Contractile Response Of Rat Aorta Research Articles

Effects of 1-(2-trifluoromethylphenyl)-imidazole (TRIM) on receptor-independent and -dependent contractile responses in rat aorta.

This study investigates whether 1-(2-trifluoromethylphenyl)-imidazole (TRIM), originally proposed as a nitric oxide synthase inhibitor and also suggested to be an inhibitor of store-operated calcium entry in mouse anococcygeal muscle, inhibits receptor-independent and -dependent responses in rat thoracic aorta. Cyclopiazonic acid- and serotonin-induced vascular responses were investigated in aortic segments isolated from male Sprague Dawley rats using isolated tissue experiments. Changes in intracellular calcium levels were also monitored via front surface fluorescence measurements in fura-2-loaded embryonic rat vascular smooth muscle cell line A7r5. TRIM inhibited serotonin-mediated vascular contractions without affecting cyclopiazonic acid-induced responses. In addition, TRIM caused a nonlinear rightward shift in the serotonin concentration-response curve, possibly via serotonin receptor modulation. TRIM may have an impact on investigation of tissue-specific receptor-independent and -dependent vascular responses. It may also be used as a lead compound in the development of selective serotonin receptor modulators.

Emerging role of angiotensin type 2 receptor (AT2R)/Akt/NO pathway in vascular smooth muscle cell in the hyperthyroidism.

Hyperthyroidism is characterized by increased vascular relaxation and decreased vascular contraction and is associated with augmented levels of triiodothyronine (T3) that contribute to the diminished systemic vascular resistance found in this condition. T3 leads to augmented NO production via PI3K/Akt signaling pathway, which in turn causes vascular smooth muscle cell (VSMC) relaxation; however, the underlying mechanisms involved remain largely unknown. Evidence from human and animal studies demonstrates that the renin-angiotensin system (RAS) plays a crucial role in vascular function and also mediates some of cardiovascular effects found during hyperthyroidism. Thus, in this study, we hypothesized that type 2 angiotensin II receptor (AT2R), a key component of RAS vasodilatory actions, mediates T3 induced-decreased vascular contraction. Marked induction of AT2R expression was observed in aortas from T3-induced hyperthyroid rats (Hyper). These vessels showed decreased protein levels of the contractile apparatus: α-actin, calponin and phosphorylated myosin light chain (p-MLC). Vascular reactivity studies showed that denuded aortic rings from Hyper rats exhibited decreased maximal contractile response to angiotensin II (AngII), which was attenuated in aortic rings pre-incubated with an AT2R blocker. Further study showed that cultured VSMC stimulated with T3 (0.1 µmol/L) for 24 hours had increased AT2R gene and protein expression. Augmented NO levels and decreased p-MLC levels were found in VSMC stimulated with T3, both of which were reversed by a PI3K/Akt inhibitor and AT2R blocker. These findings indicate for the first time that the AT2R/Akt/NO pathway contributes to decreased contractile responses in rat aorta, promoted by T3, and this mechanism is independent from the endothelium.

Molecular Interactions of Serotonin (5‐HT) and Endothelin‐1 (ET‐1) in Vascular Smooth Muscle Cells (VSMC): roles of p42/44 mitogen activated protein kinase (MAPK) and Janus Kinase (JAK) 2 pathways

Diabetes mellitus causes renal and cardiovascular disease complications and evidence suggests elevated levels of 5‐HT and ET‐1 may be involved. Previous data suggests supraphysiological concentrations of 5‐HT can potentiate ET‐1's ability to stimulate DNA synthesis and VSMC proliferation in vitro and may involve activation of p42/44 MAPK and JAK2 signaling pathways. The molecular mechanisms are not yet known. We tested the hypothesis that synergistic interaction between 5‐HT and ET‐1 at physiological concentrations would activate both p42/44 MAPK and JAK/STAT pathways to cause enhanced growth responses. Using cultured rat VSMC and Western Blot analysis our data suggest no effect of acute pre‐incubation with physiological concentrations of ET‐1 and 5‐HT on the activation of p42/44 MAPK and JAK2 proteins in normal or diabetic conditions. However, utilizing myographs to measure contractile responses of rat aorta we found that physiological levels of 5‐HT and ET‐1 each stimulate a potentiation of the contractile response. In contrast to the in vitro data, our ex vivo data suggests that physiological concentrations of 5‐HT and ET‐1 each stimulate enhanced contractile response in aorta. Hence, understanding the mechanisms of 5‐HT/ET‐1 interaction in diabetic disease conditions would provide a route for novel therapies to prevent the development of vascular injury as a complication of diabetes. (NHLBI R00 AKLB)

