Mechanisms of acetylcholine-induced vasorelaxation in high K+-stimulated rabbit renal arteries

To characterize the mechanisms of acetylcholine (ACh)-induced vasorelaxation in rabbit renal arteries precontracted with high K+ (100 mM), muscle tension and cytosolic free Ca2+ concentration ([Ca2+]i) were measured simultaneously in the fura-2-loaded arterial strips. In the artery with endothelium, high K+ increased both [Ca2+]i and muscle tension. Addition of ACh (10 microM) during high-K+ induced contraction significantly relaxed the muscle and induced additional increase in [Ca2+]i. In the presence of NG-nitro-L-arginine (L-NAME, 0.1 mM). ACh increased [Ca2+]i without relaxing the muscle. In the artery without endothelium, high K+ increased both [Ca2+]i and muscle tension although ACh was ineffective, suggesting that ACh acts selectively on endothelium to increase [Ca2+]i. 4-DAMP (10 nM) or atropine (0.1 microM) abolished the ACh-induced increase in [Ca2+]i and relaxation. However, pirenzepine (0.1 microM), AF-DX 116 (1 microM) and tropicamide (1 microM) were ineffective. The ACh-induced increase of [Ca2+li and vasorelaxation was significantly reduced by 3 microM gadolinium, 10 microM lanthanum or 10 microM SKF 96365. These results suggest that, in rabbit renal artery, ACh-evoked relaxation of 100 mM K+-induced contractions is mediated by the release of endothelial NO. ACh may stimulates the M3 subtype of muscarinic receptor in the endothelial cells, resulting in the opening of the nonselective cation channels followed by an increase of [Ca2+]i and stimulation of NO synthase.     

Mechanism of carbon monoxide-induced relaxation in the guinea pig ileal smooth muscle

The mechanism of carbon monoxide (CO)-induced relaxation were investigated in the guinea-pig ileum. CO (10%) inhibited the 40 mM KCl-induced contraction. This effect was antagonized by ODQ (1 microM), a soluble guanylate cyclase inhibitor. In contrast, CO did not inhibit the 40 mM KCl-induced increase in cytosolic Ca2+ level ([Ca2+]i). Cumulative addition of KCl induced a graded increase in both [Ca2+]i and muscle tension. In the presence of CO, the increase in muscle tension was attenuated whereas the increase in [Ca2+]i was only slightly decreased. Thus, the [Ca2+]i-tension relationship constructed by cumulative addition of KCl shifted downwards in the presence of CO. Using the patch clamp, CO was found to have little effect on the peak Ba currents (I(Ba)) when voltage was stepped from -60 mV to 0 mV. From these results, we conclude that CO inhibits contraction of guinea-pig ileum mainly by the decrease in the sensitivity of contractile elements to Ca2+ via a cyclic GMP-dependent pathway but not by the inhibition of L-type Ca2+ channel.     

Effects of high-K+, Na+-deficient solution on contractility of the smooth muscles of the bovine trachea

A high-K+, Na+-deficient (I-154 K+) solution induced contraction followed by gradual relaxation of the smooth muscles of the bovine trachea, while hyperosmotic addition of 65 mM KCl induced a large sustained contraction. Exposure of the muscle to the I-154 K+ solution induced an increase in the ratio of cellular water content and a sustained increase in oxidized flavoprotein fluorescence or reduced pyridine nucleotide fluorescence. The I-154 K+ solution also induced a sustained increase in [Ca2+]i level. Decreases in developed tension and increases in cellular water content were both prevented by the addition of sucrose or NaCl but not pyruvate. Substitution of KI for KCl in the I-154 K+ solution produced a greater inhibition of contraction, while substitution with K-propionate produced no inhibition of contraction. Moreover, decreases in developed tension and increases in cellular water content were both prevented by addition of 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), but not 10 microM bumetanide or 1 mM acetazolamide. In conclusion, I-154 K+ solution induced-relaxation in the bovine trachea may be due to swelling of smooth muscle cells and the mechanism of swelling is probably involved in DIDS-sensitive anion movement.     

