Prominent sympathetic purinergic vasoconstriction in the rabbit splenic artery: potentiation by 2,2'-pyridylisatogen tosylate

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.

Reduced Noradrenergic Signaling in the Spleen Capsule in the Absence of CB1 and CB2 Cannabinoid Receptors

The spleen is a visceral organ that contracts during hypoxia to expel erythrocytes and immune cells into the circulation. Spleen contraction is under the control of noradrenergic sympathetic innervation. The activity of noradrenergic neurons terminating in the spleen capsule is regulated by α2-adrenergic receptors (AR). Interactions between endogenous cannabinoid signaling and noradrenergic signaling in other organ systems suggest endocannabinoids might also regulate spleen contraction. Spleens from mice congenitally lacking both CB₁ and CB₂ cannabinoid receptors (Cnr1 ^-/- /Cnr2 ^-/- mice) were used to explore the role of endocannabinoids in spleen contraction. Spleen contraction in response to exogenous norepinephrine (NE) was found to be significantly lower in Cnr1 ^-/- /Cnr2 ^-/- mouse spleens, likely due to decreased expression of capsular α1AR. The majority of splenic Cnr1 mRNA expression is by cells of the spleen capsule, suggestive of post-synaptic CB₁ receptor signaling. Thus, these studies demonstrate a role for CB₁ and/or CB₂ in noradrenergic splenic contraction.

Differential effects of short- and long-term bupivacaine treatment on α1-adrenoceptor-mediated contraction of isolated rat aorta rings and the reversal effect of lipid emulsion

AIM

Arterial function is significantly influenced by bupivacaine at both clinically relevant concentrations and toxic concentrations, but the underlying mechanisms are not fully understood. In the present study we investigated the role of α1-adrenoceptors in bupivacaine effects on isolated rat aortas.

METHODS

Isolated aortic rings were prepared from rats and suspended in an organ bath. Phenylephrine (Phe)-induced vasoconstriction and acetylcholine (ACh)-induced vasodilation were recorded through an isometric force transducer connected to a data acquisition system.

RESULTS

Administration of bupivacaine (30-300 μmol/L) produced mild vasoconstriction, and this response declined with repeated administrations. Treatment of the aortic rings with bupivacaine (3-30 μmol/L) for 20 min enhanced Phe-induced vasoconstriction, while treatment for 40 min suppressed Phe-induced vasoconstriction. Both the short- and long-term bupivacaine treatment suppressed ACh-induced vasodilation. Incubation of the aortic rings with 0.2%-0.6% lipid emulsion (LE) for 100 min significantly increased the pD2 and Emax values of Phe-induced vasoconstriction, and incubation with 0.4% LE for 100 min reversed the inhibition of bupivacaine on vasoconstriction induced by Phe (30 μmol/L). In contrast, incubation with LE suppressed ACh-induced vasodilation, even at a lower concentration and with a 5-min incubation.

CONCLUSION

Bupivacaine exerts dual effects on α1-adrenoceptor-mediated vasoconstriction of isolated rat aortic rings: short-term treatment enhances the response, while long-term treatment inhibits it; the inhibition may be reversed via long-term incubation with LE.

Doxazosin selectively potentiates contraction to serotonin via 5-HT₂A receptors in longitudinal muscle strips of the rabbit gastric body

The aims of this study were to examine the effects of doxazosin on contractile responses to 5-hydroxytryptamine (5-HT), carbachol, and histamine, and to compare them with those of prazosin, alfuzosin, and terazosin, and then characterize a pharmacological profile of the 5-HT-induced contractile response using preparations of isolated longitudinal muscle strips from the rabbit gastric body. The results from these preparations showed that the contraction response to 5-HT, but not to carbachol or histamine, was found to be dose-dependently potentiated by doxazosin and its enantiomers. The specific potentiation effect on 5-HT was not observed in the preparations that were treated with prazosin, terazosin, or alfuzosin. The contractile response to 5-HT and its potentiation by doxazosin were not affected by treatment with phenoxybenzamine. However, 5-HT-induced contraction was competitively antagonized by nefazodone (with pA₂ value of 8.64 ± 0.17), and was almost completely inhibited by treatment with indomethacin. In conclusion, doxazosin, but not prazosin, alfuzosin, or terazosin, selectively potentiates 5-HT-induced contraction in the rabbit gastric body strips via an α₁-adrenoceptor-independent mechanism, without chiral recognition of its enantiomers. Additionally, the contraction to 5-HT was found to be mediated via 5-HT(₂) receptors, and was similar to PGs synthesis in the preparations.

