Comparison of the effects of arylazido aminopropionyl ATP (ANAPP3), an ATP antagonist, on responses of the smooth muscle of the guinea-pig vas deferens to ATP and related nucleotides

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.

Existence and pharmacological properties of dopamine D4 receptors in guinea pig vas deferens

This study was carried out to identify subtypes of dopamine D2-like receptors in guinea pig isolated vas deferens. Dopamine had no effect on the muscle tone in the presence of prazosin, an alpha1-adrenoceptor antagonist. However, contractile responses to adenosine triphosphate (ATP), noradrenaline and acetylcholine were potentiated in a concentration dependent manner by dopamine in the presence of prazosin. This potentiation was not inhibited by raclopride, an antagonist for dopamine D2 and D3 receptors. However, the potentiation of ATP- and noradrenaline-induced contraction was inhibited by clozapine and 8-methyl-6-(4-methyl-1-piperazinyl)-11H-pyrido[2,3b][1,4]benzodiazepine (JL-18), dopamine D4 receptor antagonists. Further, the potentiation of noradrenaline- and acetylcholine-induced contraction was also inhibited by spiperone, an antagonist for dopamine D2, D3 and D4 receptors. These results suggest that the dopamine D4 receptor is located on the postsynaptic site of guinea pig vas deferens and that activation of the dopamine D4 receptor enhances contractile responses to agonists without affecting muscle tone.

Existence of postsynaptic dopamine D2 receptor as an enhancer of contractile response in vas deferens

Effects of dopamine and (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxy-tetralin hydrochloride (N-0434), a dopamine D2 receptor agonist, in the presence of prazosin on the ATP- and acetylcholine-induced contraction were investigated in the guinea-pig vas deferens in order to test for the existence of postsynaptic dopamine receptors. The contraction induced by ATP was potentiated by dopamine and N-0434. This potentiation was antagonized by spiperone, a dopamine D2 receptor antagonist, but not by a dopamine D1 receptor antagonist and an alpha2-adrenoceptor antagonist. Similar results were also observed by acetylcholine as well as ATP. The contraction induced by transmural nerve stimulation in the presence of alpha-adrenoceptor antagonists was also potentiated by N-0434, and this potentiation was antagonized by spiperone. The results suggest that dopamine D2 receptors are located on the postsynaptic site of guinea-pig vas deferens and that the contractile responses to ATP and acetylcholine are potentiated via activation of dopamine D2 receptor.

Hydrolysis of P2-purinoceptor agonists by a purified ectonucleotidase from the bovine aorta, the ATP-diphosphohydrolase

Pharmacologists are becoming more and more aware of the possibility that certain ATP analogues currently used to classify the P2-purinoceptors are dephosphorylated by ectonucleotidases. In this study, we provide evidence that in the vascular system, these purine analogues are hydrolysed by an ATP-diphosphohydrolase (ATPDase). This enzyme is known as the major plasma membrane nucleotidase of endothelial and smooth muscle cells, and is believed to dephosphorylate extracellular triphospho- and diphosphonucleosides. Assays were conducted with a purified ATPDase from smooth muscle cells of bovine aorta. At a concentration of 250 microM, adenosine 5'-(alpha,beta-methylene) triphosphonate (alpha,beta-metATP), adenosine 5'-(beta,gamma-methylene) triphosphonate (beta,gamma-metATP), adenosine 5'-(alpha,beta-methylene) disphosphonate (alpha,beta-metADP), adenylyl 5'-(beta,gamma-imido) diphosphonate (beta,gamma-imidoATP) and adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) all resisted dephosphorylation, whereas 2-chloroadenosine triphosphate (2-chloroATP), 2-methylthioadenosine triphosphate (2-MeSATP) and 8-bromoadenosine triphosphate (8-bromo-ATP) were hydrolysed at 99, 63, and 20% of the rate of ATP hydrolysis, respectively. All the non-hydrolysable analogues tested, except alpha,beta-metADP, competed with ATP and ADP for the ATPDase catalytic site, reducing their hydrolysis by 35-50%. Apparent Km values for ATP and ADP were estimated at 14.1 and 12.0 microM, respectively, whereas apparent Km and Ki values for the purine analogues ranged from 12 to 28 microM. These results strongly support the view that (1) the ATPDase is expected to reduce substantially the P2-response induced by ATP, ADP, and some hydrolysable agonists; and (2) by competing with the hydrolysis of endogenously released ATP and ADP, non-hydrolysable analogues could alter the amplitude or direction of the cellular response induced by these natural substrates.