Influence of the Endothelium on Vascular Responses of Aortae from Endotoxic Rats

Intraperitoneal injection of endotoxin diminished the in-vitro contractile response of rat aorta to phenylephrine or clonidine, whether the intimal layer was disrupted or not. The relaxing responses to acetylcholine and sodium nitroprusside in aorta precontracted with 10(-6) M phenylephrine were similar between control and endotoxic groups. However, when the precontractile force following phenylephrine was adjusted to an equivalent level, the relaxing responses to acetylcholine and sodium nitroprusside were diminished in the endotoxic aorta compared with the controls. There was no significant difference between the two groups in the increase in cyclic GMP levels induced by acetylcholine or by sodium nitroprusside. These results suggest that aortae from endotoxic rats show decreased responsiveness to alpha-adrenoceptor stimulation not because of enhancement of the endothelium-derived relaxing factor but because of abnormality in the vascular smooth muscle which is not specific for subtypes of the alpha-adrenoceptor.

The influence of urotensin II on calcium flux in rat aorta.

The fish neuropeptide urotensin II (UII, 10 nM) caused a 51% increase in uptake of 45Ca by segments of rat aorta; this increase was abolished by the Ca channel blocking drug nitrendipine (200 nM). 45Ca efflux was unchanged in the presence of UII, but was significantly increased following washout of the peptide; again, this increase was not observed in the presence of nitrendipine. The results provide direct evidence that the nitrendipine-sensitive component of the contractile response of rat aorta to UII involves mobilization of extracellular Ca, with subsequent activation of a Ca-induced, Ca-release process intracellularly. The mechanisms responsible for the nitrendipine-resistant component of the contractile response to UII remain to be established.

Modulatory role of magnesium on the contractile response of rat aorta to several agonists in normal and calcium-free medium.

Acute withdrawal of external Mg2+ increased basal tone of rat isolated aorta incubated in the presence of Ca2+. Above normal levels of Mg2+ (1-4 mM) inhibited basal tone while much higher levels of the divalent cation (64-256 nM) evoked contractile responses regardless of the presence of Ca2+. Contractile responses to noradrenaline (1 microM) and KCl (80 mM) were inhibited by addition of cumulative concentrations of Mg2+. Acetylcholine-induced contractions in the presence of physiological concentrations of Mg2+ (1 mM) decreased gradually to the basal tone, but a sustained contraction was observed in the absence of this ion. In Ca(2+)-free medium, acetylcholine-induced phasic responses indicate the existence of an acetylcholine-sensitive Ca2+ store. KCl induced contraction only in Krebs solution, although a small residual contraction could be observed in Ca(2+)-free medium in some experiments. Mg(2+)-depletion in the extracellular medium increased contractile responses induced by acetylcholine and KCl in Ca(2+)-free medium. These results suggest that extracellular Mg2+ modulates basal tone, Ca2+ channels and responsiveness to various agents in the absence of Ca2+.

Effects of chlorbutol on 45Ca movements and contractile responses of rat aorta and its relevance to the actions of Syntocinon.

The effects of chlorbutol (0.7, 1.4 and 2.8 mM) on the contractile responses induced by KCl and noradrenaline (NA) and on 45Ca movements have been studied on rat isolated thoracic aorta. Chlorbutol decreased, in a dose-dependent manner, contractions induced by KCl and NA and this effect was observed whether it was added before or after the induced contractions. Preincubation with chlorbutol inhibited the contractile responses elicited by addition of Ca (1-5 mM) to Ca-free high-potassium solution. It also inhibited in a dose-dependent manner the 45Ca influx but increased 45Ca efflux in rat aortic strips. These results suggest that chlorbutol decreases peripheral resistance by reducing the availability of intracellular Ca to the contractile machinery in vascular smooth muscle cells. The effects of synthetic oxytocin (Syntocinon) at concentrations containing the same chlorbutol concentration were quantitatively similar from those produced by chlorbutol alone. Therefore, the inhibitory cardiovascular effects ascribed previously to synthetic oxytocin may be attributed to its preservative, chlorbutol, and not to oxytocin itself.

