Relaxation of rat resistance arteries by acetylcholine involves a dual mechanism: activation of K+ channels and formation of nitric oxide

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

The vascular endothelium masks the persistent inhibition of rat thoracic arterial tone induced by S-nitrosoglutathione

AIM

In endothelium-denuded arteries, the nitric oxide (NO) donor S-nitrosoglutathione (GSNO) induced a persistent hypo-reactivity to vasoconstrictors, and low-molecular weight thiols such as N-acetyl cysteine (NAC) produced a relaxant effect. These effects were attributed to the formation of vascular NO stores. In arteries with a functional endothelium, such long-lasting effects on arterial tone have not been well characterised. In this study, we proposed to examine the possibility of storing exogenous NO when the vascular endothelium is still able to produce its own NO.

METHODS

For this purpose, changes in isometric tension of isolated arteries were assessed in organ chambers, and nitrosothiol formation was characterised by confocal microscopy.

RESULTS

In rat aortic rings with endothelium pre-exposed to GSNO, the contractile response to norepinephrine (NE) was not attenuated in comparison with control rings, but NAC induced a relaxant effect. However, an attenuation of the response to NE was observed in GSNO-exposed, intact aortic rings after inhibition of NO synthase by N(ω)-nitro-L-arginine methylester (L-AME) or in GSNO-denuded rings. The relaxing effects of NAC were due to the mobilisation of NO from nitrosothiols after nitrosylation of protein SH residues. Moreover, the hypo-reactivity to NE and the relaxant effect of NAC were abolished by 1H-[1,2,4] oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase, and partially by the K+-sensitive channel inhibitor tetra-ethyl-ammonium (TEA).

CONCLUSION

These data show that endothelium-derived NO masked the persistent effect of GSNO in rat thoracic aorta. However, the ability of GSNO to form releasable NO stores without altering the vascular tone can be particularly useful in preventing endothelial dysfunction in which NO formation decreases.

Intracellular smooth muscle [Ca2+] in acetylcholine and nitric oxide-mediated relaxation of human small arteries

In human resistance arteries the role of intracellular calcium during receptor agonist and nitric oxide (NO)-mediated vasorelaxation is almost unknown. We examined changes in smooth muscle calcium concentration ([Ca2+]i) caused by acetylcholine and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) in isolated human subcutaneous small arteries. In arteries constricted with 50 mM KCl, acetylcholine and SNAP induced relaxation without any change in [Ca2+]i, whereas in noradrenaline constricted vessels, both acetylcholine and to a lesser degree also SNAP-mediated relaxation were associated with a decrease in [Ca2+]i. Furthermore incubation with SNAP (1 microM) induced a rightward shift in the [Ca2+]i-force relationship. These results suggest that relaxation mediated by endothelium derived hyperpolarizing factors (EDHF) is associated with reduction in [Ca2+]i, whereas NO-mediated relaxation can take place without changes in [Ca2+]i. This finding seems to be, at least partly, due to NO-mediated desensitization of the contractile apparatus to calcium.

Stress susceptibility as a determinant of endothelium-dependent vascular reactivity in rat mesenteric arteries

