Endothelium-dependent and BRL 34915-induced vasodilatation in rat isolated perfused mesenteric arteries: role of G-proteins, K+ and calcium channels

Mechanisms involved in the vasorelaxant effects produced by the acute application of amfepramone in vitro to rat aortic rings

Amfepramone (diethylpropion) is an appetite-suppressant drug used for the treatment of overweight and obesity. It has been suggested that the systemic and central activity of amfepramone produces cardiovascular effects such as transient ischemic attacks and primary pulmonary hypertension. However, it is not known whether amfepramone produces immediate vascular effects when applied in vitro to rat aortic rings and, if so, what mechanisms may be involved. We analyzed the effect of amfepramone on phenylephrine-precontracted rat aortic rings with or without endothelium and the influence of inhibitors or blockers on this effect. Amfepramone produced a concentration-dependent vasorelaxation in phenylephrine-precontracted rat aortic rings that was not affected by the vehicle, atropine, 4-AP, glibenclamide, indomethacin, clotrimazole, or cycloheximide. The vasorelaxant effect of amfepramone was significantly attenuated by NG-nitro-L-arginine methyl ester (L-NAME) and tetraethylammonium (TEA), and was blocked by removal of the vascular endothelium. These results suggest that amfepramone had a direct vasorelaxant effect on phenylephrine-precontracted rat aortic rings, and that inhibition of endothelial nitric oxide synthase and the opening of Ca2+-activated K+ channels were involved in this effect.

Regulator of G protein signaling 2 deficiency causes endothelial dysfunction and impaired endothelium-derived hyperpolarizing factor-mediated relaxation by dysregulating Gi/o signaling

Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for G(q/11)α and G(i/o)α subunits. RGS2 deficiency is linked to hypertension in mice and humans, although causative mechanisms are not understood. Because endothelial dysfunction and increased peripheral resistance are hallmarks of hypertension, determining whether RGS2 regulates microvascular reactivity may reveal mechanisms relevant to cardiovascular disease. Here we have determined the effects of systemic versus endothelium- or vascular smooth muscle-specific deletion of RGS2 on microvascular contraction and relaxation. Contraction and relaxation of mesenteric resistance arteries were analyzed in response to phenylephrine, sodium nitroprusside, or acetylcholine with or without inhibitors of nitric oxide (NO) synthase or K(+) channels that mediate endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation. The results showed that deleting RGS2 in vascular smooth muscle had minor effects. Systemic or endothelium-specific deletion of RGS2 strikingly inhibited acetylcholine-evoked relaxation. Endothelium-specific deletion of RGS2 had little effect on NO-dependent relaxation but markedly impaired EDHF-dependent relaxation. Acute, inducible deletion of RGS2 in endothelium did not affect blood pressure significantly. Impaired EDHF-mediated vasodilatation was rescued by blocking G(i/o)α activation with pertussis toxin. These findings indicated that systemic or endothelium-specific RGS2 deficiency causes endothelial dysfunction resulting in impaired EDHF-dependent vasodilatation. RGS2 deficiency enables endothelial G(i/o) activity to inhibit EDHF-dependent relaxation, whereas RGS2 sufficiency facilitates EDHF-evoked relaxation by squelching endothelial G(i/o) activity. Mutation or down-regulation of RGS2 in hypertension patients therefore may contribute to endothelial dysfunction and defective EDHF-dependent relaxation. Blunting G(i/o) signaling might improve endothelial function in such patients.

Effect of melatonin on vascular responses in aortic rings of aging rats

In old animals a marked reduction in endothelium-dependent relaxation occurs. Since there is evidence that the endothelial dysfunction associated with aging may be partly related to the local formation of reactive oxygen species, the purpose of this study was to examine the effect of the natural antioxidant melatonin (10(-5)mol/l) on in vitro contractility of aged aortic rings under conditions of increased oxidative stress (40 m mol/l glucose concentration in medium). Experiments were carried out in 18-20 months old, Wistar male rats, using adult (6-7 months old) animals as controls. A higher plasma lipid peroxidation was found in aged rats as compared to the younger ones. In a first experiment, dose-response curves for acetylcholine-induced relaxation of aortic rings were conducted. Analyzed as a main factor in a factorial ANOVA, age decreased and melatonin augmented the relaxing response to acetylcholine. melatonin's restoring effect on aortic ring relaxation was found in aged aortic rings only and was more pronounced in the presence of a high glucose medium. In a second experiment, the effect of melatonin on the contractility response to phenylephrine of intact or endothelium-denuded aortic rings obtained from aged or control rats was examined in normal or high glucose medium. A main factor analysis in the factorial ANOVA indicated that age and operation augmented, and melatonin decreased, aortic ring contractility response to phenylephrine. Melatonin's restoring effect on aortic contractility was seen in aged aortic rings. The effect of age or a high glucose medium on phenylephrine-induced contractility was more pronounced in the absence of an intact endothelium. Aging did not affect the relaxant response of intact or endothelium-denuded rings to sodium nitroprusside. The results support the improvement by melatonin of vascular response in aging rats, presumably via its antioxidant activity.

Ion channels in mesangial cells: function, malfunction, or fiction

Ion channels in glomerular mesangial cells from humans, rats, and mice have been studied by electrophysiological, molecular, and gene-knockout methods. Two channels, a large, Ca(2+)-activated K(+) channel (BK) and a store-operated Ca(2+) channel (SOCC), can be defined with respect to molecular structure and function. Human BK, comprised of a pore-forming alpha-subunit and an accessory beta1-subunit, operate as Ca(2+)-sensing feedback modulators of contractile tone. SOCC have also been characterized in a mouse cell line; they are comprised of molecules belonging to the transient receptor potential subfamily.