Berberine improves neurogenic contractile response of bladder detrusor muscle in streptozotocin-induced diabetic rats

ETHNOPHARMACOLOGICAL RELEVANCE

Coptidis Rhizoma has been used to treat diabetes mellitus for more than 1400 years in China. Berberine, one of the main alkaloids of Coptidis Rhizoma, is a principal antidiabetic component of Coptidis Rhizoma. To investigate the effects of berberine on impaired neurogenic contractility of detrusor muscle from urinary bladder of rats with early stage diabetes.

MATERIALS AND METHODS

The detrusor muscle strips were isolated from urinary bladders of streptozotocin-induced diabetic rats, 5% sucrose-induced diuretic rats or normal rats, and were placed in organ bath. The contractions induced by electrical field stimulation (EFS), carbachol, KCl, adenosine triphosphate, and the effects of berberine on those contractions were measured.

RESULTS

The EFS- or KCl-induced contraction of detrusor muscle was significantly decreased in diabetic rats as compared with diuretic or normal rats. Atropine and suramin inhibited EFS-induced contraction. In diabetic rats, the atropine sensitive components were decreased in EFS-induced contraction of detrusor muscle, and the adenosine triphosphate-induced contraction was significantly increased. The carbachol-induced contrations were not different among groups. Berberine significantly potentiated EFS-induced contractions of detrusor muscle both from normal and diabetic rats, but the potentiated effect of BBR was more sensitive to atropine in diabetic rats. Berberine also potentiated adenosine triphosphate-induced contraction of detrusor muscle, but did not change carbachol- or KCl-induced contraction.

CONCLUSION

The neurogenic contraction of urinary bladder detrusor muscle is decreased while purinergic contraction of bladder detrusor muscle is increased in rats with early stage diabetes. Berberine increases the neurogenic contractile response to EFS possibly via both presynaptic increasing neurotransmitters release and postsynaptic potentiation of purinergic transmitter-regulated response in rat urinary bladder detrusor; and in diabetic rats, berberine increases neurogenic contractile response mainly via the presynaptic increasing acetylcholine release.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Chiral recognition of doxazosin enantiomers in 3 targets for therapy as well as adverse drug reactions in animal experiments

Doxazosin used in benign prostatic hyperplasia has the side effects of causing hypotension and the risk of heart failure. The 3 targets of α1A-adrenoceptors (in the prostate), α1D-adrenoceptors (in the aorta), and an unknown mechanism (in the heart) are involved, respectively. We hypothesized that there is a chiral recognition of doxazosin enantiomers in the 3 targets. Using isolated rat aorta (α1D-adrenoceptors) and rabbit prostate (α1A-adrenoceptors), we examined pA2 and pKB values of doxazosin enantiomers. We observed chronotropic and inotropic effects of doxazosin enantiomers in isolated rat and rabbit heart tissues. (–)Doxazosin and (+)doxazosin produced a shift to the right of concentration–contraction curves for noradrenalin (aorta) and phenylephrine (prostate smooth muscle). The pA2 value of (–)doxazosin (8.625 ± 0.053) was smaller than (+)doxazosin (9.503 ± 0.051) in rat aorta, but their pKB values in rabbit prostate were the same. In rat and rabbit heart tissues, (+)doxazosin (3–30 µmol·L−1) significantly decreased atrial rate, and produced negative inotropic effects; however, (–)doxazosin did not affect the atrial rate, and produced positive inotropic effects in the atria. Thus, the chiral carbon atom of doxazosin does not affect its activity at the therapeutic target of α1A-adrenoceptors in the prostate, but significantly changes its blocking activity against α1D-adrenoceptors in the aorta, and produces opposite inotropic effects in the atria via an α1-adrenoceptor-independent mechanism.

Effects of imidazolines on neurogenic contraction in isolated urinary bladder detrusor strips from rabbit

Moxonidine and clonidine, which are imidazoline compounds, are sympathetic modulators used as centrally acting antihypertensive drugs. Moxonidine, clonidine, and agmatine produce extensive effects in mammalian tissues via imidazoline recognition sites (or receptors) or α(2)-adrenoceptors. To investigate the effects of imidazolines on the function of the urinary bladder, we tested the effects of moxonidine, clonidine, and agmatine on the neurogenic contraction induced by electric field stimulation, and on the post-synaptic receptors in isolated urinary bladder detrusor strips from rabbit. Both moxonidine at 1.0-10.0 µmol/L and clonidine at 0.1-10.0 µmol/L inhibited electric-field-stimulation-induced contraction in a concentration-dependent manner, but not agmatine (10.0-1000.0 µmol/L). Both moxonidine and clonidine failed to affect carbachol or adenosine-triphosphate-induced contractions; however, 1000.0 µmol/L agmatine significantly increased these contractions. Our study indicates that (i) moxonidine and clonidine produce a concentration-dependent inhibition of the neurogenic contractile responses to electric field stimulation in isolated detrusor strips from male New Zealand rabbits; (ii) post-synaptic muscarinic receptor and purinergic receptor stimulation are not involved in the responses of moxinidine and clonidine in this study; (iii) the inhibitory effects of these agents are probably not mediated by presynaptic imidazoline receptors.