In order to investigate the consequences of stress susceptibility on vascular function, the authors assessed the respective contributions of nitric oxide (NO), prostanoids, and endothelium-derived hyperpolarizing factor to the vascular tone in rats with a constitutionally determined high and low susceptibility to behavioral stressors. In mesenteric resistance arteries mounted in a small vessel myograph and precontracted with l-phenylephrine hydrochloride (phenylephrine), the NO-synthase inhibitor N omega-nitro-l-arginine (l-NOARG, 100 microM) elicited a smaller increase of vascular tone in apomorphine-susceptible (APO-SUS) rats (P < 0.01). Addition of indomethacin (10 microM), in the presence of l-NOARG, resulted in a smaller decrease of vascular tone in APO-SUS rats (P < 0.01). Although acetylcholine-induced relaxation in phenylephrine-precontracted arteries was not different (P > 0.1), the individual components contributing to this relaxation were. In arteries precontracted with 125 mM K+, and incubated with indomethacin, acetylcholine-induced relaxation was not significantly different (pEC(50) and E(max): P > 0.1). Sensitivity (pEC(50): P < 0.05) and maximum relaxation (E(max): P < 0.001) to sodium nitroprusside, in the presence of 125 mM K+, was more pronounced in APO-SUS rats. In phenylephrine-precontracted arteries, in the presence of l-NOARG and indomethacin, maximum relaxation to ACh was reduced in APO-SUS rats (E(max): P < 0.05). This study showed that in rats with a high susceptibility to stressors, the contribution of NO to vascular tone was decreased as was the ratio of vasoconstrictor and vasodilator cyclooxygenase products in alpha-adrenergic precontracted arteries. End-organ sensitivity to NO was greater in APO-SUS rats, possibly due to up-regulation. Moreover, the contribution of endothelium-derived hyperpolarizing factor to acetylcholine-induced vasodilation was reduced in APO-SUS rat arteries.

Endothelium-dependent and -independent vasodilator effects of eugenol in the rat mesenteric vascular bed

The possible involvement of the endothelium in the vasodilator action of eugenol was investigated in the mesenteric vascular bed (MVB) of the rat. Bolus injections of eugenol (0.2, 2 and 20 micromol) and acetylcholine (ACh; 10, 30 and 100 pmol) induced dose-dependent vasodilator responses in noradrenaline-precontracted beds that were partially inhibited by pretreatment of the MVB with deoxycholate (1 mg mL(-1)) to remove the endothelium (approximately 14% and approximately 30% of the control response remaining at the lowest doses of ACh and eugenol, respectively). The vasodilator effect of glyceryl trinitrate (1 micromol) was unaltered by deoxycholate. In the presence of either N(omega)-nitro-L-arginine methyl ester (300 microM) or tetraethylammonium (1 mM)the response to ACh was partially reduced, whereas eugenol-induced vasodilation was unaffected. Similarly the vasodilator effect of eugenol was not inhibited by indometacin (3 microM). Under calcium-free conditions the vasoconstrictor response elicited by bolus injections of noradrenaline (10 nmol) was dose-dependently and completely inhibited by eugenol (0.1-1 mM). Additionally, the pressor effects of bolus injections of calcium chloride in potassium-depolarized MVBs were greatly reduced in the presence of eugenol (0.1 mM), with a maximal reduction of approximately 71% of the control response. Our data showed that eugenol induced dose-dependent, reversible vasodilator responses in the rat MVB, that were partially dependent on the endothelium, although apparently independent of nitric oxide, endothelium-derived hyperpolarizing factor or prostacyclin. Furthermore, an endothelium-independent intracellular site of action seemed likely to participate in its smooth muscle relaxant properties.

In vivo mechanisms of acetylcholine-induced vasodilation in rat sciatic nerve

We examined the importance of nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), and neurogenic activity in agonist-induced vasodilation and baseline blood flow [i.e., nerve microvascular conductance (NMVC)] in rat sciatic nerve using laser Doppler flowmetry. Agonists were acetylcholine (ACh) and 3-morpholinosydnonimine (SIN-1). Vasodilation occurring despite NO synthase (NOS) and cyclooxygenase inhibition and showing dependence on K(+) channel activity was taken as being mediated by EDHF. NOS and cyclooxygenase inhibition with N(omega)-nitro-L-arginine (L-NNA) + indomethacin (Indo) revealed two phases of ACh-induced vasodilation: an initial, transient L-NNA + Indo-resistant vasodilation, peaking at 23 +/- 6 s and lasting 145 +/- 69 s, followed by sustained L-NNA + Indo-sensitive vasodilation. L-NNA alone did not affect sustained ACh-induced vasodilation but decreased baseline NMVC by 55%. In the presence of L-NNA + Indo, the K(+) channel blocker tetraethylammonium (TEA) inhibited transient ACh-induced vasodilation by 58% and reduced baseline NMVC by 25%. SIN-1-induced vasodilation increased fourfold in the presence of L-NNA, whereas the specific guanylyl cyclase inhibitor 1H-(1, 2, 4)oxadiazolo(4,3-alpha)quinoxalin-1-one abolished it. However, in homogenates of rat sciatic nerve, SIN-1-stimulated soluble guanylyl cyclase (sGC) activity was unaffected by L-NNA. TTX affected neither SIN-1- nor ACh-induced vasodilation. In conclusion, ACh-induced vasodilation consisted of two components, the first partially mediated by EDHF and the second by a vasodilatory prostanoid + NO. Baseline NMVC was dependent on NO and EDHF. Although L-NNA enhanced SIN-1-induced vasodilation, it had no effect on sGC-activity.

