Contractile effects of vanadate on aorta rings from virgin and pregnant rats

Multiple actions of halothane on contractile response to noradrenaline in isolated mesenteric resistance arteries

Halothane, a volatile anaesthetic, produces systemic hypotension and significantly alters organ blood flow. Isometric force was recorded in isolated rat small mesenteric arteries to investigate its action on contractile response to noradrenaline, the sympathetic neurotransmitter. Halothane (1-5%) enhanced contractile response to noradrenaline in the endothelium-intact arteries, but had little influence in the endothelium-denuded arteries. However, halothane consistently inhibited the noradrenaline response in the endothelium-denuded arteries pretreated with ryanodine (10 microM). The enhancement of the contractile response to noradrenaline in the endothelium-intact arteries was unaffected by treatment with N(G)-nitro L-arginine, tetraethylammonium, apamin, charybdotoxin, indomethacin, diclofenac, nordihydroguaiaretic acid, BQ-123, BQ-788, losartan, ketanserin, or superoxide dismutase. Halothane prolonged vasorelaxation after washout of noradrenaline in the endothelium-denuded arteries. Both ryanodine and vanadate (0.1-0.3 mM), a putative inhibitor of the plasma membrane Ca2+-ATPase, also prolonged the vasorelaxation. Halothane still prolonged the vasorelaxation in the ryanodine-treated arteries, but not in the vanadate-treated arteries. Halothane decreased the pD2 value for the pCa-force relation in the beta-escin-permeabilised, endothelium-denuded arteries. Halothane appears to influence contractile response to noradrenaline through multiple actions including endothelium-dependent enhancing, endothelium-independent enhancing, and endothelium-independent inhibitory actions. Nitric oxide, endothelium-derived hyperpolarising factor, cyclooxygenase products, lipoxygenase products, endothelin-1, angiotensin-II, serotonin, and superoxide anions are not involved in the endothelium-dependent enhancement. The endothelium-independent enhancement is presumably due to its ability to stimulate Ca2+ release from the ryanodine-sensitive intracellular stores, while the endothelium-independent inhibition is due, at least in part, to depressed Ca2+-activation of contractile proteins. Halothane may inhibit the plasma membrane Ca2+-ATPase of vascular smooth muscle cells.

Comparison of the effects of isoflurane and sevoflurane on protein tyrosine phosphorylation-mediated vascular contraction

BACKGROUND

Isoflurane induces greater effects on vasodilation and decreasing blood pressure than sevoflurane. Tyrosine kinase-catalyzed protein tyrosine phosphorylation plays an important role in regulating vascular smooth muscle contraction. The aim of the present study was to compare the effects of isoflurane and sevoflurane on tyrosine phosphorylation-mediated vascular constriction, by assessing the degree of sodium orthovanadate (Na(3)VO(4), tyrosine phosphatase inhibitor)-induced contraction and protein tyrosine phosphorylation of rat aortic smooth muscle.

METHODS

Na(3)VO(4)-induced contraction and protein tyrosine phosphorylation of rat aortic smooth muscle were measured in the presence of genistein, a tyrosine kinase inhibitor, and different concentrations of isoflurane and sevoflurane, using isometric force measurement and Western blot, respectively.

RESULTS

Na(3)VO(4) (10(-4) M) induced sustained contraction and tyrosine phosphorylation of substrates that were both markedly attenuated in the presence of genistein (5 x 10(-5) M). Isoflurane and sevoflurane dose-dependently (1, 2, 3 MAC) attenuated the Na(3)VO(4)-induced contraction (P < 0.05-0.005, n = 8), with a greater degree of inhibition by isoflurane than sevoflurane at 2 MAC (P < 0.01) and 3 MAC (P < 0.05). Both anesthetics also attenuated the total band density of the Na(3)VO(4)-induced, tyrosine-phosphorylated substrates in a concentration-dependent manner (P < 0.05-0.005, n = 4), with much greater attenuation by isoflurane than sevoflurane at 1 and 2 MAC (P < 0.05), respectively.

CONCLUSION

The results of the present study demonstrate that isoflurane exhibits a greater degree of inhibition on the Na(3)VO(4)-stimulated contraction and protein tyrosine phosphorylation of rat aortic smooth muscle compared with sevoflurane. These findings suggest that isoflurane depresses the protein tyrosine phosphorylation-mediated contraction of vascular smooth muscle to a greater degree than sevoflurane.

Vanadium and the cardiovascular functions

Inorganic and organic compounds of vanadium have been shown to exhibit a large range of insulinomimetic effects in the cardiovascular system, including stimulation of glucose transporter 4 (GLUT-4) translocation and glucose transport in adult cardiomyocytes. Furthermore, administration of vanadium compounds improves cardiac performance and smooth muscle contractility, and modulates blood pressure in various models of hypertension and insulin resistance. Vanadium compounds are potent inhibitors of protein tyrosine phosphatases. As a result, they promote an increase in protein tyrosine phosphorylation of several key components of the insulin signaling pathway, leading to the upregulation of phosphatidylinositol 3-kinase and protein kinase B, two enzymes involved in mediating GLUT-4 trans location and glucose transport. In addition, vanadium has also been shown to activate p38 mitogen-activated protein kinase and increase Ca2+levels in several cell types. The ability of vanadium compounds to activate these signaling events may be responsible for their ability to modulate cardiovascular functions.Key words: vanadium compounds, glucose transport, smooth muscle contractility, insulin signaling pathway.