Effect of melatonin on vascular reactivity in pancreatectomized rats

The present study was undertaken to assess whether the improvement of contractile performance of aortic rings by melatonin described in streptozotocin diabetic rats also occurs in another model of type I diabetes, the pancreatectomized rats. Adult male Wistar rats submitted to a subtotal pancreatectomy and exhibiting altered levels of fasting glucose and an abnormal tolerance glucose test, were used. Sham-operated laparotomized rats were employed as controls. Dose-response curves for acetylcholine-induced, endothelium-related relaxation of aortic rings (after previous exposure to phenylephrine) and for phenylephrine-induced vasoconstriction were conducted. This protocol was repeated with rings pre-incubated in a high glucose solution (44 mmol/l). Pancreatectomy decreased significantly acetylcholine-induced relaxation of aortic rings, but not phenylephrine-induced vasoconstriction, the effect being amplified by preincubation in high glucose solution. The deleterious effect of a high glucose medium was more pronounced in pancreatectomized rats. Melatonin (10(-5) M) did not modify acetylcholine-induced relaxation in normal glucose concentration but was effective to prevent the impairment of relaxation brought about by exposure to high glucose solution. The contractile response to phenylephrine of aortic rings obtained from pancreatectomized rats was not affected by melatonin. The results further support the improvement by melatonin of endothelial-mediated relaxation in blood vessels of diabetic rats.

In vivo pertussis toxin treatment reduces contraction of rat resistance arteries but not that of mouse trachea

In order to develop an in vitro method for detecting residual pertussis toxin activity in acellular pertussis vaccines, the effects of in vivo pertussis toxin treatment on contraction and relaxation properties of isolated mouse trachea and of isolated rat small mesenteric resistance arteries were studied. In vivo pertussis toxin treatment (24 or 72 microg/kg, intraperitoneally (i.p.)) did not affect contraction and relaxation properties of isolated BALB/c or NIH mouse trachea. In vivo pertussis toxin treatment (30 microg/kg, intravenously) significantly reduced noradrenaline- or KCl-induced maximal contraction and reduced sensitivity to noradrenaline in isolated male Wistar rat small mesenteric resistance arteries. However, in vivo pertussis toxin treatment did not affect relaxation properties of isolated rat small mesenteric resistance arteries. These results support the hypothesis that vasoconstriction-regulating mechanisms and not airway constriction mechanisms are involved in pertussis toxin-induced histamine sensitisation. The vasoconstriction-regulating mechanisms may provide a lead for further development of an in vitro method for measuring biologically active pertussis toxin in acellular pertussis vaccines based on mechanisms involved in the histamine sensitisation test.

Role of Ba2+-resistant K+ channels in endothelium-dependent hyperpolarization of rat small mesenteric arteries

In rat small mesenteric arteries, the influence of modulation of basal smooth muscle K+ efflux on the mechanism of endothelium-dependent hyperpolarization was investigated. The membrane potentials of the vascular smooth muscle cells were measured using conventional microelectrode techniques. Incubation of resting arteries with the gap junction uncoupler carbenoxolone (20 µM) decreased the endothelium-dependent hyperpolarization elicited by a submaximal concentration of acetylcholine (3 µM) to about 65% of the control. In the presence of Ba2+ (200 µM), which depolarized the membrane potential by 10 mV, the acetylcholine-induced membrane potential response was doubled in magnitude, reaching values not different from control. Moreover, the hyperpolarization was more resistant to carbenoxolone in these conditions. Finally, both in the absence and in the presence of carbenoxolone, the combined application of Ba2+ and ouabain (0.5 mM) did not abolish the acetylcholine response. These results suggest that gap junctional coupling plays a role in endothelium-dependent hyperpolarization of smooth muscle cells of resting rat small mesenteric arteries. Additionally, these findings show that the hyperpolarization does not rely on activation of inward rectifying K+ channels. Although a minor contribution of Na–K pumping cannot be excluded, the Ba2+ experiments show that the membrane electrical response is mediated by activation of a Ba2+-resistant K+ conductance.Key words: EDHF, carbenoxolone, potassium channels, vascular smooth muscle cell membrane potential, vasodilation.

Relaxation induced by histamine is not endothelium dependent in tail arteries of L-NAME-treated rats

The present study was carried out to evaluate the relaxation induced by histamine in tail arteries of rats after chronic inhibition of nitric oxide (NO) synthesis with the inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) compared to tail arteries of control rats. The maximum relaxation induced by histamine was greater in control (88.09% +/-5.50, n=6) than in L-NAME arteries (47.33% +/-6.40, n=6), although pD(2) values were not different between the two groups (control: 4.89+/-0.08; L-NAME: 4.81+/-0.10). After incubation with 100 microM L-NAME in vitro, the maximum relaxation induced by histamine was only reduced in the control arteries (44.93% +/-2.35, n=6), whereas it had no effect on aortas of rats pretreated with this inhibitor. The incubation with 100 microM L-NAME had the same effect as endothelium removal in both arterial groups. Furthermore, the relaxation induced by histamine was unaffected by indomethacin. The combination of L-NAME and the histamine antagonist cimetidine completely abolished the relaxation induced by histamine in both arterial groups. These results show that when NO synthesis is impaired, the relaxation induced by histamine is endothelium independent, and when NO-synthase is active, the relaxation involves both NO released from endothelial cells and an endothelium-independent mechanism that is sensitive to cimetidine.

Mechanisms of endothelium-dependent relaxation in myometrial resistance vessels and their alteration in preeclampsia

OBJECTIVE

To investigate the importance of prostacyclin and nitric oxide synthesis in endothelium-dependent relaxation in myometrial resistance vessels, and to test the hypothesis that a deficiency in nitric oxide synthesis contributes to the known alterations in endothelial function in preeclampsia.

METHODS

Thirty-six women with normal pregnancies and 14 with preeclampsia had the myometrium biopsied at cesarean section. Resistance arteries were dissected and mounted on a wire myograph. After preconstriction with vasopressin, vessels were treated cumulatively with bradykinin. The process was repeated in the presence of indomethacin and then indomethacin and NG-monomethyl-L-arginine (L-NMMA).

