Characterization and ontogeny of P1-purinoceptors on rat vas deferens

The effects of varying Mg2+ ion concentrations on contractions to the cotransmitters ATP and noradrenaline in the rat vas deferens

The vas deferens responds to a single electrical pulse with a biphasic contraction caused by cotransmitters ATP and noradrenaline. Removing Mg²⁺ (normally 1.2 mM) from the physiological salt solution (PSS) enhances the contraction. This study aimed to determine the effect of Mg²⁺ concentration on nerve cotransmitter-mediated contractions. Rat vasa deferentia were sequentially bathed in increasing (0, 1.2, 3 mM) or decreasing (3, 1.2, 0 mM) Mg²⁺ concentrations. At each concentration a single field pulse was applied, and the biphasic contraction recorded. Contractions to exogenous noradrenaline 10 μM and ATP 100 μM were also determined. The biphasic nerve-mediated contraction was elicited by ATP and noradrenaline as NF449 (10 μM) and prazosin (100 nM) completely prevented the respective peaks. Taking the contractions in normal PSS (Mg²⁺ 1.2 mM) as 100%, lowering Mg²⁺ to 0 mM enhanced the ATP peak to 170 ± 7% and raising Mg²⁺ to 3 mM decreased it to 39 ± 3%; the noradrenaline peak was not affected by lowering Mg²⁺ to 0 mM (97 ± 3%) but was decreased to 63 ± 4% in high Mg²⁺ (3 mM). Contractions to exogenous ATP, but not noradrenaline, were increased in Mg²⁺ 0 mM and both were inhibited with Mg²⁺ 3 mM. Changing Mg²⁺ concentration affects the contractions elicited by the cotransmitters ATP and noradrenaline. The greatest effects were to potentiate the contraction to ATP in Mg²⁺ 0 mM and to inhibit the contraction to both ATP and noradrenaline in high Mg²⁺. Future publications should clearly justify any decision to vary the magnesium concentration from normal (1.2 mM) values.

Purinergic signalling and development of the autonomic nervous system

Most early studies of the role of nucleotides in development have evidenced their crucial importance as carriers of energy in all organisms. However, an increasing number of studies are now available to suggest that purines and pyrimidines, acting as extracellular ligands specifically on receptors of the plasma membrane, may play a pivotal role throughout pre- and postnatal development in a wide variety of organisms including amphibians, birds, and mammals. Purinergic receptor expression and functions have been studied in the development of many organs, including the autonomic nervous system (ANS). Nucleotide receptors can induce a multiplicity of cellular signalling pathways via crosstalk with bioactive molecules acting on growth factors and neurotransmitter receptors which are fundamental for the development of a mature and functional ANS. Purines and pyrimidines may influence all the stages of neuronal development, including neural cell proliferation, migration, differentiation and phenotype determination of differentiated cells. Indeed, the normal development of the ANS is disturbed by dysfunction of purinergic signalling in animal models. To establish the primitive and fundamental nature of purinergic neurotransmission in the ontogeny of the ANS, in this review the roles of purines and pyrimidines as signalling molecules during embryological and postnatal development are considered.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Regional differences in the adenosine A(2) receptor-mediated modulation of contractions in rat vas deferens

Adenosine receptors involved in modulation of contractions were characterized in the bisected rat vas deferens by combining pharmacological and immunohistochemical approaches. In both portions, noradrenaline-elicited contractions were enhanced by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA), and inhibited by the non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) in the presence of the adenosine A(1) receptor antagonist 1,3-dipropyl-8-cyclopentyl-l,3-dipropylxanthine (DPCPX). The adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS 21680) also inhibited noradrenaline-elicited contractions but only in the prostatic portion. Contractions elicited by the stable ATP analogue alpha,beta-methyleneATP (alpha,beta-MeATP) were inhibited only by NECA in the presence of DPCPX and only in the prostatic portion. This study provides functional evidence for the presence, in both portions of the rat vas deferens, of an adenosine A(1) receptor-mediated enhancement and of an adenosine A(2) receptor-mediated inhibition of contractions. The latter effect is mediated by both A(2A) and A(2B) subtypes in the prostatic portion but only by the A(2B) subtype in the epididymal portion. This regional variation is supported by the immunohistochemical results that revealed an adenosine A(2A) receptor immunoreactivity not co-localized with nerve fibres more abundant in the prostatic than in the epididymal portion.

