Electrophysiological effects of activating the peptidergic primary afferent innervation of rat mesenteric arteries

Targeting temperature-sensitive transient receptor potential channels in hypertension: far beyond the perception of hot and cold

Transient receptor potential (TRP) channels are nonselective cation channels and participate in various physiological roles. Thus, changes in TRP channel function or expression have been linked to several disorders. Among the many TRP channel subtypes, the TRP ankyrin type 1 (TRPA1), TRP melastatin type 8 (TRPM8), and TRP vanilloid type 1 (TRPV1) channels are temperature-sensitive and recognized as thermo-TRPs, which are expressed in the primary afferent nerve. Thermal stimuli are converted into neuronal activity. Several studies have described the expression of TRPA1, TRPM8, and TRPV1 in the cardiovascular system, where these channels can modulate physiological and pathological conditions, including hypertension. This review provides a complete understanding of the functional role of the opposing thermo-receptors TRPA1/TRPM8/TRPV1 in hypertension and a more comprehensive appreciation of TRPA1/TRPM8/TRPV1-dependent mechanisms involved in hypertension. These channels varied activation and inactivation have revealed a signaling pathway that may lead to innovative future treatment options for hypertension and correlated vascular diseases.

Beneficial effect of capsaicin via TRPV4/EDH signals on mesenteric arterioles of normal and colitis mice

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Capsaicin induced vasorelaxation of human colonic submucosal arterioles in vitro and in vitro.

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Capsaicin induced an endothelium-dependent vasorelaxation of human submucosal arterioles.

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Capsaicin induced an endothelium-dependent vasorelaxation of mouse mesenteric arterioles.

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Capsaicin induced vasorelaxation minily by TRPV1-mediated endothelial nitric oxide release.

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Capsaicin induced vasorelaxation mainly by TRPV4/endothelium-dependent hyperpolarization.

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Capsaicin exerted anti-colitis action in wide-type mice, but not in TRPV4 knock-out mice.

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Capsaicin rescued the impaired endothelium-dependent vasorelaxation via TRPV4/EDH pathway.

Introduction

Although capsaicin has long been used as food additive and medication worldwide, its actions on gastrointestinal tract as its most delivery pathway have not been well addressed.

Objectives

In the present study, we aimed to study GI actions of capsaicin on mesenteric arterioles in normal and colitis mice and to elucidate the underlying mechanisms.

Methods

Vasorelaxation of human submucosal arterioles and the mesenteric arterioles from wide-type (WT) mice, TRPV1^−/− and TRPV4^−/− (KO) mice were measured. The expression and function of TRPV channels in endothelial cells were examined by q-PCR, immunostaining, Ca²⁺ imaging and membrane potential measurements.

Results

Capsaicin dose-dependently induced vasorelaxation of human submucosal arterioles and mouse mesenteric arterioles in vitro and in vivo through endothelium-dependent hyperpolarization (EDH), nitric oxide (NO), and prostacyclin (PGI₂). Using TRPV1 and TRPV4 KO mice, we found that capsaicin-induced vasorelaxation was predominately through TRPV4/EDH, but marginally through TRPV1/NO/PGI₂. Capsaicin induced hyperpolarization through activation of endothelial TRPV4 channels and intermediate-conductance of Ca²⁺-activated K⁺ channels to finally stimulate vasorelaxation. Importantly, capsaicin exerted anti-colitis action by rescuing the impaired ACh-induced vasorelaxation in WT colitis mice but not in TRPV4 KO colitis mice.

Conclusions

Capsaicin increases intestinal mucosal blood perfusion to potentially prevent/treat colitis through a novel TRPV4/EDH-dependent vasorelaxation of submucosal arterioles in health and colitis. This study further supports our previous notion that TRPV4/EDH in mesenteric circulation plays a critical role in the pathogenesis of colitis.

