Epinephrine-induced Contractions Research Articles

The novel mechanism of bitter taste receptors attenuating rat ventricular contractility

BackgroundBitter taste receptors (TAS2Rs) are a family of G‐protein coupled receptors, firstly identified in the tongue. Recent advances show the expression of TAS2Rs in extra‐oral organs, such as trachea, gastrointestinal tract, and heart. These findings raised up the interest of studying the physiological functions of TAS2Rs besides taste sensing. It has been demonstrated that the activation of TAS2Rs activates gustducin and release Gβγ, which leads to the inhibition of L‐type Ca2+ channels and the consequent airway smooth muscle relaxation.AimThis study investigates the expression of TAS2Rs in rat heart, and the mechanism of bitter compounds regulation of the contractility of cardiac muscle.MethodsRats were excised by thoracotomy and left ventricle strips were isolated. The strips were processed to study the expression levels of TAS2Rs mRNA by real time quantitative PCR (qPCR). Langendorff isolated heart models were used to record the left ventricle pressure and heart rate. Isometric tension recordings were used to measure electrical field stimulation (EFS,1 Hz – 5 V) and epinephrine‐induced contractions in combination with bitter compounds, denatonium and quinine. The conventional whole‐cell patch clamp techniques were used to record the voltage activated L‐type Ca2+ channel currents. The statistic data are expressed as mean ± SEM.ResultsWe detected the mRNA of 6 bitter receptors, TAS2R108, 120, 121, 126, 135, and 143 in the left ventricle tissues and isolated cardiomyocytes of rat, in which the mRNA level of TAS2R120 is the highest among the six followed by TAS2R143 and 121.In isolated perfused rat hearts denatonium (50 μM) reduced the heart rate from 235 ± 3 to 101 ± 12 (N=4). In isolated left ventricle strips, EFS and epinephrine induced phasic contractions which were inhibited completely by tetracaine (100 μM), a ryanodine channel blocker. Denatonium and quinine inhibited EFS‐induced ventricular contractions amplitude and frequency in a concentration dependent manner (N=6, n=6). Both of denatonium and quinine, at concentrations of 10 to 200 μM, caused a complete inhibition of epinephrine (10 μM)‐elicited phasic contractions (N=6, n=6).Nifedipine (10 μM), a selective L‐type Ca2+ channel blocker, partially inhibited EFS‐induced contractions, and the subsequent addition of denatonium (200 μM) and quinine (200 μM) cause a further and complete inhibition of the contractions (N=5, n=5). The non‐selective β‐adrenergic receptor antagonist, propranolol, did not have any effect on the EFS‐induced contractions. In isolated ventricular cardiomyocytes, denatonium (10 μM) reduced the L‐type Ca2+ channel currents density at 10 mV from 4.0 pA/pF to 1.6 pA/pF.ConclusionBitter taste receptor agonists, denatonium and quinine, induce negative ionotropic effects in rat hearts. Our data provide mechanistic evidence that TAS2Rs agonists can inhibit the cardiac contractions by attenuating the L‐type Ca2+ channels, and the consequent Ca2+ release by ryanodine receptors. TAS2Rs agonists can be useful pharmacological tools for the treatment of ventricular tachycardia.Support or Funding InformationSupported by Southwest University and the International Affair Agency of ChongqingThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Obestatin/Ghrelin System as a Novel Regulatory Mechanism of Iris Muscle Contraction

Purpose: To evaluate obestatin and ghrelin effects on iris muscle contraction. Materials and Methods: Obestatin (10−5 M) or ghrelin (10−5 M) were tested on two consecutive carbachol-or epinephrine-elicited contractions of iris rabbit sphincter or dilator muscles. Ghrelin and obestatin effects on iris muscles basal tension were also tested, and their effects on iris sphincter EFS-elicited contraction were evaluated. Results: Compared with the first, tension of the second carbachol-induced contraction of the iris sphincter decreased 11.5 ± 5.5% in the vehicle group, increased 19.0 ± 10.2% in presence of obestatin, and remained unchanged by ghrelin. Epinephrine-induced contractions were not affected by obestatin or ghrelin. EFS-elicited contractions were decreased 9.3 ± 3.2% by ghrelin. Basal tension of the iris sphincter decreased 21.7 ± 3.7% in presence of ghrelin (10−5 M), while that of the dilator decreased 14.1 ± 5.0% in presence of obestatin (10−5 M). Conclusion: This study suggests that obestatin potentiates the cholinergic contraction of the iris sphincter and relaxes the iris dilator muscles.