Maturational and maintenance effects of testosterone on terminal axon density and neuropeptide expression in the rat vas deferens

Testosterone causes growth of many pelvic ganglion cells at puberty and their maintenance during adulthood. Here we have focused on two populations of pelvic ganglion cells that project to the rat vas deferens: noradrenergic neurons that innervate the smooth muscle and synthesize neuropeptide Y, and cholinergic neurons that primarily innervate the mucosa and contain vasoactive intestinal peptide. We have assessed the muscle innervation after pre- or postpubertal castration, using immunohistochemistry to determine axon density and radioimmunoassay to quantify levels of neuropeptides in tissue extracts. Our results show that androgen deprivation in each period causes substantial effects. Noradrenergic axons in the muscle increase in density after castration, partly due to organ size being smaller than age-matched controls. However, when corrected for target size, there is an overall decrease in total number of axons. This implies that androgen exposure at puberty has a direct effect on neurons to ensure that the adult pattern of innervation is attained, and that this is not simply by matching terminal field to target size. Similar effects of pre- and postpubertal castration imply that continued exposure to testosterone is necessary to maintain normal target innervation. Castration in both time periods increased the density of axons containing vasoactive intestinal peptide, however the effects of castration on the total number of these axons in the muscle were more variable. The concentration of vasoactive intestinal peptide increased substantially following either pre- or postpubertal castration although absolute amounts per vas deferens were decreased. Effects on neuropeptide Y concentration were less pronounced but the total amount per vas deferens was decreased after pre- or postpubertal castration. Our study shows that the action of testosterone (or a metabolite) on a pelvic ganglion cell soma is likely to reflect a change in its terminal field, but that these effects are not mediated simply by testosterone influencing the size of its target organ.

Nucleotide-evoked relaxation of rat vas deferens: possible mechanisms

ATP causes relaxation of the K(+)-contracted rat vas deferens. Possible sites of action were investigated. ATP and adenosine relaxed the vas deferens precontracted with 80 mM K(+); EC(50) values and maximal relaxations averaged, respectively, 760 microM and 56% for ATP and 74 microM and 30% for adenosine. The adenosine P1 receptor antagonist 8-(para-sulfophenyl)theophylline (8-SPT) reduced relaxations caused by adenosine and low concentrations of ATP, as did the Rp-diastereomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS), an inhibitor of protein kinase A. The phosphodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724) augmented responses to adenosine and low concentrations of ATP. alpha,beta-Methylene ADP, an inhibitor of 5'-nucleotidase, reduced relaxations caused by ATP to a similar extent as did 8-SPT. In the presence of an almost saturating concentration of adenosine, ATP caused further relaxation. Conversely, in the presence of ATP, adenosine had little effect. Like ATP, UTP and other nucleoside triphosphates relaxed the vas deferens. The P2 receptor antagonists reactive blue 2, acid blue 25 and 4,4'-diisothiocyanotostilbene-2,2'-disulphonate (DIDS) attenuated the relaxation caused by ATP; suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS), Evans blue, trypan blue, reactive red 2 and brilliant blue G had no effect. Three non-selective inhibitors of protein kinases, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), staurosporine and (8R*,9S*,11S*)-(-)-9-hydroxy-9-carboxy-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinden-1-one (K-252b), markedly reduced the relaxation caused by ATP. The results indicate that adenosine, derived from enzymatic dephosphorylation, contributes to the relaxant effect of ATP, presumably by activation of a smooth muscle adenosine receptor linked to the accumulation of cAMP and activation of protein kinase A. Yet, the main part of the response to ATP is mediated by a site distinct from the adenosine receptor. The pharmacological properties of this site differ from known P2 receptor subtypes. Possibly, the nucleotide-evoked relaxation is due to a phosphoryl transfer catalyzed by an ecto-protein kinase.

