Submucosal Arterioles Research Articles

The supine venous pump function test: normal values and results in patients with the post-thrombotic syndrome: J.A.H.R. Claassen 1, M.C.H. Janssen 1, W.N.J.C. van Asten 3, H. Wollersheim 1, M.J.M. de Rooij 2, Th. Thien 1. Department of 1 Medicine, 2 Dermatology and 3 Clinical Vascular Laboratory, University Hospital, Nijmegen, Netherlands

The muscarinic receptor mediating vasolidation of resistance vessels in the rat isolated, constant-pressure perfused kidney (preconstriction by 10−7 M cirazoline) was characterized by subtype-preferring agonists and selective antagonists. The agonists produced vasolidation with the following rank order of potency: arecaidine propargyl ester (APE) > 5-methylfurtrethonium = methacholine = oxotremorine > (R)-aceclidine > arecaidine 2-butyne-1,4-diyl bisester > 4-Cl-McN-A-343 = (R)-nipecotic acid ethyl ester = N-ethyl-guvacine propargyl ester (R)-aceclidine = (S)-nipocotic acid ethyl ester > McN-A-343. Agonist-induced vasodilation disappeared after destruction of the endothelium with detergent. Highly significant correlations of agonist potencies for vasodilation were found between rat kidney and guinea-pig ileum submucosal arterioles as well as agonist potencies at smooth muscle muscarinic M3 receptors of the guinea-pig ileum. The rank order of antagonist potencies (4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > (R)-hexahydro-difenidol ∼ hexahydro-sila-difenol > pirenzepine ∼ p-fluorohexahydro-sila difenol ∼ himbacine ∼ AF-DX 384 ∼ AQ-RA 741 > (S)-hexahydro-difenidol) to attenuate vasodilation to APE in rat kidney, correlated significantly with affinities at M3 receptors in submucosal arterioles and in smooth muscle of the guinea-pig ileum, but differed from those at M1 and M2 receptors in rabbit vas deferens. The agonist and antagonist potencies suggest that vasodilation elicited by muscarinic stimuli in endothelium-intact rat renal vasculature is mediated by functional muscarinic M3 receptors.

RELATIVE CONTRIBUTIONS OF EXTRACELLULAR Ca2+ AND Ca2+ STORES TO SMOOTH MUSCLE CONTRACTION IN ARTERIES AND ARTERIOLES OF RAT, GUINEA‐PIG DOG AND RABBIT

1. These studies describe the functional effects of modulation of the sarcoplasmic reticulum (SR) Ca2+ stores at three levels of the vasculature: (i) large arteries (rat and guinea-pig aorta); (ii) small resistance arteries (rat tail artery, rabbit mesenteric artery, dog mesenteric artery); and (iii) arterioles (guinea-pig submucosal arterioles of the small intestine). 2. All tissues responded to phenylephrine (PE; 10 mumol/L) with a transient contraction in Ca(2+)-free Krebs', reflecting Ca2+ release from PE-sensitive Ca2+ stores. After pretreatment with cyclopiazonic acid (CPA; 30 mumol/L) or thapsigargin (TSG; 1 mumol/L), putative SR Ca2+ pump inhibitors, the PE-induced contraction in a Ca(2+)-free medium was significantly inhibited in arterial tissues at all levels of the vasculature. Similarly, ryanodine (RYA; 30 mumol/L), an agonist that enhances Ca2+ release from the SR, also reduced the PE contraction in a Ca(2+)-free solution. 3. CPA or TSG alone in the presence of extracellular Ca2+, caused marked and sustained contraction in the rat and guinea-pig aorta and marked but transient or no contraction in the resistance arteries. In the rat and guinea-pig aorta, RYA caused a slowly developing tension. Little increase in basal tension was produced by RYA in resistance arteries and arterioles. 4. The findings show that an agonist-releasable Ca2+ pool is present at all levels of the vasculature that is independent of the size of the vessels and suggest that under normal physiological conditions there is an intimate balance between the roles of the plasma membrane and of the SR in the maintenance of vascular contractility. It appears that the role of the SR diminishes as the arteries become smaller, while Ca2+ fluxes across the plasma membrane predominates.