Mechanisms responsible for reduced cardiotoxicity of mitoxantrone compared to doxorubicin examined in isolated guinea-pig heart preparations

We reported previously that doxorubicin, an anticancer agent that has an anthracycline structure, alters Ca2+ releasing and uptake mechanisms in the sarcoplasmic reticulum of myocardial cells. These effects of doxorubicin are apparently related to its cardiotoxicity. Mitoxantrone is a similar anticancer agent with an anthracenedion structure that has been shown to be significantly less cardiotoxic. In the present study, the effects of mitoxantrone on the functions of the sarcoplasmic reticulum were examined in isolated muscle preparations obtained from the guinea-pig heart. In electrically-stimulated left atrial muscle preparations, incubation in vitro for 4 hr with 30 or 100 microM mitoxantrone significantly prolonged the time to the peak of twitch tension, markedly increased the developed tension observed at lower stimulation frequencies, thereby attenuating the slope of positive force-frequency relationships, and increased the postrest contraction observed after a 60-sec quiescent period. In myocytes isolated from ventricular muscles, 30 microM mitoxantrone increased the peak and the size of intracellular Ca2+ concentrations ([Ca2+] i), and prolonged the time to peak [Ca2+]i. In skinned muscle fiber preparations obtained from the left ventricular muscle, 30 muM mitoxantrone significantly increased the caffeine-induced contraction without affecting the Ca2+ sensitivity of contractile proteins. These results suggest that mitoxantrone enhances Ca2+ release from the sarcoplasmic reticulum in isolated atrial muscle preparations obtained from the guinea-pig heart. Apparent enhancement of the sarcoplasmic reticulum functions, in contrast to anthracyclines that has been shown to suppress these functions, seems to explain the relative lack of marked cardiotoxicity of mitoxantrone.     

Capsaicin-induced relaxation in rabbit coronary artery.

In the present study mechanism of inhibitory effects of capsaicin on the contractility of rabbit coronary artery were studied by measurement of isometric tension and intracellular Ca2+ concentration. Capsaicin (1 microM to 30 microM) relaxed the coronary artery pre-contracted with prostaglandin (PG) F2alpha (1 microM) in a concentration-dependent manner. The PGF2alpha-induced increase in intracellular Ca2+ concentration was also inhibited. The effects of capsaicin were readily reversed by washing capsaicin from the bath. Capsaicin-induced relaxation was not attenuated by pretreatment with capsazepine (1 microM), a blocker of vanilloid receptor or ruthenium red (1 microM), a blocker of non-selective cation channel. Previous exposure to a high concentration of capsaicin (100 microM) or repeated application of capsaicin did not eliminate the relaxation response to subsequent application of capsaicin. Increasing the external K+ concentration to 80 mM significantly attenuated the capsaicin-induced relaxation with simultaneous change in intracellular Ca2+ concentration. Pretreatment with iberiotoxin (100 nM), a blocker of Ca2+-activated K+ channel, only partially inhibited the capsaicin-induced relaxation. However, application of 4-aminopyridine (4-AP, 1 mM), a blocker of delayed rectifier K+ current significantly inhibited the capsaicin-induced relaxation with concomitant attenuation of the effect on intracellular Ca2+ concentration. These results indicate that capsaicin may have a direct relaxing effect on the smooth muscle contractility, and relaxation may be due to activation of the 4-AP-sensitive, delayed rectifier K+ channels in the rabbit coronary artery.     

Tamoxifen induces vasorelaxation via inhibition of mitogen-activated protein kinase in rat aortic smooth muscle

