Dual effects of α2 -adrenoceptors in modulating myogenic tone in sheep isolated internal anal sphincter

BACKGROUND

The role of α-adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of α1 -adrenoceptors. We have used three clinically available agents, which are selective for α2 -adrenoceptors, to investigate their role in contractile and neurogenic responses on the internal anal sphincter (IAS).

METHODS

IAS strips, which had spontaneously generated tone, were used to investigate the contractile effect of lofexidine, brimonidine, and dexmedetomidine on muscle tone in the presence or absence of subtype selective antagonists. The effect of brimonidine on the magnitude and time course of neurogenic responses generated by electrical field stimulation (EFS) was also examined. The affinity of test compounds at α1 - and α2 -adrenoceptors was established by competition binding with [3H]-prazosin and [3H]-RX821002.

KEY RESULTS

All agonists caused concentration-dependent contraction of the IAS and lofexidine demonstrated an enantiomeric difference in potency with a 10-fold difference between the (-) and (+) isomers. Responses to lofexidine and dexmedetomidine were inhibited in the presence of the α1 -adrenoceptor selective antagonist prazosin, but not in the presence of RX811059 (α2 -adrenoceptor selective antagonist); brimonidine responses were inhibited by RX811059 and, to a lesser extent, by prazosin. Brimonidine affected both magnitude and duration of neurogenic responses, which was reversed in the presence of RX811059.

CONCLUSIONS & INFERENCES

We conclude that α2 -adrenoceptors can mediate contraction of IAS, although this effect is most evident with efficacious imidazoline agonists rather than the most selective ligand. In addition, this receptor subtype can directly inhibit noradrenergic contractile responses to EFS and, indirectly, enhance nitrergic relaxatory responses.

α-Tocopherol supplementation does not affect monocyte endothelial adhesion or C-reactive protein levels but reduces soluble vascular adhesion molecule-1 in the plasma of healthy subjects

Vascular monocyte retention in the subintima is pivotal to the development of cardiovascular disease and is facilitated by up-regulation of adhesion molecules on monocytes/endothelial cells during oxidative stress. Epidemiological studies have shown that cardiovascular disease risk is inversely proportional to plasma levels of the dietary micronutrients, vitamin C and vitamin E (alpha-tocopherol). We have tested the hypothesis that alpha-tocopherol supplementation may alter endothelial/monocyte function and interaction in subjects with normal ascorbate levels (> 50 microM), as ascorbate has been shown to regenerate tocopherol from its oxidised tocopheroxyl radical form in vitro. Healthy male subjects received alpha-tocopherol supplements (400 IU RRR-alpha-tocopherol/day for 6 weeks) in a placebo-controlled, double-blind intervention study. There were no significant differences in monocyte CD11b expression, monocyte adhesion to endothelial cells, plasma C-reactive protein or sICAM-1 concentrations post-supplementation. There was no evidence for nuclear translocation of NF-kappaB in isolated resting monocytes, nor any effect of alpha-tocopherol supplementation. However, post-supplementation, sVCAM-1 levels were decreased in all subjects and sE-selectin levels were increased in the vitamin C-replete group only; a weak positive correlation was observed between sE-selectin and alpha-tocopherol concentration. In conclusion, alpha-tocopherol supplementation had little effect on cardiovascular disease risk factors in healthy subjects and the effects of tocopherol were not consistently affected by plasma vitamin C concentration.

Investigation of the distribution and function of alpha-adrenoceptors in the sheep isolated internal anal sphincter

BACKGROUND AND PURPOSE

We have investigated the distribution of alpha-adrenoceptors in sheep internal anal sphincter (IAS), as a model for the human tissue, and evaluated various imidazoline derivatives for potential treatment of faecal incontinence.

EXPERIMENTAL APPROACH

Saturation and competition binding with (3)H-prazosin and (3)H-RX821002 were used to confirm the presence and density of alpha-adrenoceptors in sheep IAS, and the affinity of imidazoline compounds at these receptors. A combination of in vitro receptor autoradiography and immunohistochemistry was used to investigate the regional distribution of binding sites. Contractile activity of imidazoline-based compounds on sheep IAS was assessed by isometric tension recording.