Relationship Between Intestinal Fibrosis and Histopathologic and Morphometric Changes in Consequential and Late Radiation Enteropathy

Intestinal fibrosis is a marked feature of late radiation enteropathy. This study assessed the time dose fractionation relationships of radiation-induced fibrosis in order to elucidate possible pathogenetic mechanisms. In 290 male Sprague-Dawley rats, a loop of small bowel was transposed to the left side of the scrotum. Three weeks later, the transposed segment was irradiated with either single dose or various fractionated regimens. The animals were observed for radiation-induced intestinal complications and killed in groups, 2 and 26 weeks after completion of irradiation. A semiquantitative histopathologic radiation injury score, morphometry of the submucosa, submucosal arterioles, intestinal surface area, and relative collagen content were used as endpoints. Fibrosis, measured by collagen assay and radiation injury score, increased with total dose, increasing fraction size and reduction in overall treatment time. This paralleled the results of morphometric assessment of mucosal surface area. Differences in vascular morphometry were only statistically significant in response to changes in total dose and fraction size and not with changes in overall treatment time. We conclude that fibrosis increases with increasing observation time, increasing fraction size, increasing total dose, and reduction of interfraction interval. Consequential injury, occurring as a result of disruption of mucosal integrity, seems to be an important mechanism for development of intestinal fibrosis. In contrast, vascular injury is relatively independent of this mechanism.

Electronic‐endoscopic Observation of ET‐1 Induced Mucosal Ischemia and Preventive Effect of ETA Recepter Selective Antagonist, FR139317, in Rat Distal Colon

Abstract:For investigation of the effects of Endothelin‐1 (ET‐1) on colonic mucosa, ET‐1 (10–160 nmol/kg) was sprinkled on rat colonic mucosa under observation with a new electronic endoscopic system (TOSHIBA TRE‐3000, Japan). A high dose of ET‐1 induced complete obstruction of submucosal arterioles, but not venules, without affecting arterial blood pressure. The ET‐1 ‐induced contraction of both vessel types was maintained for 50 min and was followed by intramucosal dot hemorrhages. In this experimental model, the effects of ET‐1 on the mucosal microcirculation were further analyzed with a laser doppler blood flowmeter (LDF) during endoscopic observation. The maximum decrease in mucosal blood flow was to 20 % of the control value and this decrease was maintained for 20 min at a dose of 80 nmol/kg. An ETA receptor selective antagonist, FR139317 (800 nmol/kg), inhibited ET‐1 ‐induced changes in endoscopic findings and LDF, shifting the concentration‐response curve of LDF to the right. FR139317 is a potentially useful new therapy for inflammatory bowel diseases, conditions which have been reported to be associated with high local concentrations of ET‐1. Our electronic‐endoscopic system was demonstrated to be a useful basic research tool for studying gastrointestinal pathophysiology.

Interaction of endogenous nitric oxide and CGRP in sensory neuron-induced gastric vasodilation.

Stimulation of capsaicin-sensitive sensory nerves induces gastric mucosal hyperemia, which is mediated in part by both calcitonin gene-related peptide (CGRP) and nitric oxide (NO). In the present study, we used in vivo microscopy in anesthetized rats to determine 1) whether these agents were released locally at the submucosal level and, if so, 2) whether CGRP dilates arterioles via release of endothelium-derived NO. Intragastric capsaicin (160 microM) dilated submucosal arterioles from 25 +/- 3 to 67 +/- 8 microns. The intragastric capsaicin-induced vasodilation was markedly reversed not only by intravenous administration of the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME) but also by submucosal suffusion of either L-NAME or the CGRP receptor antagonist human CGRP-(8-37). The latter findings indicate that both NO and CGRP are released locally at the submucosal level. Submucosal application of CGRP induced dose-dependent dilation of gastric submucosal arterioles, which was significantly attenuated by L-NAME. However, at the same degree of vasodilation (42 microns), the dilation induced with submucosal CGRP was much less attenuated by NO synthesis inhibition (-28%) compared with that induced with intragastric capsaicin (-79%). This indicates that endothelium-derived NO released by CGRP was not the only source of submucosal NO in the latter response. There must be another as yet undetermined source of submucosal NO, e.g., possibly nitroxidergic nerves.