The contribution of the mitogen-activated protein kinase (MAPK) pathway to the relaxation induced by tamoxifen, a synthetic non-steroidal anti-estrogen, was examined in rat vascular smooth muscle. Tamoxifen (0.1-300 microM) inhibited the contraction induced by endothelin-1 (ET-1, 3 nM) in aortic smooth muscle in a concentration-dependent manner. The inhibitory effect of tamoxifen was not attenuated by 10 microM ICI 182,780, a selective antagonist of estrogen receptors. In the Ca(2+) channel inhibitor verapamil (1 microM)-pretreated strips, tamoxifen also inhibited the contraction induced by ET-1. Both PD098059 and SB203580, inhibitors of MAPK/extracellular signal-regulated kinase (ERK) kinase and p38 MAPK, respectively, inhibited ET-1-induced contraction in aortic smooth muscle. In Western blot analysis with anti-phosphorylated MAPK antibodies, ET-1 (3 nM) enhanced activities of both ERK1/2 and p38 MAPK in aortic muscle strips, which were not attenuated by the treatment with 4 mM EGTA. Tamoxifen (100 microM) inhibited the activities of ERK1/2 and p38 MAPK induced by ET-1 without significant changes in the expression of these kinases. These results suggest that tamoxifen induces relaxation of rat vascular smooth muscle, and that this is, at least in part, mediated by the inhibition of the Ca(2+)-independent MAPK pathway.     

Cdc42 contributes to phorbol ester-induced Ca2+-independent contraction of pulmonary artery smooth muscle

We determined the contribution of the Rho family of low molecular GTP-binding proteins to phorbol ester-induced contraction in swine pulmonary artery smooth muscle. In Ca2+-free medium containing 1 mM EGTA, 12-deoxyphorbol 13-isobutyrate (DPB, 1 microM), a protein kinase C (PKC) activator, elicited sustained contractions, which were not inhibited by treatment with verapamil, a voltage-dependent Ca2+ channel antagonist, and Y27632, a Rho-associated kinase inhibitor. Immunoblot analysis showed three PKC isoforms (alpha, epsilon, and zeta) and two Rho GTPases (RhoA and Cdc42) in both cytosolic and the membrane fractions from quiescent strips. DPB (1 microM) significantly induced PKCalpha and epsilon to translocate from the cytosolic to the membrane fraction in Ca2+-free medium. DPB also elicited the translocation of Cdc42, but not RhoA to the membrane fraction. Similarly, in the experiment for measurement of Rho GTPase activity by pull-down assay, DPB (1 microM) significantly increased the activity of Cdc42 in Ca2+-free medium. Norepinephrine (NE, 10 microM) stimulated the redistribution of RhoA from the cytosolic to the membrane fraction in swine pulmonary artery smooth muscle. In contrast, NE did not alter the subcellular distributions of Cdc42 and the PKC isoforms. These results indicate that phorbol ester evokes PKC-mediated Ca2+-independent contraction via a Rho GTPase pathway, especially Cdc42, in smooth muscle from swine pulmonary arteries.     

Spontaneous contractions of intestinal smooth muscle re-aggregates from the new-born rat triggered by thromboxane A2

Isolated smooth muscle cells from the small intestine of new-born rats were prepared by enzymatic digestion. These cells re-aggregate after 1 day in culture to clusters. The re-aggregates show spontaneous rhythmical contractions at 37 degrees C with a frequency (13.1 +/- 0.8 min-1, n = 49), which is similar to that of the intact smooth muscle layer. The cholinergic agonist carbachol (5 x 10(-5) mol l-1) caused an increase in the frequency of the spontaneous contractions often ending in a permanent contraction. A similar effect was achieved with the thromboxane A2 (TXA2) agonist, U-46619 (10(-5) mol l-1). In contrast, both the TXA2 receptor blocker, Bay u3405 (5 x 10(-4) mol l-1), as well as the Ca2+ channel blocker, verapamil (5 x 10(-5) mol l-1), suppressed the spontaneous contractions. The observed contractility was insensitive against the neuronal blocker tetrodotoxin (10(-6) mol l-1). These analyses of video images were supported by the measurement of relative changes in the intracellular Ca2+ concentration with the Ca(2+)-sensitive dye, fura-2. Spontaneous contractions were paralleled by spikes in the intracellular Ca2+ concentration, which were abolished by Bay u3405, but stimulated by U-46619 or carbachol. In summary, these results obtained at re-aggregates of intestinal smooth muscle cells support the hypothesis of a role of TXA2 in the generation of spontaneous intestinal smooth muscle contractions in vitro.     