KEY RESULTS

Saturation binding confirmed the presence of both alpha(1)- and alpha(2)-adrenoceptors, and subsequent characterization with sub-type-selective agents, identified them as alpha(1A)- and alpha(2D)-adrenoceptor sub-types. Autoradiographic studies with (3)H-prazosin showed a positive association of alpha(1)-adrenoceptors with immunohistochemically identified smooth muscle fibres. Anti-alpha(1)-adrenoceptor immunohistochemistry revealed similar distributions of the receptor in sheep and human IAS. The imidazoline compounds caused concentration-dependent contractions of the anal sphincter, but the maximum responses were less than those elicited by l-erythro-methoxamine, a standard non-imidazoline alpha(1)-adrenoceptor agonist. Prazosin (selective alpha(1)-adrenoceptor antagonist) significantly reduced the magnitude of contraction to l-erythro-methoxamine at the highest concentration used. Both prazosin and RX811059 (a selective alpha(2)-adrenoceptor antagonist) reduced the potency (pEC(50)) of clonidine.

CONCLUSIONS AND IMPLICATIONS

This study shows that both alpha(1)- and alpha(2)-adrenoceptors are expressed in the sheep IAS, and contribute (perhaps synergistically) to contractions elicited by various imidazoline derivatives. These agents may prove useful in the treatment of faecal incontinence.

Evidence for the expression of multiple uracil nucleotide-stimulated P2 receptors coupled to smooth muscle contraction in porcine isolated arteries

BACKGROUND AND PURPOSE

The uracil nucleotides UDP and UTP have been reported to activate P2Y2, P2Y4 and P2Y6 receptors to cause vasoconstriction. We have performed a comparative analysis of these receptors in endothelium-denuded smooth muscle from porcine isolated coronary and ear arteries, using pharmacological and molecular tools.

EXPERIMENTAL APPROACH

Tissue segments were used to construct non-cumulative concentration response curves for UTP and UDP, in the absence and presence of the P2 receptor antagonists PPADS or suramin. RT-PCR and immunoblot analyses were employed to define gene expression and immunoreactivity for P2Y2, P2Y4 and P2Y6 receptors.

KEY RESULTS

In the coronary artery, UTP-evoked contractile responses were reduced in the presence of suramin, but not PPADS, while the smaller responses to UDP were unaffected by either antagonist. In the ear artery, contractile responses to UDP were much smaller than those to UTP; responses to UTP were inhibited by both PPADS and suramin. RT-PCR suggested predominant expression of P2Y2 receptors in the coronary artery, while P2Y4 and P2Y6 receptor gene expression appeared equivalent in both tissues. Immunoblot analyses provided evidence for P2Y6 receptors in both tissues, with equivocal evidence of P2Y2 and P2Y4 receptor immunoreactivities.

CONCLUSIONS AND IMPLICATIONS

We conclude that UTP-evoked contraction of porcine coronary artery smooth muscle appears to be predominantly P2Y2-mediated, while the ear artery appears to express a uracil nucleotide-sensitive P2 receptor(s) which fails to fit readily into the current classification.

A novel mechanism of vasoregulation: ADP-induced relaxation of the porcine isolated coronary artery is mediated via adenosine release

In this study, we have investigated the mechanism of ADP-induced relaxation of porcine coronary artery (PCA) rings. The P2Y receptor agonists ADP and ADPbetaS produced concentration-dependent relaxation of endothelium-denuded PCA smooth muscle with pD2 values of 5.3 and 4.9, respectively. RT-polymerase chain reaction (RT-PCR) and immunoblotting demonstrated mRNA and protein expression of P2Y1 and A2A adenosine receptors in the PCA. The nonselective P2 antagonist PPADS or the P2Y1-selective antagonist MRS2179 failed to alter ADP- or ADPbetaS-induced relaxations. Relaxations to ADP were, however, blocked by the A2A adenosine receptor-selective antagonists ZM241385 and SCH58261 (apparent pK(B) values of 9.2 and 8.9, respectively). We excluded roles for direct occupancy of A2A adenosine receptors by ADP or ADPbetaS as well as metabolism to adenosine as mechanisms for ADP-evoked relaxations. However, ADP responses were significantly enhanced in the presence of the ENT1 nucleoside transporter inhibitors dipyridamole and NBTI and were significantly inhibited by adenosine deaminase, indicating a role for extracellular adenosine. Suprafusion of [3H]-adenine-labeled PCA segments showed that ADP induced the release of a number of purines, including adenosine. These data suggest that ADP mediates relaxation of the PCA via a novel mechanism that involves adenine nucleotide-evoked adenosine release and the subsequent activation of A2A receptors.

