Contraction Of Aortic Smooth Muscle Research Articles

Feverfew and vascular smooth muscle: extracts from fresh and dried plants show opposing pharmacological profiles, dependent upon sesquiterpene lactone content.

Preparations of fresh or dried feverfew (Chrysanthemum parthenium) are widely consumed in the U.K. as a remedy for arthritis and migraine, but the pharmacological basis for this has not been established. We have, therefore, compared the properties of extracts of fresh plants with those of dried powdered leaves available commercially from health food shops. The two extracts differed radically in their content of alpha-methylbutyrolactones and in their pharmacological profile when tested in vitro on the rabbit aortic ring and rat anococcygeus preparations. Extracts of fresh leaves caused does- and time-dependent inhibition of the contractile responses of aortic rings to all receptor-acting agonists so far tested; the effects were irreversible and may represent a toxic modification of post-receptor contractile function in the smooth muscle. The presence of potentially -SH reactive parthenolide and other sesquiterpene alphamethylenebutyrolactones in these extracts, and the close parallelism of the actions of pure parthenolide, suggest that the inhibitory effects are due to these compounds. In contrast , chloroform extracts of dried powdered leaves were not inhibitory but themselves elicited potent and sustained contractions of aortic smooth muscle that were not antagonised by ketanserin (5-HT2 receptor antagonist). These extracts did not contain parthenolide or butyrolactones according to a chemical-HPLC assay, We conclude that there are marked differences in the pharmacological potency and profiles between preparations from fresh and dried feverfew and that this may relate to their lactone content. As the effects of the lactones are potentially toxic, it will be necessary to compare the clinical profiles and side effects of preparations obtained from the two sources.

An endothelin B receptor-selective antagonist: IRL 1038, [Cys 11-Cys 15]-endothelin-1(11–21)

In the inhibition of specific binding or [ 125]endothelins (ETs) to membranes from various tissues of rats, guinea pigs, pigs and humans, [Cys 11-Cys 15]-ET-1(11–21), IRL 1038, has a much higher affinity for ET B receptors ( K i = 6–11 nM) than for ET A receptors ( K i = 0.4–0.7 μM). In contraction assays, with ET-3 as a stimulant, 3 μM IRL 1038 antagonized the ET B receptor-mediated contraction of guinea pig ileal and tracheal smooth muscle without any significant agonistic activity, but did not effect the ET A receptor-mediated contraction of rat aortic smooth muscle. IRL 1038 is, therefore, considered to be the first antagonist selective to the ET B receptor.

Mechanisms of caffeine-induced contraction and relaxation of rat aortic smooth muscle.

1. Using front-surface fluorimetry and Fura-2, we determined the effects of caffeine on cytosolic calcium concentration ([Ca2+]i) and on tension of strips of the rat thoracic aorta. We also determined the effects of caffeine on 45Ca2+ influx into the strips. The objective was to elucidate the mechanism of contraction and relaxation in vascular smooth muscle, as induced by caffeine. 2. In normal physiological salt solution (PSS), caffeine induced a transient tension development, while it induced a biphasic change in [Ca2+]i. The initial transient peak in [Ca2+]i which coincided with tension development was followed by a sustained increase. Thus, changes in tension did not follow changes in [Ca2+]i. In Ca(2+)-free PSS, both the caffeine-induced contraction and the increase in [Ca2+]i were transient. It was suggested that in both the presence and absence of extracellular Ca2+, the transient increase in [Ca2+]i was due to the release of Ca2+ from the intracellular store. Although the sustained increase in [Ca2+]i depended on extracellular Ca2+, it was not affected by diltiazem, a Ca2+ antagonist. 3. Caffeine inhibited the increase in [Ca2+]i and tension development during 118 mM-K+ depolarization, in a concentration-dependent manner. The extent of reduction in tension (relaxation) was greater than that expected from the reduction in [Ca2+]i based on the [Ca2+]i-tension relationship observed with K+ depolarization. Pretreatment of the strips with ryanodine did not alter the inhibitory effects of caffeine. 4. Caffeine inhibited the increased [Ca2+]i and developed tension during stimulation by 10(-5) M-noradrenaline, in a concentration-dependent manner. 5. Dibutyryl cAMP (10(-4) M) inhibited both high K(+)-induced and noradrenaline-induced tension development. Inhibition of an increase in [Ca2+]i in relation to the inhibition of tension during noradrenaline stimulation was much greater than that in 118 mM-K+ depolarization. 6. Although caffeine per se had no effect on 45Ca2+ influx in the strips in normal PSS, caffeine did inhibit the increase in 45Ca2+ influx stimulated by 118 mM-K+ or by 10(-5) M-noradrenaline, to a similar extent and with similar IC50 values. 7. The characteristic features of the effects of caffeine on vascular smooth muscle, i.e. the transient nature of contraction and the relaxation of precontracted strips could be explained as follows: caffeine is able to reduce [Ca2+]i after releasing Ca2+ from intracellular stores; however, this may play a minor role. Independent of the [Ca2+]i reduction, the second messenger, cAMP, might directly influence the [Ca2+]i-tension relationship, and if so, would play a major role.