Age-related changes in contractions of chicken aorta and ischiadic artery

The characteristics of the contraction of vascular smooth muscle were examined in thoracic aorta and ischiadic artery of chickens aged 3, 6, 10 and 18 weeks. High K⁺ solution induced a sustained contraction in smooth muscle preparations of aorta and ischiadic artery in vitro . The contraction of the ischiadic artery became greater with age, whereas the contraction of the aortic preparation did not. In the ischiadic artery, the magnitude of the contraction divided by the weight of the muscle preparation was constant at all ages studied. However, those in the aortic preparation decreased with age. These results suggest that the changes in the contractile responses of vascular smooth muscle owing to the age of chickens vary widely according to the preparations of blood vessels, and that the functional smooth muscle cells in the thoracic aorta of chicken do not increase with age.     

Phagocytotic activation of muscularis resident macrophages inhibits smooth muscle contraction in rat ileum

Intestinal muscularis resident macrophages distributed in myenteric region may play an important role in the immunological host defense against infection. In this study, we investigated the phagocytic stimulation of resident macrophages on cyclooxygenase-2 (COX-2) expression and smooth muscle contraction in the small intestine of rat. After the injection of FITC-dextran to rat, phagocytosed macrophages could be detected in the myenteric plexus. FITC-positive macrophages were also immunostained with COX-2 antibody. The number of COX-2 immunopositive cells increased in a time-dependent manner reaching its maximum at 4 hr after the injection, which then decreased gradually but considerable number of cells were still remained on 7 days. The injection of FITC-dextran, however, did not change the population of ED2-positive resident macrophages even on 7 days. Production of PGE2 was significantly higher in the dextran treated tissue as compared to control tissue. In the smooth muscle tissue phagocytosed dextran, carbachol-induced contraction was significantly decreased. The suppression of the carbachol-induced contraction was completely restored by COX inhibitor, indomethacin. Finally we demonstrated that, in freshly isolated macrophage cells, addition of dextran induced a slow and sustained increase in intracellular Ca2+ concentration. These results indicate that phagocytotic activation of muscularis resident macrophages induces COX-2 gene expression and then results in production of PGE2 to suppress the smooth muscle contractile activity.     

Effects of trypsin on cytosolic calcium levels in the rat aortic endothelium

The effect of trypsin on vascular tone and the cytosolic calcium concentration ([Ca(2+)](i)) of endothelial and smooth muscle cells were examined in the rat aorta. A calcium indicator, fura-PE3, was used to measure [Ca(2+)](i) simultaneously with vascular tone. In the endothelium-intact rat aorta, carbachol and trypsin increased [Ca(2+)](i) in a dose-dependent manner. In the endothelium-denuded rat aorta, carbachol did not change [Ca(2+)](i), but trypsin slightly increased it. Addition of trypsin to the norepinephrine-stimulated rat aorta relaxed the muscle with an additional increase in [Ca(2+)](i). Under calcium-free conditions, trypsin induced a transient increase in [Ca(2+)](i). Trypsin-induced endothelium-dependent relaxation was inhibited by preincubation with l-NMMA, an endothelial NO synthase inhibitor, U-73122, a phospholipase C inhibitor, cyclopiazonic acid, a sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase blocker, and lanthanum, a nonselective Ca(2+) channel blocker. However, indomethacin, a nonselective cyclooxygenase inhibitor, and SKF-96365, a store-operated Ca(2+)-channel blocker, had no effect on the trypsin-induced relaxation. These results suggest that trypsin increases [Ca(2+)](i) in the endothelial cells through SKF-96365-insensitive Ca(2+) channels and regulates the release of NO, which results in relaxation of the rat aorta.     