Effects of adenine nucleosides and nucleotides on neuromuscular transmission to the prostatic stroma of the rat

1. The aim of this study was to investigate the effects of adenine nucleosides and nucleotides on contractility of the smooth muscle of rat prostate gland. 2. Nerve terminals within rat isolated prostatic tissues were electrically field stimulated (60 V, 0.5 ms, 10 Hz, 20 pulses every 60 s). Adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine had no effect on baseline smooth muscle tone but concentration-dependently inhibited electrically-evoked contractile responses. The relative order of potency was ATP congruent with AMP congruent with adenosine>ADP. 3. The inhibition by ATP and adenosine of field stimulation-induced contractions in the rat prostate was antagonized by 8-phenyltheophylline (10 microM), but not by suramin (100 microM) and only slightly by reactive blue 2 (5 microM). 4. The adenosine metabolizing enzyme adenosine deaminase (0.1 unit ml(-1)) inhibited the inhibitory effects of ATP and adenosine. The P2 purinoceptor agonist 2-methylthio ATP (10 nM - 0.1 mM), had no effect on field stimulation-induced contractions of the rat prostate. 5. ATP and adenosine did not modify the contractile responses of the rat prostate to exogenously added noradrenaline (10 microM). 6. Inhibitory concentration-response curves to a number of adenosine analogues with differing stabilities and selectivities for the different adenosine receptors yielded a relative rank order of agonist potency of: N(6)-cyclopentyladenosine (CPA)>N(6)-cyclohexyladenosine (CHA) congruent with (-)-N(6)-(2-phenylisopropyl)-adenosine (R-PIA) congruent with 5'-(N-ethylcarboxamido)-adenosine (NECA)>(+)-N(6)-(2-phenylisopropyl)-adenosine (S-PIA)>2-p-[2-carboxyethyl]phenethyl-amino-5'-N-ethylcarboxamido-ade nosine (CGS 21680). 7. These results indicate that adenine nucleoside and nucleotide induced inhibition of electrically-evoked contractions in the rat prostate occurs through activation of adenosine but not ATP receptors. The relative order of potency of adenosine analogues is consistent with activation of receptors of the A(1)-adenosine receptor subtype. These receptors appear to be prejunctional.

Effects of noradrenaline, the calcium ionophore A23187, forskolin, sodium nitroprusside and glibenclamide on the degradation of extracellular adenosine 5'-triphosphate by the rat isolated vas deferens

1. The effects of noradrenaline (NA), the calcium ionophore A23187, forskolin, sodium nitroprusside (SNP) and the K+-channel blocker glibenclamide on the degradation by ectonucleotidases of extracellular adenosine 5'-triphosphate (ATP) were studied in the rat vas deferens. 2. ATP (100 microM) was rapidly broken down by the rat vas deferens with a half-life of 5.83 +/- 0.40 min via adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP), with the final degradation product being inosine and with little adenosine being detected in the samples. 3. Preincubation for 1 h with NA (10 microM), A23187 (10 microM), or glibenclamide (100 microM) had no significant effect on the breakdown of ATP or the production of metabolites. However, both forskolin (10 microM) and SNP (1 microM) significantly increased the concentrations of AMP detected with time. In the case of SNP (1 microM) there was also a significant reduction in the rate of production of inosine, while in the case of forskolin (10 microM) there was a significant increase in the rate of removal of ATP. 4. These results suggest that preincubation with SNP may inhibit 5'-nucleotidase and so reduce the metabolism of AMP, while preincubation with forskolin may increase the activity of the ectonucleotidases responsible for production of AMP from ATP.

Species-dependent effects of adenosine receptor agonists on contractile responses of vas deferens to ATP

1. Experiments were carried out to examine the postjunctional actions of adenosine receptor agonists on the smooth muscle of the vas deferens of the guinea-pig and rabbit. 2. Although they produced neither contraction nor relaxation by themselves, adenosine analogues enhanced contractions of the guinea-pig vas deferens induced by 10 microm ATP. The rank order of potency was N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > adenosine > CGS 21680. Dose-response curves for NECA were shifted to the right by the nonselective adenosine receptor antagonist 8(p-sulphophenyl)theophylline (8-SPT; 100 microM) and by the selective A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 1 mM). 3. In the rabbit vas deferens, contractions induced by ATP (1 mM) were inhibited rather than facilitated by NECA. Neither CPA, R(-)-N6-(2-phenyl isopropyl)-adenosine (R-PIA) nor CGS 21680 had any effect. 4. The results indicate that the smooth muscle of the guinea-pig vas deferens expresses facilitatory adenosine A1 receptors but not adenosine A2 receptors. In contrast, in rabbit there are postjunctional inhibitory adenosine A2A receptors but not adenosine A1 receptors.