Synergistic actions of nitrovasodilators and isoprenaline on rat aortic smooth muscle

Previous studies have established that nitrovasodilators potentiate the inhibition of platelet function by activators of adenylyl cyclase, but uncertainty exists as to whether a comparable effect is seen in vascular smooth muscle. We initially studied the effects of the nitrovasodilators, sodium nitroprusside (SNP) and 3-morpholinosydnonimine (SIN-1), on the relaxation by isoprenaline of rat aortic smooth muscle that had been precontracted by phenylephrine. Concentrations of SNP (0.25 nM) and SIN-1 (30 nM) that relaxed aortic smooth muscle less than 30% alone, caused significant (3-fold) decreases in the IC 50 values for isoprenaline. The cAMP phosphodiesterase inhibitors, cilostamide (20 nM) and Ro 20–1724 (10 μM), caused comporable reductions in the IC 50 values for isoprenaline. At these concentrations, each of the four compounds also increased the maximum relaxation achieved with isoprenaline. Even more marked synergistic interactions were observed between isoprenaline and either the nitrovasodilators or the cAMP phosphodiesterase inhibitors when these compounds were added simultaneously before contraction of aortic smooth muscle by phenylephrine. Thus, concentrations of SNP (5 nM), SIN-1 (1 μM), cilostamide (1 μM) and Ro 20–1724 (100 μM) that inhibited contraction by less than 30% decreased the IC 50 values for isoprenaline by 8- to 10-fold. At the above concentrations, these compounds each caused a supra-additive inhibition of contraction when added with 100 nM isoprenaline. Thus, synergism between nitrovasodilators and isoprenaline, an activator of adenylyl cyclase, could be detected in vascular smooth muscle and was particularly marked when inhibition of contraction was studied. This action of nitrovasodilators resembled that of inhibitors of cAMP phosphodiesterase.

O-199 - Glibenclamide-lnduced relaxation and contraction of rabbit aortic smooth muscle.

O-199 - Glibenclamide-lnduced relaxation and contraction of rabbit aortic smooth muscle.

Nitroprusside enhances isoprenaline-induced increases in cAMP in rat aortic smooth muscle

Low concentrations of sodium nitroprusside (SNP) and of isoprenaline acted synergistically to inhibit the phenylephrine-induced contraction of rat aortic smooth muscle. In experiments with these concentrations, SNP enhanced the increases in smooth muscle cAMP caused by isoprenaline by 4- to 5-fold, whereas the SNP-induced increases in tissue cGMP were unaffected by isoprenaline. We conclude that cAMP is likely to mediate the synergistic inhibition of the contraction of rat aortic smooth muscle by these compounds.

α1-Adrenergic partial agonists utilize cytosolic Ca2+ more effectively for contraction in aortic smooth muscle

Fura 2 loaded thoracic aorta strips from rabbits were used. Norepinephrine, phenylephrine, clonidine, and tizanidine induced an increase in cytosolic Ca2+ concentration [( Ca2+]i) and muscle tension in a concentration-dependent manner. A positive correlation between [Ca2+]i and tension development owing to the agonists was noted. The slope of regression lines between [Ca2+]i and tension development for clonidine and tizanidine, alpha 1-adrenergic partial agonists, were significantly steeper than those for norepinephrine and phenylphrine, alpha 1-adrenergic full agonists. The intrinsic activities of the partial agonists obtained from tension development were greater than those from changes in [Ca2+]i. These results suggest that the partial agonists cause a greater muscle tension than the full agonists at the same level of [Ca2+]i.

Inhibition of the contraction of the isolated longitudinal muscle of the guinea-pig ileum by botulinum C2 toxin: evidence for a role of G/F-actin transition in smooth muscle contraction.