Digoxin- and monensin-induced changes of intracellular Ca2+ concentration in isolated guinea-pig ventricular myocyte

This study was undertaken to determine the possible mechanisms of actions of monensin and digoxin by using isolated guinea-pig ventricular myocytes. Since Ca2+ is the major signal for triggering contraction of cardiac muscle, the objective of this study was to determine whether monensin and digoxin affect the [Ca2+]i of cardiac myocytes and if so is this effect due to an increase in [Na+]i. Three different concentrations of digoxin (0.3, 1 and 3 micromol/l) and three different concentrations of monensin (0.3, 1 and 3 micromol/l) were used. Each treatment was monitored for two hours by using computerized fluoroscopy. Both digoxin and monensin increased the [Ca2+]i and accelerated the onset time of [Ca2+]i increase in a dose-dependent manner. Normal myocytes (loaded with fura-2 for 30 min before the treatment) were also compared with 'weakened' myocytes (loaded with fura-2 for 3 h before the treatment to create a 'weakened' condition). It was found that although 0.3 micromol/l monensin and digoxin did not change the [Ca2+]i in normal myocytes, they increased the [Ca2 +]i in 'weakened' myocytes. Finally, a Na+-free medium was used to demonstrate the effect of [Na+]o on both monensin- and digoxin-induced increases in [Ca2+]i. It was found that digoxin did not increase the [Ca2+]i in the Na+-free medium. Although monensin increased the [Ca2+]i in the Na+-free solution, this increase was not as large as in the Na+-containing medium. The results of the study led to the conclusion that the positive inotropic effect of digoxin depends on [Na+]o. However, monensin increases [Ca2+]i in Na+-dependent and -independent ways. An addition conclusion was that 'weakened' myocytes are more sensitive to the monensin and digoxin treatment than normal myocytes.     

Lysophosphatidylcholine decreases delayed rectifier K+ current in rabbit coronary smooth muscle cells

Lysophosphatidylcholine (LPC), which exists abundantly in lipid fraction of oxidized low density lipoprotein, has been implicated in enhanced agonist-induced contraction and increase of intracellular Ca2+. The effect of LPC on the activity of delayed rectifier K+ current (I(dK)), which is a major determinant of membrane potential and vascular tone under resting condition, was examined in rabbit coronary smooth muscle cells using whole cell patch clamping technique. Application of LPC to the bath solution caused a concentration-dependent inhibition of I(dK), and the concentration to produce half-maximal inhibition was 1.51 microM. This effect of LPC on I(dK) was readily reversed after washout of LPC in the bath. The steady-state voltage dependence of I(dK) was shifted to positive direction by both extra- and intracellular application of LPC. Staurosporine (100 nM) pretreatment significantly suppressed the LPC-induced inhibition of I(dK). These results suggest that LPC inhibits I(dK) in rabbit coronary smooth muscle cells by a pathway that involves protein kinase C, and the LPC-induced inhibition of I(dK) may be, at least in part, responsible for the abnormal vascular reactivity in atherosclerotic coronary artery.     

奶牛输卵管平滑肌前列腺素类受体的分析研究

本研究选用了前列腺素类化合物(Prostaglandin E2（PGE2）、Prostaglandin F2α（PGF2α）、Prostaglandin D2（PGD2)）和受体选择性激动剂（Butaprost、U-46619）,用多道生理信号采集系统测定其对体外分离的奶牛输卵管峡部及壶腹部平滑肌收缩的影响,以此初步揭示受体的种类。结果发现,PGE2和Butaprost均抑制了奶牛输卵管峡部、壶腹部的自发性收缩运动,呈现明显的浓度依存性舒张反应;PGF2α在高浓度(1～10 μmol/L)下,使峡部和壶腹部的自发性收缩运动有了显著增强;PGD2在低浓度时轻微抑制了壶腹部平滑肌的收缩,但在高浓度(1～10 μmol/L)时加强了峡部和壶腹部平滑肌的收缩;U-46619从低浓度便引起了输卵管峡部和壶腹部平滑肌的强烈收缩。结果表明,EP4、EP2、FP、DP和TP受体同时存在于奶牛输卵管平滑肌上,能与内源性前列腺素类结合并参与调节输卵管平滑肌的自发性收缩。     

Effects of induction of capacitative calcium entry on equine laminar microvessels