Vascular Response of Current and Potential Dental Etchants

This study investigated the hemostatic properties of current and potential dentin conditioners by observing the contraction of blood vessels using the rat carotid artery model. Four different agents (3M Scotchbond etchant, NRC, RC-Prep, File-Eze) were used. Dose-dependent contractions/relaxations of the test materials were first compared with epinephrine, followed by administration of papaverine, which was used to reverse epinephrine-induced contractions. In all sequences, the contraction or relaxation forces produced by the test and control materials were recorded using a force displacement transducer. RC-Prep was the only agent to produce epinephrine-like contractions (vasoconstriction) in the rat carotid artery. No contraction could be achieved with 3M etchant. Dose-dependent relaxations were observed with 3M etchant, which eventually led to collapse of the vessel wall; afterward, no response could be achieved with administration of adrenalin. File-Eze and NRC also did not cause vasoconstriction. Both materials caused dose-dependent relaxations in the smooth muscle. However, subsequent administration of adrenalin and papaverine caused dose-dependent contractions and relaxations, respectively, showing that these etchant effects did not lead to collapse of the vessel wall, as did 3M etchant.

Peptidergic control of gastrointestinal blood flow in the estuarine crocodile, Crocodylus porosus.

Peptidergic mechanisms influencing the resistance of the gastrointestinal vascular bed of the estuarine crocodile, Crocodylus porosus, were investigated. The gut was perfused in situ via the mesenteric and the celiac arteries, and the effects of different neuropeptides were tested using bolus injections. Effects on vascular resistance were recorded as changes in inflow pressures. Peptides found in sensory neurons [substance P, neurokinin A, and calcitonin gene-related peptide (CGRP)] all caused significant relaxation of the celiac vascular bed, as did vasoactive intestinal polypeptide (VIP), another well-known vasodilator. Except for VIP, the peptides also induced transitory gut contractions. Somatostatin and neuropeptide Y (NPY), which coexist in adrenergic neurons of the C. porosus, induced vasoconstriction in the celiac vascular bed without affecting the gut motility. Galanin caused vasoconstriction and occasionally activated the gut wall. To elucidate direct effects on individual vessels, the different peptides were tested on isolated ring preparations of the mesenteric and celiac arteries. Only CGRP and VIP relaxed the epinephrine-precontracted celiac artery, whereas the effects on the mesenteric artery were variable. Somatostatin and NPY did not affect the resting tonus of these vessels, but somatostatin potentiated the epinephrine-induced contraction of the celiac artery. Immunohistochemistry revealed the existence and localization of the above-mentioned peptides in nerve fibers innervating vessels of different sizes in the gut region. These data support the hypothesis of an important role for neuropeptides in the control of the vascular bed of the gastrointestinal tract in C. porosus.

Antiaggregating and vasodilatory effects of a new nitroderivative of acetylsalicylic acid