RESULTS

The vessels from women with normal pregnancies showed endothelium-dependent relaxation to bradykinin which was not significantly altered by the presence of indomethacin. The addition of L-NMMA significantly, but only partially, reduced the relaxation to bradykinin in the presence of indomethacin (p = 0.03). The vessels from women with preeclampsia showed markedly reduced relaxation to bradykinin (p < 0.0001), as compared to vessels from normal pregnant women. Relaxation of vessels from women with preeclampsia was increased by the addition of indomethacin (p = 0.03) but was virtually eradicated by the presence of L-NMMA.

CONCLUSIONS

Eicosanoid synthesis plays little part in the relaxation of normal pregnant myometrial vessels to bradykinin. Nitric oxide synthesis mediates part but not all of the endothelium-dependent relaxation. In preeclampsia, relaxation to bradykinin is reduced; inhibition of eicosanoid synthesis allows increased relaxation, and nitric oxide synthesis appears to mediate a greater proportion of the relaxation than in normal pregnant women.

Role of K+ channels and sodium pump in the vasodilation induced by acetylcholine, nitric oxide, and cyclic GMP in the rabbit aorta

The endothelium-dependent relaxation caused by acetylcholine (ACh) in rabbit aorta segments was reduced by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester and by blockade of: Na+ pump with ouabain, large-conductance Ca2+-activated K+ (BK(Ca)) channels with charybdotoxin (ChTx), or voltage-dependent K+ (Kv) channels with 4-aminopyridine (4-AP). ACh relaxation was unaltered by glibenclamide, apamin, and Ba2+, blockers of ATP-sensitive K+ channels, small-conductance Ca2+-activated K+ channels, and inward rectifier K+ channels, respectively. The relaxation induced by exogenous NO and 8-bromocyclic GMP (8-BrcGMP) was similar in intact and endothelium-denuded segments, and it was reduced or unaltered by the same drugs used in the case of ACh. However, a 4-AP concentration 20-fold higher was necessary to reduce exogenous NO relaxation. These data suggest a resemblance in the mechanisms implicated in the relaxation elicited by ACh, exogenous NO, and 8-BrcGMP. Therefore, the relaxation caused by ACh is mainly mediated by endothelial NO, which in turn, enhances cGMP levels; this messenger appears to be the major one responsible for the smooth muscle cell hyperpolarization in the relaxation elicited by ACh, which is mediated by activation of the Na+ pump and ChTx- and 4-AP-sensitive K+ channels, likely BK(Ca) and Kv channels.

Nitric oxide-dependent and -independent mechanisms in the relaxation elicited by acetylcholine in fetal rat aorta

The aim of the present study was to analyze the mechanisms involved in the relaxation induced by 1 microM acetylcholine (ACh) in aortic segments from fetal rats at term precontracted with 3 microM prostaglandin F2alpha (PGF2alpha) and incubated with 1 microM indomethacin. The endothelium-dependent relaxation caused by ACh was reduced by the nitric oxide (NO) synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 0.1 mM), such an effect was reversed by 0.1 mM L-arginine (L-Arg). After precontraction of segments with 50 mM KCl the relaxant response to ACh was smaller than that after precontraction with PGF2alpha; this reduction was increased by L-NMMA, whereas L-NMMA plus L-Arg potentiated the relaxation. Thiopentone sodium (0. 1 mM), ouabain (10 microM), tetraethylammonium (TEA, 0.5 mM) and apamin (1 microM), inhibitors of cytochrome P450 monooxygenases, Na+ pump, Ca2+-activated (KCa) and small-conductance (SKCa) K+ channels, respectively, reduced the relaxation to ACh, which was unaffected by charybdotoxin (0.1 microM) and glibenclamide (1 microM), inhibitors of large-conductance BKCa and ATP-sensitive K+ channels. The L-NMMA/indomethacin-resistant relaxation to ACh was markedly reduced by thiopentone sodium, and similarly decreased by either ouabain or TEA. The endothelium-independent relaxation induced by exogenous NO (10 microM) in segments precontracted with PGF2alpha was unaltered by ouabain, glibenclamide, TEA and after precontraction with 50 mM KCl, and potentiated by L-NMMA. The potentiation of NO responses by L-NMMA was also observed in segments precontracted with KCl. These results suggest that ACh relaxes the fetal rat aorta by endothelial release of both NO and endothelium-derived hyperpolarizing factor (EDHF), a metabolite derived from cytochrome P450 monooxygenases, that hyperpolarizes smooth muscle cells by activation of KCa, essentially SKCa channels, and Na+ pump. It seems that when the effect of EDHF is abolished, the formation of NO could be increased.

Involvement of K+ channel permeability changes in the L-NAME and indomethacin resistant part of adenosine-5'-O-(2-thiodiphosphate)-induced relaxation of pancreatic vascular bed