Zinc drives vasorelaxation by acting in sensory nerves, endothelium and smooth muscle

Zinc, an abundant transition metal, serves as a signalling molecule in several biological systems. Zinc transporters are genetically associated with cardiovascular diseases but the function of zinc in vascular tone regulation is unknown. We found that elevating cytoplasmic zinc using ionophores relaxed rat and human isolated blood vessels and caused hyperpolarization of smooth muscle membrane. Furthermore, zinc ionophores lowered blood pressure in anaesthetized rats and increased blood flow without affecting heart rate. Conversely, intracellular zinc chelation induced contraction of selected vessels from rats and humans and depolarized vascular smooth muscle membrane potential. We demonstrate three mechanisms for zinc-induced vasorelaxation: (1) activation of transient receptor potential ankyrin 1 to increase calcitonin gene-related peptide signalling from perivascular sensory nerves; (2) enhancement of cyclooxygenase-sensitive vasodilatory prostanoid signalling in the endothelium; and (3) inhibition of voltage-gated calcium channels in the smooth muscle. These data introduce zinc as a new target for vascular therapeutics.

Capsaicin Causes Vasorelaxation of Rat Aorta through Blocking of L-type Ca2+ Channels and Activation of CB1 Receptors

The aim of this work was to determine whether Capsaicin may exert a vascular regulation through the activation of CB₁ and/or CB₂ receptors causing vasorelaxation in the rat aorta. Our results show the location of TRPV1 mainly in the endothelial and smooth muscle cells membrane. Nevertheless, Capsaicin caused vasorelaxation of this artery through a mechanism independent of TRPV1, since the specific antagonists Capsazepine and SB-366791 did not block the effect of Capsaicin. Because the significant expression of CB₁ and CB₂ receptors has been previously reported in the rat aorta, we used antagonists for these two receptors prior to the addition of Capsaicin. In these experiments, we found that the inhibition of CB₁ using AM281, decreases the vasorelaxant effect caused by Capsaicin. On the other hand, the vasorelaxant effect is not altered in the presence of the CB₂ receptor antagonist AM630. Furthermore, a partial decrease of the effect of Capsaicin was also seen when L-type calcium channels are blocked. A complete block of Capsaicin-induced vasorelaxation was achieved using a combination of Verapamil and AM281. In accordance to our results, Capsaicin-induced vasorelaxation of the rat aorta is neither dependent of TRPV1 or CB₂ receptors, but rather it is strongly suggested that a tandem mechanism between inactivation of L-type calcium channels and the direct activation of CB₁ receptors is involved. These findings are supported by CB₁ docking simulation which predicted a binding site on CB₁ receptors for Capsaicin.

Angiotensin II Receptor-Neprilysin Inhibitor Sacubitril/Valsartan Improves Endothelial Dysfunction in Spontaneously Hypertensive Rats

BACKGROUND

We have previously demonstrated that antihypertensive treatment with renin-angiotensin system inhibitors restores the impaired endothelium-dependent hyperpolarization (EDH)-mediated responses in spontaneously hypertensive rats (SHRs). Herein, we investigated whether the angiotensin II receptor-neprilysin inhibitor sacubitril/valsartan (LCZ696) would improve reduced EDH-mediated responses and whether LCZ696 would exert additional effects on endothelium-dependent and endothelium-independent vasorelaxation compared with an angiotensin II type 1 receptor blocker alone during hypertension.

METHODS AND RESULTS

SHRs were treated for 3 months with either LCZ696 or valsartan, from the age of 8 to 11 months. Age-matched, untreated SHRs and Wistar-Kyoto rats served as controls. Membrane potentials and contractile responses were recorded from the isolated superior mesenteric arteries. Acetylcholine-induced, EDH-mediated responses were impaired in untreated SHRs compared with Wistar-Kyoto rats. EDH-mediated responses were similarly improved in the LCZ696- and valsartan-treated SHRs. No difference was observed in acetylcholine-induced, nitric oxide-mediated relaxations among the 4 groups. Endothelium-independent relaxations in response to a nitric oxide donor, sodium nitroprusside, and those to levcromakalim, an ATP-sensitive K+-channel opener, were similar among the 4 groups; however, the sensitivities to levcromakalim were significantly higher in both LCZ696- and valsartan-treated SHRs.