Effects of tamoxifen on contractile responses of rat aorta

Effects of tamoxifen on contractile responses of rat aorta

Differential roles of rho‐kinase and protein kinase C (PKC) in contractile responses of rat aorta and mesenteric artery

Rho-kinase and protein kinase C (PKC) have each been shown to mediate vasoconstriction via calcium sensitization in response to G protein-coupled receptor (GPCR) stimulation. However, the relative contributions of rho-kinase and PKC to vascular contraction, and whether their roles vary between large and small blood vessels, are not well understood. We therefore assessed the relative roles of rho-kinase and PKC in mediating vascular contraction in isolated rings of aorta (Ao), superior mesenteric artery (MA) and second order branches of the superior mesenteric artery (BMA), from male Sprague-Dawley rats. We tested the effects of the rho-kinase inhibitor, Y-27632 (1μM), and the PKC inhibitor, Ro 31-8220 (5μM) either alone or in combination, on contractile responses to phenylephrine (PE). Maximum responses of Ao and MA to PE were reduced by Y-27632 (n=3-10; P<0.05), whereas responses of BMA were unaffected. Ro 31-8220 partly reduced contractile responses to PE in Ao and MA (n=3-10; P<0.05). Responses of BMA were completely abolished by Ro 31-8220 (n=6-7; P<0.05), and endothelial removal did not alter this effect. Co-treatment with Y-27632 and Ro 31-8220 abolished contractile responses in all arteries. Thus, contractile responses of Ao and MA to PE can involve both rho-kinase and PKC, to varying degrees. In BMA, PKC appears to exclusively mediate these contractile responses, with no apparent role for rho-kinase.

Estrogen potentiates vasopressin-induced contraction of female rat aorta by enhancing cyclooxygenase-2 and thromboxane function

To determine the roles of estrogen and constrictor prostanoids in vasopressin (VP)-induced contraction of female rat aorta, vascular reactivity to VP was determined in thoracic aortas of intact, ovariectomized, and ovariectomized + estrogen-replaced female rats in the presence of indomethacin (Indo), NS-398, SQ-29,548, or vehicle control. The effects of estrogen on vascular reactivity to the thromboxane A(2) analog U-46619 were also examined. Maximal contractile response to VP in intact female rats (5,567 +/- 276 mg/mg of aortic ring wt) was markedly attenuated by ovariectomy (2,485 +/- 394 mg; P < 0.001) and restored by estrogen replacement with 17beta-estradiol (5,059 +/- 194 mg; P > 0.1). Indo and NS-398 significantly attenuated maximal responses to VP in intact female rats to a similar extent [3,176 +/- 179 (P < 0.0001) and 3,258 +/- 152 mg (P < 0.0001), respectively]. Ovariectomy abolished and estrogen replacement restored the inhibitory effects of Indo, NS-398, and SQ-29,548. Contractile responses of rat aorta to U-46619 were significantly greater (P < 0.0001) in females (5,040 +/- 238 mg) than in males (3,679 +/- 96 mg). Ovariectomy markedly attenuated (3,923 +/- 84 mg; P < 0.01) and estrogen replacement restored (5,024 +/- 155 mg; P > 0.1) responses to U-46619 in female aortas. These data reveal that estrogen is an important regulator of the contractile responses of female rat aorta to VP, which appears to potentiate both cyclooxygenase-2 and constrictor prostanoid function in the vascular wall.

Taurine depletion alters vascular reactivity in rats.