Postnatal development of purinoceptors in rat visceral smooth muscle preparations

1. Adenosine and ATP have well-established functions as neuromodulator and neurotransmitter, respectively, in smooth muscle preparations, and purinergic control may be an early form of autonomic control in both evolution and ontogenesis. 2. This review describes the postnatal development of responses mediated by the various receptors for adenosine and for nucleotides in the rat duodenum, colon, urinary bladder and vas deferens and considers the implications that this development may have for the importance of purinergic control in neonates and adults.

A1 adenosine receptor modulation of electrically-evoked contractions in the bisected vas deferens and cauda epididymis of the guinea-pig

1. The effects of adenosine receptor agonists upon both electrically-evoked and phenylephrine-induced contractile responses were investigated in the bisected vas deferens and the cauda epididymis of the guinea-pig. Electrical field-stimulation (10 s trains of pulses at 9 Hz, 0.1 ms duration, supramaximal voltage) elicited biphasic and monophasic contractile responses from preparations of bisected vas deferens and cauda epididymis, respectively; these responses were abolished by tetrodotoxin (300 nM). 2. In the prostatic half of the vas deferens the A1 selective adenosine receptor agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) and the non-selective A1/A2 adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) inhibited electrically-evoked contractions (pIC50+/-s.e.mean values 6.15+/-0.24, 5.99+/-0.26 and 5.51+/-0.24, respectively). The responses to CPA were blocked by the A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine, DPCPX (100 nM). 3. In the epididymal half of the vas deferens NECA potentiated (at < or = 100 nM) and inhibited (at > or = 1 microM) electrically-evoked contractions. In the presence of the non-selective alpha-adrenoceptor antagonist phentolamine (3 microM), the alpha1-adrenoceptor antagonist, prazosin (100 nM), or at a reduced train length (3 s) NECA inhibited electrically-evoked contractions (pIC50 values 6.05+/-0.25, 5.97+/-0.29 and 5.71 +/-0.27, respectively). CPA (at 10 microM) also inhibited electrically-evoked contractions in this half of the vas deferens. In the presence of prazosin (100 nM), CPA also inhibited electrically-evoked contractions (pIC50 6.14+/-0.67); this effect was antagonized by DPCPX (30 nM, apparent pK(B) 8.26+/-0.88). In the presence of the P2 purinoceptor antagonist, suramin (300 microM), CPA (up to 1 microM) potentiated electrically-evoked contractions. 4. NECA, CPA and APNEA potentiated electrically-evoked contractions in preparations of cauda epididymis (pEC50 values 7.49+/-0.62, 7.65+/-0.74 and 5.84+/-0.86, respectively), the response to CPA was competitively antagonized by DPCPX (100 nM) with an apparent pK(B) value of 7.64+/-0.64. 5. The alpha1-adrenoceptor agonist phenylephrine elicited concentration-dependent contractile responses from preparations of bisected vas deferens and cauda epididymis. NECA (1 microM) potentiated responses to phenylephrine (< or = 1 microM) in the epididymal, but not in the prostatic half of the vas deferens. In preparations of epididymis NECA (1 microM) shifted phenylephrine concentration response curves to the left (4.6 fold). In the presence of a fixed concentration of phenylephrine (1 microM), NECA elicited concentration-dependent contractions of preparations of the epididymal half of the vas deferens and of the epididymis (pEC50 values 7.57+/-0.54 and 8.08+/-0.18, respectively). NECA did not potentiate responses to ATP in either the epididymal half of the vas deferens or the epididymis. 6. These studies are consistent with the action of stable adenosine analogues at prejunctional A1 and postjunctional A1-like adenosine receptors. The prejunctional A1 adenosine receptors only inhibit the electrically-evoked contractions of purinergic origin (an effect predominant in the prostatic half of the vas deferens). At the epididymis, where electrically-evoked contractions are entirely adrenergic, the predominant adenosine receptor agonist effect is a potentiation of alpha1-adrenoceptor-, but not of ATP-induced contractility.