Vitamin C supplementation in normal subjects reduces constitutive ICAM-1 expression

Regulation of monocyte adhesion molecule gene expression is via redox sensitive transcription factors. We have investigated whether dietary antioxidant supplementation with vitamin C (250 mg/day) can modulate monocyte ICAM-1 expression in healthy male subjects with low plasma vitamin C at baseline. In a randomised, double-blind, crossover study, monocyte ICAM-1 mRNA was analysed using quantitative reverse transcriptase PCR. Protein was determined by flow cytometry (monocytes) and ELISA (plasma). Monocyte numbers were unaltered by supplementation. Subjects with low plasma vitamin C (<50 microM) prior to supplementation expressed higher levels of monocyte ICAM-1mRNA, and showed a significant (50%) reduction in ICAM-1mRNA expression after 6 weeks of 250 mg/day vitamin C supplementation (p<0.05). This was paralleled by a reduction in sICAM-1 (p<0.05). For the first time, these results show that dietary vitamin C can modulate monocyte ICAM-1 gene expression in vivo, where regulation of gene expression represents a novel mechanism for benefit from dietary antioxidants.

Identification of a gene sequence encoding a putative pyruvate oxidoreductase in Serpulina pilosicoli

Serpulina pilosicoli is a recently described species of intestinal spirochaete which can be identified using a species-specific monoclonal antibody BJL/AC1 reactive with a 29-kDa protein located in the cell envelope. A genomic library of the type strain of S. pilosicoli P43/6/78T was created in lambda zap express and screened using BJL/AC1. Single positive clones were isolated and excised into the phagemid vector pBK-CMV. Phagemid DNA was purified and a single clone was selected for sequencing. The size of spirochaetal DNA insert was determined by digestion with restriction endonucleases EcoRI and PstI as being approximately 2.6 kb. The nucleotide sequence of the gene encoding the protein with which the antibody reacted was determined by cycle sequencing. The insert contained an open reading frame of 285 nucleotides. Translation of the nucleotide sequence into amino acid (aa) residues showed a sequence of 275 aa. Comparison of this sequence with databases revealed homology to pyruvate oxidoreductases from various organisms found in the gastroinestinal tract. These included the pyruvate ferredoxin oxidoreductase (POR) alpha submit of Helicobacter pylori (38.8% identity in 250 aa), pyruvate-flavodoxin oxidoreductase of Escherichia coli (28.7% identify in 258 aa) and Giardia intestinalis (25.1% identity in 251 aa). A significant level of homology was also observed with hyperthermophilic bacteria such as the POR of Thermatoga maritima (38.6% in 254 aa) and the 2-ketovalerate-ferredoxin oxidoreductase of Pyrococcus furiosus (34% in 262 aa).

Sub-specific differentiation of intestinal spirochaete isolates by macrorestriction fragment profiling

Macrorestriction fragment profile analysis by PFGE was used to distinguish intestinal spirochaetes, some of which were isolated from cases of swine dysentery and intestinal spirochaetosis in humans, pigs, mice, chickens and dogs. Macrorestriction fragment profiles using SmaI and SacII restriction enzymes were produced and used in statistical analysis. This permitted the division of the isolates into two major clusters. One cluster contained isolates which were identified as Serpulina pilosicoli and the second cluster contained isolates identified as Serpulina hyodysenteriae by immunoblotting with species-specific mAbs. Both species contained sub-specific groups, although these rarely correlated with the source of the isolates. We conclude that PFGE is capable of sub-specific differentiation of intestinal spirochaetes, but that the current species contain a large variety of genotypes among which cross-species transmission may be feasible.