OBJECTIVE: To determine the effects of induction of capacitative Ca2+ entry on tone in equine laminar arteries and veins. SAMPLE POPULATION: Laminar arteries and veins from 6 adult mixed-breed horses. PROCEDURE: Arteries and veins were isolated and mounted on small vessel myographs for the measurement of isometric tension. Capacitative Ca2+ entry was induced by incubating the vessels with the specific Ca2+-ATPase inhibitor thapsigargin (100nM) in a Ca2+-free physiologic salt solution. Capacitative Ca2+ entry-associated contractile responses were determined by the subsequent addition of 2mM Ca2+ to the solution bathing the vessels; in some experiments, either the voltage-gated Ca2+ blocker diltiazem (10microM) or the putative capacitative Ca2+ entry inhibitor trifluoromethylphenylimidazole (300microM) was added to the bathing solution 15 minutes prior to a second 2mM Ca2+ exposure. The Sr2+ permeability of the capacitative Ca2+ entry pathway in laminar vessels was assessed by exposing the vessels to 4mM Sr2+ after induction of capacitative Ca2+ entry with thapsigargin. RESULTS: Induction of capacitative Ca2+ entry elicited robust contractile responses in laminar veins but did not increase tone in laminar arteries. In laminar veins, capacitative Ca2+ entry-induced contractile responses were unaffected by preincubation with diltiazem, attenuated by trifluoromethylphenylimidazole, and were impermeable to Sr+. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that induction of capacitative Ca2+ entry elicits vasoconstriction in equine laminar veins but not in laminar arteries and should therefore be considered a potential mechanism by which selective venoconstriction occurs in horses during the development of acute laminitis.     

Photodynamic stimulation causes sustained increase in intracellular calcium concentration in cells of small cell lung carcinoma

Photodynamic agents, due to their selective uptake by tumor cells and photon-dependent selective activation, have immense implications for cancer treatment. The present study provided direct evidence that the photon activation of chloro-aluminum phthalocyanine sulphonate (A1PcS4) in the presence of extracellular Ca2+ caused a rapid increase followed by a sustained increase in intracellular concentration of calcium ion ([Ca2+]i) in a small cell lung carcinoma (SCLC) cell line, SBC-3. The [Ca2+]i increase by photodynamic stimulation was completely inhibited by the removal of extracellular Ca2+ and reintroduction of extracellular Ca2+ immediately led to a rapid elevation of [Ca2+]i. However, the increase was not inhibited by application of Ni2+, nifedipine, or SK&F 96365, a receptor-mediated and voltage-dependent Ca2+ entry blocker. The photosensitizer A1PcS4 alone or light alone (4 min) had no effect on [Ca2+]i. Cytotoxicity examination by trypan blue exclusion test, however, suggested photodynamic stimulation-induced cell injury which was observed in both the presence and the absence of extracellular Ca2+. These results indicate that [Ca2+]i increase may not be mandatory for photodynamic stimulation-induced cell injury. Whether [Ca2+]i increase can accelerate, at least in part, cell death under the physiological condition, whether the mechanism(s) of cell death can be different in the presence and the absence of extracellular Ca2+, and whether [Ca2+]i increase can be totally unrelated to cell death await further work.     

Effects of substance P on nicotine-induced intracellular Ca2+ dynamics in bovine adrenal chromaffin cells

Substance P (SP) is colocalized with ACh in splanchnic nerves that innervate into adrenal medulla and the peptide has been shown to inhibit nicotinic agonists-induced catecholamine secretion. To elucidate the effects of SP on cytosolic Ca2+ dynamics, the present study was conducted using fura-2-loaded isolated bovine adrenal chromaffin cells. Stimulation of the cells with nicotine (10-100 microM) produced a rapid rise of cytosolic Ca2+ concentration ([Ca2+]i), the peak level of which increased in a dose-dependent manner, followed by a gradual decay. In the presence of 10 microM SP, the dose-response relationship of the peak levels shifted downward. Quantitative analyses implied that SP inhibits the nicotine-induced Ca2+ influx in a noncompetitive manner. Nicotinic acetylcholine receptor is composed of two major functional domains: an agonist-binding site and an ionophore or channel domain. Agonist binding activates ionophore/channel domain and causes mainly Na+ influx. This Na+ influx depolarizes the cell and activates voltage-dependent Ca2+ channels. Based on this fact, the present results indicate that SP dose not block nicotine binding sites but interferes with other sites of nicotinic receptor/channel molecule, most probably a channel domain. It was suggested that SP colocalized with ACh in splanchnic nerves functions as a physiological modulator of catecholamine secretion by non-competitively suppressing ACh-induced cytosolic Ca2+ dynamics in bovine adrenal chromaffin cells.     