1. We have previously demonstrated that adenosine-5'-O-(2-thiodiphosphate) (ADPbetaS), a potent P2Y-purinoceptor agonist, relaxed pancreatic vasculature not only through prostacyclin (PGI2) and nitric oxide (NO) release from the endothelium but also through other mechanism(s). In this study, we investigated the effects of an inhibitor of the Na+/K+ pump, of ATP-sensitive K+ (K(ATP)) channels and of small (SK(Ca)) or large (BK(Ca)) conductance Ca2+-activated K+ channels. Experiments were performed at basal tone and during the inhibition of NO synthase and cyclo-oxygenase. 2. In control conditions, ADPbetaS (15 microM) induced an initial transient vasoconstriction followed by a progressive and sustained vasodilatation. In the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME, 200 microM) the transient vasoconstriction was reversed into a one minute vasodilator effect, which was then followed by a progressive and sustained vasodilatation similar to that observed with ADPbetaS alone. The addition of indomethacin (10 microM) did not significantly modify the profile of ADPbetaS-induced vasodilatation. 3. Ouabain (100 microM) decreased basal pancreatic flow rate and did not modify ADPbetaS-induced relaxation. This inhibitor of the Na+/K+ pump increased the pancreatic vasoconstriction induced by L-NAME or by the co-administration of L-NAME and indomethacin. Ouabain did not modify either the L-NAME or the L-NAME/indomethacin resistant part of the ADPbetaS vasodilatation. 4. The K(ATP) inhibitor tolbutamide (185 microM) did not significantly modify basal pancreatic flow rate and ADPbetaS-induced relaxation. This inhibitor which did not change L-NAME-induced vasoconstriction, significantly diminished the L-NAME resistant part of ADPbetaS-induced vasodilatation. Tolbutamide intensified the vasoconstriction induced by the co-administration of L-NAME and indomethacin. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS vasodilatation was not changed by the closure of K(ATP). 5. The SK(Ca) inhibitor apamin (0.1 microM) did not significantly change pancreatic vascular resistance whatever the experimental conditions (in the absence or in presence of L-NAME or L-NAME/indomethacin). In the presence of L-NAME, the closure of SK(Ca) channels changed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and thereafter modified only the beginning of the second part of the L-NAME-resistant part of the ADPbetaS-induced vasodilatation. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation remained unchanged in the presence of apamin. 6. Charybdotoxin (0.2 microM), an inhibitor of BK(Ca), increased pancreatic vascular resistance in the presence of L-NAME/indomethacin. In the presence of L-NAME, the closure of BK(Ca) channels reversed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and drastically diminished the sustained vasodilatation. In contrast the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation was not modified by the presence of charybdotoxin. Under L-NAME/indomethacin/charybdotoxin/apamin infusions, ADPbetaS evoked a drastic and transient vasoconstriction reaching a maximum at the second minute, which was followed by a sustained increase in the flow rate throughout the ADPbetaS infusion. The maximal vasodilator effect of ADPbetaS observed was not modified by the addition of apamin. 7. The results suggest that the L-NAME-resistant relaxation induced by ADPbetaS in the pancreatic vascular bed involves activation of BK(Ca), K(ATP) and to a lesser extent of SK(Ca) channels, but the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation is insensitive to the closure of K(ATP), SK(Ca) and BK(Ca) channels.

Effects of homocysteine on acetylcholine- and adenosine-induced vasodilatation of pancreatic vascular bed in rats

1. Epidemiological and experimental data have shown that homocysteine may provoke vascular lesions and that moderate homocysteinaemia may constitute an independent risk factor for vascular disease. It is now documented that homocysteine damages human endothelial cells in culture, possibly by producing hydrogen peroxide in an oxygen-dependent reaction. 2. In this study, we have examined the direct effect of this sulphur amino acid on pancreatic vascular resistance. Experiments were performed on the vascular bed of the rat isolated pancreas perfused at constant pressure; thus, any change in pancreatic vascular resistance resulted in a change in the flow rate. D,L-Homocysteine perfused for one hour at three different concentrations (200 microM, 2 mM, 20 mM) did not induce any significant change in the flow rate per se. Homocysteine infusion for 30 min at a concentration of 200 microM or 2 mM abolished the endothelium-dependent vasodilatation induced by acetylcholine (0.05 microM), but did not modify adenosine (1.5 microM)-induced vasodilatation. 3. The effect of D,L-homocysteine (200 microM or 2 mM) cannot be ascribed to a direct antimuscarinic effect since 30 min pretreatment of rat ileum with these concentrations did not significantly change the contractile effect of increasing concentrations of acetylcholine (0.015-15 microM). 4. Preincubation of human umbilical vein endothelial cells with D,L-homocysteine (0.2-5.0 mM) had no significant effect on overall cell number or viability during 18 h of incubation; the endothelial cells exposed to concentrations up to 5 mM exhibited a spindle-shaped, whirled pattern. This pattern was reversed 48 h after the removal of homocysteine. A cytotoxic effect was seen after 18 h incubation in 10 mM D,L-homocysteine. 5. In conclusion, an acute infusion of homocysteine altered acetylcholine endothelium-induced vasodilation, whereas the adenosine vasodilatator effect was insensitive to the deleterious action of homocysteine in vitro.

Loss of acetylcholine-induced relaxation by M3-receptor activation in mesenteric arteries of spontaneously hypertensive rats

This study investigated which subtype of muscarinic receptor in mesenteric arteries from SHRs mediates the loss of the release of nitric oxide (NO) or endothelium-derived hyperpolarizing factor (EDHF) in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. After SHRs and WKY rats were killed, their superior mesenteric arteries were isolated to perform the vascular relaxation study. Acetylcholine (ACh, 1 nM-1 microM)-induced relaxation was in a concentration-dependent manner and was impaired in mesenteric arterial rings obtained from the SHR group, whereas nitroglycerin (10 nM-1 microM)-induced relaxation was not affected in endothelium-denuded rings obtained from the hypertensive rat. In the presence of N(omega)-nitro-L-arginine (L-NNA; 0.1 mM) or methylene blue (10 microM), the ACh-induced relaxation was partially attenuated in both SHRs and WKY rats. The relaxation of ACh was partially inhibited by 4-aminopyridine (4-AP; 1 mM), apamin (APM; 1 microM), or charybdotoxin (CTX; 0.1 microM) in WKY rats, whereas that relaxation was not affected by 4-AP, APM, or CTX in SHRs. However, the L-NNA-resistant relaxation of ACh was further inhibited by APM or CTX in WKY rats but not in SHRs. When arterial rings were precontracted by 60 mM K+, the ACh-induced relaxation was not significantly different in SHRs and WKY rats. However, this relaxation was abolished by L-NNA (0.1 mM) in both strains. In addition, the M3 muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine was the most potent in inhibiting the relaxation of ACh, being more potent than pirenzepine and methoctramine, which preferentially block M1 and M2 receptors in the mesenteric artery of WKY rats, respectively. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate (1 microM) almost abolished the relaxation of ACh, but not of A23187, in SHRs and WKY rats. These results suggest that NO and EDHF are released from the endothelium of rat mesenteric artery on the activation of muscarinic M3-receptor by ACh, and the loss of ACh-induced relaxation in small arteries of SHRs is mainly through reducing the release or activity or both of EDHF.