The effect of botulinum C2 toxin was studied on the contractions of the guinea pig ileum myenteric plexus longitudinal muscle preparation. Botulinum C2 toxin inhibited the muscle contraction induced by electrical stimulation (60 V; 0.5 ms; 0.33 Hz) in a time and concentration dependent manner. The inhibitory effect occurred with a time lag of about 1 h, and depended on the presence of both toxin components. After 4 h of incubation with 1.7 micrograms/ml of component I and 6.7 micrograms/ml of component II of botulinum C2 toxin, the smooth muscle contraction was inhibited by about 60%. At these toxin concentrations, about 55% of the modifiable smooth muscle actin was ADP-ribosylated. Smooth muscle contraction induced by bradykinin and bethanechol were similarly inhibited. Moreover, the C2 toxin inhibited muscle contraction induced by Ba2+, and by direct muscle membrane depolarization (60 V; 10 ms; 0.33 Hz) after suppression of acetylcholine release by normorphine. Also cytochalasin D inhibited the electrically evoked contraction of the ileum longitudinal muscle. In contrast to botulinum C2 toxin, inhibition of contractility by cytochalasin D occurred without a lag phase, and was reversed by washing off the toxin. In contrast of guinea pig ileum longitudinal muscle, botulinum C2 toxin did not reduce the contraction of the rabbit aortic smooth muscle stimulated by K+-depolarization or noradrenaline.

Time- and use-dependent inhibition by ryanodine of caffeine-induced contraction of guinea-pig aortic smooth muscle

We examined how ryanodine interfered with Ca 2+-releasing action of caffeine in guinea-pig aorta. Ryanodine (10μM) decreased the caffeine-induced contraction depending on the time of pretreatment with the agent. The development of ryanodine-effect with time was neither due to a slow access to its binding site nor due to the depletion of stored Ca 2+. Ryanodine more potently inhibited the second or the subsequent contraction due to caffeine than the first one even after the agent was removed from the bath after first caffeine. The enhancement of ryanodine-effect depended on the history of Ca 2+ release but not on an increase in cytoplasmic Ca 2+. The data suggest that an opening of Ca 2+ release channel enhances the interaction of ryanodine with the channel.

Cytochalasin-B and phloretin depress contraction and relaxation of aortic smooth muscle

Cytochalasin-B (20 μM) and phloretin (100 μM) blocked by more than 80% the contractile responses to calcium ions in partially depolarized rabbit aortic strips. Both also blocked, but only by approximately 50%, the responses to noradrenaline and histamine in normal calcium medium. The responses to these agonists in calcium-free EGTA medium were also blocked partially. Cytochalasin-B partially blocked the acetylcholine-induced relaxation of aortic strips precontracted with phenylephrine but not the relaxation due to nitroglycerine or compound A 23187. The relaxation due to isoprenaline was potentiated by cytochalasin B. Since both compounds are known to block hexose transport but share no other known effects, it is suggested that a glycolytic intermediate could be required for the contraction in response to calcium. However, the concentrations of cytochalasin-B required for these effects were somewhat greater than those usually required to block hexose transport.

Ca2+ regulation not associated with phosphorylation of myosin light chain in aortic intima smooth muscle. II. Effects of regulatory proteins on the actin-myosin interaction.

Contractile and regulatory proteins were prepared from bovine aortic intima, and actin from bovine stomach smooth and rabbit skeletal muscles. In the desensitized and reconstituted actomyosin system, the superprecipitation activity was measured by the turbidity method. Superprecipitation of each system was not exhibited even in the presence of Ca ions, but was observable only in the presence of tropomyosin and Ca ions, while 20,000-dalton light chain of myosin remained dephosphorylated during the reaction. Addition of tropomyosin to the reconstituted acto-myosin digest system (trypsin-digested myosin was devoid of 20,000-dalton light chain) also restored the Ca2+-sensitivity. These results indicate that the phosphorylation of myosin light chain is not a crucial step in the contraction of aortic intima smooth muscle. For full activation of the actin-myosin-ATP interaction, additional factors other than the myosin light chain kinase are required, although some contribution of the kinase to the full activation cannot be ruled out.

Effects of ketamine on vascular smooth muscle function.