We studied in vitro the effects on platelet aggregation and vascular tone of a new nitrocompound (nitroxy-butyl-acetylsalicylate: NO-ASA). In order to elucidate any possible activity due to the release of nitric oxide or the inhibition of platelet cyclo-oxygenase we compared NO-ASA to acetylsalicylic acid. NO-ASA 1 mM inhibited arachidonic acid-induced platelet aggregation (basal 75.4 ± 2.35%; NO-ASA 22 ± 3.46%; M ± SEM; P<0.001; n=6), but proved less active than acetylsalicylic acid (complete inhibition at 2 × 10 −5 M). NO-ASA also significantly reduced thrombin-induced (0.04–0.08 U/ml) platelet aggregation in acetylsalicylic acid-treated platelets (basal 70.5 ± 1.7%; NO-ASA 35.4 ± 2.2%; P<0.001; n=10; IC 50 7 × 10 −5 M). Methylene blue reduced the effects of NO-ASA on thrombin-induced (NO-ASA 46.7 ± 5.25%; NO-ASA + MB 59.1 ± 4.3%; P<0.01; n=8), but not arachidonic acid-induced platelet aggregation. The inhibitory effects of NO-ASA on platelet aggregation were partially removed by oxyhaemoglobin. Platelet thromboxane A 2 production (TXB 2 concentration in the supernatant of the aggregate 35.38 ± 7.81 ng/ml; n=8), was totally abolished by acetylsalicylic acid (0.17 ± 0.04 ng/ml; P<0.001; n=8) and reduced by NO-ASA (8.3 ± 4.05 ng/ml; P<0.01; n=8). In vitro studies on isolated rat aortic rings showed NO-ASA 10 −3 M, but not ASA up to 10 −3 M, induce a dose dependent vasorelaxation (100% of epinephrine-induced contraction) both in intact and endothelium denuded arteries (IC 50 5 × 10 −5 M). Addition of methylene blue reversed this relaxation. In conclusion these data demonstrate that NO-ASA acts through a double mechanism: a) by inhibiting cyclo-oxygenase and b) by releasing NO active on guanylyn cyclase both in platelets and in vascular smooth muscle cells.

Effects of Cirsiliol, a Flavone Isolated from Achillea fragrantissima, on rat isolated smooth muscle

AbstractThe effects of the flavone cirsiliol on rat isolated phenylephrine-precontracted proximal aorta, acetylcholine-pre-contracted trachea, urinary bladder, and uterus were studied. Cirsiliol (10−8-3×10−4M) caused concentration-dependent relaxation of the rat isolated phenylephrine-precontracted proximal aorta, acetylcholine-precontracted trachea, urinary bladder and inhibited the phasic contractions and the tone of the uterus. Cirsiliol (3×10−5, 1×10−4 and 3×10−4M) shifted to the right epinephrine concentration-effect curves on the proximal aorta and significantly inhibited the maximum contractions induced by epinephrine in this preparation. In Ca2+-free, depolarizing salt solution, cirsiliol (3×10−5, 1×10−4 and 3×10−4M) also shifted to the right CaCl2 concentration-effect curves on the proximal aorta and inhibited the maximum contractions induced by 3×10−2M CaCl2 whereas it had no effect on epinephrine-induced contractions in Ca2+-free salt solution on the same preparation. These observations suggest...

Medical Management of Benign Prostatic Hyperplasia: A Canine Model Comparing the in Vivo Efficacy of Alpha-1 Adrenergic Antagonists in the Prostate

Medical management of benign prostatic hyperplasia (BPH) is an alternative to surgical treatment of this disease. A major target for pharmacologic therapy is the alpha-1 adrenergic receptor, since activation of this receptor by endogenous catecholamines is thought to contribute to outlet obstruction. In the present study, we compared the potency of various alpha-1 adrenergic antagonists against epinephrine-induced contraction of the canine prostate. The drugs tested were dibenzyline, prazosin, terazosin and YM617. The rank order of potency, comparing inhibitory constants (Ki’s), was found to be YM617>> prazosin> terazosin> dibenzyline. This study is the first to compare all of these drugs directly in the prostate in vivo. The rank order of potency of the drugs is similar to the rank order of potency at other alpha-1 receptors. These results demonstrate that 1) our model is useful in confirming activity of drugs at the alpha-1 receptor and 2) the prostate alpha-1 receptor is similar to other alpha-1 receptors. Whether activity at the alpha-1 adrenergic receptor is a sufficient determinant of clinical efficacy of these drugs remains to be determined.