The effect of aging on adrenergic and nonadrenergic receptor expression and responsiveness in canine skeletal muscle

We tested the hypothesis that adrenergic and nonadrenergic receptor responsiveness and protein expression would be altered with advancing age. Young (n = 6; 22 ± 1 mo; mean ± SE) and old (n = 6; 118 ± 9 mo) beagles were instrumented with flow probes and an indwelling catheter for continuous measurement of external iliac blood flow and arterial blood pressure. Vascular conductance (VC) was calculated as hindlimb blood flow/mean arterial pressure. Selective agonists for α-1, α-2, neuropeptide-Y (NPY), and purinergic (P2X) receptors were infused at rest and during treadmill running at moderate (2.5 mph) and heavy (4 mph with 2.5% grade) exercise intensities. Feed arteries were dissected from gracilis muscles, and α-1D, α-1B, α-2A, P2X-4, P2X-1, and NPY-Y1 receptor protein expression was determined. Phenylephrine produced similar decreases (P > 0.05) in VC in young and old beagles at rest (young: -62 ± 5%; old: -59 ± 5%) and during moderate (young: -67 ± 5%; old: -62 ± 4%) and heavy (young: -54 ± 4%; old: -49 ± 3%) exercise. Clonidine caused similar (P > 0.05) decreases in VC in old compared with young dogs at rest (young: -59 ± 8%; old: -70 ± 6%) and during moderate (young: -52 ± 6%; old: -47 ± 5%)- and heavy (young: -42 ± 5%; old: -43 ± 5%)-intensity exercise. NPY infusion resulted in a similar decline in VC in young and old beagles at rest (young: -40 ± 7%; old: -39 ± 9%) and during moderate (young: -47 ± 6%; old: -40 ± 6%)- and heavy (young: -40 ± 3%; old: -38 ± 4%)-intensity exercise. α-β-Methylene-ATP also produced similar decreases in VC in young and old beagles at rest (young: -36 ± 6%; old: -40 ± 8%) and during exercise at moderate (young: -42 ± 5%; old: -40 ± 9%) and heavy (young: -47 ± 5%; old: -42 ± 8%) intensities. α-1B receptor protein expression was elevated (P < 0.05) in old compared with young dogs, whereas there were no age-related differences in α-1D or α-2A receptor expression and nonadrenergic P2X-4, P2X-1, and NPY-Y1 receptor expression. The present findings indicate that postsynaptic adrenergic and nonadrenergic receptor responsiveness was not altered by advancing age. Moreover, the expression of adrenergic and nonadrenergic receptors in skeletal-muscle feed arteries was largely unaffected by aging.

Relaxant and contractile responses of detrusor muscle strips obtained from bladder outlet-obstructed rats treated with doxazosin enantiomers

(-)Doxazosin, one of (±)doxazosin enantiomers, was speculated to have a pharmacological enantioselectivity between the cardiovascular system and the urinary system by comparison with (+)doxazosin. Therefore, to evaluate the potential benefits of (-)doxazosin in the treatment of benign prostate hyperplasia, we compared the effects of the 3 agents, using rat mesenteric artery preparations and obstructed bladder strips. Concentration-response curves for carbachol (contractile response) and isoprenaline (relaxant response) in detrusor muscle strips of the bladder outlet obstruction (BOO) rats were shifted to the left, with significant increases in the Emax values, and significant decreases in the EC50 values by comparison with the sham-operated rats (P < 0.05, n = 10). The enhanced responses in detrusor muscle strips of the BOO rats treated with (±)doxazosin and its enantiomers at 3 mg·(kg body mass)(-1)·day(-1) for 2 weeks returned to normal levels, and the 3 agents inhibited the enhanced responses to carbachol and isoprenaline to the same extent. On the other hand, the 3 agents uncompetitively inhibited the vasoconstrictive response curves for NA in the rat isolated mesenteric artery, and the pKB value of (-)doxazosin at vascular α1-adrenoceptors was significantly smaller (P < 0.05, n = 6) than that of (+)doxazosin or (±)doxazosin. In conclusion, although (-)doxazosin inhibits vascular functional α1-adrenoceptors more weakly than (+)doxazosin, both agents equally ameliorate the enhanced responses in detrusor muscle of BOO rats, suggesting that the chiral carbon atom in the molecular structure of doxazosin does not affect its beneficial effects in the bladder smooth muscle of BOO rats.