Adenosine analogues relax guinea-pig taenia caeci via an adenosine A2B receptor and a xanthine-resistant site

In this study we have sub-classified the adenosine A2 receptor mediating relaxation in the guinea-pig taenia caecum using the adenosine A2A receptor-selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine) and the adenosine A2A receptor-selective antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl)phenol). CGS 21680 did not elicit relaxations, and a pKB value of 7.80 was obtained for ZM 241385 against 5'-N-ethylcarboxamidoadenosine suggesting the presence of adenosine A2B receptors. Relaxations are also mediated via a xanthine-resistant site. In this study relaxations to the adenosine A3 receptor agonist IB-MECA (N6-(3-iodo-benzyl)adenosine-5'-N-methyluronamide) were blocked by neither 8-sulphophenyltheophylline (100 microM) nor the adenosine A3 receptor antagonist BW-A1433 (1,3-dipropyl-8-(4-acrylate)phenylxanthine, 100 microM), suggesting that this site is not an adenosine A3 receptor.

Differential distribution of adenosine A2 receptors in the epididymal and prostatic portions of the rat vas deferens

In the rat vas deferens there are prejunctional A1 receptors mediating inhibition of transmitter release and post-junctional A1 and A2 receptors mediating enhancement and inhibition of contractions respectively. In this study the distribution of adenosine receptors in the prostatic and epididymal portions of the bisected rat vas deferens was investigated. The pre- and post-junctional A1 receptors were present on both portions of the bisected tissue. However, post-junctional A2 receptors appear to be present only in the prostatic region, showing that adenosine receptors are differentially distributed along the length of the rat vas deferens.

Selective enhancement by an adenosine A1 receptor agonist of agents inducing contraction of the rat vas deferens

The adenosine analogue N6-cyclopentyladenosine (CPA), acting via postjunctional A1 receptors, has been shown to enhance contractions of the rat vas deferens induced by adenosine 5'-triphosphate (ATP), the sympathetic cotransmitter in this tissue. The aim of the present study was to examine the ability of CPA to enhance contractions induced by other contractile agents. CPA (0.01-0.3 microM) enhanced contractions induced by exogenous ATP (10 microM), 5-hydroxytryptamine (5-HT) (3 microM), tyramine (10 microM), 2-methyl-5-hydroxytryptamine (2-Me-5-HT) (10 microM) and KCl (35 mM) and this enhancement was blocked by an A1-selective concentration (3 nM) of 1, 3-dipropyl-8-cyclopentylxanthine (DPCPX). CPA failed to enhance contractions induced by exogenous noradrenaline (NA) (1 microM or 10 microM), bradykinin (0.1 microM), phenylephrine (3 microM) or carbachol (10 microM). The contractions induced by ATP (10 microM), 5-HT (3 microM), 2-Me-5-HT (10 microM) and KCl (35 mM) were unaffected by tetrodotoxin (1 microM) as well as by desensitisation of the P2x-purinoceptors with the ATP analogue adenosine 5'-(alpha, beta-methylene) triphosphonate. The contractions induced by tyramine (10 microM) and 2-Me-5-HT (10 microM) were blocked by prazosin (100 nM) or by imipramine (1 microM). Ketanserin (10 nM) antagonised the response to 5-HT giving a dose-ratio of 12.9 corresponding to an apparent pA2 of 9.1. In conclusion, the A1-mediated effect was clearly selective for certain contractile agents and not due to a non-specific increase in contractility of the tissue. CPA enhanced contractions induced by both ATP and indirect sympathomimetics which release endogenous NA, and this enhancement of the two sympathetic cotransmitters may have a functional significance, and demonstrates the complexity of the neuromodulatory effects of adenosine in the rat vas deferens.