We recently showed that chronic taurine supplementation is associated with attenuation of contractile responses of rat aorta to norepinephrine and potassium chloride. However, the potential involvement of endogenous taurine in modulation of vascular reactivity is not known. Therefore, we examined the effect of beta-alanine-induced taurine depletion on the in vitro reactivity of rat aorta to selected vasoactive agents. The data indicate that both norepinephrine- and potassium-chloride-induced maximum contractile responses of endothelium-denuded aortae were enhanced in taurine-depleted rats compared with control animals. However, taurine depletion did not affect tissue sensitivity to either norepinephrine or potassium chloride. By contrast, sensitivity of the endothelium-denuded aortae to sodium nitroprusside was attenuated by taurine depletion. Similarly, taurine deficiency reduced the relaxant responses of endothelium-intact aortic rings elicited by submaximal concentrations of acetylcholine, and this effect was associated with decreased nitric oxide production. Taken together, the data suggest that taurine depletion augments contractility but attenuates relaxation of vascular smooth muscle in a nonspecific manner. Impairment of endothelium-dependent responses, which is at least in part associated with reduced nitric oxide generation, may contribute to the attenuation of the vasorelaxant responses. These vascular alterations could be of potential consequence in pathological conditions associated with taurine deficiency.

Vascular reactivity in diabetic rats: effect of melatonin.

The aim of the present study was to evaluate the in vitro contractile response of rat aorta in mild and severe type I diabetes and the effect of melatonin on it. Aortic rings were obtained from male Wistar rats injected with streptozotocin 8-12 wks earlier. Rats were divided into three groups: non-diabetic rats (NDR), mildly diabetic rats (MDR) and severely diabetic rats (SDR). Dose-response curves for acetylcholine-induced, endothelium-related relaxation of aortic rings (after previous exposure to phenylephrine) and for serotonin-induced vasoconstriction were conducted in the presence or absence of 10-5 mol/L melatonin. This protocol was repeated with rings preincubated in a high glucose solution (44 mmol/L). The contractile response to phenylephrine decreased in SDR, an effect counteracted by preincubation with high glucose. Melatonin decreased phenylephrine-induced vasoconstriction in MDR and counteracted the effect of high glucose in SDR. Acetylcholine-evoked relaxation decreased significantly after exposure to a high glucose in SDR, this effect being counteracted by melatonin. Serotonin-induced vasoconstriction decreased in SDR and augmented in MDR, but only after exposure to high glucose. Melatonin reduced the maximal tension of aortic contraction after serotonin in MDR, both under basal conditions and after preincubation in a high glucose solution. The results support the existence of differences in vasomotor responses as a function of the diabetes state and of an improvement of contractile performance in diabetic rats after exposure to melatonin at a pharmacological concentration (in terms of circulating melatonin levels but not necessarily for some other fluids or tissues).

Lysophosphatidylcholine potentiates vascular contractile responses in rat aorta via activation of tyrosine kinase.

We previously reported that while lysophosphatidylcholine (LPC) does not itself produce contraction, it significantly potentiates the contractile responses induced by high-K(+), UK14,304 (a selective alpha(2)-adrenoceptor agonist) and phorbol ester in the endothelium-denuded rat aorta. To further investigate this phenomenon, we examined the effects of genistein and tyrphostin B42 (both tyrosine kinase inhibitors) on the LPC-induced potentiation of the contractile responses to high-K(+) and UK14,304 in the endothelium-denuded rat aorta. Although genistein (3 x 10(-6) M, 10(-5) M) did not affect the high-K(+)-induced contractile response, it selectively inhibited the potentiating effect of LPC on the contraction and it strongly inhibited the LPC-induced augmentation of the associated increases in [Ca(2+)](i). Genistein also attenuated the LPC-induced augmentation effects on both the increase in [Ca(2+)](i) and contractile response induced by the UK14,304. In contrast, daidzein (10(-5) M) did not inhibit the potentiating effect of LPC. Tyrphostin B42 (3 x 10(-5) M) attenuated the potentiating effect of LPC on high K(+)-induced contractions. Western blot analysis showed that LPC increased the tyrosine phosphorylation of a number of proteins, including 42 and 44 kDa proteins and 53 - 64 kDa proteins. These protein phosphorylations were inhibited by genistein. Sodium orthovanadate (10(-4) M), a tyrosine phosphatase inhibitor, also markedly enhanced the high-K(+)-induced contractile responses. This enhancing effect was attenuated by genistein. These results suggest that the LPC-induced augmentation of contractile responses in the rat aorta is due to activation of tyrosine kinase, which in turn regulates Ca(2+) influx.