Differences in the responses to purinergic nerve stimulation and applied ATP in the guinea-pig vas deferens

The responses to sympathetic nerve stimulation and to applied ATP in the guinea-pig vas deferens were compared. Nifedipine (10 microM) markedly reduced the non-adrenergic neural contraction but only partially blocked the contractions produced by bath-applied ATP. Suramin (300 microM) also markedly reduced the contractile responses produced by nerve stimulation, but had no significant effect on the contractions produced by bath-applied ATP. Using intracellular recording techniques, nerve stimulation was shown to produce an excitatory junction potential which was abolished by suramin (1 microM). Ionophoretic application of ATP and bath-applied ATP also produced a depolarization. Suramin (1 microM) failed to abolish the response to bath-applied ATP and enhanced the ionophoretically induced depolarization. These results suggest either that ATP is not a transmitter in the vas deferens or that two classes of purinoceptor are present, one suramin-sensitive receptor which produces a contraction via the opening of voltage-dependent Ca2+ channels, and another which is suramin-resistant and produces a contraction by another means.

Potentiating and inhibitory effects of periodate-oxidized ATP analogs on contractions of vas deferens to ATP

Previous studies have shown that treatment of guinea-pig isolated vas deferens with the affinity label periodate-oxidized ATP (2',3'-dialdehyde ATP), results in two irreversible effects on biphasic contractile responses to ATP, i.e., potentiation of the P2X purinoceptor-mediated first phase and inhibition of the ecto-kinase-mediated second phase. The present experiments were designed to evaluate whether periodate-oxidized ADP, periodate-oxidized AMP, and periodate-oxidized adenosine, produce similar effects. Periodate-oxidized ATP and periodate-oxidized ADP (10(-2) M) elicited contraction of the vas deferens (periodate-oxidized ATP > periodate-oxidized ADP; periodate-oxidized AMP and periodate-oxidized adenosine had no agonist activity. After incubation of the preparations for 5 min with 10(-2) M periodate-oxidized ATP, periodate-oxidized ADP, periodate-oxidized AMP or periodate-oxidized adenosine, the first phase of contraction to submaximal ATP concentrations was potentiated. Simultaneously, periodate-oxidized ATP, periodate-oxidized ADP and periodate-oxidized AMP inhibited the second contractile phase, whereas periodate-oxidized adenosine did not. The results indicate that the requirement for 5'-phosphate to produce potentiation and inhibition is different: 5'-phosphate is not needed to potentiate the first phase of contraction to ATP, but at least one 5'-phosphate is required to inhibit the second phase of contraction.

Effects of suramin on contractions of the guinea-pig vas deferens induced by analogues of adenosine 5'-triphosphate

1. Adenosine 5'-triphosphate (ATP) and some of its analogues contract the guinea-pig vas deferens, acting via receptors which have been classified as P2X-purinoceptors. We have recently shown, however, that the effects of ATP are enhanced, rather than inhibited, by the non-selective P2 antagonist, suramin, and that this enhancement could not easily be explained in terms of inhibition by suramin of the breakdown of ATP. We therefore investigated the effects of suramin on contractions induced by ATP analogues, to define the structure-activity relationships of the suramin-resistant response. 2. In the absence of suramin, the order of potency for ATP analogues was adenosine 5'-(alpha,beta-methylene)triphosphonate (AMPCPP) = P1,P5-diadenosine pentaphosphate (Ap5A) = adenosine 5'-tetraphosphate (Ap4) > adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) = adenylyl 5'-(beta,gamma-methylene) diphosphonate (AMPPCP) > P1,P5-diadenosine tetraphosphate (Ap4A) > adenosine 5'-O-(2- thiodiphosphate) (ADP beta S) > 2-methylthioadenosine 5'-triphosphate (MeSATP) > or = ATP > adenosine 5'-diphosphate (ADP). This is generally in agreement with previously reported structure-activity relationships in this tissue. 3. In the presence of suramin (1 mM), responses to Ap5A, Ap4A, AMPPCP, ADP beta S and ADP were abolished or greatly reduced, and contractions induced by AMPCPP, Ap4 and ATP gamma S were inhibited. Contractions induced by MeSATP however, like those induced by ATP itself, were not reduced, but at concentrations above 100 microM were enhanced. In the presence of suramin (1 mM) the order of potency of analogues was therefore AMPCPP = Ap4> ATP = MeSATP> ATP gamma S, with all other analogues tested being essentially inactive at concentrations up to 500 microM.4. Contractile responses of the vas deferens to transmural nerve stimulation (1-50 Hz) in the presence of the alpha-adrenoceptor antagonist, phentolamine (10 microM), were abolished by suramin (1 mM). This is in agreement with previous reports that suramin inhibits the excitatory junction potential, a response thought to be mediated by P2 purinoceptors. It is however hard to reconcile the evidence implicating ATP as the non-adrenergic transmitter responsible for this response with the failure of suramin to inhibit the contractions induced by ATP itself while abolishing nerve-mediated contractions.5. In conclusion, these results confirm our previous findings of a suramin-resistant component to the ATP-induced contraction in the guinea-pig vas deferens, and show that the structure-activity relationships of this response are not identical to those of any known P2-purinoceptor subclass. Although the inhibition by suramin of the breakdown of ATP may contribute to the suramin-resistance of some of the ATP analogues, it does not appear to provide the full explanation.

