Rho kinase inhibitors reduce neurally evoked contraction of the rat tail artery in vitro

Functional and morphological study of the effects of carvacrol on smooth muscle of the thoracic aorta in the rat

Background

The leaves of Origanum are widely used in herbal medicine hence of having many beneficial ingredients, one of these important compounds is Carvacrol. The inhibitory effect of Carvacrol was the core of this study by applying different kinds of stimulants to smooth muscles in the wall of thoracic aorta in rats.

Aim

To investigate the pharmacological effects of Carvacrol, the main active ingredient present in the medicinal plant Origanum, on the contractile activity and morphology of the smooth muscle of the rat thoracic aorta.

Materials and Methods

After the thoracic aorta arteries were isolated and prepared for the experiments, each thoracic aorta was cut into 5-mm ring segments; different stimulants were used (Potassium Chloride, Norepinephrine, U46619, and α,β-methylene ATP) in the presence and absence of Carvacrol on four groups of rats. The isolated rings were placed and connected to a force transducer which in turn linked to a data acquisition system via an amplifier to record the effect of each stimulant. GraphPad Prism version 5.02 for Windows, one-way analysis of variance followed by Dunnett's multiple comparison test.

Results

It was found out that Carvacrol obstructs the contractile responses elicited by exogenous NA, KCl, U46619, and α,β-methylene ATP in a concentration dependent manner.

Conclusion

The addition of Carvacrol in the experimental rats showed an increase in the thickness of tunica media as evident by the number of smooth muscle layers and laminae of elastic fibers. It was found that Carvacrol reduced the vascular smooth muscle contractility in the rat thoracic aorta. The mechanism of action is presumed to be achieved through interfering with the mobilization of both intracellular and extracellular Ca2⁺ through different receptors. Furthermore, it might be suggested that Carvacrol in high doses stimulates smooth muscles in the wall of aorta leading to an increase in the thickness of tunica media layer.

ATP as a cotransmitter in sympathetic and parasympathetic nerves - another Burnstock legacy

Geoff Burnstock created an outstanding scientific legacy that includes identification of adenosine 5'-triphosphate (ATP) as an inhibitory neurotransmitter in the gut, the discovery and characterisation of a large family of purine and uridine nucleotide-sensitive ionotropic P2X and metabotropic P2Y receptors and the demonstration that ATP is as an excitatory cotransmitter in autonomic nerves. The evidence for cotransmission includes that: 1) ATP is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle tissues, including the vas deferens and most arteries. 2) When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to elicit depolarisation, Ca²⁺ influx, Ca²⁺ sensitisation and contraction. 3) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder, where it stimulates postjunctional P2X1 receptors, and a second, as yet unidentified site to evoke contraction of detrusor smooth muscle. In both systems membrane-bound ecto-enzymes and soluble nucleotidases released from postganglionic nerves dephosphorylate ATP and so terminate its neurotransmitter actions. Currently, the most promising potential area of therapeutic application relating to cotransmission is treatment of dysfunctional urinary bladder. This family of disorders is associated with the appearance of a purinergic component of neurogenic contractions. This component is an attractive target for drug development and targeting it may be a rewarding area of research.

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

ATP as a cotransmitter in the autonomic nervous system

The role of adenosine 5'-triphosphate (ATP) as a major intracellular energy source is well-established. In addition, ATP and related nucleotides have widespread extracellular actions via the ionotropic P2X (ligand-gated cation channels) and metabotropic P2Y (G protein-coupled) receptors. Numerous experimental techniques, including myography, electrophysiology and biochemical measurement of neurotransmitter release, have been used to show that ATP has several major roles as a neurotransmitter in peripheral nerves. When released from enteric nerves of the gastrointestinal tract it acts as an inhibitory neurotransmitter, mediating descending muscle relaxation during peristalsis. ATP is also an excitatory cotransmitter in autonomic nerves; 1) It is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle preparations, such as the vas deferens and most arteries. When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to evoke depolarisation, Ca(2+) influx, Ca(2+) sensitisation and contraction. 2) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder and again acts at postjunctional P2X1 receptors, and possibly also a P2X1+4 heteromer, to elicit smooth muscle contraction. In both cases the neurotransmitter actions of ATP are terminated by dephosphorylation by extracellular, membrane-bound enzymes and soluble nucleotidases released from postganglionic nerves. There are indications of an increased contribution of ATP to control of blood pressure in hypertension, but further research is needed to clarify this possibility. More promising is the upregulation of P2X receptors in dysfunctional bladder, including interstitial cystitis, idiopathic detrusor instability and overactive bladder syndrome. Consequently, these roles of ATP are of great therapeutic interest and are increasingly being targeted by pharmaceutical companies.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