Physiological significance of P2X receptor-mediated vasoconstriction in five different types of arteries in rats

P2X(1) receptors, the major subtype of P2X receptors in the vascular smooth muscle, are essential for α,β-methylene adenosine 5'-triphosphate (α,β-MeATP)-induced vasoconstriction. However, relative physiological significance of P2X(1) receptor-regulated vasoconstriction in the different types of arteries in the rat is not clear as compared with α(1)-adrenoceptor-regulated vasoconstriction. In the present study, we found that vasoconstrictive responses to noncumulative administration of α,β-MeATP in the rat isolated mesenteric arteries were significantly smaller than those to single concentration administration of α,β-MeATP. Therefore, we firstly reported the characteristic of α,β-MeATP-regulated vasoconstrictions in rat tail, internal carotid, pulmonary, mesenteric arteries, and aorta using single concentration administration of α,β-MeATP. The rank order of maximal vasoconstrictions for α,β-MeATP (E (max·α,β-MeATP)) was the same as that of maximal vasoconstrictions for noradrenaline (E (max·NA)) in the internal carotid, pulmonary, mesenteric arteries, and aorta. Moreover, the value of (E (max·α,β-MeATP)/E (max·KCl))/(E (max·NA)/E (max·KCl)) was 0.4 in each of the four arteries, but it was 0.8 in the tail artery. In conclusion, P2X(1) receptor-mediated vasoconstrictions are equally important in rat internal carotid, pulmonary, mesenteric arteries, and aorta, but much greater in the tail artery, suggesting its special role in physiological function.

Is tonic sympathetic vasoconstriction increased in the skeletal muscle vasculature of aged canines?

We tested the hypothesis that tonic adrenergic and nonadrenergic receptor-mediated sympathetic vasoconstriction would increase at rest and during exercise with advancing age. Young (n = 6; 22 ± 1 mo; means ± SE) and old (n = 6; 118 ± 9 mo) beagles were studied. Selective antagonists for alpha-1, alpha-2, neuropeptide Y (NPY), and purinergic (P(2x)) receptors were infused at rest and during treadmill running at 2.5 mph and 4 mph with 2.5% grade. Prazosin produced similar increases in vascular conductance in young and old beagles at rest (Young: 158 ± 34%; Old: 98 ± 19%) and during exercise at 2.5 mph (Young: 80 ± 10%; Old: 58 ± 12%) and 4 mph and 2.5% grade (Young: 57 ± 5%; Old: 26 ± 4%). Rauwolscine caused similar (P > 0.05) increases in vascular conductance in old compared with young dogs at rest (Young: 119 ± 25%; Old: 64 ± 22%) and at 2.5 mph (Young: 86 ± 13%; Old: 60 ± 7%) and 4 mph with 2.5% grade (Young: 61 ± 5%; Old: 43 ± 7%). N2-(diphenylacetyl)-N-[4-hydroxyphenyl)methyl]-d-arginine amide (BIBP) caused a smaller increase (P < 0.05) in vascular conductance in old compared with young dogs at rest (Young: 179 ± 44%; Old: 91 ± 22%), whereas similar increases (P > 0.05) of experimental limb vascular conductance in young and old dogs occurred following BIBP during exercise at 2.5 mph (Young: 56 ± 16%; Old: 50 ± 12%) and 4 mph and 2.5% grade (Young: 45 ± 10%; Old: 25 ± 7%). Pyridoxal-phosphate-6-azophenyl-2'-4'-disulfonic acid infusion produced a larger increase in vascular conductance in old compared with young beagles at rest (Young: 88 ± 14%; Old: 191 ± 58%), whereas similar increases were observed at 2.5 mph (Young: 47 ± 18%; Old: 31 ± 11%) and 4 mph with 2.5% grade (Young: 26 ± 13%; Old: -18 ± 8%). At rest, NPY receptor-mediated restraint of skeletal muscle blood flow was reduced with advancing age, whereas P(2x) receptor-mediated restraint of skeletal muscle blood flow was increased. During exercise, the magnitude of adrenergic and nonadrenergic sympathetic vasoconstriction was not different between young and old dogs. Overall, these data demonstrate that adrenergic receptor-mediated vasoconstriction was not elevated at rest, but nonadrenergic sympathetic vasoconstriction was altered under basal conditions in aged beagles.