Subtypes of purinoceptors in rat and dog urinary bladder smooth muscles

1. Both adenosine and adenosine 5'-triphosphate (ATP) (10 microM and 100 microM) relaxed 10 microM acetylcholine (ACh)-induced contraction of rat bladder strips, which was completely antagonized by 100 microM 8-(p-sulphophenyl) theophylline. In dog bladder neither adenosine nor ATP inhibited ACh-induced contraction. 2. P2x-purinoceptor agonists contracted both rat and dog bladder strips with the potency order of alpha,beta-MeATP > ATP > ADP. 3. Alpha,beta-MeADP (100 microM) induced a contraction of the rat bladder strip even after desensitization of P2x-purinoceptors but failed to contract the dog bladder strip. 4. 2-MeSATP (1 microM to 300 microM) concentration-dependently induced contraction of rat bladder strips; this contraction was significantly inhibited after desensitization of P2x-purinoceptors. Cibacron blue 3GA (100 microM) antagonized the drug at concentrations lower than 30 microM, whereas it augmented the response to the drug at concentrations above 30 microM. 5. ADP beta S (1 microM to 1 mM) concentration-dependently induced contraction of rat bladder strips after desensitization of P2x-purinoceptors; a contraction which was significantly antagonized by cibacron blue 3GA (100 microM). 6. It is concluded that three subtypes of purinoceptors, P1 (mediating relaxation), and P2x and another type of P2 (mediating contraction), exist in rat urinary bladder smooth muscle, whereas a single subtype of the receptor, P2x-purinoceptor (mediating contraction) occurs in dog urinary bladder smooth muscle.

Stimulation of lumbar sympathetic nerves evokes contractions of cat colon circular muscle mediated by ATP and noradrenaline