Pacing-Induced Heart Failure in the Dog: Evaluation of Peripheral Vascular α-Adrenoceptor Subtypes

Postjunctional alpha-adrenoceptor characteristics were evaluated in canine dorsal pedal arterial and saphenous vein rings studied before and after development of severe pacing-induced heart failure (CHF). Before CHF, all agonists produced concentration-dependent increases in tension of both blood vessels. After development of CHF, the responsiveness and sensitivity of the vessels to the alpha 1-agonists and the mixed agonists were significantly increased as compared with control. The maximum responses to BHT 920 and BHT 933 remained unaltered after CHF, but both vessels showed decreased sensitivity to BHT 920. Before CHF, the rank order of potency with respect to norepinephrine (NE) for the dorsal pedal artery was as follows: NE greater than epinephrine greater than methoxamine greater than BHT 933 greater than BHT 920, and for the saphenous vein was epinephrine greater than NE greater than BHT 933 greater than methoxamine greater than BHT 920. At peak CHF, the rank order of potency for the artery was epinephrine greater than NE greater than methoxamine greater than BHT 933 greater than BHT 920, whereas in the vein BHT 920 was approximately 80 times less potent than NE (as compared with being only five times less potent before CHF). Prazosin was a potent, competitive antagonist (pA2 values of 9.2 and 9.0 for the artery and the vein) of methoxamine-induced contractions before development of CHF. Prazosin had a 10-fold lower potency against epinephrine-induced contractions in the dorsal pedal artery, whereas it was not competitive against epinephrine in the saphenous vein. Against the selective alpha 2-agonists, prazosin either showed no antagonism or was not competitive. After CHF, prazosin was non competitive against all agonists tested. Yohimbine was a potent, competitive antagonist against BHT 920 both before and at CHF. Yohimbine had intermediate antagonism against epinephrine and produced no antagonism of methoxamine-induced contractions. We conclude that increased reactivity and sensitivity of the peripheral vasculature to alpha 1-agonists occurs at CHF.

Characterization of α-adrenoceptors in the late pregnant ovine uterine artery

Ring segments of uterine artery from pregnant ewes within two weeks of term were used to study alpha-adrenoceptor-mediated contractions in vitro. The order of potency for the agonists giving the same maximum contractile effect was norepinephrine greater than epinephrine greater than phenylephrine much greater than methoxamine; whereas the maximum contraction to clonidine was about 42% of that to norepinephrine. Medetomidine caused inconsistent responses and B-HT 920 was ineffective. Phentolamine (10(-7) M) and prazosin (10(-7) M), significantly (P less than 0.005) inhibited epinephrine-induced contractions, whereas yohimbine (10(-7) M) was ineffective. Clonidine-induced contractions were blocked by prazosin (10(-8) and 10(-7) M) but were insensitive to yohimbine (10(-7) M). The dissociation constant (KB) of the competitive antagonist prazosin was determined with phenylephrine. The slope of the Schild plot was not significantly different from unity. The pA2 value of prazosin with phenylephrine was 9.1 +/- 0.2. The dissociation constant (KA) and the relationship between receptor occupancy and response for the full agonists norepinephrine and phenylephrine were analyzed using the irreversible antagonist dibenamine. The KA values of norepinephrine and phenylephrine were 1.5 +/- 0.4 x 10(-6) M and 2.5 +/- 0.8 x 10(-6) M, respectively. The occupancy response relationship was nearly linear and the half-maximal response to norepinephrine or phenylephrine was obtained with a 50% receptor occupancy. These results suggest that contraction to alpha-adrenoceptor agonists in the late pregnant ovine uterine artery is mediated primarily by alpha 1-adrenoceptors, and there appears to be no substantial alpha 1-adrenoceptor reserve in this tissue. The mediation of contraction via alpha 2-adrenoceptors is not prominent in this tissue.