ATP overflow in skeletal muscle 1A arterioles

The purpose of this study was to investigate the sources of ATP in the 1A arteriole, and to investigate age-related changes in ATP overflow. Arterioles (1A) from the red portion of the gastrocnemius muscle were isolated, cannulated and pressurized in a microvessel chamber with field stimulation electrodes. ATP overflow was determined using probes specific for ATP and null probes that were constructed similar to the ATP probes, but did not contain the enzyme coating. ATP concentrations were determined using a normal curve (0.78 to 25 micromol l(-1) ATP). ATP overflow occurred in two phases. Phase one began in the first 20 s following stimulation and phase two started 35 s after field stimulation. Tetrodotoxin, a potent neurotoxin that blocks action potential generation in nerves, abolished both phases of ATP overflow. alpha1-Receptor blockade resulted in a small decrease in ATP overflow in phase two, but endothelial removal resulted in an increase in ATP overflow. ATP overflow was lowest in 6-month-old rats and highest in 12- and 2-month-old rats (P<0.05). ATP overflow measured via biosensors was of neural origin with a small contribution from the vascular smooth muscle. The endothelium seems to play an important role in attenuating ATP overflow in 1A arterioles.

Potentiation by yohimbine of alpha-adrenoceptor-mediated vasoconstriction in response to clonidine in the rabbit ear vein

We investigated the pharmacological profile of the vasoconstrictive response to clonidine in the isolated rabbit ear vein, and compared the characteristics of clonidine with those of noradrenaline and moxonidine. The maximal vasoconstrictive responses to clonidine and moxonidine in the rabbit ear vein were 35.94+/-11.18% and 88.78+/-11.54% of the maximum response to noradrenaline, respectively. Prazosin 0.1 microM inhibited the vasoconstriction induced by lower concentrations of noradrenaline, and the concentration-dependent response curve for noradrenaline was significantly shifted to the right by 1 microM prazosin. Yohimbine (0.1 and 0.5 microM) only decreased the vasoconstrictive response to lower concentrations of noradrenaline, but did not affect the response to higher concentrations. Vasoconstrictive responses to lower but not higher concentrations of clonidine and moxonidine were inhibited by 0.1 microM yohimbine. In contrast, the same concentration of yohimbine significantly potentiated the maximal response to a high concentration of clonidine by 24.06%. In isolated rabbit ear vein pretreated with 0.1 microM yohimbine, prazosin competitively inhibited the concentration-response curve for clonidine with a pA(2) value of 8.05+/-0.06. We conclude that clonidine acts mainly on alpha(2)-adrenoceptors to produce vasoconstriction in the rabbit ear vein; however, in the preparation pretreated with yohimbine, the clonidine-induced vasoconstriction is mediated via alpha(1)-adrenoceptors and its maximal vasoconstriction is significantly potentiated.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

ATP is the predominant sympathetic neurotransmitter in rat mesenteric arteries at high pressure

Most studies of neurovascular transmission in isolated small mesenteric arteries have used either isometric recording techniques or measured vasoconstriction in vessels with no distending pressure. Here we have used pressure myography to assess the contribution of noradrenaline and ATP to sympathetic neurotransmission in rat second-order mesenteric arteries. In arteries pressurized to 30 or 90 mmHg, activation of sympathetic axons with trains of electrical stimuli (50 pulses, 0.5-10 Hz) evoked frequency-dependent vasoconstrictions that increased in amplitude at higher pressure. In the presence of the P2-receptor antagonist suramin (0.1 mM), the amplitude of vasoconstrictions to trains at 2 and 10 Hz did not differ at 30 and 90 mmHg. In contrast, in the presence of the alpha(1)-adrenoceptor antagonist prazosin (0.1 microm) vasoconstrictions at 90 mmHg were larger than those at 30 mmHg. At both pressures, the combination of prazosin and suramin virtually abolished constrictions. The purinergic component of vasoconstriction (prazosin-resistant) was almost abolished by the L-type Ca(2+) channel antagonist nifedipine (1 microm). Increasing pressure from 30 to 90 mmHg decreased the resting membrane potential and increased the amplitude of purinergic excitatory junction potentials. These findings indicate that the contribution of ATP to neurovascular transmission increases when the pressure is raised from 30 to 90 mmHg, which is similar to the pressure second-order mesenteric arteries experience in vivo, and that Ca(2+) influx through L-type Ca(2+) channels is largely responsible for purinergic activation of the vascular smooth muscle.