Role of cyclic nucleotide phosphodiesterase isoenzymes in contractile responses of denuded rat aorta related to various Ca2+ sources.

We have examined the cyclic nucleotide phosphodiesterase isoforms (PDE) involved in the contractile response of rat aorta to different agonists and different experimental procedures for use in functional studies. The inhibitory effect of AAL 05 on the different PDEs isolated from bovine aortic smooth muscle was examined. Compound AAL 05 appeared to be a selective PDE3 inhibitor. We analyzed the ability of the non-selective inhibitor IBMX (3-isobutyl-1-methylxanthine) and the isoenzyme selective inhibitors nimodipine (type 1), AAL 05 (6-(N-methyl-N-cyclohexyl butyl carboxamide) quinolin-2-one) and SK&F 94120 (5-(4-acetamidophenyl) pyrazin-2(1H)-one; type3), rolipram (type4) and zaprinast (type5) to affect the contractile responses of denuded rat aortic rings to KCl (80 mM) and noradrenaline (NA, 1 microM) in the presence or absence of extracellular Ca2+. Rolipram (10-100 microM) and zaprinast (1-100 microM) failed to relax the aortic strips, but IBMX (0.1-30 microM), nimodipine (1 fM10 microM), AAL 05 (0.01-100 microM) and SK&F 94120 (0.1-100 microM) produced a concentration-dependent relaxation or inhibition of contractile responses to the different agonists, but the pIC50 obtained for each inhibitor was different depending on the experimental procedure. Except for nimodipine (a Ca2+ channel blocker), all the PDE inhibitors showed the following rank of potency: pIC50 on NA-induced contractions in Ca2+-free medium > pIC50 on NA-induced contractions in Ca2+-containing solution > pIC50 on depolarizing solution-induced contraction. This ranking apparently depends on the differences in the Ca2+ sources. We obtained a good correlation between the pKi of PDE3 inhibitors in biochemical studies and the pIC50 on NA-induced contraction in Ca2+-free medium. In conclusion, PDE1 and PDE3 isoenzymes play an important role as modulators of rat aortic smooth muscle contractility regardless of the experimental procedure used. Since intracellular mechanisms are more dependent on PDE activity, experimental procedures performed in absence of extracellular calcium are the most suitable for analyzing the modulatory role of PDE inhibitors.

Effects of canrenone on aorta and right ventricle of the rat.

Canrenone is a major active metabolite of spironolactone and, in addition to the antimineralocorticoid effect, shares with the parent compound the action as a partial agonist with respect to ouabain on the Na+-K+ ATPase. We have investigated whether canrenone, through its action on Na+-K+ ATPase, reverses rat aorta contractions induced by ouabain and has vasorelaxant properties unrelated to its interaction with ouabain. Contractile responses of endothelium-deprived aorta to 1 mM ouabain, 0.1 microM phenylephrine, 10 microM serotonin, and 60 mM K+ were relaxed by canrenone (50-250 microM), with maximum inhibitions of 85.3%, 55.3%, 56.7%, and 64.2%, respectively. Canrenone shifted to the right the concentration-response curve for Ca2+ in depolarized aorta and did not affect the response to 10 mM caffeine. In rat right ventricular strips driven at 0.1 Hz, canrenone exerted negative inotropic effect. The relaxation of ouabain-induced contraction may be due, at least in part, to an interaction between canrenone and ouabain on the Na+-K+ ATPase. Inhibition of calcium entry through calcium channels either in aorta or ventricles is the most parsimonious hypothesis of mechanism underlying the effect of canrenone on contractile responses of rat aorta to agonists and high K+ and the negative inotropic effect on ventricular strips.