An in Vivo Evaluation of Alpha Adrenergic Receptors in Canine Prostate

The role of alpha2 adrenergic receptors in the prostate in vivo is unknown. A model was developed to measure canine prostatic urethral pressure in vivo, and to assess the ability of various alpha adrenergic blocking agents to affect prostatic pressure. In this model, an esophogeal pressure catheter is inserted into the prostatic urethra to record prostatic urethral pressure. We investigated the effects of alpha adrenergic agonists and antagonists on prostatic pressure using this model. Dose-response curves were generated for epinephrine, and were then repeated in the presence of either prazosin (alpha1 antagonist), yohimbine (alpha2 antagonist) or SK&F-86466 (alpha2 antagonist). Prazosin was the most potent of the three drugs in competitively blocking epinephrine-induced contraction of the prostate, with an inhibition constant of 0.24µg./kg. calculated from the double reciprocal plot. Clonidine, an alpha2 adrenergic agonist, caused contraction of the prostate, which was also blocked by prazosin. Furthermore, the specific alpha2 agonist BHT-920 was totally inactive in our system. These results demonstrate that the increase in urethral pressure caused by alpha-adrenergic agonists can be blocked by alpha adrenergic antagonists. However, the specific alpha1 antagonist, prazosin, is more potent than alpha2 antagonists, and is also effective against an alpha2 agonist, clonidine. This suggests that blockade of alpha1 receptors may be a more useful strategy for causing relaxation of the prostate than blockade of alpha2 receptors.

Unusual Effects of Histamine Antagonists on Canine Nasal Blood Vessels

This paper documents some of the lesser-known effects of antihistamines on nasal blood vessel smooth muscle. An in vitro muscle tension detecting technique was utilized. Three different H1 receptor antagonists (pyrilamine maleate, chlorpheniramine maleate, promethazine hydrochloride), and the H2 antagonist metiamide were used. Besides their basic antihistaminic effect, H1 antagonists (classical antihistamines) induced a contraction, enhanced norepinephrine- and epinephrine-induced contractions, and relaxed the sustained contraction induced by methoxamine hydrochloride or high concentrations of potassium. Although these effects were quite similar to those induced by histamine, the mechanism of each effect was apparently different. Metiamide did not produce these effects except for the relaxation of the sustained contraction. The actions of histamine antagonists seem to be much more complicated than expected.

H1 and H2 histamine receptors in the in vitro nasal mucosa.

The effect of histamine was studied on canine nasal blood vessels with an in vitro muscle tension measuring technique. Observations suggest that histamine has at least four different effects: 1) inhibits the contraction induced by nerve terminal stimulation, 2) relaxes the smooth muscle under sustained contraction, 3) contracts the smooth muscle, 4) enhances (nor)epinephrine-induced contraction. Both H1 and H2 histamine receptors were identified in the mucosal blood vessels of the canine nasal septum. H1 receptors were present at the surface of vascular smooth muscle and mediated the contractile response (3) due to opening calcium influx channels. H2 receptors existed at both sympathetic nerve terminals and the muscle cell membrane. Stimulation of either H2 receptor caused muscle relaxation. The presynaptic H2 receptors mediated vasodilation through the inhibition of norepinephrine release from the nerve endings. Postsynaptic H2 receptors caused direct muscle relaxation possibly through an increase in cyclic AMP. Both H1 and H2 antagonists worked well, whereas agonists did not properly produce theoretically expected results in these specimens.

Comparative effects of nitroprusside and nitroglycerin; actions on phasic and tonic components of arterial smooth muscle contraction

Nitroprusside shares many of the pharmacological actions of nitroglycerin. We investigated the characteristics of the relaxant action of these two vasodilators on vascular smooth muscle. Both drugs completely reduced, in a dose-related fashion, the maximal tension developed in rabbit aortic strips isometrically contracted with angiotensin-II or a submaximal epinephrine contraction (70% of maximal). The ED 50 values for the two drugs were similar, but their dose-response curves were not parallel. For aortic strips maximally contracted with epinephrine, only 70% relaxation was achieved with single maximal effective doses of nitroprusside or nitroglycerin, and the ED 50 was increased 25- and 630-fold respectively. Complete relaxation could not be induced by combined effects of single doses of the two drugs although cumulative exposure to nitroprusside, but not to nitroglycerin, caused complete relaxation. Pretreatment of aortic strips with nitroprusside or nitroglycerin decreased their reactivity to epinephrine (decrease of potency and intrinsic activity) and completely blocked angiotensin contractions. Contraction velocity analysis revealed that nitroprusside and nitroglycerin reduced the tonic contribution to total epinephrine contraction by 64 and 47% respectively; the phasic contribution was reduced by approximately 50% by either vasodilator. Angiotensin-induced contraction was reduced also by nitroprusside and nitroglycerin. It is postulated that the limitation of both phasic and tonic modalities of contraction by nitroprusside and nitroglycerin is due to a membranal action of the vasodilators to promote not only less uptake but also a loss of Ca 2+ which could explain the attenuation of phasic (angiotensin) contraction and in addition the decrease observed for the tonic component of epinephrine-induced contraction.