Frequency and pattern dependence of adrenergic and purinergic vasoconstriction in rat skeletal muscle arteries

Sympathetic nerves fire in bursts followed by brief periods of quiescence. Periods of quiescence may be a valuable part of coding for different neurotransmitters. We compared adrenergic- and non-adrenergic-mediated vasoconstriction with repeating burst patterns versus constant frequency stimulation. Seventeen rats were killed, and the femoral arteries dissected out and mounted in organ tissue baths at 37 degrees C and pH 7.4. Field stimulation was applied to artery rings from five rats at constant frequencies of 2-6 Hz for 144 impulses. In 12 rats, artery rings were stimulated with two burst pattern protocols consisting of repeating pairs, triplets, quadruplets or sextuplets performed using either 8 or 30 Hz as the instantaneous frequency for a total of 144 impulses. All protocols were repeated with the P2 purinergic antagonist pyridoxal-phosphate-6-azophenyl-2'4'-disulphonic acid (PPADs; 0.42 m) or the alpha(1)-antagonist prazosin (1.59 microM). Tension was decreased by the addition of the P2 antagonist PPADs (P < 0.05). Prazosin abolished tension at all constant frequencies (P < 0.05). P2 and alpha(1)-antagonism decreased tension with 8 and 30 Hz burst pattern field stimulation. However, the magnitude of decrease in tension with prazosin was less with burst patterns compared to the same average constant frequencies (P < 0.05). It appears that P2X receptors and alpha(1)-receptors in the femoral artery are sensitive to frequency and patterns of electrical stimulation.

Changes in purinergic signalling in developing and ageing rat tail artery: importance for temperature control

This study aimed to examine the expression and function of P2 receptors of the rat tail and mesenteric arteries during maturation and ageing (4, 6 and 12 weeks, 8 and 24 months). Functional studies and receptor expression by immunohistochemistry revealed a heterogeneous phenotype of P2 receptor subtypes depending on artery age. The purinergic component of nerve-mediated responses in the tail artery was greater in younger animals; similarly responses to ATP and alpha,beta-meATP and the expression of P2X1 receptors decreased with age. Contractile responses to 2-MeSADP decreased with age, and were absent at 8 and 24 months; P2Y1 receptor expression followed this pattern. UTP-induced contractions and P2Y2 receptor expression also decreased with age. The mesenteric artery contracted to UTP, responses at 4 and 6 weeks were larger than at other ages although P2Y2 receptor expression did not significantly differ with age. 2-MeSADP induced relaxation of the mesenteric artery, responses being greatest at 6 weeks and decreased thereafter, which was mimicked by the P2Y1 receptor immunostaining. We speculate that the dramatic changes in expression of P2 receptors in the rat tail artery, compared to the mesenteric artery, during development and ageing are related to the role of the tail artery in temperature regulation.

Non-adrenergic inhibition at prejunctional sites by agmatine of purinergic vasoconstriction in rabbit saphenous artery

We investigated the effects of agmatine, clonidine, xylazine and moxonidine on the purinergic vasoconstriction induced by electrical stimulation in the rabbit isolated saphenous artery without endothelium. Transmural electrical stimulations induced reproducible responses in the arterial preparations, which were abolished by tetrodotoxin at 0.1 microM or pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS, 30 microM), but were not affected by 1 microM prazosin. Clonidine, xylazine and moxonidine induced transient and concentration-independent vasoconstriction, with threshold concentrations of 1, 3 and 30 microM, respectively. Agmatine, in contrast, did not produce any vascular response even at 1 mM. Lower concentrations of clonidine, xylazine and moxonidine (0.01-0.3 microM) concentration-dependently decreased vasoconstrictor responses to electrical stimulation, whereas agmatine (0.1-1 mM) induced an inhibitory followed by a facilitatory effect on electrically evoked responses. Agmatine, clonidine and moxonidine but not xylazine significantly enhanced the vasoconstriction elicited by 1 mM ATP. The concentration-response curve for NA was shifted to the left slightly by 1 mM agmatine, but not affected by 0.3 microM of other three agonists. Phenoxybenzamine did not affect the vasoconstrictive responses to 1mM ATP and to electrical stimulations, but abolished those to NA. Agmatine at 1mM evoked only an inhibitory effect on electrical stimulation-induced vasoconstriction in the preparation pretreated with phenoxybenzamine, and the inhibitory action was enhanced to 38.6% from the control value (without treatment with phenoxybenzamine) of 22.5%. The non-imidazoline compound xylazine at 0.3 microM lost its inhibitory effect on the neurogenic vasoconstriction in the presence of phenoxybenzamine. In conclusion, agmatine produces a biphasic effect on the purinergic vasoconstriction induced by sympathetic nerve stimulation in the rabbit isolated saphenous artery. The monophasic inhibition of agmatine in the artery treated with phenoxybenzamine is due to an alpha-adrenoceptor-independent mechanism at prejunctional sites, and the potentiation effect of agmatine is mainly dependent on its enhancement of vasoconstriction at postjunctional sites.