CONCLUSIONS

LCZ696 appears to be as effective as valsartan in improving the impaired EDH-mediated responses during hypertension. LCZ696 and valsartan exert similar beneficial effects on endothelium-independent relaxation via enhanced sensitivity of the ATP-sensitive K+ channel. However, the dual blockade of renin-angiotensin system and neutral endopeptidase with LCZ696 does not appear to provide additional benefit over valsartan alone on vasomotor function in mesenteric arteries of SHRs.

Vasorelaxation to the Nitroxyl Donor Isopropylamine NONOate in Resistance Arteries Does Not Require Perivascular Calcitonin Gene-Related Peptide

Nitroxyl (HNO) donors offer considerable therapeutic potential for the treatment of hypertension-related cardiovascular disorders, particularly heart failure, as they combine an inotropic action with peripheral vasodilation. Angeli's salt is the only HNO donor whose mechanism has been studied in depth, and recently, Angeli's salt vasodilation was suggested to be indirect and caused by calcitonin gene-related peptide (CGRP) released from perivascular nerves after HNO activates TRPA1 (transient receptor potential cation channel subfamily A member 1) channels. We investigated resistance artery vasorelaxation to the HNO donor, isopropylamine NONOate (IPA/NO), one of the structures providing a template for therapeutic development. Wire myography in combination with measurements of smooth muscle membrane potential was used to characterize the effect of IPA/NO in mesenteric resistance arteries. Immunohistochemistry was assessed in pressurized arteries. IPA/NO concentration dependently hyperpolarized and relaxed arteries precontracted with the α1-adrenoreceptor agonist, phenylephrine. These effects were blocked by the soluble guanylyl cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) but not by the KATP channel inhibitor, glibenclamide. Vasorelaxation persisted in the presence of raised [K+]o, used to block hyperpolarization, capsaicin to deplete perivascular CGRP, or HC030031 (2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4 isopropylphenyl) acetamide) to block TRPA1 receptors. Without preconstriction, hyperpolarization to IPA/NO was suppressed by glibenclamide, capsaicin, or HC030031. Hyperpolarization but not vasorelaxation to exogenous CGRP was inhibited with glibenclamide. Thus, vascular hyperpolarization is not necessary for vasorelaxation to the HNO donor IPA/NO, even though both effects are cGMP dependent. The reduced hyperpolarization after depletion of perivascular CGRP or block of TRPA1 receptors indicates some release of CGRP, but this does not contribute to HNO vasorelaxation. Therefore, HNO-TRPA1-CGRP signaling does not seem important for vasodilation to IPA/NO in resistance arteries.

Interaction of Perivascular Adipose Tissue and Sympathetic Nerves in Arteries From Normotensive and Hypertensive Rats

The inhibitory action of perivascular adipose tissue (PVAT) in modulation of arterial contraction has been recently recognized and contrasted with the prohypertensive effect of obesity in humans. In this study we demonstrated that PVAT might have opposing effect on sympatho-adrenergic contractions in different rat conduit arteries. In superior mesenteric artery isolated from normotensive Wistar-Kyoto rats (WKY), PVAT exhibited inhibitory influence on the contractions to exogenous noradrenaline as well as to endogenous noradrenaline released from arterial sympathetic nerves during transmural electrical stimulation or after application of tyramine. In contrast, the abdominal aorta with intact PVAT responded with larger contractions to transmural electrical stimulation and tyramine when compared to the aorta after removing PVAT; the responses to noradrenaline were similar in both. This indicates that PVAT may contain additional sources of endogenous noradrenaline which could be responsible for the main difference in the modulatory effect of PVAT on adrenergic contractions between abdominal aortas and superior mesenteric arteries. In spontaneously hypertensive rats (SHR), the anticontractile effect of PVAT in mesenteric arteries was reduced, and the removal of PVAT completely eliminated the difference in the dose-response curves to exogenous noradrenaline between SHR and WKY. These results suggest that in mesenteric artery isolated from SHR, the impaired anticontractile influence of PVAT might significantly contribute to its increased sensitivity to adrenergic stimuli.