Differential Effects of p-Chloromercuriphenyl Sulfonate on Arterial Responses to Epinephrine and Serotonin

SummaryThe purpose of this study was to gain insight into the role of sulfhydryl (SH) groups in arterial receptors for vasoactive simulants. We studied the effects of p-chloromercuriphenyl sulfonate (PCMBS, 10-6 to 10-4 M), a potent and specific SH binding agent, on contractile responses produced with 10-5M epinephrine and 10-5M serotonin in helical strips of arterial smooth muscle prepared from tail arteries of the rat. Responses to epinephrine were rapidly, markedly, and reversibly inhibited with low concentrations of PCMBS. In contrast, responses to serotonin were slowly inhibited when high concentrations of PCMBS were tested. The inhibitory effect of PCMBS (10-5M) on epinephrine-induced contractions was prevented by 10-3 M cysteine. Alpha-adrenergic blockade with phentol-amine (10-4 or 5 × 10-6 M) virtually abolished responses to epinephrine and significantly decreased responses to serotonin. These findings are consistent with the view that arterial receptors for contractile responses to epinephrine a...

Rabbit testicular contractions: bimodal interaction of prostaglandin E1 with other agonists.

Contractions of rabbit testicular capsules in vitro and in vivo in response to prostaglandin F2a, epinephrine, and acetylcholine were measured in the presence of prostaglandin E1 (PGE1). Contractions induced by these agents were markedly inhibited at 10-7 M concentrations of PGE1 in both preparations. Even so, lower levels of PGE1 (10-8 or 10-9 M) potentiated epinephrine- or acetylcholine-induced stimulation of increased tonus in vovo. Epinephrine, PGE1, and PGF2a caused increased tonus and contractions in vitro and in vovo; however, acetylcholine stimulated the testicular capsule much more in vivo than in vitro. The control log dose-response curve for epinephrine stimulation showed a slight decrease in the upper concentration range. Propranolol, a beta-adrenergic blocking agent, more than overcame this decrease and also prevented a theophylline inhibition of epinephrine-induced contraction. Prostaglandin E1's bimodal effect gives it the ability to modulate testicular contractions by increasing stimulation by other agonists in vovo at low concentrations of PGE1 but inhibiting it at higher concentrations. Thus PGE1 is a potential regulator of testicular capsular motility and possibly sperm transport and hemodynamics.

Adenylcyclase-phosphodiesterase system in arterial smooth muscle

Isoproterenol, epinephrine, and norepinephrine increase cyclic AMP formation in rat aorta through their stimulatory effect on adenyl cyclase. Isoproterenol, and epinephrine and norepinephrine in the presence of phentolamine, have a relaxing effect on the rat aortic strip. Propranolol prevents the rise in cyclic AMP with the three catecholamines and modifies the epinephrine-induced contraction. Inhibition of phosphodiesterase is also accompanied by relaxation of arterial smooth muscle.

Studies of the chemical nature of the α-adrenergic receptor—V: validity of the receptor protection approach

The uptake of [ 14C]phenoxybenzamine into rabbit aortic strips bas been measured at bath concentrations sufficient to abolish epinephrine-induced contractions. The relationship between the degree of protection conferred by different doses of epinephrine against [ 14C]dibenamine blockade of α-adrenergic receptors and the count differences between protected strips and unprotected controls has been investigated. The results support the view that epinephrine combines with a large number of non-specific sites as well as α-adrenergic receptors.