2,2'-Pyridylisatogen tosylate antagonizes P2Y1 receptor signaling without affecting nucleotide binding

The effect of 2,2'-pyridylisatogen tosylate (PIT) on the human P2Y(1) receptor and on other recombinant P2Y receptors has been studied. We first examined the modulation by PIT of the agonist-induced accumulation of inositol phosphates. PIT blocked 2-methylthio-ADP (2-MeSADP)-induced accumulation of inositol phosphates in 1321N1 astrocytoma cells stably expressing human P2Y(1) receptors in a non-competitive and concentration-dependent manner. The IC(50) for reduction of the maximal agonist effect was 0.14microM. In contrast, MRS2179, a competitive P2Y(1) receptor antagonist, parallel-shifted the agonist concentration-response curve to the right. PIT also concentration-dependently blocked the P2Y(1) receptor signaling induced by the endogenous agonists, ADP and ATP. A simple structural analogue of PIT was synthesized and found to be inactive as a P2Y(1) receptor antagonist, suggesting that the nitroxyl group of PIT is a necessary structural component for P2Y(1) receptor antagonism. We next examined the possible modulation of the binding of the newly available antagonist radioligand for the P2Y(1) receptor, [3H] MRS2279. It was found that PIT (0.01-10microM) did not inhibit [3H] MRS2279 binding to the human P2Y(1) receptor. PIT (10microM) had no effect on the competition for [3H] MRS2279 binding by agonists, ADP and ATP, suggesting that its antagonism of the P2Y(1) receptor may be allosteric. PIT had no significant effect on agonist activation of other P2Y receptors, including P2Y(2), P2Y(4), P2Y(6), P2Y(11) and P2Y(12) receptors. Thus, PIT selectively and non-competitively blocked P2Y(1) receptor signaling without affecting nucleotide binding.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Unusual absence of endothelium-dependent or -independent vasodilatation to purines or pyrimidines in the rat renal artery

BACKGROUND

Adenosine triphosphate (ATP) is a cotransmitter with noradrenaline (NA) in sympathetic perivascular nerves. It has a dual role in the maintenance of vascular tone as ATP, released from endothelial cells during shear stress or hypoxia, induces vasodilatation via endothelial P2Y receptors or by direct action on smooth muscle. The role and distribution of P2 receptors is well characterized for many blood vessels but not for the rat renal artery. This study aims to determine whether ATP is a vasoconstrictor cotransmitter with NA and whether ATP induces vasodilatation via the endothelium or smooth muscle.

METHODS

On isolated rat renal arteries, electrical field stimulation (EFS) in the absence and presence of antagonists to P2X receptors and alpha1-adrenoceptors was examined. Concentration-response curves were constructed to NA, ATP, alpha,beta-methylene ATP (alpha,beta-meATP), uridine triphosphate (UTP), and 2-methylthio ADP (2-MeSADP) on low tone. Curves to acetylcholine (ACh), 2-MeSADP, and UTP were constructed on raised tone. Immunofluorescent localization of P2X and P2Y receptor subtypes was performed.

RESULTS

Electrical field stimulation induced vasoconstriction, partially inhibited by the P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and predominantly by prazosin. Exogenous NA and ATP mimicked EFS; immunostaining for P2X1 and P2X2 receptors was expressed on vascular smooth muscle. Unusually, ATP, 2-MeSADP, and UTP failed to induce vasodilatation. Acetylcholine induced vasodilatation. alpha,beta-meATP, 2-MeSADP, and UTP induced vasoconstriction via P2X1, P2Y1, and P2Y2 receptors, respectively. Immunostaining for P2X1, P2Y1, and P2Y2 receptors was expressed on the vascular smooth muscle.

CONCLUSION

Adenosine triphosphate and NA are cotransmitters in sympathetic nerves supplying the rat renal artery, NA being the dominant partner. The novel feature of this vessel is that purines and pyrimidines do not produce either endothelium-dependent or -independent vasodilatation; P2X1, P2Y1, and P2Y2 receptors on the smooth muscle all mediate vasoconstriction.