The ontogenetic profiles of the pre- and postjunctional adenosine receptors in the rat vas deferens

1. The ontogenetic profiles of the prejunctional A1 and postjunctional A1 and A2 receptors on the rat vas deferens were investigated, using a combination of functional and radioligand binding assays to follow the A1 receptors and functional assays alone to follow the development of the A2 receptors. 2. The prejunctional A1 receptor, assessed by the inhibitory action of N6-cyclopentyladenosine (CPA) (3 nM-3 microM) on nerve-mediated contractions, was present from day 15 onwards, day 15 being the earliest age at which nerve-mediated contractions could be detected. The potency of CPA was constant across the ages studied, with pD2 values ranging from 6.4-7.1, not significantly different from that previously observed in adult rat vas deferens. 3. The postjunctional A2 receptors, assessed by the inhibitory action of 5'-N-ethylcarboxamidoadenosine (NECA) (10 nM-30 microM) on KCl-induced contractions were present from day 10 onwards, day 10 being the earliest age at which responses to KCl could be observed. The potency of NECA remained constant with an increase in age, with potency values, expressed as pEC25 values, ranging from 6.5-7.0. 4. The postjunctional A1 receptor displayed a different development profile from that of the prejunctional A1 and postjunctional A2 receptors. Postjunctional A1 receptors were identified by the enhancement of KCl-induced contractions by CPA (10 nM-0.3 microM). At 10 and 15 days, CPA failed to enhance KCl-induced contractions. From day 20 to day 40, this enhancement increased with an increase in age and the level of enhancement achieved statistical significance from day 30. 5. Radioligand binding studies using 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX) revealed binding sites characteristic of A1 receptors on the vas deferens from rats aged 20 days onwards. The density (Bmax) of A1 receptors expressed relative to protein content was greatest at day 20 (153 +/- 33 fmol mg-1 protein) and declined at day 30 (43.9 +/- 3.7 fmol mg-1 protein) to a level commensurate with that previously determined in adult rat vas deferens (43.3 +/- 12 fmol mg-1 protein). However, when expressed relative to tissue wet weight little variation in receptor density was observed between these ages (Bmax 0.13 +/- 0.02 fmol mg-1 wet weight at 20 days; 0.17 +/- 0.01 fmol mg-1 wet weight at 30 days). The binding affinity (KD) remained constant with an increase in age and was similar to the KD value previously generated for adult rat vas deferens (approximately 1 nM). At ages 10 and 15 days no reproducible binding could be detected. 6. These results show the differential development of the adenosine receptors on the rat vas deferens with postjunctional A1 receptors demonstrating delayed development, while prejunctional A1 and postjunctional A2 receptors were present from the earliest ages studied. In addition, comparison of binding studies and functional studies suggests that the binding studies detect only the A1 receptors present on the smooth muscle and not those present on the nerve terminals.

The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine to adenosine A1 receptors in rat smooth muscle preparations

1. The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), an antagonist radioligand selective for adenosine A1 receptors, was studied in rat duodenum, colon muscularis mucosae and longitudinal muscle, urinary bladder and vasa deferentia. 2. [3H]-DPCPX bound with high affinity to a single site in all membrane preparations studied with the exception of the rat urinary bladder in which no specific binding was detected. The affinity (Kd) of the binding site for [3H]-DPCPX was similar in all membrane preparations, the colon longitudinal muscle (1.18 +/- 0.47 nM), colon muscularis mucosae (0.84 +/- 0.15 nM), duodenum (1.59 +/- 0.18 nM) and vasa deferentia (0.93 +/- 0.17 nM). The density of [3H]-DPCPX binding sites was similar in the duodenum (38.8 +/- 4 fmol mg-1 protein), muscularis mucosae (43 +/- 3.5 fmol mg-1 protein) and vasa deferentia (43.3 +/- 12.2 fmol mg-1 protein), but in the longitudinal muscle 6-7 fold more binding sites (295 +/- 70 fmol mg-1 protein) were identified. 3. Inhibition studies using DPCPX (0.1-100 nM), N6-cyclopentyladenosine (CPA) (0.1-100 nM), 5'-N-ethylcarboxamidoadenosine (NECA) (2 nM-10 microM) and (R)-N6-phenylisopropyladenosine (R-PIA) (1 nM-1 microM) to displace the binding of [3H]-DPCPX at a concentration around the Kd value (1 nM), demonstrated an order of potency of displacement in all tissues of DPCPX > or = CPA > R-PIA > NECA. This potency order is characteristic of an A1 receptor, indicating that [3H]-DPCPX binds to adenosine A1 receptors in the rat duodenum, colon and vasa deferentia. Two site analysis revealed that the agonists bind to both a high and low affinity state of the receptor.4. The existence of Al binding sites in the rat vasa deferentia, colon muscularis mucosae and duodenum, and their absence in the urinary bladder, is consistent with previous functional studies.However, in contrast to the findings of the [3H]-DPCPX binding assay, no functional response mediated by adenosine Al receptors could be detected by measuring contractile or relaxant responses to CPA in the colon longitudinal muscle. The functional significance of the binding sites in this tissue has therefore yet to be determined.