1. The action of the lumbar sympathetic nerves to cat colon was studied in vitro using isolated muscle strips with attached lumbar colonic nerves (LCN) orientated in the axis of circular muscle layer. Electrical stimulation of LCN caused frequency-dependent increases in resting tension and in amplitude of spontaneous contractions. Contractile responses were abolished by tetrodotoxin (3 microM) and by guanethidine (30 microM), indicating that they were neurogenic, involving the release of neurotransmitter from sympathetic fibres. 2. Propranolol (1-9 microM), a beta-adrenoceptor antagonist, caused a concentration-dependent potentiation of LCN-evoked contractile responses. Propranolol (3 microM) potentiated contractile responses to exogenously applied noradrenaline but not to phenylephrine. 3. Phentolamine (1-9 microM), an alpha-adrenoceptor antagonist, and prazosin (1-9 microM), an alpha 1-adrenoceptor antagonist, caused a concentration-dependent reduction of amplitude but did not abolish LCN-evoked contractile responses. Prazosin (3 microM) or phentolamine (3 microM) antagonized contractile responses to noradrenaline and phenylephrine. 4. Desensitization of purinoceptors with the P2x-receptor agonist, alpha,beta-methylene ATP, caused a decrease in amplitude of LCN-evoked contractile responses and abolished contractile responses to ATP. In muscle strips where alpha 1-adrenoceptors were blocked with prazosin (3 microM) and P2-purinoceptors were desensitized with alpha,beta-methylene ATP, the amplitude of contractile responses was reduced by 82-100%. 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and suramin, affected LCN-evoked contractile responses. ANAPP3 (50-100 microM) caused a concentration-dependent reduction in the amplitude of contractile response. Suramin (100 microM) caused a small reduction in amplitude of contractile responses but potentiated their amplitude at a concentration of 500 microM. 6. ANAPP3 (100 microM) irreversibly inhibited contractions to alpha,beta-methylene ATP or ATP. Suramin (100-500 microM) inhibited contractions to alpha,beta-methylene ATP (0.5-1 microM) or low concentrations of ATP (10-50 microM) but potentiated contractions at higher concentrations. ANAPP3 (100 microM) and suramin (100, 500 microM) had no effect on contractile responses to noradrenaline. 7. Clonidine (0.05-1 microM), a selective alpha 2-adrenoceptor agonist, caused a concentration-dependent reduction in amplitude of LCN-evoked contractile responses, at 10 Hz, while yohimbine (0.1-1 microM), a selective alpha 2-adrenoceptor antagonist, increased them. At 1 microM, both compounds affected LCN-evoked contractions at all frequencies. This suggests that prejunctional alpha 2-receptors are involved in autoinhibition at sympathetic terminals. 8. In summary, LCN-evoked contractile responses involve the corelease of noradrenaline and ATP or a related purine nucleotide from sympathetic fibres. It is likely that the neurogenic responses are mediated through excitatory postjunctional alpha 1-adrenoceptors, excitatory suramin-sensitive and suramin-insensitiveP2X-purinoceptors and inhibitory beta-adrenoceptors. Also, autoinhibitory prejunctional alpha2-adrenoceptors regulate the LCN excitatory pathway to cat colon circular muscle.

Interaction of adenine nucleotides, UTP and suramin in mouse vas deferens: suramin-sensitive and suramin-insensitive components in the contractile effect of ATP

Effects of various nucleotides, nucleosides and noradrenaline on smooth muscle tension were studied in the isolated mouse vas deferens. alpha, beta-Methylene-ATP, ATP gamma S, noradrenaline, ATP and UTP elicited contraction, with potency decreasing in that order; there was no contractile response to adenosine or uridine (up to 100 mumol/l). Prolonged incubation with alpha, beta-methylene-ATP (concentration increased stepwise from 0 to 15 mumol/l) selectively reduced contractions induced by ATP and UTP but not those induced by noradrenaline, and there was cross-tachyphylaxis between ATP and UTP. Suramin (10-300 mumol/l) did not alter the response to noradrenaline but shifted the concentration-response curves for alpha, beta-methylene-ATP, ATP gamma S, UTP and lower concentrations of ATP (0.1-1 mumol/l) to the right. The pA2-values of suramin were 5.2 against alpha, beta-methylene-ATP, 4.8 against ATP gamma S, 5.1 against UTP and 5.4 against lower concentrations of ATP. The effects of higher concentrations of ATP were largely resistant to suramin. The results indicate that the mouse vas deferens possesses contraction-mediating smooth muscle P2x-receptors. UTP also acts at this receptor, and there is no evidence for a separate UTP receptor. The selective inhibition of nucleotide- but not noradrenaline-induced contractions by suramin confirms the view that suramin is a selective P2-antagonist. The resistance against suramin of part of the effect of ATP suggests that ATP activates a suramin-insensitive site in addition to the P2x-receptor.

Possible subtypes of ATP receptor producing contraction of rat vas deferens, revealed by cross-desensitisation

1. Adenosine triphosphate (ATP), alpha beta-methylene ATP and the dinucleotides P1P4diadenosine tetraphosphate (AP4A) and P1P5-diadenosine pentaphosphate (AP5A) have been applied to preparations of the rat vas deferens in vitro in preliminary experiments designed to assess the feasibility of using such nucleotides to detect subtypes of the ATP receptor causing smooth muscle contractions. 2. Desensitisation to the dinucleotides induced a change in the profile of the response to ATP with the preferential loss of an early transient component. 3. Desensitisation to low concentrations of alpha beta-methylene ATP produced a greater loss of the later, slower component of contraction to ATP. 4. It is suggested that these components may therefore involve pharmacologically distinguishable nucleotide receptors.