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

The relaxation of rat mesenteric resistance arteries to acetylcholine was studied in vessels (normalised internal diameter 230-330 microns) mounted in an isometric myograph and contracted with noradrenaline (5 microM). Removal of the endothelium abolished acetylcholine-induced vasorelaxation, whereas pretreatment with NG-nitro-L-arginine (500 microM) only inhibited the response partly. The relaxation was, however, completely inhibited by NG-nitro-L-arginine when the arteries were contracted with 80 mM K+. Acetylcholine-induced vasorelaxation was also attenuated by pretreatment with the K+ channel blocker, iberiotoxin (100 nM), and the combined pretreatment with iberiotoxin+NG-nitro-L-arginine completely blocked vasorelaxation to acetylcholine. Further, vasorelaxation to acetylcholine was attenuated by tetraethylammonium (5 mM), 4-aminopyridine (1 mM), and BaCl2 (100 microM), respectively, whereas glibenclamide (1 microM) and indomethacin (10 microM) were devoid of effect. Vasorelaxation to the nitric oxide donor sodium nitroprusside was not influenced by iberiotoxin. We conclude that in rat mesenteric resistance arteries, there is a significant nitric oxide-independent component of acetylcholine-induced vasorelaxation, which is mediated by activation of several types of K+ channels, in particular large conductance Ca(2+)-dependent K+ channels.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Disruption of the rat mesenteric arterial bed endothelial function by air perfusion

The impact of air perfusion on the endothelial function of the rat mesenteric arterial bed (MAB; perfused with Krebs' bicarbonate plus indomethacin) was compared to that of the NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME). Air shifted the dose-response curve for the alpha-adrenoceptor agonist, norepinephrine (NE) to the left (ED50%: 2.9+/-0.7 to 0.9+/-0.7 microg, P < 0.05); maximal vasoconstriction did not change. L-NAME produced a similar increase in midrange sensitivity (ED50% 1.4+/-0.7 microg, P < 0.05) and a 20% increase in maximum (152+/-6 to 183+/-7 mmHg, P < 0.05). Electromechanical stimulation with potassium chloride (KCl) was not modified by reserpine. Neither air nor L-NAME modified midrange sensitivity to KCl. L-NAME produced a 17% increase in maximum (91+/-4 to 107+/-5 mmHg, P < 0.05); reserpine abolished the latter effect. Air and L-NAME diminished endothelium-dependent vasodilation elicited by carbachol. Air did not modify endothelium-dependent vasodilation elicited by sodium nitroprusside; this response was potentiated by L-NAME. In summary, air and L-NAME produced similar effects on receptor-dependent activation of the endothelial L-arginine nitric oxide (NO) pathway. Potentiation by L-NAME of the maximal electromechanical response suggests the existence of a tone-dependent NO system. Abolition of the latter response by reserpine suggests that this system is of sympathetic origin.

Role of Na+/K+ ATPase on the relaxation of rabbit ear and femoral arteries

The role of Na+/K+ ATPase in vascular relaxation has been studied by determining its inhibitory effects on 2-mm segments from rabbit central ear and femoral arteries, mounted for isometric tension recording. Acetylcholine (10(-8)-10(-4) M), the nitric oxide donor sodium nitroprusside (10(-8)-3 x 10(-4) M), the potassium channel agonist cromakalim (10(-8) x 10(-5) M), histamine (10(-8)-10(-4) M) in the presence of the H1 antagonist chlorpheniramine (10(-5) M), and papaverine (10(-6)-3 x 10(-4) M) all produced arterial relaxation in ear and femoral arteries precontracted with endothelin 1. Addition of potassium (6 x 10(-3)-1.2 x 10(-2) M) caused relaxation of the same arteries preincubated in potassium-free medium. Ouabain (10(-5) M) an inhibitor of Na+/K+ ATPase, reduced the relaxation of ear arteries, but not of femoral arteries, in response to acetylcholine; it also reduced the response to sodium nitroprusside, cromakalim or histamine, and abolished the relaxation to potassium, but did not modify the response to papaverine, in both types of artery. These results suggest that Na+/K+ ATPase might play a role in the relaxation of ear and femoral arteries to nitrovasodilators, to potassium channel openers and to activation of histamine receptors, and that Na+/K+ ATPase might play a role in the cholinergic relaxation of ear, but not femoral arteries, suggesting that the mechanism of cholinergic relaxation might differ in each type of artery.

Study of the mechanisms involved in adenosine-5'-O-(2-thiodiphosphate) induced relaxation of rat thoracic aorta and pancreatic vascular bed

1. The endothelium-dependent relaxation of blood vessels induced by P2Y-purinoceptor activation has often been shown to involve prostacyclin and/or nitric oxide (NO) release. In this work, we have investigated the mechanisms involved in the relaxant effect of the P2Y agonist, adenosine -5'-O-(2-thiodiphosphate) (ADP beta S) using two complementary preparations: rat pancreatic vascular bed and aortic ring. 2. On the pancreatic vascular bed, ADP beta S (1.5 and 15 microM) infused for 30 min induced a concentration-dependent vasodilatation; it was progressive during the first 10 min (first period) and sustained from 10 to 30 min (second period). Indomethacin (10 microM) delayed ADP beta S-induced vasodilatation (1.5 and 15 microM) by about 6 min. N omega-nitro-L-arginine methyl ester (L-NAME) (200 microM) suppressed the relaxation for about 5 min but thereafter ADP beta S at the two concentrations progressively induced an increase in the flow rate. Even the co-administration of L-NAME and indomethacin did not abolish the ADP beta S-induced vasorelaxation. 3. On 5-hydroxy tryptamine (5-HT) precontracted rings mounted in isometric conditions in organ baths, we observed that ADP beta S induced a concentration-dependent relaxation of rings with a functional endothelium; this effect was stable for 25 min. The ADP beta S relaxant effect was strongly inhibited by Reactive Blue 2 (30 microM) and was suppressed by pretreatment of rings with saponin (0.05 mg ml-1 for 30 min), which also abolished the acetylcholine-induced relaxation. 4. ADP beta S-induced relaxation of 5-HT precontracted rings is largely inhibited by indomethacin (100 or 10 microM) or L-NAME (100 microM). 5. We conclude that: the ADP beta S-induced relaxation is endothelium-dependent, mediated by P2Y-purinoceptors, and at least in part linked to NO and prostacyclin release, depending on the preparation used. Furthermore, on the pancreatic vascular bed, (an)other mechanism(s) than prostacyclin and NO releases may be involved in ADP beta S-induced vasodilatation.