Influence of pressure on adenosine triphosphate function as a sympathetic neurotransmitter in small mesenteric arteries from the spontaneously hypertensive rat

OBJECTIVES

Enhanced sympathetic neurotransmission contributes to hypertension in the spontaneously hypertensive rat (SHR). We recently reported a method for studying sympathetic neurotransmission in pressurized small arteries, demonstrating a major role of adenosine triphosphate (ATP) as a sympathetic neurotransmitter under these physiological conditions. We have now used this methodology to assess the role of ATP as a sympathetic neurotransmitter in small mesenteric arteries isolated from SHRs.

METHODS

Small arteries were mounted in a suction electrode, cannulated and pressurized to either 30 or 90 mmHg. Nerve-evoked alterations in membrane potential were assessed using sharp microelectrodes. Neurally evoked vasoconstrictor responses were measured in the absence and presence of the α1-adrenoceptor antagonist, tamsulosin (0.1 μmol/l), or the P2 purinoceptor antagonist suramin (0.1 mmol/l).

RESULTS

At 30 mmHg the P2X-receptor-mediated excitatory junctional potential (EJP) was larger in arteries from SHRs (7.9 ± 0.9 mV) than Wistar-Kyoto (WKY) rats (3.2 ± 0.4 mV, P < 0.05). Increasing pressure increased the amplitude of the EJP, which again, was larger in SHRs. At 90 mmHg, activation of the perivascular nerves produced a larger vasoconstriction in arteries isolated from SHRs compared with WKY rats. The vasoconstrictor response in SHRs was abolished by either suramin or tamsulosin.

CONCLUSION

These data provide electrophysiological evidence for enhanced purinergic function in the SHR and show that ATP is fundamentally important in contributing to the vasoconstriction produced after activation of the perivascular nerves in pressurized arteries from the SHR. This involves a synergistic interaction with noradrenaline to causes enhanced mesenteric arterial vasoconstriction, which may contribute to the hypertension in this model.

Boron-doped diamond nano/microelectrodes for biosensing and in vitro measurements

Since the fabrication of the first diamond electrode in the mid 1980s, repid progress has been made on the development and application of this new type of electrode material. Boron-doped diamond (BDD) electrodes exhibit outstanding properties compared to oxygen-containing sp2 carbon electrodes. These properties make BDD electrodes an ideal choice for use in complex samples. In recent years, BDD microelectrodes have been applied to in vitro measurements of biological molecules in tissues and cells. This review will summarize recent progress in the development and applications of BDD electrodes in bio-sensing and in vitro measurements of biomolecules. In the first section, the methods for BDD diamond film deposition and BDD microelectrodes preparation are described. This is followed by a description and discussion of several approaches for characterization of the BDD electrode surface structure, morphology, and electrochemical activity. Further, application of BDD microelectrodes for use in the in vitro analysis of norepinephrine (NE), serotonin (5-HT), nitric oxide (NO), histamine, and adenosine from tissues are summarized and finally some of the remaining challenges are discussed.

Inhibitory effects of zingerone, a pungent component of Zingiber officinale Roscoe, on colonic motility in rats

Ginger (rhizome of Zingiber officinale Roscoe) is an herbal medicine for the treatment of gastrointestinal disorders including constipation and diarrhea. Zingerone is a likely active constituent responsible for the antidiarrheal activity of ginger. The current study was designed to characterize pharmacological actions of zingerone on colonic motility. To evaluate pharmacological effects of zingerone on colonic motility, we used isolated colonic segments from rats, in which mechanical responses were recorded in the longitudinal direction. In addition, we evaluated the effects on colonic motility in vivo by measuring intraluminal pressure changes and expelled fluid volume from the colon in anesthetized rats. Zingerone was applied to the lumen of the colon to allow the drug to access from the mucosal side. Zingerone inhibited spontaneous contractile movements in the isolated colonic segments in a dose-dependent manner. The inhibitory effects of zingerone on colonic movements were not affected by pretreatment with capsazepine, a typical antagonist of transient receptor potential vanilloid 1. In addition, tetrodotoxin, a blocker of voltage-dependent sodium channels on neurons, did not affect the suppression of colonic movements by zingerone, suggesting that zingerone acts on the smooth muscles directly. Zingerone also attenuated colonic motility in vivo without affecting blood pressure and heart rate. The effects were reversible and reproducible. Our findings suggest that zingerone can inhibit colonic motility via direct action on smooth muscles. Zingerone might exert beneficial therapeutic effects on hypermotility-induced diarrhea by abrogating excessive gastrointestinal motility.

