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R Bültmann

Publications and source records attributed to R Bültmann.

At least 37 records · Page 2Linked to original sources
PubMed1996

Evaluation of P2-purinoceptor antagonists at two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli.

The guinea-pig taenia coli possesses two relaxation-mediating receptors for nucleotides: a prototypic P2Y-purinoceptor, which is activated by adenosine 5'-O-(2-thio-diphosphate) (ADP beta S), and a separate receptor for alpha, beta-methylene ATP (alpha,beta-MeATP). Effects of several as yet incompletely characterized P2-purinoceptor antagonists at these receptors were examined. The concentration-relaxation curve of ADP beta S was shifted to the right by reactive blue 2, suramin, 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulp honic acid (XAMR0721; at 1000 microM only), pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (iso-PPADS), pyridoxal 5-phosphate, trypan blue and Evans blue (at 320 microM only). Schild plots for the antagonism of reactive blue 2, suramin, iso-PPADS and pyridoxal 5-phosphate against ADP beta S had slopes < 1. The concentration-relaxation curve of alpha,beta-MeATP was shifted to the right by reactive blue 2, suramin, XAMR0721, iso-PPADS, pyridoxal 5-phosphate and trypan blue but not by Evans blue (320 microM). Schild plots for the antagonism of suramin, XAMR0721 and iso-PPADS against alpha,beta-MeATP had slopes > 1. Only XAMR0721 differed clearly in potency against the two nucleotides: it was considerably more potent against alpha,beta-MeATP than against ADP beta S. 2-Methylthio ATP (MeSATP; 1 microM) and ATP (100 microM) were degraded by pieces of taenia coli. All antagonists except trypan blue attenuated the degradation of either or one of the two nucleotides. The selective effect of XAMR0721 against alpha,beta-MeATP confirms the existence of two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli. Comparison of the apparent affinities of the antagonists for the two taenia coli receptors with affinities for the P2X-purinoceptor of the rat vas deferens shows that reactive blue 2, suramin, iso-PPADS, pyridoxal 5-phosphate and trypan blue have little selectivity for any of the three receptors. XAMR0721, which has been shown to possess relatively high affinity for the P2Y-purinoceptor in turkey erythrocytes, was very weak at the P2Y-receptor of the taenia, thus supporting the existence of pharmacologic P2Y-receptor subtypes. Evans blue, with little effect in the taenia coli but a marked effect in the rat vas defrens, is the most selective P2X- (versus P2Y-) purinoceptor antagonists presently known, although its effect on the degradation of nucleotides must be kept in mind.

Adrenergic alpha-Agonists↗
PubMed1995

Two relaxation-mediating P2-purinoceptors in guinea-pig taenia caeci.

2-Methylthio ATP, noradrenaline, adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), alpha, beta-methylene ATP, ATP and adenosine caused relaxation of guinea-pig isolated taenia caeci precontracted by carbachol, with potency decreasing in the order indicated. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonate (DIDS) 10, 32 and 100 microM shifted the concentration-response curve of alpha, beta-methylene ATP increasingly to the right, by the factor 141 at DIDS 100 microM. Concentration-response curves of the other agonists were not shifted to the right by DIDS 100 microM, except for the curve of ADP beta S which was shifted by the factor 2.4. The relaxation produced by 2-methylthio ATP faded rapidly. When the fade was complete, further addition of 2-methylthio ATP or ATP did not elicit relaxation, whereas the relaxant effect of alpha, beta-methylene ATP was unchanged. The results indicate that there are at least two relaxation-mediating P2-purinoceptors in guinea-pig taenia caeci, the P2Y-purinoceptor which is relatively insensitive to DIDS and a distinct receptor for alpha, beta-methylene ATP which is very sensitive to DIDS.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1995

Reactive red 2: a P2y-selective purinoceptor antagonist and an inhibitor of ecto-nucleotidase.

Effects of reactive red 2 and its parent compound acid red 33 were studied in rat vas deferens and guinea-pig taenia coli. In rat vas deferens, reactive red 2(1 to 10 microM) shifted the concentration-response curve for the P2X-purinoceptor-mediated contraction effect of alpha, beta-methylene ATP slightly to the right and progressively decreased the maximum (apparent antagonist Kd value 0.42 microM). Acid red 33 (1000 microM) shifted the curve to the right without changing the maximum (apparent Kd 386 microM). The concentration-contraction curve of noradrenaline was not altered by reactive red 2. In the carbachol-precontracted guinea-pig taenia coli, reactive red 2 (0.1 to 10 microM) shifted the concentration-response curve for the P2Y-purinoceptor-mediated relaxation effect of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) progressively to the right; only at the highest concentration of antagonist (10 microM) was the maximum slightly depressed; a pA2 value of 7.55 (Kd 0.028 microM) was derived from the shift. Acid red 33 (1000 microM) shifted the concentration-relaxation curve of ADP beta S to the right without changing the maximum (apparent Kd 171 microM). Reactive red 2 (1 to 10 microM) also shifted the concentration-response curve for the relaxation effect of alpha, beta-methylene ATP, which is mediated by an unclassified P2-purinoceptor, progressively to the right but simultaneously decreased the maximum (apparent Kd 1.6 microM). The concentration-relaxation curve of 2-chloroadenosine was not altered by reactive red 2. Pieces of vas deferens and taenia coli degraded 76 and 66% of added ATP (10 microM) within 30 min, respectively. Reactive red 2(0.1 to 100 microM) progressively reduced this degradation by up to 95%, with IC50 values of 3.9 +/- 0.6 and 3.9 +/- 2.3 microM, respectively. Acid red 33 (1000 microM) reduced the degradation by 30 and 20%, respectively. The results indicate that reactive red 2 is a relatively potent antagonist at both P2X-purinoceptors in rat vas deferens and P2Y-purinoceptors in guinea-pig taenia coli, with a 15 fold selectively for the P2Y-purinoceptor. It inhibits ecto-nucleotidase in both tissues. The dichloro-triazine residue that distinguishes the compound from acid red 33 greatly enhances the potency at both receptor subtypes as well as at the nucleotidase. As regards P2-purinoceptor subtypes, the results confirm the existence of two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli.