PHARMACOLOGICAL EVIDENCE OF HYPOTENSIVE ACTIVITY OF MARRUBIUM VULGARE AND FOENICULUM VULGARE IN SPONTANEOUSLY HYPERTENSIVE RAT

The hypotensive effects of the water extract of Marrubium vulgare L. and Foeniculum vulgare L. were investigated in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto rats (WKY). Oral administration of Marrubium or Foeniculum extract lowered the systolic blood pressure of SHR but not of WKY. In SHR, Foeniculum but not Marrubium treatment increased water, sodium and potassium excretion. Ex vivo as well as in vitro, Marrubium extract inhibited the contractile responses of rat aorta to noradrenaline and to KCl (100 mM). Inhibition was greater in aorta from SHR compared to WKY and was not affected by the NO synthase inhibitor N-nitro-L-arginine. Vascular effects of Foeniculum extract were less pronounced than those of Marrubium and were blocked by N-nitro-L-arginine. These results indicate that hypotensive activity of Marrubium and Foeniculum extracts seems to be mediated through different pathways: Foeniculum appeared to act mainly as a diuretic and a natriuretic while Marrubium displayed vascular relaxant activity.

Mechanisms of methyclothiazide-induced inhibition of contractile responses in rat aorta

Methyclothiazide (MCTZ), a thiazide diuretic, inhibits the contractile response induced by norepinephrine in aortic rings from 12-week-old spontaneously hypertensive rats (SHR). Although not modified by indomethacin, this inhibition was attenuated by either mechanical removal of the endothelium or Nω-nitro- l-arginine (NOLA) treatment. These results suggest that the MCTZ effects on the norepinephrine-evoked vascular response are mediated by an endothelium-dependent mechanism involving endothelium-dependent relaxing factor (EDRF)/nitric oxide (NO) release. MCTZ was also found to alter the contractile response induced by the addition of Ca 2+ to a depolarizing solution, and this inhibitory effect was partially abolished by NOLA application. Our data led us to propose that MCTZ relaxes aortic rings, resulting in an endothelium-dependent relaxation phenomenon that could even be reinforced under high-K + depolarizing conditions.

Effect of divalent cations on KCl- and noradrenaline-induced contractile responses in rat aorta after nifedipine treatment

Nifedipine (1 μM) relaxed the sustained contractile responses induced by 1 μM noradrenaline or 60 mM KCl in rat aortic strips. After washing, a second addition of the spasmogens gave smaller tonic contractions than the first one. Even more, a third addition of KCl also gave a smaller contraction than the first one, but a complete recovery of the contractile response to noradrenaline was obtained by a third addition of this agonist. Application of cumulative amounts of Ca 2+ or Ba 2+ (2.4–24 mM) on the residual contraction in response to these agents after nifedipine treatment, but in the absence of the blocker, restored the magnitude of the contractile responses. Addition of cumulative amounts of Mg 2+ (2.4–24 mM) did not modify or even relax the contractile responses to KCl and noradrenaline, respectively.

Magnesium Modulates Contractile Responses of Rat Aorta to Thiocyanate: A Possible Relationship to Smoking-Induced Atherosclerosis