Raised tone reveals ATP as a sympathetic neurotransmitter in the porcine mesenteric arterial bed

The relative importance of ATP as a functional sympathetic neurotransmitter in blood vessels has been shown to be increased when the level of preexisting vascular tone or pressure is increased, in studies carried out in rat mesenteric arteries. The aim of the present study was to determine whether tone influences the involvement of ATP as a sympathetic cotransmitter with noradrenaline in another species. We used the porcine perfused mesenteric arterial bed and porcine mesenteric large, medium and small arteries mounted for isometric tension recording, because purinergic cotransmission can vary depending on the size of the blood vessel. In the perfused mesenteric bed at basal tone, sympathetic neurogenic vasocontractile responses were abolished by prazosin, an α1-adrenoceptor antagonist, but there was no significant effect of α,β-methylene ATP, a P2X receptor-desensitizing agent. Submaximal precontraction of the mesenteric arterial bed with U46619, a thromboxane A2 mimetic, augmented the sympathetic neurogenic vasocontractile responses; under these conditions, both α,β-methylene ATP and prazosin attenuated the neurogenic responses. In the mesenteric large, medium and small arteries, prazosin attenuated the sympathetic neurogenic contractile responses under conditions of both basal and U46619-raised tone. α,β-Methylene ATP was effective in all of these arteries only under conditions of U46619-induced tone, causing a similar inhibition in all arteries, but had no significant effect on sympathetic neurogenic contractions at basal tone. These data show that ATP is a cotransmitter with noradrenaline in porcine mesenteric arteries; the purinergic component was revealed under conditions of partial precontraction, which is more relevant to physiological conditions.

Perspective on the role of P2X-purinoceptor activation in human vas deferens contractility

The contractile actions of α,β-methylene ATP (α,β-meATP) and ATP and the effects of K(+) channel blockers in longitudinal and circular muscles of human vas deferens were investigated with a view to clarifying the functional importance of P2X(1)-purinoceptor activation and K(+) channels in modulating contractility of the tissues. The results provide an experiment-based perspective for resolving differing reports on purinergic activation of the tissues and uncertain roles of large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated delayed rectifier K(+) (K(V)) channels. α,β-Methylene ATP (3-100 μm) evoked suramin-sensitive contractions of longitudinal muscle but rarely of circular muscle. ATP (0.1-3 mm) less reliably activated only longitudinal muscle contractions. These were enhanced by ARL 67156 (100 μm), but a different ectonucleotidase inhibitor, POM 1, was ineffective. Both muscle types were unresponsive to ADP-βS (100 μm), a P2Y-purinoceptor agonist. Longitudinal muscle contractions in response to α,β-meATP were enhanced by FPL 64176 (1 μm), an L-type Ca(2+) agonist, TEA (1 mm), a non-specific K(+) channel blocker, 4-aminopyridine (0.3 mm), a selective blocker of K(V) channels, and iberiotoxin (0.1 μm), a selective blocker of BK(Ca) channels. Quiescent circular muscles responded to α,β-meATP reliably in the presence of FPL 64176 or iberiotoxin. Apamin (0.1 μm), a selective blocker of small conductance Ca(2+)-activated K(+) (SK(Ca)) channels had no effect in both muscle types. Y-27632 (1-10 μm) reduced longitudinal muscle contractions in response to α,β-meATP, suggesting involvement of Rho-kinase-dependent contractile mechanisms. The results indicate that P2X(1)-purinoceptor stimulation elicits excitatory effects that: (a) lead to longitudinal muscle contraction and secondary activation of 4-aminopyridine-sensitive (K(V)) and iberiotoxin-sensitive (BK(Ca)) K(+) channels; and (b) are subcontractile in circular muscle due to ancillary activation of BK(Ca) channels. The novel finding of differential action by P2X(1)-purinoceptor agonists in the muscle types has functional implication in terms of the purinergic contribution to overall contractile function of human vas deferens. The modulatory effects of K(V) and BK(Ca) channels following P2X(1)-purinoceptor activation may be pivotal in providing the crucial physiological mechanism that ensures temporal co-ordination of longitudinal and circular muscle contractility.