The fade of the purinergic neurogenic contraction of the guinea-pig vas deferens: analysis of possible mechanisms

The purinergic response of the guinea-pig vas deferens to long trains of pulses at high frequency consists of an initial twitch followed by a much lower plateau. Mechanical, neurochemical and electrophysiological techniques were used to examine the reason for the fade. Mechanical measurements. In tissues stimulated by trains of 180 pulses/10 Hz and treated with prazosin to suppress the noradrenergic contraction component, the response to alpha, beta-methylene ATP and to exogenous ATP was as high during the secondary plateau of the purinergic neurogenic contraction as it was outside electrical stimulation periods; the response to 50 pulses/100 Hz was also unchanged during the low plateau. The plateau was not increased by reactive blue 2,8-(p-sulphophenyl)theophylline, propranolol or capsaicin. Neurochemical measurements. In tissues preincubated with [3H]-noradrenaline, electrical stimulation elicited an overflow of tritium and of ATP. In the absence of drugs as well as in the presence of prazosin and suramin to suppress contractions, the overflow of tritium per pulse decreased slightly in the course of trains of 90 pulses/10 Hz; the overflow of ATP per pulse decreased to a greater extent on average, but the decrease was not statistically significant. In the presence of prazosin and nifedipine, also to suppress contractions, the overflow of tritium per pulse again decreased slightly in the course of trains of 105 pulses/10 Hz, but the overflow of ATP per pulse if anything tended to increase. Electrophysiological measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Post-junctional excitatory adenosine A1 receptors in the rat vas deferens

1. At concentrations between 1 nM and 1 microM, the A1-selective agonists N6-cyclopentyladenosine (CPA) and (R)-N6-phenylisopropyladenosine (R-PIA) each enhanced contractions of the rat vas deferens induced by ATP (10 microM), and this enhancement was blocked by an A1-selective concentration (1 nM) of the antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). 2. No such enhancement was observed with the non-selective agonists adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) at concentrations between 1 nM and 100 microM, which instead inhibited the contractions. 3. These results show that in addition to the previously demonstrated inhibitory A1 and A2 adenosine receptors, the rat vas deferens also possesses post-junctional excitatory A1 adenosine receptors.

Differential effects of suramin on P2-purinoceptors mediating contraction of the guinea-pig vas deferens and urinary bladder