Possible role of P-450-derived metabolites in endothelium-dependent relaxation of rat small mesenteric arteries

We reported previously that acetylcholine (ACh)-induced endothelium-dependent relaxation of rat mesenteric microvessels depended both on nitric oxide (NO) and on a charybdotoxin (CTX)-sensitive endothelium-derived hyperpolarizing vasodilator. Cytochrome P450 (CYP)-dependent arachidonic acid metabolites act in some systems as hyperpolarizing vasodilators. We sought to quantitate contributions of such metabolites to the CTX-sensitive component of ACh-induced vasodilation in isolated rat mesenteric resistance arteries. ACh relaxed these vessels nearly completely (93.3+/-1.2%, n = 71); cyclooxygenase inhibition with indomethacin did not diminish this response (94.3+/-11.4%, n = 9). NO synthase inhibition with Nitro-L-arginine (NNLA) reduced relaxation by 30% (n = 54, p<0.05). Pretreatment of vessels with CYP inhibitors, either clotrimazole (CTM) or 17-octadecynoic acid (17-ODYA), or with selective K+ channel inhibitors, either tetraethyammonium acetate (TEA) or CTX, each led to similar small reductions in maximal relaxation (17%, 22%, 16%, and 9% respectively, n = 3-6). Combined pretreatment with NNLA + either (CTM or 17-ODYA) or (TEA or CTX) each led to similar maximal relaxations (52.2+/-4.8%, 50.6+/-9.2, 37.6+/-8.6%, and 44.1+/-11.5%, respectively, n = 6-35; p<0.05 for NNLA+[CTM or TEA or CTX] vs NNLA alone). Combined pretreatment with NNLA+CTM+(CTX or TEA) led to similar maximal relaxations (43.0+/-7.3%, 43.7+/-15%, n = 6-11) that did not differ from values in vessels pretreated with either NNLA+CTM or NNLA+(CTX or TEA). We conclude that the ACh-induced vasodilation was insensitive to cyclooxygenase inhibition, partially sensitive to NO synthase inhibition, and that the K+ channel blockers TEA and CTX identified the same minor component of ACh relaxation as did the CYP inhibitor CTM.

Effect of cholecalciferol treatment on the relaxant responses of spontaneously hypertensive rat arteries to acetylcholine

We studied the effect of oral cholecalciferol treatment on the endothelium-dependent vascular relaxation and hyperpolarization induced by acetylcholine (ACh), which is impaired in spontaneously hypertensive rats (SHR). Adult female SHR and normotensive Wistar-Kyoto rat (WKY) controls received 125 microg of cholecalciferol per kilogram body weight per day for 6 weeks. The responses to ACh of the isolated mesenteric vascular bed and mesenteric artery rings were measured, as well as the smooth muscle cell membrane potential. After cholecalciferol treatment, the systolic blood pressure and basal perfusion pressure of the mesenteric vascular bed of the SHR fell to control levels. The relaxant and hyperpolarizing effects of ACh, which are reduced in SHR, were also brought to control levels after cholecalciferol treatment. These effects of ACh were inhibited by N(omega)-nitro-L-arginine in SHR and by apamin in WKY. After cholecalciferol treatment, SHR hyperpolarizing responses showed the same inhibition pattern as those of WKY. This indicates that, after cholecalciferol treatment, SHR vascular mesenteric preparation responses to ACh are mediated by endothelium-derived hyperpolarizing factor, which induces activation of Ca(2+)-dependent K(+) channels, as in WKY. In untreated SHR, the ACh-mediated response is entirely due to ACh acting via the release of nitric oxide.