Role of potassium channels in endothelium-dependent relaxation resistant to nitroarginine in the rat hepatic artery

1. In the presence of indomethacin (IM, 10 microM) and N omega-nitro-L- arginine (L-NOARG, 0.3 mM), acetylcholine (ACh) induces an endothelium-dependent smooth muscle hyperpolarization and relaxation in the rat isolated hepatic artery. The potassium (K) channel inhibitors, tetrabutylammonium (TBA, 1 mM) and to a lesser extent 4-aminopyridine (4-AP, 1 mM) inhibited the L-NOARG/IM-resistant relaxation induced by ACh, whereas apamin (0.1-0.3 microM), charybdotoxin (0.1-0.3 microM), iberiotoxin (0.1 microM) and dendrotoxin (0.1 microM) each had no effect. TBA also inhibited the relaxation induced by the receptor-independent endothelial cell activator, A23187. 2. When combined, apamin (0.1 microM) + charybdotoxin (0.1 microM), but not apamin (0.1 microM) + iberiotoxin (0.1 microM) or a triple combination of 4-AP (1 mM) + apamin (0.1 microM) + iberiotoxin (0.1 microM), inhibited the L-NOARG/IM-resistant relaxation induced by ACh. At a concentration of 0.3 microM, apamin + charybdotoxin completely inhibited the relaxation. This toxin combination also abolished the L-NOARG/ IM-resistant relaxation induced by A23187. 3. In the absence of L-NOARG, TBA (1 mM) inhibited the ACh-induced relaxation, whereas charybdotoxin (0.3 microM) + apamin (0.3 microM) had no effect, indicating that the toxin combination did not interfere with the L-arginine/NO pathway. 4. The gap junction inhibitors halothane (2 mM) and 1-heptanol (2 mM), or replacement of NaCl with sodium propionate did not affect the L-NOARG/IM-resistant relaxation induced by ACh. 5. Inhibition of Na+/K(+)-ATPase by ouabain (1 mM) had no effect on the L-NOARG/IM-resistant relaxation induced by ACh. Exposure to a K(+)-free Krebs solution, however, reduced the maximal relaxation by 13% without affecting the sensitivity to ACh. 6. The results suggest that the L-NOARG/IM-resistant relaxation induced by ACh in the rat hepatic artery is mediated by activation of K-channels sensitive to TBA and a combination of apamin + charybdotoxin. Chloride channels, Na+/K(+)-ATPase and gap junctions are probably not involved in the response. It is proposed that endothelial cell activation induces secretion of an endothelium-derived hyperpolarizing factor(s) (EDHF), distinct from NO and cyclo-oxygenase products, which activates more than one type of K-channel on the smooth muscle cells. Alternatively, a single type of K-channel, to which both apamin and charybdotoxin must bind for inhibition to occur, may be the target for EDHF.

Cryptolepine-induced vasodilation in the isolated perfused kidney of the rat: role of G-proteins, K+ and Ca2+ channels

The isolated perfused kidney of the rat was used to examine the contribution by guanosine triphosphate (GTP)-binding (G-) proteins, K+ and Ca2+ channels to the vasodilator actions of cryptolepine (5-methylquindoline). In normal Krebs-Henseleit buffer (4.7 mM KCl), cryptolepine elicited dose-dependent reductions in perfusion pressure of phenylephrine-preconstricted kidneys. The reductions in perfusion pressure by cryptolepine at bolus doses of 2.5, 5, and 10 micrograms were -18.0 +/- 3.4, -30.6 +/- 5.3, and -38.3 +/- 6.8 mm Hg, respectively (n = 19). In K(+)-free (0 mM KCl) Krebs-Henseleit solution, the vasodilator response to cryptolepine was reduced by 44.7 +/- 5.7% (n = 5; P < 0.01). The addition of ouabain (10(-4) M) further reduced cryptolepine-induced vasodilation to 63.0 +/- 7.2% (n = 11: P < 0.01) of the control. A combination of both conditions did not abolish the vasodilator responses to cryptolepine, suggesting the involvement of additional mechanisms. In 80, as opposed to 20 mM KCl, the reductions in perfusion pressure by cryptolepine, 2.5, 5, and 10 micrograms were markedly reduced to -0.8 +/- 0.8, -2.3 +/- 1.4, and -4.0 +/- 2.1 mm Hg, respectively (P < 0.01; n = 6). Responses to acetylcholine and diazoxide, an adenosine triphosphate (ATP)-dependent K+ channel activator, were also markedly reduced, suggesting the involvement of K+ channels for these agents. Furthermore, tetraethylammonium (5 and 10 mM), a non-specific K+ channel blocker, inhibited the vasodilator responses to cryptolepine (n = 5; P < 0.01) and to diazoxide and acetylcholine in a dose-related manner. However, glibenclamide (5 and 10 microM), an ATP-sensitive K+ channel blocker, inhibited the vasodilator responses to diazoxide and acetylcholine but was without effect on cryptolepine-induced vasodilation. This suggests that cryptolepine activates K+ channels which are tetraethyl ammonium- but not glibenclamide-sensitive. In pertussis toxin-treated rats, the vasodilator response to cryptolepine was not affected while that to acetylcholine and especially diazoxide was markedly inhibited. This suggests that, unlike diazoxide and acetylcholine, the K+ channels activated by cryptolepine are not coupled to pertussis toxin-sensitive G-proteins. In the presence of verapamil (5 microM) and cobalt chloride (1 mM), Ca2+ channel blockers, the vasodilator response to cryptolepine was inhibited (n = 5; P < 0.01), suggesting that Ca2+ flux across membranes is also involved in cryptolepine-induced vasodilation in the rat kidney.