Sympathetic nerve-mediated release of ATP from the guinea-pig vas deferens is unaffected by reserpine

The release of ATP from the guinea-pig vas deferens was measured using the luciferin-luciferase assay. The effects of reserpine, tetrodotoxin, 6-hydroxydopamine and guanethidine on this efflux were investigated. Reserpine, which produced extensive loss of noradrenaline (NA) (about 99%) and eliminated the second phase of the nerve-mediated contraction, failed to impair either ATP release from the vas deferens or the concomitant 'twitch' contraction; in fact both were increased. Therefore the neurotransmitter mediating the twitch cannot be NA, thus excluding the gamma-receptor hypothesis. Furthermore, since the release of ATP is unaffected by reserpine, it is unlikely that ATP is being released from smooth muscle as a consequence of the post-junctional actions of NA. Tetrodotoxin, 6-hydroxydopamine and guanethidine substantially reduced or abolished the release of ATP and both phases of the nerve-mediated response. To conclude, in the guinea-pig vas deferens, ATP co-released with NA from sympathetic nerves, mediates the twitch phase of the contraction and its underlying electrical events.

Possible interaction of cholinergic nerves with two different (pre and post) sites of the neuromuscular junction in guinea-pig vas deferens

Effects of various drugs on the mechanical responses of the longitudinal smooth muscle of guinea-pig vas deferens evoked by ACh and field nerve stimulation were examined. ACh (28-280 microM) produced a contraction consisting of two phases. The first phase of the contraction was suppressed by guanethidine and by nicotinic antagonists. The second phase was suppressed only by atropine. Both phases were unaffected by TTX or prazosin. Field stimulation (0.1 msec, 40 Hz) evoked contractions which also consisted of two (early and late) phases. Guanethidine (0.1-1 microM) suppressed both phases whilst prazosin (1 microM) suppressed only the late phase. Atropine (0.1 microM) suppressed both phases whilst physostigmine (5 microM) potentiated both phases of field stimulation-evoked contractions. Pentolinium suppressed both phases of field stimulation-evoked contractions at low concentrations (2-10 microM), but potentiated them at a higher concentration (100 microM). dTC at a low concentration (0.5 microM) suppressed the early phase, but slightly enhanced the late phase of field stimulation responses. At a higher concentration (20 microM), dTC potentiated both phases of the response. Pentolinium and dTC did not affect the contractions induced by 90 mM-K ions, ATP or NA. These results suggest that cholinergic nerves possess an excitatory action not only directly at the smooth muscle but also at the noradrenergic nerve terminals. The role of each receptor is discussed further.

Further comparison of contractions of the smooth muscle of the guinea-pig isolated vas deferens induced by ATP and related analogs

Some structural requirements which are important for the contractile activity of adenine nucleotides at P2-purinergic receptors in the guinea-pig isolated vas deferens were examined. The consequences of deletions, substitutions and isosteric rearrangements in 1-N, C-2, C-8, N-9 and N6 of adenine, in the ribose hydroxyls, and in the polyphosphate chain were examined. Several kinds of effects on activity were observed, including potentiation of responses, losses in activity, modifications of response duration, and transitions from biphasic concentration-response curves, which are characteristic of ATP, to monophasic curves. Generally, deletions and substitutions to adenine reduced activity. Ribose modifications were better-tolerated. The presence of 5'-phosphate or phosphorothioate moieties was required for maximum activity. A comparison of the present results with previous reports indicates that certain modifications to ATP result in similar effects in both the vas deferens and urinary bladder, which are opposite those which occur in the taenia coli.