Cardiovascular Responses to Glibenclamide During Endotoxaemia in the Pig

Vanelli, G., Hussain, S.N.A., Dimori, M. and Aguggini, G., 1996. Cardiovascular responses to glibenclamide during endotoxaemia in the pig. Veterinary Research Communications, 21(3), 187-200The effects of blockading the ATP-sensitive potassium channel (K+ATP channels) on endotoxin-induced vascular derangements was studied. Escherichia coli endotoxin was infused (20 µg/kg per h) intravenously for 180 min into anaesthetized, mechanically ventilated, indomethacin-treated pigs. After 150 min of endotoxaemia, glibenclamide (a K+ATP channel blocker) was infused intravenously at 2 mg/kg per min for 5 min. The cardiovascular parameters were recorded before (control), every 30 min up to 150 min during endotoxaemia, and then at 5, 15 and 30 min after administration of glibenclamide. Infusion of endotoxin reduced the systemic arterial pressure to 60.6% ± 3.7% (p < 0.01) and increased the pulmonary arterial pressure by 75.9% ± 11.0% (p < 0.01) of the control values. Within 5 min, infusion of glibenclamide transiently but significantly reversed the systemic hypotension by raising the systemic vascular resistance, whereas the increased pulmonary arterial pressure was further augmented. Glibenclamide infusion did not influence the cardiac output. Within 30 min, the cardiovascular parameters had returned to the values induced by endotoxin, except for the systemic vascular resistance. Infusion of glibenclamide into normal pigs did not change the systemic pressure or resistance, but the pulmonary pressure and resistance were augmented transiently. These data suggest that, in pigs, the ATP-sensitive K+ channels may be one factor playing a role in the vascular changes due to endotoxaemia, especially in the systemic circulation.     

Palmar digital vessel relaxation in healthy horses and in horses given carbohydrate

OBJECTIVE: To compare in vitro smooth muscle relaxation of palmar digital vessels from healthy horses with those from horses in the prodromal stage of experimentally (carbohydrate) induced laminitis. ANIMALS: 16 adult horses. PROCEDURE: Segments of palmar digital vessels were obtained from 5 healthy horses and 6 horses given carbohydrate. Vascular rings from the palmar digital artery and vein were suspended in individual organ baths containing buffer solution and indomethacin; isometric tension was recorded, and contraction and relaxation were compared. Smooth muscle contraction in response to cumulative addition of phenylephrine was recorded in the absence and presence of 1 microM NG-nitro-L-arginine methyl ester (L -NAME). After wash out, vascular rings were preconstricted with phenylephrine (0.3 microM), and cumulative endothelium-dependent (acetylcholine-induced) and independent (nitroprusside-induced) smooth muscle relaxations were recorded in the absence or presence of L -NAME. RESULTS: Phenylephrine increased vascular smooth muscle tone in ring preparations of palmar digital arteries and veins. Addition of acetylcholine or nitroprusside induced relaxation of palmar digital artery and vein ring preparations. Use of L-NAME (1 microM) significantly reduced maximal relaxation induced by acetylcholine, but not by nitroprusside. Maximal relaxation induced by acetylcholine, but not by nitroprusside, was reduced in vascular rings prepared from carbohydrate-overloaded horses. CONCLUSION AND CLINICAL RELEVANCE: Reduced endothelium-dependent relaxation of palmar digital vessels may have a role in the pathophysiology of acute laminitis after carbohydrate overload in horses.     

Effects of xylazine on canine coronary artery vascular rings

OBJECTIVE: To determine the effects of xylazine on canine coronary artery smooth muscle tone. SAMPLE POPULATION: Hearts of 26 healthy dogs. PROCEDURE: Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation. RESULTS: Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.