Neuropeptide Y and CGRP concentrations in the rat tail artery: Effects of age and two types of diabetes

The changes in concentrations of two neuropeptides, neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP) in different segments of the rat tail artery have been investigated (a) after 12 and 16 weeks of streptozotocin (STZ)-induced hyperglycemia that has been induced at the age of 10 weeks, and (b) in 52-week-old Goto-Kakizaki (GK) rats that were intolerant of glucose, and age-matched Wistar controls. In the control animals at 22, 26 and 52 weeks of age, the concentration of CGRP was significantly greater in distal, relative to proximal, segments of normal arteries, and this contrasted with the pattern of distribution of NPY, which was consistently greater in the proximal than the distal segments. STZ-induced diabetes caused significant reductions in the concentrations of NPY and CGRP in the middle and distal segments of the vessel after 12 and 16 weeks of hyperglycemia. In the glucose-intolerant Goto-Kakizaki (GK) rats, the noradrenalin and adrenalin levels increased significantly in the distal segment of the artery relative to controls; in contrast there was a significant fall in dopamine concentration. The only significant change in the level of NPY in 52-week-old GK rats was an increase in the proximal segment, suggesting that in Type II pre-diabetes, noradrenalin and its co-transmitter NPY are affected independently. The concentration of CGRP increased significantly in all segments of the artery of the 12-month-old GK rats relative to controls. The similarities and differences between these measurements in Type I and Type II diabetic models are discussed.

Raised tone reveals purinergic-mediated responses to sympathetic nerve stimulation in the rat perfused mesenteric vascular bed

Noradrenaline and ATP are sympathetic co-transmitters. In rat isolated mesenteric small arteries, activation of sympathetic nerves can produce a vasoconstrictor response mediated by ATP. In contrast, the rat perfused mesenteric bed displays vasoconstrictor responses that are blocked solely by alpha1-adrenoceptor antagonists. This study assessed the effect of raising tone with a vasoconstrictor on purinergic and noradrenergic responses to sympathetic nerve stimulation in the rat perfused mesentery. Rat mesenteric vascular beds were perfused with physiological salt solution and responses to nerve stimulation, or P2X-receptor agonists, were determined under basal conditions and after raising tone with endothelin-1. The contribution of noradrenaline and ATP to sympathetic nerve-mediated responses was assessed using the alpha1-adrenoceptor antagonist, prazosin and the P2X-receptor desensitizing agent, alpha,beta-methyleneATP. The effect of endothelin-1 on excitatory junction potentials generated in response to nerve stimulation in isolated mesenteric arteries was also assessed. Under baseline conditions, responses to nerve stimulation were mediated solely by activation of alpha1-adrenoceptors. After raising perfusion pressure with endothelin-1 or the thromboxane mimetic 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha (U44619), sympathetic nerve-mediated responses were larger than under basal conditions and the response was partly sensitive to P2X-receptor desensitization. Responses to exogenous P2X-receptor agonists were enhanced after treatment with endothelin-1, while endothelin-1 decreased the amplitude of excitatory junction potentials. These results indicate that ATP acts as an important, functional, sympathetic neurotransmitter in the perfused mesentery under raised tone conditions, where the perfusion pressure is closer to that found in vivo. This effect is due to a postjunctional enhancement of purinergic function.