1In vitro studies were undertaken on rat aortic strips and portal vein segments to determine whether or not the amine-type anaesthetic, ketamine, can exert direct actions on vascular smooth muscle.2 Ketamine was found to inhibit development of spontaneous mechanical activity and lower basal tension. This action took place with ketamine concentrations found in anaesthetic plasma concentrations, i.e., 1 x 10(-5) to 2 x 10(-4) M.3 Ketamine (10(-5) to 10(-3) M) dose-dependently attenuated contractions induced by adrenaline, noradrenaline, angiotensin II, vasopressin and KCl. These inhibitory actions were observed with ketamine added either before or after the induced contractions.4 Ca(2+)-induced contractions of K(+)-depolarized aortae and portal veins were also attenuated, dose-dependently, by ketamine.5 In contrast to the above inhibitory actions, ketamine (2 x 10(-6) to 1 x 10(-4) M) was found to potentiate specifically 5-hydroxytryptamine(5-HT)-induced contractions of both aortic and venous smooth muscle. However, this was only observed if ketamine was added after 5-HT had initiated a contractile response.6 All of the inhibitory, as well as 5-HT-potentiating, effects were completely, and almost immediately, reversed upon washing out the anaesthetic from the organ baths.7 A variety of pharmacological antagonists failed to mimic or affect the inhibitory effects induced by ketamine.8 These data suggest that rat plasma concentrations of ketamine commonly associated with induction of surgical anaesthesia can induce, directly, relaxation and contractile potentiation of vascular muscle.9 These diverse findings may aid in explaining the well-known biphasic pressor actions of ketamine.

Effect of Clonidine on the Contractile Responsiveness of Aortic Smooth Muscle to Norepinephrine

A length-tension relationship was established for aortic smooth muscle of rats by means of membrane depolarization with KCl. Maximal tension was developed by aortic rings when the preload initial tension was 8--9 g. Two aortic rings (4 mm thick) were removed from segments of the aorta cut 1.5 (upper) and 2.4 cm (lower) below the aortic arch. Upper rings appeared to develop a greater tension than lower rings during depolarization with KCl. However, in spite of this quantitative difference between rings from upper and lower segments of the aorta, there were no qualitative differences observed. Clonidine, an antihypertensive agent, induced a contraction in aortic smooth muscle, apparently by way of alpha-adrenoreceptor stimulation. Clonidine also attenuated significantly the development of active tension by norepinephrine but did not inhibit significantly the development of active tension following membrane depolarization with KCl. This suggests that there is a partial antagonism between norepinephrine and clonidine, presumably at the site of the receptors on smooth muscle, and that clonidine does not interfere with the contractile mechanism, per se.

Acetaldehyde on vascular smooth muscle: Possible role in vasodilator action of ethanol

Previous studies on intact and isolated blood vessels indicate that ethanol can exert depressant actions on vascular smooth muscle. This study, using isolated rat aortic strips and portal veins, was designed to ascertain whether acetaldehyde (ACT), a major metabolite of ethanol, could exert similar effects. The results indicate that ACT can: (a) inhibit spontaneous mechanical activity and lower baselin tension in aortic strips; (b)_depending upon concentration, enhance (abolished by phentolamine) or inhibit such spontaneous contractions in portal veins; (c) dose-dependently attenuate contractions induced by epinephrine, angiotensin, vasopressin, serotonin and KCl; (d)_cause non-competitive displacement of the concentration—effect curves of these vasoactive compounds; (e) relax drug-induced contractions of aortic and venous smooth muscle; (f) attenuate Ca 2+-induced contractions of K +-depolarized aortas and portal veins. These profound depressant actions of ACT are not attenuated, prevented or mimicked by α-adrenoreceptors, or release of prostaglandin-like substances. The direct vasodepressant actions of ACT on vascular smooth muscles may play significant roles in alcohol-induced peripheral vasodilatation and hypotension, and cardiovascular collapse noted in the alcohol-Antabuse reaction.

Magnesium withdrawal and contraction of arterial smooth muscle: effects of EDTA, EGTA, and divalent cations.

The divalent cation chelators, CaEDTA and EGTA, were demonstrated to exert opposite effects on contractions of rat aortic smooth muscle induced by withdrawal of external magnesium ([Mg2+]0). Addition of CaEDTA potentiated such contractions more than 100%, while EGTA promoted rapid relaxation. Rapid relaxation of contractions induced by withdrawal of [Mg2+]0 could also be induced by Mn, Ni, and Cd but not Sr. Using EC50's, a relative descending order of contractile inhibition was noted for the divalent cations: Mn greater than Cd greater than Mg greater than Ni. The ability of CaEDTA to potentiate contractions produced by withdrawal of [Mg2+]0, as well as the ability of divalent cations to relax contractions of aortic smooth muscle, appears to be related to actions on the transmembrane flux of Ca2+. These findings thus lend support to the view that Mg ions either play an important role in regulating membrane permeability to [Ca2+]0 or occupy membrane sites which are exchangeable with membrane-bound Ca in certain types of aterial smooth muscle.