Purinergic and adrenergic cotransmission in canine isolated and perfused gastroepiploic arteries

1. The vasoconstrictor responses of canine gastroepiploic artery to periarterial electrical nerve stimulation (PNS; 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) were observed in a frequency dependent manner. The PNS-induced vasoconstrictions were abolished by tetrodotoxin (1 micromol/L) and mostly depressed but not completely by guanethidine (10 micromol/L). 2. Vasoconstrictor responses to administered noradrenaline were antagonized significantly by prazosin (0.1 micromol/L), an alpha1-adrenoceptor antagonist, but were not significantly affected by suramin (100 micromol/L), a P2 purinoceptor antagonist, or alpha,beta-methylene ATP (1 micromol/L), a P2X receptor desensitizing agent. Exogenous ATP-induced responses were clearly depressed by suramin or alpha,beta-methylene ATP, but were not significantly affected by prazosin. 3. The vasoconstrictor responses to PNS at a low frequency (2 and 4 Hz) of stimulation were markedly inhibited by suramin (100 micromol/L) and by alpha,beta-methylene ATP (1 micromol/L). The remaining responses after suramin or alpha,beta-methylene ATP were abolished by subsequent application of prazosin (0.1 micromol/L). At a high frequency (8 Hz) of stimulation, the vascular response was not significantly inhibited by suramin or alpha,beta-methylene ATP, but it was abolished by prazosin. 4. Injection of xylazine (0.3-30 nmol/L), an alpha2-adrenoceptor agonist, did not induce any clear vasoconstriction. The exposure of tissues to rauwolscine (0.1-0.3 micromol/L), an alpha2-adrenoceptor antagonist, dose-dependently increased PNS-induced vasoconstrictions at all frequencies tested. 5. The present results indicate that ATP acts as a cotransmitter with noradrenaline and is responsible for post-junctional vasoconstrictor responses at low frequencies of sitmulation, whereas the effect of noradrenaline is dominant at high-frequency stimulation in canine gastroepiploic artery. Prejunctional alpha2-adrenoceptor autoinhibition may modulate the release of either noradrenaline or ATP from sympathetic nerve terminals.

Different sensitivities to alpha1 adrenoceptor blockade on periarterial sympathetic nerve-induced constriction by low and high frequencies in canine isolated splenic arteries

The periarterial nerve electrical stimulation at 4 and 10 Hz induced a monophasic vasoconstriction of the canine splenic artery in a pulse number-related manner (1-30 pulses). The responses at 4 Hz were not significantly affected by 0.1 microM prazosin, but abolished by 1 microM alpha, beta-methylene ATP. Prazosin (0.1 microM) partially but significantly inhibited responses at 10 Hz, and the remaining responses were blocked by 1 microM alpha, beta-methylene ATP. It indicates that the monophasic vasoconstrictor response to short pulses of stimulation at a low frequency is mediated by P2X-receptors, whereas the response at a high frequency may be due to activation of not only P2X-receptors but also alpha1 adrenoceptors.

Pharmacological analysis of vasoconstrictor responses to periarterial purinergic nerve stimulation

1. Periarterial electrical nerve stimulation at a low frequency (1 Hz) readily induced a vasoconstrictor response of the canine splenic artery in a pulse number-related manner (1-30 pulses of trains). The vasoconstrictor response to trains of up to 10 pulses at 1 Hz of stimulation appeared to be monophasic, whereas it became clearly distinguished into two phases at a longer train of 30 pulses. 2. The monophasic vasoconstrictor responses to trains of 1, 3 or 10 pulses were not modified by an alpha1-adrenoceptor blocking agent, prazosin (0.1 microM), but were completely inhibited by the P2X receptor desensitization with alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-methylene ATP; 1 microM). The 1st phase of vasoconstriction induced by a train length of 30 pulses was not influenced by the treatment with prazosin, but was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and the remaining response of this phase was blocked by alpha,beta-methylene ATP. 3. Rauwolscine (0.3 microM), an alpha2-adrenoceptor antagonist, enhanced the vasoconstrictor responses to trains of 1, 3 or 10 pulses. Particularly at 10 pulses of electrical stimulation, the vasoconstrictor responses were significantly potentiated. The blockade of neuronal uptake of noradrenaline with imipramine (1 microM) did not affect the vasoconstrictor responses to trains of 1, 3 or 10 pulses. 4. It is concluded that short pulse trains of stimulation at a low frequency may selectively activate a purinergic component of sympathetic cotransmission, and the prejunctional alpha2-adrenergic feedback mechanism may tonically participate into the modulation of ATP release. Imipramine-sensitive neuronal uptake mechanism may not play an important role in regulating vascular responses to periarterial purinergic nerve stimulation.