Smooth muscle in the annulus fibrosus of the tympanic membrane: physiological effects on sound transmission in the gerbil

In a wide variety of mammals, the rim of the tympanic membrane (annulus fibrosus) has an array of contractile elements, either smooth muscle [Henson and Henson, J. Assoc. Res. Otolaryngol. 1 (2000) 25-32] or myofibroblasts [Kuijpers et al., Hear. Res. 128 (1999) 80-88]. These elements are anchored peripherally to the bony tympanic ring and centrally to incoming fibers of the pars tensa. Their arrangement suggests that they are involved in the control of tympanic membrane tension. In this study, cochlear microphonic (CM) threshold changes were recorded in gerbils to study the physiological effects of these contractile elements. It was demonstrated that the application of substances known to make smooth muscle contract (vanadate and norepinephrine) caused concentration-dependent elevations in CM thresholds. Maximum changes of 8-9 dB occurred with the lowest frequency tested (2.16 kHz). The application of muscle-relaxing drugs reversed these effects. Controls showed that the threshold changes were not induced by effects on middle or inner ear structures. These results add to emerging evidence that the tympanic membrane has intrinsic control of tension and is potentially able to have some control over energy levels reaching the cochlea.

Insulin-mimetic action of vanadate: role of intracellular magnesium

The insulin-mimetic effect of vanadate is well established, and vanadate has been shown to improve insulin sensitivity in diabetic rats and humans. Although the exact mechanism(s) remain undefined, we have previously demonstrated a direct relation of intracellular free magnesium (Mg(i)) levels to glucose disposal, to insulinemic responses following glucose loading, and to insulin-induced ionic effects. To investigate whether the insulin-mimetic effects of vanadate could similarly be mediated by Mg(i), we utilized (31)P-nuclear magnetic resonance spectroscopy to measure Mg(i) in erythrocytes from normal (NL, n=10) and hypertensive (HTN, n=12) subjects, before and after incubation with insulin and with different doses of sodium vanadate. In NL, vanadate elevated Mg(i) levels, with maximum efficacy at 50 7 micromol/L (186+/-6 to 222+/-6 7micromol/L, P>0.01), as did physiologically maximal doses of insulin, 200 7microU/mL (185+/-6 to 222+/-8 7micromol/L, P<0.01). In HTN, only vanadate, but not insulin, increased Mg(i) (insulin: 173+/-7 to 180+/-9 7micromol/L, P=NS; vanadate: 170+/-7 to 208+/-10 7micromol/L, P<0.01). Mg(i) responses to insulin (r=0.637, P<0.001), but not to vanadate (r=0.15, P=NS), were closely and directly related to basal Mg(i) levels. We conclude that (1) both vanadate and insulin stimulate erythrocyte Mg(i) levels; (2) cellular Mg(i) responses to insulin, but not to vanadate, depend on basal Mg(i) content-the lower the basal Mg(i), the less the Mg(i) response to insulin. As such, (3) Mg(i) responses to vanadate were equivalent among HTN and NL, whereas HTN cells exhibited blunted Mg(i) responses to insulin, and (4) the ability of vanadate to improve insulin sensitivity clinically may be mediated, at least in part, by its ability to increase Mg(i) levels, which in turn, helps to determine cellular insulin action.

Attenuation of rat diaphragm low-frequency fatigue by vanadate in vitro

Sodium vanadate inhibits protein tyrosine phosphatases, including in skeletal muscle. Vanadate increases contractile force of airway, vascular and gastrointestinal smooth muscle. The present study tested the hypothesis that vanadate augments skeletal muscle contractility. Rat diaphragm muscle strips (n=26 from 12 animals) were studied in vitro at 37 degrees C. Muscles contracted isometrically while stimulated supramaximally with one of two protocols: 30 min of continuous 0.1 Hz stimulation, or 5 min of intermittent 20 Hz stimulation (duty cycle 0.33). Vanadate (500 microM)-treated muscle strips were compared with untreated muscle. Vanadate did not affect force or isometric twitch kinetics of otherwise quiescent muscle. During prolonged 0.1 Hz stimulation, force of control muscles declined by 17 +/- 4% over 30 min, whereas muscles incubated with vanadate maintained force virtually unchanged. Force over time was significantly greater with than without vanadate (P = 0.03), with values being significantly different during the last 10 min of the 30 min stimulation period. In the absence of vanadate force declined at a rate of approximately 0.6% per min, whereas with vanadate the rate of force decline was less than 0.1% per min (P < 0.02). During intermittent 20 Hz stimulation, the degree of force decline was not affected by vanadate at any time over a course of 5 min. Isometric contractile kinetics were not altered by vanadate during either 0.1 or 20 Hz stimulation. These data suggest that vanadate ameliorates low- but not higher-frequency fatigue in diaphragm, suggesting a role for protein tyrosine phosphorylation in the regulation of muscle fatigue resistance.