Catechol-O-methyl transferase inhibition and potentiation of epinephrine responses by desmethylpapaverine

Papaverine was demethylated chemically to yield desmethylpapaverine (papaveroline). In low and intermediate concentrations it moderately potentiated contractions, of isolated aortic strip of rabbit, induced by epinephrine, angiotensin, and potassium (nonspecific potentiation); in higher concentrations it markedly potentiated the epinephrine-induced contractions only (specific potentiation). On the blood pressure of the anaesthetized cat desmethylpapaverine potentiated epinephrine-induced pressor responses and prolonged isoproterenol-induced depressor responses. In vitro studies of catechol-O-methyl transferase (COMT) indicate that desmethylpapaverine is a potent inhibitor of the enzyme. It is suggested that the specific potentiation produced by desmethylpapaverine may be due to inhibition of COMT.

Role of calcium in contractile excitation of vascular smooth muscle by epinephrine and potassium.

A technique of constant-flow perfusion of isolated segments of dog intestinal arteries was used to investigate the role of calcium in contractile excitation of vascular smooth muscle by epinephrine and potassium. Contractile responses of the arterial muscle to epinephrine, perfused at constant concentration, and to potassium, perfused at high concentration, were both promptly prevented by calcium deionizing accomplished by simultaneous perfusion with solutions containing ethylenediaminetetraacetate (EDTA) for the chelation of available calcium ions. Potassium-induced muscular contracture was removed in a few minutes and epinephrine-induced contractions of potassium-depolarized muscle were soon prevented by perfusion with solutions containing ionic species of EDTA which chelated calcium ions. In muscle depolarized by potassium and calcium deionized by perfusion with solution containing MgK 2 EDTA, the contractile responses to epinephrine were immediately restored by combined perfusate injection of calcium ion and epinephrine. Injections of isosmotic calcium chloride solution of subthreshold or threshold contractile strength, when the artery had been perfused with sodium chloride-rich solution and the muscle had been relaxed, produced much greater contractions during both potassium and epinephrine excitation. The contractions from injected calcium chloride solution were increased by epinephrine excitation of muscle already depolarized by the external application of potassium sulfate, as they were in the previously polarized or resting muscle excited by epinephrine. The thesis is advanced that the relative muscular contractions induced by standard injected volumes of isosmotic calcium chloride solution were a relative index of the calcium influx and calcium permeability of the arterial smooth muscle cells in the various experimental conditions prevailing. Equimolar injections of isosmotic calcium iodide solution produced greater muscular contractions than did injections of isosmotic calcium chloride solution. The possibility is discussed that transmembranous passage of calcium into vascular muscle cells occurs significantly in the form of associated ion pairs. A dual effect of potassium on vascular smooth muscle behavior was observed in that a previously contractile-inducing amount of injected potassium exerted a relaxing effect on the arterial segment perfused with low-potassium solution when the muscle was contracted by epinephrine. It is concluded that: (a) calcium is essentially involved in excitation-contraction coupling of vascular smooth muscle; (b) the calcium permeability and calcium influx of vascular smooth muscle is greatly increased during both epinephrine and potassium excitation of the muscle cells, and epinephrine accomplishes these calcium changes even in muscle already depolarized by potassium; (c) adrenergic neurohormones exert their vasoexcitatory contractile effect by a membrane reaction which is basically nonelectrical and which primarily triggers an increased permeability and influx of calcium into the vascular myoplasm; and (d) the migrated calcium activates intracellularly the myoplasmic events of vascular smooth muscle contraction.

Effect of L-3,3'5-triiodothyronine on epinephrine-induced contraction of isolated rabbit aortic strip.

SummaryL-3,3′,5-Triiodothyronine (T3) potentiated epinephrine-induced contractions of isolated rabbit aortic strip only under the following conditions: 1) copper acetate (2 × 10-7) addition to the bath or 2) when trace metal contaminants were occasionally present in the bath, as suggested by short duration control responses to epinephrine. EDTA, a potent chelating agent, similarly potentiated epinephrine contractions. It was further demonstrated that T3 delays metal-catalyzed oxidation of epinephrine in separate incubate experiments. This effect appears to be the major factor in potentiation of epinephrine contractions by T3. It is further concluded that T3 probably acts by virtue of its metal-chelating effect.