Dual excitatory actions of acetylcholine at the neuromuscular junction in the guinea-pig vas deferens

The action of acetylcholine (ACh) on the smooth muscle of guinea-pig vas deferens was studied using the sucrose-gap method. ACh, when applied at a concentration of 10(-6) M, evoked a depolarization of the smooth muscle membrane which was slow in time course (slow depolarization). When ACh was applied at higher concentrations, another depolarization which was fast in time course (fast depolarization) occurred, overlapping the early part of the slow depolarization. The magnitudes of both depolarizations were concentration-dependent on ACh. TTX and adrenergic receptor antagonists had little effect on either depolarizations, while guanethidine and nicotinic receptor antagonists mainly suppressed the fast depolarization. In contrast, atropine suppressed the slow depolarization. The membrane conductance observed by current application, was reduced during the slow depolarization, and the reversal potential of the depolarization was 18.3 mV negative to the resting membrane potential. Whereas, the reversal potential of the fast depolarization was 27.6 mV positive to the resting membrane potential. This reversal potential was quite similar to that of the adenosine triphosphate (ATP)-induced depolarization, previously observed in the same tissue. From these observations, it is suggested that in the guinea-pig vas deferens, ACh acts on nicotinic receptors at the sympathetic postganglionic nerve terminal, causing the release mostly of a non-adrenergic transmitter, probably ATP. In addition, ACh also acts on muscarinic receptors on the smooth muscle membrane, inducing membrane depolarization resulting from a reduction of the membrane conductance to potassium ions.

Effects of chronic drug treatment on the sensitivity of mouse vas deferens to drugs

Chronic treatment of mice with clonidine or morphine reduced the sensitivity of the isolated, field-stimulated, vas deferens to the inhibitory effects of acutely administered clonidine and morphine. Chronic treatment of mice with clonidine produced a postsynaptic non-specific supersensitivity to noradrenaline and carbachol, but not to potassium chloride. Chronic treatment with morphine produced a specific supersensitivity to noradrenaline. Chronic treatment of mice with naloxone increased the sensitivity of the isolated, field stimulated, vas deferens to the inhibitory effects of acutely administered morphine, but not to clonidine. Naloxone treatment of mice did not affect the sensitivity of the isolated vas deferens to noradrenaline, carbachol or potassium chloride. These results suggest a physiological role for the opioid receptors in the mouse vas deferens and an apparent cross-tolerance between clonidine and morphine. The possible mechanisms are discussed.

Interaction of [3H]arylazido aminopropionyl ATP ([3H]ANAPP3) with P2-purinergic receptors in the smooth muscle of the isolated guinea-pig vas deferens

Following its photolysis in the presence of the isolated guinea-pig vas deferens, the ATP photoaffinity label ANAPP3 produces a specific antagonism of adenine nucleotide-induced contractile responses which are mediated by P2-purinergic receptors. To characterize the site of covalent photoincorporation of ANAPP3, intact vasa deferentia were treated with [3H]ANAPP3 and samples of homogenate, cytosol and a crude membrane fraction were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Photolysis of [3H]ANAPP3 (10(-5) M; 3.0 mu Ci/ml) resulted in the incorporation of radioactivity into cellular components with apparent molecular weights of 54-66 and 43-57 kilodaltons. The photoincorporation of [3H]ANAPP3 was associated with the crude membrane fraction and not the cytosol, was reduced in the presence of ATP in an ATP-concentration-dependent manner, was lessened following pretreatment of the tissues with photolyzed nonradiolabeled ANAPP3, and was unaffected by the nucleoside transport inhibitor, dipyridamole. In tension studies on the same tissues the presence of ATP resulted in a concentration-dependent reduction in the initial contractile response to [3H]ANAPP3 the response to 3H was antagonized in tissues which had been pretreated with nonradiolabeled ANAPP3, and dipyridamole had no effect on the contractile response to [3H]ANAPP3. According to several criteria these findings indicate that the antagonism by photolyzed ANAPP3 of adenine nucleotide-induced responses is a direct result of the covalent insertion at or near the recognition site of cell-surface P2-purinergic receptors.

Evidence for an increase in membrane conductance during adenosine triphosphate-induced depolarization in the guinea-pig vas deferens

The effects of exogenously applied adenosine triphosphate (ATP) on the smooth muscle of guinea-pig vas deferens were studied with the double sucrose-gap method. ATP evoked a membrane depolarization which was associated with a decrease in the size of electrotonic potentials. Conditioning hyperpolarization induced by current application caused an increase in the magnitude of the ATP-induced depolarization; the larger the conditioning hyperpolarization, the greater the ATP-induced depolarization. These results are discussed with respect to the ionic mechanism of the electrical event in response to ATP in this tissue.

Is there a basis for distinguishing two types of P2-purinoceptor?