Characterization of endothelin vasomotor responses in submucosal arterioles in the guinea pig intestine: [formula omitted], S. Vanner. GI Diseases Research Unit, Queen's University, Kingston, Canada

Characterization of endothelin vasomotor responses in submucosal arterioles in the guinea pig intestine: [formula omitted], S. Vanner. GI Diseases Research Unit, Queen's University, Kingston, Canada

Characterization of P 2 receptor-mediated constriction of submucosal arterioles in guinea pig ileum : [formula omitted], A. Herring and T. Harpstead. Department of Pharmacology & Toxicology, Michigan State University., E. Lansing, MI 48824

Characterization of P 2 receptor-mediated constriction of submucosal arterioles in guinea pig ileum : [formula omitted], A. Herring and T. Harpstead. Department of Pharmacology & Toxicology, Michigan State University., E. Lansing, MI 48824

Histamine H 1 and H 3 vasodilator mechanisms in the guinea pig ileum

Background/Aims : Histamine dilates gastrointestinal blood vessels. Whether this is caused by direct activation of vascular histamine receptors or by activation of enteric neurons is not known. The aim of this study was to determine which of these pathways is activated by histamine and to examine the cellular mechanisms involved. Methods : The effects of histamine were studied in in vitro submucosal preparations from the guinea pig ileum using videomicroscopy to monitor changes in submucosal arteriolar diameter. Results : Histamine caused a tetrodotoxin-insensitive dose-dependent dilation (median effective concentration [EC 50], 1 μmol/L), showing direct activation of vascular histamine receptors. The H 1 antagonist pyrilamine, but not the H 2 blocker ranitidine, competitively inhibited the histamine dilatation. The nitric oxide synthase inhibitor N G -monomethyl- l-arginine ( l-NMMA) inhibited histamine vasodilations by 66%. Indomethacin alone did not alter histamine vasodilations but, when combined with l-NMMA, caused a significantly greater inhibition of the histamine response compared with l-NMMA alone. l-Arginine prevented the actions of l-NMMA. In the presence of both H 1 and H 2 antagonists, periarteriolar stimulation of sympathetic nerves evoked a tetrodotoxin-sensitive vasoconstriction, which was inhibited by histamine (EC 50, 0.8 μmol/L). This histamine action was blocked by the H 3 antagonist thioperamide. Conclusions : Histamine can produce vasodilation of submucosal arterioles by two distinct mechanisms: activation of vascular H 1 receptors resulting in release of nitric oxide from endothelium and activation of H 3 receptors on sympathetic nerve terminals resulting in presynaptic inhibition of vasoconstrictor tone.

Corelease of neuropeptides from capsaicin-sensitive afferents dilates submucosal arterioles in guinea pig ileum.

Extrinsic sensory afferent nerves projecting to the guinea pig intestinal submucosal arterioles contain both substance P (SP) and calcitonin gene-related peptide (CGRP); selective stimulation of these nerves causes vasodilation. However, it is not known whether either or both of these neuropeptides may be responsible for this neurogenic vasodilation. To examine this question, the actions of selective SP and CGRP antagonists on vasodilations evoked by SP, CGRP, and selective stimulation of extrinsic sensory afferents with capsaicin were measured in isolated submucosal arteriolar preparations with videomicroscopy. The SP receptor antagonist CP-96,345 (200 nM) abolished the vasodilation produced by half-maximal concentration (EC50) of SP and was without effect on the vasodilation produced by EC50 concentration of CGRP. Conversely, the CGRP receptor antagonist, CGRP-(8--37) (1 microM) abolished the vasodilation to CGRP but did not alter the SP-induced vasodilation. Neither antagonist altered the muscarinic vasodilation or nerve-evoked sympathetic vasoconstriction. Maximum inhibition of the capsaicin-induced vasodilation by CP-96,345 (600 nM) was 67%, and maximum inhibition of this response by CGRP-(8--37) (2 microM) was 53%. Complete inhibition of the capsaicin-induced vasodilation occurred when both antagonists were present. It is concluded that the vasodilation in response to activation of extrinsic sensory afferents innervating submucosal arterioles is due to the corelease of SP and CGRP.

