sensitive G proteins and the release of NO from the endothelium or enhancement of the actions of endogenous NO. At high concentrations (i.e., 500 nM), CGRP also directly activates the smooth muscle independent of NO. Both mechanisms of activation ultimately cause the PKA-mediated opening of K+(ATP) channels and resultant hyperpolarization.

Mesenteric arterial relaxation to calcitonin gene-related peptide is increased during pregnancy and by sex steroid hormones

The present study investigated whether pregnancy and circulatory ovarian hormones increase the sensitivity of the mesenteric artery to calcitonin gene-related peptide (CGRP)-induced relaxation and possible mechanisms involved in this process. Mesenteric arteries from young adult male rats or female rats (during estrous cycle, after ovariectomy, at Day 20 of gestation, or Postpartum Day 2) were isolated, and the responsiveness of the vessels to CGRP was examined with a small vessel myograph. The CGRP (10(-10) to 10(-7) M) produced a concentration-dependent relaxation of norepinephrine-induced contractions in mesenteric arteries of all groups. Arterial relaxation sensitivity to CGRP was significantly (P < 0.05) greater in female rats compared with male rats. Pregnancy increased the sensitivity to CGRP significantly (P < 0.05) compared to ovariectomized and Postpartum Day 2 rats. In pregnant rats, CGRP-receptor antagonist, CGRP(8-37), inhibited the relaxation responses produced by CGRP. The CGRP-induced relaxation was not affected by N(G)-nitro-l-arginine methyl ester (nitric oxide inhibitor, 10(-4) M) but was significantly (P < 0.05) attenuated by an inhibitor of guanylate cyclase (1H-[1 , 2 , 4 ]oxadizaolo[4 , 3 -a]quinoxalin-1-one, 10(-5) M). Relaxation responses of CGRP on mesenteric arteries were blocked (P < 0.05) by a cAMP-dependent protein kinase A inhibitor, Rp-cAMPs (10(-5) M). The CGRP-induced vasorelaxation was significantly (P < 0.05) attenuated by calcium-dependent (tetraethylammonium, 10(-3) M), but not ATP-sensitive (glybenclamide, 10(-5) M), potassium channel blocker. Therefore, the results of the present study suggest that mesenteric vascular sensitivity to CGRP is higher during pregnancy and that cAMP, cGMP, and calcium-dependent potassium channels appear to be involved. Therefore, we propose that CGRP-mediated vasodilation may be important to maintain vascular adaptations during pregnancy.

Calcitonin gene-related peptide dilates the pregnant rat uterine vascular bed via guanylate cyclase, ATP- and Ca-sensitive potassium channels and gap junctions

We studied the mechanism of calcitonin gene-related peptide (CGRP)-induced vasorelaxation in isolated uterine vascular beds of pregnant rats. The vascular beds were perfused in situ with Krebs buffer containing dextran and indomethacin, an inhibitor of cyclooxygenase. Baseline perfusion pressure was maintained with norepinephrine. When applied as a bolus, CGRP caused a decreased perfusion pressure in uterine vascular beds that was dose-dependent and equal in both mid-pregnant and late-pregnant rats. The non-selective inhibitor of nitric oxide synthase (NOS), Nomega-nitro-L-arginine methyl ester (L-NAME), did not significantly affect CGRP-induced vasodilatation in vascular beds of either group. CGRP-induced vasodilatation was not influenced by preincubation with the inhibitors of adenylate cyclase (SQ 22536 or MDL 12330A), but was significantly attenuated by the selective inhibitor of soluble guanylate cyclase (ODQ). The vasorelaxant effect of CGRP was not significantly influenced by the inhibitor of voltage-gated potassium (KV) channels (4-aminopyridin), but was significantly attenuated by an inhibitor of calcium-regulated potassium (KCa) channels (tetraethylammonium) and by an inhibitor of adenosine triphosphate-sensitive potassium (KATP) channels (glibenclamide). The gap junction uncoupling agent (carbenoxolone) also significantly attenuated the CGRP-induced decrease in perfusion pressure. We conclude that vasorelaxation induced by CGRP in the pregnant rat uterine vascular bed is not dependent on endothelial nitric oxide. In the uterine circulation of late-pregnant rats, the CGRP effect involves activation of soluble guanylate cyclase, but not adenylate cyclase, and does involve KCa and KATP channels and gap junctions.