Oxidizing effects of vanadate on calcium mobilization and amylase release in rat pancreatic acinar cells

The effects of vanadate were examined by monitoring intracellular free calcium concentration ([Ca2+]i) and amylase secretion in collagenase-dispersed rat pancreatic acinar cells. Vanadate increased [Ca2+]i by mobilizing calcium from agonist-releasable intracellular calcium stores, since this increase was observed in the absence of extracellular calcium and vanadate failed to increase [Ca2+]i after treatment with thapsigargin in calcium-free medium. Moreover, pretreatment of acinar cells with vanadate prevented the cholecystokinin octapeptide (CCK-8)-induced signal of [Ca2+]i, whereas co-incubation with CCK-8 potentiated the plateau phase of calcium response to CCK-8 without modifying the transient calcium spike. The effects of vanadate on calcium mobilization were reversed by the presence of the sulfhydryl reducing agent dithiothreitol. Vanadate also activated the calcium influx, since an additional enhancement of calcium influx induced by thapsigargin-evoked intracellular store depletion was observed and vanadate reversed the inhibitory effect of lanthanum (an inhibitor of calcium entry) into acinar cells. In addition, vanadate evoked a concentration-dependent release of amylase from pancreatic acinar cells and moreover, reduced the secretory response to CCK-8. We conclude that, in pancreatic acinar cells, vanadate releases calcium from the agonist-releasable intracellular calcium pool and consequently induces amylase secretion. These effects are likely due to the oxidizing effects of this compound.

Role of tyrosine phosphorylation in the regulation of cerebral vascular tone in newborn pig in vivo

The role of tyrosine phosphorylation was investigated using protein tyrosine phosphatase inhibitors in newborn pigs equipped with a cranial window in vivo. We tested the hypothesis that cyclooxygenase and nitric oxide (NO) synthase are physiological targets for tyrosine phosphorylation in cerebral circulation. Phenylarsine oxide dilated pial arterioles and increased prostacyclin and prostaglandin E2 in cortical periarachnoid fluid; these responses were inhibited by indomethacin. Nomega-nitro-L-arginine methyl ester (L-NAME) and Nomega-nitro-L-arginine (L-NNA) inhibited the vasodilation to phenylarsine oxide; the effects of NO synthase inhibitors and indomethacin were additive. Cyclooxygenase-mediated vascular responses were assessed using topical application of arachidonic acid. Phenylarsine oxide and sodium orthovanadata potentiated vasodilation and prostanoid synthesis in response to arachidonic acid. Nomega-nitro-L-arginine methyl ester and Nomega-nitrol-arginine did not affect vasodilation or prostanoid production in response to arachidonic acid, indicating no cross talk between cyclooxygenase and NO synthase. These data indicate that cyclooxygenase and NO synthase are physiological targets for tyrosine phosphorylation in the cerebral circulation of newborn pigs.

Decavanadate possesses alpha-adrenergic agonist activity and a structural motif common with trans-beta form of noradrenaline

Decavanadate, an inorganic polymer of vanadate, produced contraction of rat aortic rings at a relatively high concentration compared to phenylephrine, an agonist of alpha-adrenergic receptor. This effect was blocked by two known alpha-adrenergic receptor antagonists, prazosin and phenoxybenzamine. Decavanadate, formed by possible dimerization of V5 under acid conditions, possessed a structural feature of two pairs of unshared oxygen atoms at a distance of 3.12 A, not found in its constituents of V4 or V5. A structural motif of O..O..O using such oxygen atoms is recognized in decavanadate. This matches with a similar motif of N..O..O that uses the essential amino and hydroxyl groups of the side-chain and the m-hydroxyl group in trans-beta form of noradrenaline. The interaction of such a structural motif with the membrane receptor is likely to be the basis of the unusual noradrenaline-mimic action of decavanadate.

Protein tyrosine phosphorylation in cardiovascular system

Protein tyrosine phosphorylation is believed to play a central role in signaling pathways initiated by growth factor receptor activation. Recent studies have shown that various vasoactive peptides, in addition to eliciting a contractile response, also serve as growth factors for vascular smooth muscle ans stimulate tyrosyl phosphorylation of several endogenous proteins. Some of these proteins have been identified and are similar to those stimulated by growth factor receptor activation. Furthermore, evidence is also accumulating to support an involvement of protein tyrosine phosphorylation in acute action of growth factors and vasoactive peptides on smooth an muscle contractility. This review still briefly summarize the recent work on vasoactive peptide-mediated protein tyrosine phosphorylation in cardiovascular tissues and its potential functional significance.