Impaired vasodilation in response to perivascular nerve stimulation in mesenteric arteries of TRPV1-null mutant mice

BACKGROUND

The role of the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in perivascular sensory nerves in the regulation of vascular reactivity is largely unknown. This study was designed to test the hypothesis that vasodilation induced by electrical field stimulation (EFS) of perivascular sensory nerves is mediated by the TRPV1 via release of sensory neurotransmitters in wild-type (WT) mice, and this effect is abolished in gene-targeted TRPV1-null mutant (TRPV1(-/-)) mice.

METHODS

Isolated mesenteric resistance arteries from WT and TRPV1(-/-)) mice were perfused and pretreated with guanethedine and atropine to block sympathetic and parasympathetic nerve activity, respectively. After precontracting with phenylephrine, changes of vascular diameters induced by EFS were monitored in the absence or presence of the TRPV1 receptor antagonist capsazepine; the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP8-37; or the substance P (SP) receptor antagonist, RP67580.

RESULTS

EFS-induced vasodilation was significantly reduced in arteries in TRPV1(-/-)) mice when compared to that of WT mice. Capsazepine and CGRP8-37 attenuated vasodilation induced by EFS in WT but not TRPV1(-/-)) mice. In contrast, RP67580 had no effect on the EFS-induced vasodilation in WT or TRPV1(-/-)) mice. The release of CGRP in the face of EFS challenge was significantly increased in both WT and TRPV1(-/-)) arteries, which was attenuated by capsazepine in WT but not TRPV1(-/-)) arteries. Exogenous CGRP caused dose-dependent vasodilation to a similar degree in WT and TRPV1(-/-)) arteries.

CONCLUSIONS

Our data show that in WT mice transmural stimulation of perivascular sensory nerves activates the TRPV1, leading to CGRP release from sensory nerve endings; and blockade of CGRP, but not SP, receptors abolishes TRPV1-mediated vasodilation during EFS. All these effects are impaired in TRPV1(-/-)) mice, indicating that TRPV1 plays a key role in modulating perivascular sensory nerve-mediated vasodilation.

Methanandamide hyperpolarizes gastric arteries by stimulation of TRPV1 receptors on perivascular CGRP containing nerves

Endogenous as well as synthetic cannabinoids have potent vasodilatory actions in a variety of vascular preparations. Their precise mechanism of action is as yet unclear, but several studies point to the activation of type 1 vanilloid (TRPV1) receptors on primary afferent perivascular nerves, stimulating the release of calcitonin gene related peptide (CGRP). Given the documented gastroprotective function of these nerves, and the various gastrointestinal effects reported for cannabinoids, we explored a possible link between these systems in the gastric circulation by comparing responses of small gastric arteries to cannabinoids and to calcitonin gene related peptide using conventional microelectrode techniques. Exposure of small gastric arteries to the stable endocannabinoid analogue methanandamide caused a hyperpolarization of the vascular smooth muscle cells, which was completely abolished by the vanilloid receptor antagonist capsazepine (P < 0.01). Exposure to exogenous calcitonin gene related peptide evoked fully reproducible (P > 0.05) hyperpolarizations with similar time course, unaffected by capsazepine. Preincubation with glibenclamide, an inhibitor of ATP-sensitive potassium (KATP) channels, reversed both responses to methanandamide (P < 0.01) and calcitonin gene related peptide (P < 0.05). Similar results were found in rat mesenteric arteries. These findings show that cannabinoids stimulate TRPV1 receptors, presumably causing the release of calcitonin gene related peptide, which hyperpolarizes the smooth muscle cells by activation of KATP channels. Because membrane hyperpolarization is a powerful mediator of vasorelaxation, this novel pathway might prove to be an important mechanism affording gastroprotection.