Immunohistochemical demonstration of calbindin-containing nerve endings in the rat esophagus

Immunoreactivity for calbindin was found in nerve endings with irregular laminar shapes in the rat esophagus. In the myenteric ganglia, laminar endings of a range of sizes formed a complex network and appeared to lie at the surface of the ganglion. The myenteric ganglia that contained nerve endings were most abundant in the upper portion of the esophagus, their number decreasing orally to anally. Calbindin-immunoreactive nerve cell bodies were scattered throughout the esophagus. Laminar terminals were found in the connective tissue of the lamina propria immediately beneath the epithelium and in the muscularis mucosae. Occasional nerve branches formed a network of aborizing endings that surrounded part of the submucosal arterioles. Immunoreactive nerve endings in the mucosa and submucosa were present only in the upper part of the cervical esophagus. Unilateral vagotomy caused a remarkable decrease in the number of the myenteric ganglia containing the calbindin-immunoreactive laminar endings after 15 days or survival; in some of ganglia, the laminar structures disappeared and nerve endings showing weak immunoreactivity had an indistinct appearance, so that the outline of the ganglia became obscure. In operated rats at 24 days, the number of innervated ganglia was about half that in normal rats. However, there was no change in the morphology and the occurrence of the immunoreactive laminar structures in the mucosa and submucosa after denervation. The results show that many of the laminar endings that are immunoreactive for calbindin in the myenteric ganglia are derived from the vagus nerve. Thus, the calbindin-immunoreactive nerve endings with laminar expansions that are found in the rat esophageal wall could be sensory receptors.

Effect of capsaicin on distribution of binding sites for tachykinins and calcitonin gene-related peptide in rat urinary bladder: A quantitative autoradiographic study

The autoradiographic localization of binding sites for [ 125I]BH-[Sar 9,Met(O 2) 11]SP, [ 125I]NKA, and [ 125I]CGRP was investigated in adjacent sections of urinary bladder body, from adult rats pretreated 14 days before with capsaicin or vehicle. Location of silver grains was assessed both qualitatively and quantitatively using computerized densitometry. Dense labeling of smooth muscle was seen with both [ 125I]BH-[Sar 9,Met(O 2) 11]SP ([ 125I]BHSar-SP) and [ 125I]NKA; in addition, [ 125I]BHSar-SP labeled submucosal blood vessels. For these radioligands, no differences were apparent between sections from capsaicin- and vehicle-pretreated rats. Specific binding of [ 125I]CGRP was observed over the epithelium and weakly over submucosal arterioles, but not over smooth muscle. The density of [ 125I]CGRP binding sites on the epithelium, but not blood vessels, was increased ( p < 0.05) by 22% after chronic capsaicin pretreatment, suggesting receptor upregulation. This study demonstrates that although all three peptides are colocalized in primary afferent sensory fibers in rat urinary bladder, the receptors for these neuropeptides are located on different cell types and may be subject to different neural influences.

Nitric oxide synthase distribution in the rat intestine: A histochemical analysis

Background: Nitric oxide is an inhibitory transmitter of nonadrenergic, noncholinergic neurons and is purported to be an endothelium-derived relaxant-type factor in the mammalian gut. This study aimed to provide a complete report on the distribution of NO synthase in the rat small and large intestine. Methods: NO synthase was visualized histochemically through this enzyme's reduced nicotinamide adenine dinucleotide phosphate diaphorase activity and the distribution of staining within the gut wall. Results: The presence of NO synthase activity in myenteric neurons and their efferents to the circular muscle was confirmed. The largest proportion of stained cells per ganglion was found in the ileum, and the smallest proportion was in the colon. Stained neural elements were also found within the submucosa throughout the intestine. Stained cells within the myenteric and submucous nerve plexi displayed both type I and type II morphologies, with the latter being more numerous. In addition to neural staining, submucosal arterioles showed a regular pattern of small patches of staining unrelated to any perivascular innervation. Conclusions: These findings indicate an extensive neural and vascular localization of NO generation potential throughout the wall of the rat intestine, thus providing a structural basis for the functional diversity of NO.