Endothelium-dependent relaxation to acetylcholine in bovine oviductal arteries: mediation by nitric oxide and changes in apamin-sensitive K+ conductance

1. Mechanisms underlying the relaxant response to acetylcholine (ACh) were examined in bovine oviductal arteries (o.d. 300-500 microns and i.d. 150-300 microns) in vitro. Vascular rings were treated with indomethacin (10 microM) to prevent the effects of prostaglandins. 2. ACh elicited a concentration-related relaxation in ring segments precontracted with noradrenaline (NA), which was abolished by endothelium denudation. 3. The ACh-induced relaxation was attenuated but not abolished by NG-nitro-L-arginine (L-NOARG, 1 microM-1 mM), an inhibitor of nitric oxide (NO) formation. The inhibition caused by L-NOARG (10 microM) was reversed by addition of excess of L-arginine but not D-arginine (1 mM). 4. In high K+ (40-60 mM)-contracted rings, ACh was a much less effective vasodilator and its relaxant response was completely abolished by L-NOARG (100 microM). 5. In NA (10 microM)-contracted rings, ACh induced sustained and concentration-dependent increases in cyclic GMP, which were reduced below basal values by L-NOARG (100 microM), while potent relaxation persisted. Similar increases in cyclic GMP were evoked by ACh in high K+ (50 mM)-treated arteries and under these conditions, both cyclic GMP accumulation and relaxation were L-NOARG-sensitive. 6. S-nitroso-L-cysteine (NC), a proposed endogenous precursor of endothelial NO, also induced cyclic GMP accumulation in NA-contracted oviductal arteries. 7. Methylene blue (MB, 10 microM), a proposed inhibitor of soluble guanylate cyclase, inhibited both endothelium-dependent relaxation to ACh and endothelium-independent response to exogenous NO, whereas relaxation to NC remained unaffected. 8. The L-NOARG-resistant response to ACh was not affected by either ouabain (0.5 mM), glibenclamide (3 microM), tetraethylammonium (TEA, 1 mM) or charybdotoxin (50 nM), but was selectively blocked by apamin (0.1-1 microM). However, apamin did not inhibit either relaxation to ACh in high K(+)-contracted rings or endothelium-independent relaxation to either NO or NC. 9. Apamin and MB inhibited ACh-induced relaxation in an additive fashion, suggesting the involvement of two separate modulating mechanisms. 10. These results suggest that ACh relaxes bovine oviductal arteries by the release of two distinct endothelial factors: a NO-like substance derived from L-arginine, which induces cyclic GMP accumulation in smooth muscle, and another non-prostanoid factor acting by hyperpolarization mechanisms through alterations in apamin-sensitive K+ conductance.

Renal vasodilation to histamine in vitro: roles of nitric oxide, cyclo-oxygenase products and H2 receptors

The aim of this study was to evaluate the roles of nitric oxide (NO) and prostanoids in vasodilation to histamine in the preconstricted isolated perfused rat kidney. Kidneys were excised from Hypnorm/Hypnovel-anaesthetised Wistar rats and perfused at constant flow in vitro. Renal perfusion pressure was elevated similarly with methoxamine (3 microM) or modified Krebs Henseleit solution containing high KCl (30 mM) and vasodilation to histamine (10, 30 nmol) and papaverine (30, 100 nmol) was then examined before and during perfusion with the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 0.3 mM) or the cyclo-oxygenase inhibitor, indomethacin (10 microM). Furthermore, the vasodilator response to 30 nmol histamine was examined in the presence of the H2 receptor antagonist, ranitidine (0.1-10 microM). Vasodilation to histamine (10, 30 nmol) was found to be unaffected by L-NAME (0.3 mM) or indomethacin (10 microM), while ranitidine (0.1-10 microM) antagonised vasodilation to 30 nmol histamine with an estimated pA2 of 6.67. Vasodilation to histamine in the isolated perfused rat kidney is therefore probably independent of NO and prostanoids and mediated by H2 receptors.

Cerebral vascular smooth muscle potassium channels and their possible role in the management of vasospasm

One of the promising therapeutic uses of the potassium channel openers is in the management of cerebral vasospasm, a prolonged vasoconstriction of major cerebral arteries which follows aneurysmal subarachnoid haemorrhage. In this review, we first summarize the properties of potassium channels in cerebral vascular smooth muscle. Calcium-activated and ATP-dependent potassium channels are the major potassium channels identified in the cerebrovascular smooth muscle and both are believed to play a role in the regulation of cerebrovascular smooth muscle tone. The calcium-activated potassium channels can be activated by depolarization, by elevation of internal calcium and by some vasodilators. Some neuropeptides and potassium channel openers open the ATP-dependent potassium channels and produce vasodilation. We then review the effects of both synthetic and endogenous potassium channel openers in the cerebrovascular system, discuss their efficacy in the management of models of cerebrovascular spasm, and outline the clinical promise of these agents.

