Cholinergic receptors in the human vas deferens

Regulation of nerve-evoked contractions of rabbit vas deferens by acetylcholine

Stimulation of intramural nerves in the vas deferens of many species yields a classical biphasic contraction comprised of an initial fast component, mediated by P2X receptors and a second slower component, mediated by α1-adrenoceptors. It is also recognized that sympathetic nerve-mediated contractions of the vas deferens can be modulated by acetylcholine (Ach), however there is considerable disagreement in the literature regarding the precise contribution of cholinergic nerves to contraction of the vas deferens. In this study we examined the effect of cholinergic modulators on electric field stimulation (EFS)-evoked contractions of rabbit vas deferens and on cytosolic Ca(2+) levels in isolated vas deferens smooth muscle cells (VDSMC). The sustained component of EFS-evoked contractions was inhibited by atropine and by the selective M3R antagonist, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP). EFS-evoked contractions were potentiated by Ach, carbachol (Cch), and neostigmine. The sustained phase of the EFS-evoked contraction was inhibited by prazosin, an α1-adrenoceptor antagonist and guanethidine, an inhibitor of noradrenaline release, even in the continued presence of Ach, Cch or neostigmine. The soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one enhanced the amplitude of EFS-evoked contractions and reduced the inhibitory effects of 4-DAMP. Isolated VDSMC displayed spontaneous Ca(2+) oscillations, but did not respond to Cch. However, the α1-adrenoceptor agonist, phenylephrine, evoked a Ca(2+) transient and contracted the cells. These data suggest that EFS-evoked contractions of the rabbit vas deferens are potentiated by activation of M3 receptors and reduced by activation of a sGC-dependent inhibitory pathway.

Expression and function of G-protein-coupled receptorsin the male reproductive tract

This review focuses on the expression and function of muscarinic acetylcholine receptors (mAChRs), α1-adrenoceptors and relaxin receptors in the male reproductive tract. The localization and differential expression of mAChR and α1-adrenoceptor subtypes in specific compartments of the efferent ductules, epididymis, vas deferens, seminal vesicle and prostate of various species indicate a role for these receptors in the modulation of luminal fluid composition and smooth muscle contraction, including effects on male fertility. Furthermore, the activation of mAChRs induces transactivation of the epidermal growth factor receptor (EGFR) and the Sertoli cell proliferation. The relaxin receptors are present in the testis, RXFP1 in elongated spermatids and Sertoli cells from rat, and RXFP2 in Leydig and germ cells from rat and human, suggesting a role for these receptors in the spermatogenic process. The localization of both receptors in the apical portion of epithelial cells and smooth muscle layers of the vas deferens suggests an involvement of these receptors in the contraction and regulation of secretion.

High spatial resolution studies of muscarinic neuroeffector junctions in mouse isolated vas deferens

It is acknowledged that neurotransmission in the mouse vas deferens is predominantly mediated by ATP and noradrenaline (NA) released from sympathetic nerves while cholinergic transmission in the rodent vas deferens is often overlooked despite early literature. Recently we have characterized a cholinergic component of neurogenic contraction of mouse isolated vas deferens. In the present paper, by confocal imaging of Ca(2+) dynamics we detected acetylcholine (ACh) action at muscarinic cholinergic neuroeffector junctions at high-resolution. Experiments were carried out in the presence of prazosin (100 nM) and alpha,beta methylene ATP (alpha,beta-MeATP) (1 microM) to inhibit responses to NA and ATP respectively. Exogenous ACh (10 microM) elicited Ca(2+) transients, an effect blocked by the muscarinic receptor antagonist, cyclopentolate (1 microM). Ca(2+) transients were evoked by electrical stimulation of intrinsic nerves in the presence of the cholinesterase inhibitor neostigmine (10 microM). Stimulation produced a marked increase in the frequency and number of Ca(2+) transients. Cyclopentolate reduced the frequency of occurrence of spontaneous and evoked events to control levels. The alpha(2)-adrenoceptor antagonist yohimbine (300 nM) did not affect the spontaneous Ca(2+) transients, but increased the frequency of occurrence of evoked transients, an effect inhibited by cyclopentolate. The postjunctional effects of neuronally-released ACh are limited by the action of cholinesterase. Release of ACh appears to be tonically inhibited by NA released from sympathetic nerve terminals through action at prejunctional alpha(2)-adrenoceptors. Tetrodotoxin (TTX, 300 nM) abolished the nerve-evoked Ca(2+) events, with no effect on Ca(2+) transients elicited by exogenous ACh. In conclusion, the presence of spontaneous and evoked cholinergic Ca(2+) transients in smooth muscle cells of the mouse isolated vas deferens has been revealed. These events are mediated by ACh acting at M(3) muscarinic receptors. This action stands in marked contrast to the lack of effect of neuronally-released NA on smooth muscle Ca(2+) dynamics in this tissue.