Participation of Rho-associated kinase in electrical stimulated and acetylcholine-induced contraction of feline esophageal smooth muscle

The RhoA/Rho-associated kinase (ROCK) signaling pathway has been known to play a critical role in Ca(2+)-sensitization of smooth muscle contraction. In this study, we investigated the role of ROCK in feline esophageal body smooth muscle contraction induced by electrical field stimulation and exogenous acetylcholine in vitro. Y-27632 [(+)-(R)-trans-4-(1-aminoethyl)-(4-pyridyl) cyclohexanecarboxamide dihydrochloride], ROCK inhibitor, and specific antibodies to ROCK1 and ROCK2 proteins, which are two isoforms of ROCK, were used. Electrical field stimulation induced off-contraction and on-contraction in the presence of N(G)-nitro-L-arginine methylester, originating from the cholinergic nerve. Y-27632 inhibited both excitatory contractions in a concentration-dependent manner. Exogenous acetylcholine concentration-dependently induced two types of contractions: an initial contraction which occurred immediately after the addition of acetylcholine during short periods, and a sustained contraction which sluggishly continued after the initial contraction. Maximal initial and sustained contractions were reached at 10(-5) M acetylcholine. Y-27632 significantly inhibited both acetylcholine-induced contractions in a concentration-dependent manner. Western blot analysis revealed that acetylcholine maximally increased the level of phosphorylation in the 20 kDa regulatory light chain of myosin II (MLC(20)) at Ser(19) from 0.25 min to 1 min, and then declined after 2 min. The level changes of MLC(20) phosphorylation during the 5 min paralleled with those of acetylcholine-induced contractions. The expression of ROCK1 and ROCK2 in membrane fractions of muscle was increased by acetylcholine; more specifically, ROCK2 continually expressed up to 5 min. Taken together, ROCK may be involved in neural-evoked and acetylcholine-induced contraction via translocation to the membrane in feline esophageal smooth muscle.

Impact of alpha1-adrenoceptor expression on contractile properties of vascular smooth muscle cells

Low-frequency blood pressure oscillations (Mayer waves) are discussed as a marker for sympathetic modulation of vascular tone. However, the factors that determine the frequency response of the vasculature to sympathetic stimuli are not fully understood. Possible mechanisms include functions related to alpha(1)-adrenergic receptors (alpha(1)-AR) and postreceptor processes involved in the vascular contractile response. The purpose of the present study was to examine the hypothesis that expression levels of alpha(1)-AR and their subtype distribution determine velocity and magnitude of alpha(1)-AR-mediated vascular smooth muscle cell (VSMC) contraction. alpha(1A)-, alpha(1B)-, and alpha(1D)-AR subtypes were transfected into VSMCs from rat aorta and characterized immunocytochemically via confocal microscopy. Functional studies in isolated cells were performed using video microscopy. The alpha(1)-AR agonist phenylephrine produced dose-dependent contractions of VSMCs. All transfected groups were more sensitive to phenylephrine compared with controls. Maximal contraction velocity almost doubled in transfected cells. However, no differences in observed parameters were found between the three transfected groups. Contractile properties in response to membrane depolarization with KCl were similar in all groups. In conclusion, alpha(1)-AR density determines velocity and sensitivity of alpha(1)-AR-mediated contraction in VSMCs. alpha(1)-AR subtype distribution does not appear to influence vasoconstriction to sympathetic stimuli.

(±)3,4-Methylenedioxyamphetamine elicits action potential bursts in a central snail neuron

The effects of (+/-)3,4-methylenedioxyamphetamine (MDA) were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials. Extracellular or intracellular application of MDA elicited action potential bursts of the central RP4 neuron. The action potential bursts elicited by MDA were not blocked when neurons were immersed in high-Mg2+ solution, Ca2+-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, sulpiride or methiothepin. Notably, the induction of action potential bursts was blocked by pretreatment with protein kinase C (PKC) inhibitors, chelerythrine and Ro 31-8220, but not by protein kinase A (PKA) inhibitors, KT-5720 and H89, nor by the phospholipase C (PLC) inhibitor, U73122. PKC activators, i.e., phorbol 12,13-dibutyrate (PDBu) and 1-oleoyl-2-acety-sn-glycerol (OAG; a membrane-permeant DAG analog), facilitate the induction of action potential bursts elicited by MDA. Voltage-clamp studies revealed that MDA decreased the delayed rectifying K+ current (I(KD)) of the RP4 neuron. Further, although Ro 31-8220 did not affect the I(KD), Ro 31-8220 decreased the inhibitory effect of MDA on the I(KD). These results suggest that the generation of action potential bursts elicited by MDA was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic, dopaminergic or serotoninergic receptors of the excitable membrane. Instead, the MDA-elicited action potential bursts are closely related to PKC activity and the inhibitory effects on the I(KD).