The effects of perfusion rate and NG-nitro-L-arginine methyl ester on cirazoline- and KCl-induced responses in the perfused mesenteric arterial bed of rats

1. The purpose of this study was to characterize the effect of NG-nitro-L-arginine methyl ester (L-NAME) on the perfusion rate/pressure relations, and on the pressor responses induced to cirazoline and KCl in isolated, perfused mesenteric arterial beds from normotensive and spontaneously hypertensive rats. 2. The basal perfusion pressure of arterial beds perfused with either physiological salt solution (PSS) or PSS containing 1% polyvinylpyrrolidone increased as the perfusion rate increased. L-NAME, in concentrations up to 100 microM, failed to alter the basal pressure regardless of the perfusion rate and viscosity; however, at 5 microM, it potentiated cirazoline-induced vasoconstriction at each of the perfusion rates. 3. L-NAME but not D-NAME caused a leftward shift of cirazoline concentration-response curves with a marked increase in the maximal response. The potentiating action of L-NAME was abolished in arterial beds perfused with a Ca(2+)-free physiological salt solution and also in beds denuded of endothelium by an infusion of distilled water for 5 min. 4. In endothelium-intact and -denuded preparations, L-NAME potentiated KCl pressor responses; the endothelium-independent potentiation of KCl pressor activity was stereospecific, time-independent and was not prevented by the presence of dexamethasone (0.5 microM) in the perfusion medium. However, L-NAME failed to potentiate vasoconstriction obtained to KCl in arterial beds denervated by cold storage (4-5 degrees C) for 2 days. 5. The absence of K+ in the perfusate did not inhibit the ability of L-NAME to potentiate alpha-adrenoceptor-mediated pressor responses, and nor did L-NAME inhibit KCl-induced vasodilatation in preconstricted arteries. It was thus concluded that L-NAME does not affect Na+/K(+)-ATPase activity. 6. No differences in the potentiating ability of L-NAME on either cirazoline- or KCl-mediated pressor responses were apparent between normotensive Sprague Dawley (SD), Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats.7. Our data thus provide evidence that: the presence of a vasoconstrictor is required for basal nitricoxide (NO) release in the mesenteric arterial bed from either normotensive or spontaneously hypertensive rats; L-NAME causes potentiation of cirazoline- and KCl-induced vasoconstriction respectively by inhibiting endothelial and neuronal NO synthase(s). Furthermore, our data indicate that NO synthase activity is not impaired in the mesenteric arterial bed of spontaneously hypertensive rats.

Elevation of cytosolic calcium by cholinoceptor agonists in SH-SY5Y human neuroblastoma cells: estimation of the contribution of voltage-dependent currents

1. Muscarinic but not nicotinic receptor stimulation in SH-SY5Y human neuroblastoma cells induces a concentration-dependent increase in [3H]-inositol phosphate formation and a biphasic increase in [Ca2+]i. The latter involves release from both an intracellular store and Ca2+ entry across the plasma membrane. Here we examine the possibility that this agonist-stimulated Ca2+ entry occurs indirectly, as a consequence of depolarization. 2. Electrophysiological characterization, by whole cell patch-clamp techniques revealed that SH-SY5Y cells possess a tetrodotoxin-sensitive inward sodium current, a dihydropyridine-insensitive calcium current and an outward potassium current which was blocked by tetraethylammonium, 4-aminopyridine and intracellular caesium ions. The outward potassium current showed voltage-dependent activation and inactivation, similar to that seen for A-currents. 3. Application of nicotinic agonists evoked an inward current in cells voltage-clamped at negative holding potentials, but this current rectified, resulting in little or no outward current flow at positive potentials. The mean amplitude at a holding potential of -60 mV was -1.14 nA. Extrapolation of the current-voltage relation gave a reversal potential of +8 mV, indicative of a non-specific cationic permeability. 4. Application of muscarinic agonists had no detectable effect in most of the cells tested. However, in one third of cells studied, a small slowly activating inward current was observed. The mean amplitude of this current at a holding potential of -60 mV was -8.3 pA.5. This study confirms that SH-SY5Y cells possess voltage-dependent sodium, potassium and calcium currents. In addition, these cells are strongly depolarized by nicotinic agonists, which produce little change in [Ca2t]1. On the other hand, muscarinic agonists produce profound changes in [Ca2+1J with only a small inward current (depolarization). The contrasting effects of these two cholinoceptor agonists strongly implies that the Ca2+ entry after muscarinic receptor activation is not primarily due to activation of voltage-dependent calcium channels.

Comparison of the actions of acetylcholine and BRL 38227 in the guinea-pig coronary artery

1. The contractile and electrical responses to acetylcholine (ACh) in isolated segments of guinea-pig and rabbit coronary arteries were compared to those of the putative adenosine 5'-triphosphate (ATP)-dependent K+ channel opener, BRL 38227. 2. Both ACh and BRL 38227 produced concentration-dependent relaxation of vessel segments contracted with the H1-receptor agonist, 2-(2-aminoethyl)pyridine. 3. An IC90 of either vasodilator also produced 17-20 mV of hyperpolarization of the guinea-pig coronary artery. 4. Glibenclamide (1-35 microM) depolarized the guinea-pig coronary artery by 8-12 mV and antagonized BRL 38227- but not ACh-induced relaxation and hyperpolarization. 5. In the guinea-pig coronary artery, the K+ channel blockers phencyclidine (PCP, 100 microM), tetraethylammonium (TEA, 10 mM) and scorpion venom (8.7 micrograms ml-1) all significantly reduced ACh-induced relaxation and hyperpolarization whereas only PCP was an effective antagonist of both relaxation and hyperpolarization with BRL 38227. 6. Similar effects of glibenclamide and scorpion venom on ACh- and BRL 38227-induced relaxation were observed in the rabbit coronary artery. 7. Apamin (3.5 microM) was without effect on either the ACh- or BRL 38227-induced relaxation in the guinea-pig coronary artery. 8. In conclusion, the actions of BRL 38227 in coronary artery are compatible with its proposed effects on ATP-dependent K+ channels. In contrast, the results with ACh suggest that some step between the initial binding of ACh to endothelial muscarinic receptors and the final relaxation of the smooth muscle depends upon the opening of Ca(2+)-activated K+ channels.