Pharmacological characterization of muscarinic receptor subtypes mediating vasoconstriction of human umbilical vein

The present study attempted to pharmacologically characterize the muscarinic receptor subtypes mediating contraction of human umbilical vein (HUV).HUV rings were mounted in organ baths and concentration-response curves were constructed for acetylcholine (ACh) (pEC50: 6.16+/-0.04; maximum response 80.00+/-1.98% of the responses induced by serotonin 10 microM). The absence of endothelium did not modify the contractile responses of ACh in this tissue. The role of cholinesterases was evaluated: neither neostigmine (acetylcholinesterase inhibitor) nor iso-OMPA (butyrylcholinesterase inhibitor) modified ACh responses. When both enzymes were simultaneously inhibited, a significantly but little potentiation was observed (control: pEC50 6.33+/-0.03; double inhibition: pEC50 6.57+/-0.05). Atropine, nonselective muscarinic receptors antagonist, inhibited ACh-induced contraction (pKB 9.67). The muscarinic receptors antagonists pirenzepine (M1), methoctramine (M2) and pFHHSiD (M3) also antagonized responses to ACh. The affinity values estimated for these antagonists against responses evoked by ACh were 7.58, 6.78 and 7.94, respectively. On the other hand, PD 102807 (M4 selective muscarinic receptors antagonist) was ineffective against ACh-induced contraction.In presence of a blocking concentration of pirenzepine, pFHHSiFD produced an additional antagonism activity on ACh-induced responses. The M1 muscarinic receptors agonist McN-A-343 produced similar maximum but less potent responses than ACh in HUV. The calculated pA2 for pirenzepine against McN-A-343 induced responses was 8.54. In conclusion, the data obtained in this study demonstrate the role of M1 muscarinic receptor subtypes and suggest the involvement of M3 muscarinic receptor subtypes in ACh-induced vasoconstriction in HUV rings. In addition, the vasomotor activity evoked by ACh does not seem to be modulated by endothelial factors, and their enzymatic degradation appears to have little functional relevance in this tissue.

Impairment of male fertility induced by muscarinic receptor antagonists in rats

Male rats were treated with a muscarinic receptor antagonist at 3, 10, and 100mg/kg/day for 4 weeks prior to mating with untreated females and their reproductive status was determined on gestation days (GD) 15-17. Treatment-related decreases in the pregnancy rate were observed at 100mg/kg/day without any effects on mating performance. Impairment of male fertility by this compound was also observed after treatment for 1 week, but there were no effects after a 1-week withdrawal period suggesting reversibility of the effect. There were no treatment-related effects on sperm production or motility, or testicular histopathology in any group. In order to determine whether the reduced fertility was a class effect of muscarinic receptor antagonists, atropine was examined. Males received atropine for 1 week at 62.5 and 125 mg/kg/day and were mated with untreated females. A low pregnancy rate associated with a decrease in the number of implantations was observed at 125 mg/kg/day. The effect on implantation was also observed at 62.5mg/kg/day. These findings suggest that the impairment of fertility in male rats induced by muscarinic receptor antagonists is a class effect, and has a relatively short onset of effect and is quickly reversible.

Evidence for a M(1) muscarinic receptor on the endothelium of human pulmonary veins

1. To characterize the muscarinic receptors on human pulmonary veins associated with the acetylcholine (ACh)-induced relaxation, isolated venous and arterial preparations were pre-contracted with noradrenaline (10 microM) and were subsequently challenged with ACh in the absence or presence of selective muscarinic antagonists. 2. ACh relaxed venous preparations derived from human lung with a pD(2) value of 5.82+/-0.09 (n=16). In venous preparations where the endothelium had been removed, the ACh relaxations were abolished (n=4). ACh relaxed arterial preparations with a pD(2) value of 7. 06+/-0.14 (n=5). 3. Atropine (1 microM), the non selective antagonist for muscarinic receptors, inhibited ACh-induced relaxations in human pulmonary veins. The affinity value (pK(B) value) for atropine was: 8.64+/-0.10 (n=5). The selective muscarinic antagonists (darifenacin (M(3)), himbacine (M(2),M(4)), methoctramine (M(2)) and pFHHSiD (M(1),M(3))) also inhibited ACh-induced relaxations in venous preparations. The pK(B) values obtained for these antagonists were not those predicted for the involvement of M(2 - 5) receptors in the ACh-induced relaxation in human pulmonary veins. 4. The pK(B) value for darifenacin (1 microM) was significantly greater in human pulmonary arterial (8.63+/-0.14) than in venous (7.41+/-0.20) preparations derived from three lung samples. 5. In human pulmonary veins, the pK(B) values for pirenzepine (0.5 and 1 microM), a selective antagonist for M(1) receptors, were: 7.89+/-0.24 (n=7) and 8.18+/-0.22 (n=5), respectively. In the venous preparations, the pK(B) values derived from the functional studies with all the different muscarinic antagonists used were correlated (r=0.89; P=0.04; slope=0.78) with the affinity values (pK(i) values) previously published for human cloned m1 receptors in CHO cells. 6. These results suggest that the relaxations induced by ACh are due to the activation of M(1) receptors on endothelial cells in isolated human pulmonary veins.

Muscarinic receptor subtypes in the bisected vas deferens of the rat

1. The nature of muscarinic receptor subtypes in the isolated prostatic and epididymal segments of the vas deferens of the rat were studied. 2. Presynaptic receptors were characterized in segments under neurogenic transmural stimulation; postsynaptic receptors in segments without stimulation. 3. The present work suggests that the potency of ACh required to activate muscarinic receptors is higher in the prostatic than in the epididymal segment. 4. McN-A-343 was only able to induce dose-dependent contractions in the prostatic segment. 5. The pA2 value for 4-DAMP suggests that in the prostatic segment the postsynaptical ACh receptors seem to be pharmacologically similar to the ACh-M3 subtype. 6. Antagonism of the presynaptic ACh receptor subtype by pirenzepine supports the evidence that these receptors belong to the ACh-M1 subtype.