Central role of heterocellular gap junctional communication in endothelium-dependent relaxations of rabbit arteries

1. The contribution of gap junctions to endothelium-dependent relaxation was investigated in isolated rabbit conduit artery preparations pre-constricted by 10 microM phenylephrine (PhE). 2. Acetylcholine (ACh) relaxed the thoracic aorta by approximately 60 % and the superior mesenteric artery (SMA) by approximately 90 %. A peptide possessing sequence homology with extracellular loop 2 of connexin 43 (Gap 27, 300 microM) inhibited relaxation by approximately 40 % in both artery types. Gap 27 also attenuated the endothelium-dependent component of the relaxation induced by ATP in thoracic aorta but did not modify force development in response to PhE. 3. NG-nitro-L-arginine methyl ester (L-NAME, 300 microM), an inhibitor of NO synthase, attenuated ACh-induced relaxation by approximately 90 % in the aorta but only by approximately 40 % in SMA (P < 0.05). Residual L-NAME-insensitive relaxations were almost abolished by 300 microM Gap 27 in aorta and inhibited in a concentration-dependent fashion in SMA (approximately 50 % at 100 microM and approximately 80 % at 10 mM). Gap 27 similarly attenuated the endothelium-dependent component of L-NAME-insensitive relaxations to ATP in aorta. 4. Responses to cyclopiazonic acid, which stimulates endothelium-dependent relaxation through a receptor-independent mechanism, were also attenuated by Gap 27, whereas this peptide exerted no effect on the NO-mediated relaxation induced by sodium nitroprusside in preparations denuded of endothelium. 5. ACh-induced relaxation of 'sandwich' mounts of aorta or SMA were unaffected by Gap 27 but completely abolished by L-NAME. 6. We conclude that direct heterocellular communication between the endothelium and smooth muscle contributes to endothelium-dependent relaxations evoked by both receptor-dependent and -independent mechanisms. The inhibitory effects of Gap 27 peptide do not involve homocellular communication within the vessel wall or modulation of NO release or action.

Acetylcholine causes dose dependent increase in pulmonary flow in patients with chronic heart failure and elevated pulmonary vascular resistance

Elevated pulmonary vascular resistances occur to a variable degree in patients with chronic congestive heart failure (CHF). These might be caused by increased levels of endogenous vasoconstrictors, defective endothelial vasodilatory mechanisms or structural vascular abnormalities. To determine the contribution of defective endothelial mediated vasodilation, we tested 10 patients with CHF due to coronary artery disease (n = 4) or dilated cardiomyopathy (n = 5), and congenital corrected transposition of the great arteries (n = 1) (median pulmonary artery pressure 36 mmHg, range of pulmonary vascular resistance 0.94-10.7 WE). Patients were in median functional class NYHA III, median left ventricular ejection fraction was 21%, median oxygen uptake at the anaerobic threshhold was 8.25 ml/kg/min. Flow was measured by a flow wire (0.018 inch) positioned in a pulmonary artery branch with a diameter of 3-8 mm determined by intravascular ultrasound before. Acetylcholine infusion was adjusted to 10(-6), 10(-5) and 10(-4) molar concentrations in the pulmonary artery branch. A dose dependent increase in flow between 64 to 140% was seen in 8 out of 10 patients. We conclude: Acetylcholine mediated vasodilation is found in patients with CHF and elevated pulmonary vascular resistances.