Role of L-arginine-derived nitric oxide in cholinergic dilation of gastric arterioles

The role of L-arginine-derived nitric oxide (NO) in cholinergic vasodilation of resistance vessels was studied in the intact stomach of the rat, utilizing an in vivo microscopy technique. Two L-arginine analogues, NG-monomethyl-L-arginine (L-NMMA) and nitro-L-arginine methyl ester (L-NAME), were used to block NO synthesis. Cholinergic dilation of gastric submucosal arterioles was induced by topical application of various concentrations of acetylcholine (ACh) (10(-7)-10(-4) M). Intravenous but not topical administration of L-NMMA and L-NAME caused an increase in arterial pressure. Intravenous or topical L-NAME reduces resting arteriolar diameter. These findings support the contention that NO formation modulates basal vascular tone and suggest that NO release may play a significant role in the regulation of the gastric circulation. L-Arginine analogues attenuated the arteriolar dilating effect of ACh but not adenosine or nitroglycerin. Substantial arteriolar responses to ACh remained after systemic or topical treatment with either L-NMMA or L-NAME. These results indicate that the L-arginine-NO pathway accounts only in part for ACh-induced vasodilation in gastric resistance vessels in vivo.

Mechanism of action of capsaicin on submucosal arterioles in the guinea pig ileum.

Vasomotor neurons in the enteric nervous system release acetylcholine to dilate submucosal arterioles, but it is not known whether sensory nerves that project to these vessels also can provide a vasodilator innervation. This possibility was examined by determining the mechanism of action of capsaicin on guinea pig ileal submucosal arterioles in vitro. Capsaicin dilated all vessels that had been preconstricted with prostaglandin F2 alpha; mean effective concentration was 11 nM, and maximal dilation occurred at 60-200 nM. The vasodilation showed marked desensitization upon repeated applications of capsaicin. Tetrodotoxin blocked the capsaicin-induced vasodilation but not the desensitization observed upon repeated application. Muscarinic receptor antagonists did not affect the actions of capsaicin. Capsaicin did not dilate arterioles whose extrinsic sensory afferent fibers had been surgically removed. Substance P and human calcitonin gene-related peptide II dilated arterioles; these dilations were not inhibited after desensitization of the capsaicin-induced vasodilation. Thus capsaicin dilates submucosal arterioles by selectively activating extrinsic afferent fibers that release vasodilator transmitter substances onto these vessels.

Characterization of muscarinic receptorsmediating vasodilation in rat perfused kidney

The muscarinic receptor mediating vasolidation of resistance vessels in the rat isolated, constant-pressure perfused kidney (preconstriction by 10 −7 M cirazoline) was characterized by subtype-preferring agonists and selective antagonists. The agonists produced vasolidation with the following rank order of potency: arecaidine propargyl ester (APE) > 5-methylfurtrethonium = methacholine = oxotremorine > (R)-aceclidine > arecaidine 2-butyne-1,4-diyl bisester > 4-Cl-McN-A-343 = (R)-nipecotic acid ethyl ester = N-ethyl-guvacine propargyl ester (R)-aceclidine = (S)-nipocotic acid ethyl ester > McN-A-343. Agonist-induced vasodilation disappeared after destruction of the endothelium with detergent. Highly significant correlations of agonist potencies for vasodilation were found between rat kidney and guinea-pig ileum submucosal arterioles as well as agonist potencies at smooth muscle muscarinic M 3 receptors of the guinea-pig ileum. The rank order of antagonist potencies (4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > (R)-hexahydro-difenidol ∼ hexahydro-sila-difenol > pirenzepine ∼ p-fluorohexahydro-sila difenol ∼ himbacine ∼ AF-DX 384 ∼ AQ-RA 741 > (S)-hexahydro-difenidol) to attenuate vasodilation to APE in rat kidney, correlated significantly with affinities at M 3 receptors in submucosal arterioles and in smooth muscle of the guinea-pig ileum, but differed from those at M 1 and M 2 receptors in rabbit vas deferens. The agonist and antagonist potencies suggest that vasodilation elicited by muscarinic stimuli in endothelium-intact rat renal vasculature is mediated by functional muscarinic M 3 receptors.