Thiocyanate anions (SCN−) as the end products of tobacco smoke and found in the blood of cigarette smokers have been implicated in atherogenesis and heart diseases. Magnesium deficiency has also been implicated in the etiology of atherogenesis. The contractile responses of rat aorta to SCN−and the modulation of extracellular magnesium ions ([Mg2+]o) on the effect of SCN−were, therefore, studied in isolated rat aortic rings. SCN−exposure at a range of concentrations (from 10−5to 5 × 10−2M) induces contractile responses of isolated rat aortic rings with and without endothelium in a concentration-dependent manner. Significant differences in responsiveness to SCN−were found in rat aortic ring segments with and without endothelial cells. Preincubation of these vessels with low [Mg2+]omarkedly shifted the contractile concentration–effect curves to the left, and the contractile effects of SCN−in rat aortic rings were potentiated. In contrast to lowering [Mg2+]o, increasing [Mg2+]oto 2.4 mM was found to dramatically attenuate the contractile responses to SCN−. In the absence of extracellular Ca2+([Ca2+]o), SCN−-induced contractions were, however, almost abolished after exposure to Mg2+-free medium.In order to investigate the mechanisms of [Mg2+]omodulation of SCN−-induced contractile response of rat aorta, changes in intracellular Ca2+([Ca2+]i) were measured in cultured primary smooth muscle cells isolated from rat aorta. The resting level of [Ca2+]iin the rat aortic smooth muscle cells was 80.6 ± 6.6 nM. Exposure of these cells to SCN−(5 × 10−5to 5 × 10−3M) produced rises in [Ca2+]iin a concentration-dependent manner. Preincubation of these cells with low [Mg2+]o(0 or 0.3 mM, the lowest physiological range) for 24 h significantly potentiated increments in [Ca2+]iinduced by SCN−. These rises in [Ca2+]iinduced by SCN−were completely inhibited by pretreating the cells with 2.4 mM [Mg2+]ofor 24 h. These results support a hypothesis whereby cigarette smoking or exposure to smoking can induce cardiovascular diseases, at least partly, probably by causing spasm and thicknening of arterial blood vessels as a consequence of large rises in [Ca2+]iin vascular smooth muscle cells. The chronic presence of or exposure to both thiocyanate and low Mg2+in the blood of smokers can result in rapid flux of Ca2+into vascular smooth muscle cells, thus accelerating or initiating atherosclerotic processes in smokers.

Differential Effects of Methanol on Rat Aortic Smooth Muscle

LI, W., B. T. ALTURA AND B. M. ALTURA. Differential effects of methanol on rat aortic smooth muscle. Alcohol 16(3) 221–229, 1998.—The effects of methanol on isolated segments of rat aorta were investigated. In the absence of any vasoactive agent, methanol (5–675 mM) failed to alter basal tension. In rat aortic rings precontracted with high K + (30 mM), methanol elicited a concentration-related relaxation at concentrations of from 5 to 675 mM. The K +-induced contraction in the presence of endothelium was more strongly inhibited by methanol than in the absence of endothelium. The effective concentration producing approximately 50% of the maximal relaxation response (ED 50) to methanol was about 96 mM. Methanol-induced relaxations could not be abolished either by 5 × 10 −5 M N-nitro- l-arginine methyl ester ( l-NAME) or N G-nitro- l-arginine ( l-NNA), both selective inhibitors of nitric oxide (NO) formation; these relaxations were not potentiated by addition of excess l-arginine. An inhibitor of prostanoid synthesis, indomethacin (10 −5 M), had no effects on methanol-induced relaxation. Removal of extracellular Ca 2+ ([Ca 2+] o) resulted in almost complete inhibition of the relaxant effects of methanol on rat aortic ring segments. Marked attenuation of the relaxation responses of intact arteries to methanol was obtained after buffering intracellular Ca 2+ ([Ca 2+] i) with 10 μM BAPTA-AM. In 5-hydroxytryptamine (5-HT, 2.5 μM)- or phenylephrine (PE, 0.1 μM)-precontracted rat aortic rings, methanol amplified contractile responses to 5-HT and PE; these increased responses were concentration dependent. No significant differences in these methanol potentiated responses were found between aorta with or without endothelial cells. The amplified rat aortic smooth muscle responses induced by methanol after PE could be modified only by phentolamine, an antagonist of PE, while responses to 5-HT could be inhibited by methysergide (an antagonist of 5-HT) and by phentolamine, diphenhydramine, and haloperidol. Pretreatment with 50, 200, and 500 mM methanol increased rat aortic contractile responses induced by 5-HT and PE. Our results suggest that: (a) acute methanol exposure relaxes rat aortic smooth muscle contractile responses induced by high K +, leading to vessel relaxation. This relaxation effect of methanol is endothelium-dependent, clearly Ca 2+ dependent, and independent of endogenous vasodilators such as acetylcholine, histamine, catecholamines, serotonin, or PG. (b) Methanol seems to increase potassium current by shifting the potential towards more negative values in depolarized vascular muscle cell membranes, probably inducing hyperpolarization of the cell membranes leading to a repolarization. (c) In contrast to the relaxant responses, methanol protentiates contractile response of rat aorta to 5-HT and PE.