Capsaicin-induced, capsazepine-insensitive relaxation of the guinea-pig ileum

The mechanisms underlying transient receptor potential vanilloid receptor type 1 (TRPV1)-independent relaxation elicited by capsaicin were studied by measuring isometric force and phosphorylation of 20-kDa regulatory light chain subunit of myosin (MLC(20)) in ileum longitudinal smooth muscles of guinea-pigs. In acetylcholine-stimulated tissues, capsaicin (1-100 microM) and resiniferatoxin (10 nM-1 microM) produced a concentration-dependent relaxation. The relaxant response was attenuated by 4-aminopyridine and high-KCl solution, but not by capsazepine, tetraethylammonium, Ba(2+), glibenclamide, charybdotoxin plus apamin nor antagonists of cannabinoid receptor type 1 and calcitonin-gene related peptide. A RhoA kinase inhibitor reduced the relaxant effect of capsaicin at 30 microM. Capsaicin and resiniferatoxin reduced acetylcholine- and caffeine-induced transient contractions in a Ca(2+)-free, EGTA solution. Capsaicin at 30 microM for 20 min did not alter basal levels of MLC(20) phosphorylation, but abolished an increase by acetylcholine in MLC(20) phosphorylation. It is suggested that the relaxant effect of capsaicin at concentrations used is not mediated by TRPV1, but by 4-aminopyridine-sensitive K(+) channels, and that capsaicin inhibits contractile mechanisms involving Ca(2+) release from intracellular storage sites. The relaxation could be explained by a decrease in phosphorylation of MLC(20).

Enhanced neurally evoked responses and inhibition of norepinephrine reuptake in rat mesenteric arteries after spinal transection

In patients with high thoracic spinal lesions that remove most of the central drive to splanchnic preganglionic neurons, visceral or nociceptive stimuli below the lesion can provoke large increases in blood pressure (autonomic dysreflexia). We have examined the effects of T4 spinal transection on isometric contractions of mesenteric arteries isolated from spinalized rats. Nerve-evoked contractions involved synergistic roles for norepinephrine and ATP. At 7 wk after spinal transection, responses to perivascular stimulation at 1-5 Hz were enhanced fivefold, whereas the alpha1-adrenoceptor antagonist prazosin (10 nM) produced a twofold larger reduction in contraction (to 20 pulses at 10 Hz) than in unoperated controls. In contrast, the reduction in nerve-evoked contractions by the P2-purinoceptor antagonist suramin (0.1 mM) and the responses to the P2-purinoceptor agonist alpha,beta-methylene ATP or to high K+ concentration did not greatly differ between groups, indicating that arteries from spinalized rats were not generally hyperreactive. Sensitivity to the alpha1-adrenoceptor agonist phenylephrine was enhanced in arteries from spinalized rats, and the difference from controls was abolished by the norepinephrine uptake blocker desmethylimipramine. Sensitivity to the alpha1-adrenoceptor agonist methoxamine, which is not a substrate for the neuronal norepinephrine transporter, was similar among the groups. Thus the increased neurally evoked response after spinal transection appeared to be due to a reduction in neuronal uptake of released norepinephrine, a mechanism that did not explain the enhanced response of tail arteries after spinal transection that we previously reported. The findings provide further support for potentiated neurovascular responses contributing to the genesis of autonomic dysreflexia.

Hyperinnervation of mesenteric arteries in spontaneously hypertensive rats by sympathetic but not primary afferent axons

Hypertrophy of the perivascular plexus is thought to play a role in the development of hypertension in spontaneously hypertensive rats (SHR). However, it is not known whether the sympathetic varicosities are more numerous or larger, or form more neurovascular junctions. Further, a parallel hypertrophy of primary afferent terminals around the vessels might modulate any effects of hypertrophied sympathetic terminals. We have investigated the perivascular plexus around second-order mesenteric arteries of SHR and Wistar-Kyoto (WKY) rats by electron microscopy. Noradrenergic terminals were identified by the presence of small granular vesicles after chromaffin fixation, and substance P (SP+) afferent axons were identified by immunohistochemistry. The numbers of noradrenergic axon and varicosity profiles were higher (48 and 25%, respectively) in SHR than in WKY rats, and the majority lay closer to the medio-adventitial border. In contrast, there was no difference in the numbers of SP+ axons. Sympathetic and SP+ varicosities were indistinguishable in size, shape, vesicle content and mitochondrion content between each other and between the strains. However, both the number of neuromuscular junctions and the proportion of varicosities that formed them in SHR arteries were more than double those in WKY vessels. The data clearly show that hyperinnervation in SHR is specific for noradrenergic axons.