Mucosal stimulation evokes vasodilation in submucosal arterioles by neuronal and nonneuronal mechanisms.

The effects of mechanical stimulation of intestinal mucosa on submucosal arterioles of guinea pig ileum were examined using video microscopy of in vitro preparations consisting of submucosal plexus with adjacently attached mucosa. Mucosal stimulation did not alter the diameter of relaxed vessels but dilated arterioles preconstricted with phenylephrine or the prostaglandin analogue U-46619. Tetrodotoxin (TTX) or muscarinic receptor antagonists inhibited the vasodilation evoked by mucosal stimulation in 60% of preparations examined from normal and extrinsically denervated animals. The TTX-sensitive vasodilation to mucosa stimulation was partially inhibited by the 5-hydroxytryptamine3 (5-HT3) receptor antagonist ICS 205930. The TTX-insensitive vasodilation was largely prevented when the histamine receptor antagonists cimetidine and pyrilamine and the prostaglandin synthesis inhibitor indomethacin were applied. This study has demonstrated a reflex vasodilation to mucosal stimulation in an isolated submucosal plexus preparation that involves both neuronal and nonneuronal pathways. The neuronal pathway converges on cholinergic vasodilator neurons in the submucosal ganglia. The nonneuronal pathway involves the release of 5-HT, histamine, and prostaglandins from mucosal elements; 5-HT excites cholinergic vasodilator neurons, whereas histamine and prostaglandins dilate submucosal arterioles directly.

Cholera toxin-sensitive neurons in guinea pig submucosal plexus.

Cholera toxin (CT) increases intestinal secretions by direct stimulation of mucosal enterocytes; enteric neurons also may play a role. We tested the latter possibility by retrograde labeling of mucosal terminals in guinea pig small intestine with the B subunit of CT (B-CT) and by intracellular recordings from submucosal neurons during superfusion with CT. All vasoactive intestinal peptide (VIP)-positive neurons, and only VIP-positive neurons, were labeled with B-CT. Fluorogold (FG) was used to retrogradely label nerve terminals in submucosal arterioles in preparations in which B-CT labeled mucosal terminals; colocalization of B-CT with FG was observed in neurons up to 3 mm from the site of FG application. CT selectively depolarized neurons known to contain VIP. We conclude that all VIP-containing neurons, and only VIP neurons, in guinea pig submucosal plexus possess B-CT binding sites and can be activated by CT. Some of these neurons provide a dual innervation to both arterioles and mucosa. We suggest that one functional consequence of CT may be to activate vasodilator nerves, thus increasing vascular perfusion of the mucosa to further stimulate intestinal secretions.

Subtype classification of presynaptic α2-adrenoceptors

1. 1. Presynaptic α 2-adrenoceptors in a few tissues have recently been pharmacologically classified in functional studies. 2. 2. Autoreceptors are of α 2 A -subtype in rabbit occipito-perietal cortex, rat cerebral cortex, vas deferens, submandibular gland, kidney, guinea-pig ileum submucosal arterioles and urethra. 3. 3. Heteroreceptors are of α 2 A -subtype in rat cerebral cortex, vas deferens, guinea-pig ileum submucosal plexus and Auerbach's plexus. 4. 4. In rat atria autoreceptors have been shown to be of α 2 B -subtype. 5. 5. Classification is done mainly with the α 2 A -adrenoceptor-selective oxymetazoline, WB 4101 and BRL 44408, and the α 2 B -adrenoceptor-selective prazosin, AR-C 239, chlorpromazine and BRL 41992. 6. 6. With four α 2-adrenoceptor subtypes to consider, a larger number of subtype-selective compounds may have to be characterized in the classification in the many tissues, where presynaptic α 2-adrenoceptors are found.