It is suggested that the P2-purinoceptor may be separated into two subtypes largely on the basis of the rank order of agonist potency of structural analogues of ATP and also on the activity of antagonists at the P2-purinoceptor: Subtype 1 (designated P2X), potency order: alpha, beta-methyleneATP, beta, gamma-methyleneATP greater than ATP = 2 methylthioATP; antagonism by ANAPP3 and selectively desensitisation following administration of alpha, beta-methyleneATP; present in the vas deferens and urinary bladder of guinea-pig and rat, frog and rat ventricle, and also in the smooth muscle of the rat femoral artery and rabbit central ear artery, where they mediate excitation. Subtype 2 (designated P2Y), potency order: 2-methylthioATP much greater than ATP greater than alpha, beta-methyleneATP, beta, gamma-methyleneATP; weak antagonism by ANAPP3 and desensitisation following administration of alpha, beta-methyleneATP; present in the guinea-pig taenia coli and the longitudinal muscle layer of the rabbit portal vein, where they mediate relaxation and also on the vascular endothelial cells of the rat femoral artery and pig aorta (where occupation leads to the production of endothelium-derived relaxing factor). Differences in the structure of the P2-purinoceptor in various tissues may be useful in the development of drugs for the treatment of vascular, gastrointestinal and urinoglenital disorders.

Comparison of the effects of apamin, a Ca2+-dependent K+ channel blocker, and arylazido aminopropionyl ATP (ANAPP3), a P2-purinergic receptor antagonist, in the guinea-pig vas deferens

Apamin, which blocks Ca2+-dependent increases in K+ permeability, antagonizes ATP-induced relaxation of several smooth muscles. The ATP photoaffinity label arylazido aminopropionyl ATP (ANAPP3), following its photolysis in the presence of the guinea-pig vas deferens, antagonizes contractile responses to ATP. This study was conducted to determine whether apamin antagonizes ATP-induced responses in the guinea-pig vas deferens, and also to evaluate whether ANAPP3 antagonizes responses to ATP by interfering with Ca2+-dependent K+ permeability changes. Apamin (10(-6) M) potentiated ATP-induced contractions. This potentiation was nonspecific in that responses to norepinephrine, histamine and acetylcholine also were enhanced; responses to KCl were unaffected. To evaluate the possible interactions between the two agents at the same cellular site, the effect of apamin was examined in ANAPP3-treated tissues. In such tissues apamin did not potentiate the residual responses to ATP; however, apamin was nevertheless able to potentiate responses of ANAPP3-treated tissues to norepinephrine, histamine and acetylcholine, and responses to KCl remained unaffected. These studies provide additional support for the view that ANAPP3 antagonizes ATP-induced responses of the guinea-pig vas deferens by blocking P2-purinergic receptors. The antagonism by ANAPP3 is not attributable to a blockade of Ca2+-dependent K+ permeability changes.

Comparison of photoaffinity labeling of P2-purinergic receptors of isolated guinea-pig vas deferens by arylazido aminopropionyl ATP and by arylazido aminobutyryl ATP

Two chemically related arylazido photoaffinity analogs of ATP (arylazido aminopropionyl ATP (ANAPP3) and arylazido aminobutyryl ATP (ANABP3)), which have been reported in the literature to differ in their ability to inhibit myosin ATPase, were compared for their ability to antagonize contractile responses of the isolated guinea-pig vas deferens to ATP. During photolysis in organ chambers the photoconversion (delta A260/delta t) of ANAPP3 occurred with greater than first order kinetics or was multiexponential and t1/2 = 7.5 min, while delta A260/delta t for ANABP3 was first order and t1/2 = 2.25 min. After photolysis of these compounds in the presence of the guinea-pig vas deferens, using irradiation periods which caused 80% consumption of the compounds, ANABP3 was 2-3 times more potent than ANAPP3 in antagonizing contractions to ATP, which are mediated by P2-purinergic receptors. A comparison of concentration-response curves obtained for nonphotolyzed ANAPP3 and ANABP3 used as agonists suggested that the greater antagonism produced by photolyzed ANABP3 is not attributable to a greater potency. The results suggest that the longer 3'-hydroxyl-arylazide bridge length of ANABP3 places the arylnitrene intermediate in a position at or near the P2-receptor which is more favorable for covalent insertion.