1. The effect of the P2-purinoceptor antagonist, suramin, was investigated on contractions of the guinea-pig vas deferens and urinary bladder induced by adenosine 5'-triphosphate (ATP) and by the other naturally occurring nucleoside triphosphates. 2. ATP, guanosine 5'-triphosphate (GTP), cytidine 5'-triphosphate (CTP), inosine 5'-triphosphate (ITP) and uridine 5'-triphosphate (UTP) (0.1-500 microM) each contracted both the guinea-pig bladder and the guinea-pig vas deferens. In the vas deferens the order of potency of the nucleotides was ATP >> CTP > GTP > or = UTP = ITP, and in the bladder it was ATP >> CTP = GTP > UTP = ITP, although maximal responses to these agonists were not achieved in either tissue. 3. Suramin (30 microM-1 mM) dose-dependently inhibited ATP-induced contractions of the bladder in an apparently non-competitive manner, causing a reduction in the slope of the concentration-response curve to ATP. In contrast, suramin (5 microM-1 mM) had little inhibitory effect on ATP-induced contractions of the vas deferens, and indeed at concentrations of 100 microM and above markedly potentiated high concentrations of ATP (100-500 microM). The contractions induced by CTP, GTP, UTP and ITP (1-500 microM) were, however, abolished by suramin (1 mM) in each tissue. 4. Desensitization of the P2X purinoceptors in the guinea-pig vas deferens with adenosine 5'-alpha,beta-methylenetriphosphonate (AMPCPP) (300 microM) abolished contractions induced by ATP (1 microM-1 mM) in the absence of suramin. However, the contractions induced in the presence of suramin were unaffected by prior desensitization, indicating that they were not mediated by P2X-purinoceptors.5. ATP (100 MicroM) was dephosphorylated by both isolated tissue preparations under the conditions of these experiments, breakdown products being detectable after 2 min, with the major breakdown product in the bladder being inosine whereas that in the vas deferens was adenosine. Approximately 35% of the ATP remained intact after incubation for 30 min with the bladder, and approximately 45% remained after incubation for 30 min with the vas deferens. In each tissue this degradation was inhibited by suramin (1 mM), so that after incubation of ATP (100 MicroM) in the presence of suramin for 30 min,approximately 50% remained in the case of the bladder and approximately 65% remained in the vas deferens. However, inhibition of the production of the inhibitory agonist, adenosine by suramin did not appear to be responsible for the potentiation observed in the vas deferens, as the PI-purinoceptor antagonist 8-sulphophenyltheophylline (100 MicroM) did not reduce this potentiation.6. Chelation of divalent cations did not appear to account for the enhancement by suramin of ATP-induced contractions of the vas deferens, as the enhancement was still observed when Mg2+ was omitted from the buffer or when its concentration (normally 1.2 mM) was increased ten fold to 12 mM,or when the concentration of Ca2+ (normally 2.5 mM) was reduced to 0.83 mM. Even in the absence of Mg2+ and with the Ca2+ concentration reduced to 0.83 mM, no inhibition by suramin (1 mM) of ATP-induced contractions was observed.7. The most likely explanation for the potentiation by suramin of the ATP-induced contractions of the vas deferens is the co-existence of inhibitory P2Y-purinoceptors. However, no consistent relaxations to ATP (1-100 MicroM) or to the more potent P2Y-purinoceptor agonist 2-methylthioadenosine 5'-triphosphate(2-MeSATP) (0.01-100 MicroM) could be detected in the vas deferens precontracted with KCl (35 mM), even after desensitization of P2x-purinoceptors with AMPCPP (300 MicroM). Similarly, ATP (1-100 MicroM) or 2-MeSATP (0.01-1100 MicroM) added before KCI (35 mM), carbachol (10 JM) or noradrenaline (10 MicroM) did not reduce subsequent contractions to these agents.8. The differential effect of suramin on the contractions induced by ATP in the bladder and the vas deferens was unexpected, and shows that the receptor populations by which ATP acts in these tissues may not be identical. The failure of suramin to inhibit responses to ATP in the vas deferens suggests that this tissue, in addition to possessing P2x-purinoceptors may also possess a suramin-insensitive contractile ATP receptor revealed in the presence of suramin.

Adenosine receptor subtypes

The numerous and widespread effects of adenosine provide both an opportunity for the development of novel therapeutic agents acting via adenosine receptors and the challenge of achieving selectivity of action. The feasibility of achieving selectivity is enhanced if receptor subtypes can be identified. Biochemical, functional and receptor-cloning studies are beginning to provide convergent data supporting the existence of A1, A2A, A2B and A3 receptors. However, studies of the functional significance of these receptors in intact tissues both in vitro and in vivo have lagged behind the biochemical studies. In this article, Michael Collis and Susanna Hourani review the current status of adenosine receptor classification and propose that ligands with greater selectivity need to be evaluated in a wide range of functional preparations if the therapeutic potential of this area is to be realized.