Structural and morphometric analysis of murine small intestine after indomethacin administration.

Indomethacin, a nonsteroidal anti-inflammatory drug, induces the formation of gastrointestinal ulceration both in experimental animals and in humans. A study of indomethacin-induced ulcers in the mouse showed that two doses of indomethacin, each administered subcutaneously at 85 mg/kg body weight, induced well-defined gastrointestinal ulcers in C57 mice, accompanied by inflammatory and vascular changes in the stomach and small intestine. Maximal damage was observed 20 h after the second dose of indomethacin. Morphometric analysis identified changes in all compartments of the small intestine. There was a marked reduction in the length of the small intestine, intestinal dilatation, a significant decrease in villous height, with the formation of subepithelial blisters or blebs within villi, and submucosal vascular dilatation. There was no change in the number of villi or of submucosal arterioles or in the total amount of muscle present in the wall of the intestine. The tissue changes identified in this study may have implications for gut function at specific periods during indomethacin treatment.

Histamine H 1 receptors mediate vasodilation in guinea-pig ileum resistance vessels: characterization with computer-assisted videomicroscopy and new selective agonists

Histamine receptors on guinea-pig ileum submucosal arterioles (outside diameter 40–80 μm) were studied in vitro using a computer-assisted videomicroscopy system (Diamtrak ®). Histamine receptor agonists investigated in this study were histamine, the H 1 receptor-selective compound, 2-[2-(3-fluorophenyl)-4-imidazolyl]ethanamine (VZ 20), the H 2 receptor-selective compounds, dimaprit, impromidine, (±)-N 1 [3-(4-fluorophenyl)-3-(pyridin-2-yl)propyl]-N 2-[3-(1H-imidazol-4-yl)Dpropyllguanidine (arpromidine) and (±)-N 1-[3-(3,4-difluorophenyl)-3-(pyridin2-yl)propyl]-N 2-[3-(1H-imidazol-4-yl)propyl]guanidine (BU-E-75), as well as the H 3 receptor-selective drug, (R)-α-methylhistamine ((R)-α-MeHA). Applied to vessels at resting tone, the agonists (1 nM-300 μM) did not change arteriolar diameter. Vessels prcconstrictcd by 10 μM noradrenaline showed similar concentration-dependent vasodilations with histamine and VZ 20 (pD 2 = 5.38 and 5.36, respectively). This histamine-induced vasodilation was not affected by tetrodotoxin (0.5 μM) or indomethacin (1 μM). but was completely abolished in the presence of 1μM of the H 1 receptor antagonist, mepyramine. Calculation of the antagonist affinity of mepyramine for the histamine receptors in submucosal arterioles yielded a pA 2 of 9.46. In contrast to histamine and VZ 20, the H 2 receptor agonist, dimaprit, and the H 3 receptor agonist, (R)-α-MeHA, were ineffective at preconstrictcd arterioles. The guanidine-type H 2 receptor agonists, impromidine, arpromidine and BU-E-75, produced vasodilation at noradrenaline-preconstricted arterioles (−log (EC 50 = 4.47, 5.30 and 5.39, respectively) but, in contrast to histamine, were ineffective at arterioles preconstricted by U-46619 (300 nM). The −log EC 50 were significantly correlated to antagonist affinities (pA 2 values) of the compounds determined at α 1-adrenoceptors in rat vas deferens and using urapidil as reference α 1-receptor antagonist (impromidine = 4.73, arpromidine = 5.66 and BU-E-75 = 6.15: urapidil pA 2 = 7.52 and −log EC 50) = 6.56 at guinea-pig ileum submucosal arterioles). Thus, vasodilation due to the guanidine-typ H 2 receptor agonists in noradrenalinc-preconstricted arterioies was due to concomitant α 1-adrenoceptor antagonism of the compounds. The data suggest that submucosal arterioles in guinea-pig ileum possess only functional histamine H 1 receptois; activation of which dilates the blood vessels.