Adenosine Triphosphatases↗
PubMed1995

Functional consequences of inhibition of nucleotide breakdown in rat vas deferens: a study with Evans blue.

The effect of Evans blue on nucleotide breakdown, nucleotide-evoked contractions and electrically evoked contractions, overflow of ATP and overflow of tritium (after labelling with [3H]-noradrenaline) was studied in rat vas deferens. Pieces of vas deferens degraded 83 to 85% of added ATP, ADP and 2-methyl-thio ATP (all 100 microM) over 30 min. Evans blue (100 microM) reduced this degradation to 22 to 26%. Nucleotides elicited contraction with potency declining in the order alpha,beta-methylene ATP > 2-methylthio ATP > ATP > ADP. Evans blue (100 microM) shifted the concentration-response curve of alpha, beta-methylene ATP to the right and increased the maximum. Concentration-response curves of ATP, ADP and 2-methylthio ATP, in contrast, were shifted to the left and responses were much potentiated. In the presence of Evans blue, the rank order of potency was ATP > 2-methylthio ATP > alpha, beta-methylene ATP > ADP. Electrical field stimulation (100 pulses at 10 Hz) elicited contraction and an overflow of tritium and ATP. Evans blue (100 microM) did not alter the contraction and the evoked overflow of tritium but increased 24-fold the evoked overflow of ATP. The results indicate that Evans blue may serve as an-albeit impure-ecto-nucleotidase inhibitor in functional experiments. Such experiments demonstrate that the low potency of ATP (and also ADP and 2-methylthio ATP) in eliciting contraction, and the small size of the overflow of ATP upon sympathetic nerve stimulation, are due to rapid breakdown.

Adenosine Triphosphate↗
PubMed1994

Antinociceptive effect of intrathecally administered P2-purinoceptor antagonists in rats.

To investigate whether ATP participates in spinal nociceptive transmission, effects of intrathecally applied P2-purinoceptor antagonists and agonists in the tail-flick and the formalin test were studied in rats. In the tail-flick assay, the P2 antagonists suramin (12-120 micrograms), Evans blue (0.1-10 micrograms), Trypan blue (1-30 micrograms) and Reactive blue 2 (1-30 micrograms) but not pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 0.03-30 micrograms) caused moderate antinociception up to a doubling of the response latency. In contrast, the P2 agonists alpha,beta-methylene ATP (alpha,beta-mATP, 0.3-30 micrograms) and 2-methylthio-ATP (3-30 micrograms) decreased the tail-flick latency by up to about 50%. When co-injected with alpha,beta-mATP, suramin (120 micrograms) or Evans blue (10 micrograms) prevented the effect of alpha,beta-mATP 3 micrograms but not of alpha,beta-mATP 30 micrograms. In the formalin test, pretreatment with suramin (3-90 micrograms) 60 min prior to testing caused significant antinociception by decreasing the weighted pain intensity score by up to about 80%. alpha,beta-mATP (30 micrograms), applied 30 min prior to testing, was without effect. The results indicate that endogenous ATP, acting through P2-purinoceptors, may contribute to nociceptive information processing in the spinal cord.

Adenosine Triphosphate↗
PubMed1994

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)

Action Potentials↗
PubMed1994

Release of ATP in rat vas deferens: origin and role of calcium.

Release of endogenous ATP elicited by electrical (neural) stimulation and exogenous agonists was studied in the rat isolated vas deferens. The aims were to dissect neural and postjunctional contributions to the nerve activity-evoked overflow of ATP and to clarify the role of transmitter receptors and calcium in postjunctional ATP release. In tissues preincubated with [3H]-noradrenaline, electrical stimulation (100 pulses/10 Hz) elicited contraction and an overflow of tritium and ATP. Contractions as well as ATP overflow were reduced by prazosin 0.3 microM and even more so by prazosin 0.3 microM combined with suramin 300 microM. They were also reduced by nifedipine 10 microM and even more so by nifedipine 10 microM combined with ryanodine 20 microM (the additional effect of ryanodine on ATP overflow was not significant). In tissues not pretreated with [3H]-noradrenaline, exogenous noradrenaline 10 microM and alpha,beta-methylene ATP 10 microM elicited contraction and an overflow of ATP. Responses to noradrenaline were blocked by prazosin 0.3 microM but not suramin 300 microM and were greatly reduced by nifedipine 10 microM and in Ca(2+)-free medium. Responses to alpha,beta-methylene ATP were blocked by suramin 300 microM but not prazosin 0.3 microM, were reduced by nifedipine 10 microM (effect on ATP overflow not significant) and were reduced even more in Ca(2+)-free medium. Neuropeptide Y 0.3 microM caused only very small contraction and ATP overflow. The electrically as well as the agonist-evoked ATP overflow correlated well with the contraction responses except in experiments with suramin which retarded the removal, by vas deferens tissue, of ATP from the medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1994

P2-purinoceptor antagonists discriminate three contraction-mediating receptors for ATP in rat vas deferens.

The sites of action at which ATP elicits contraction of the rat vas deferens were studied by means of the P2-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (iso-PPADS), suramin and reactive blue 2. Increasing concentrations of iso-PPADS (up to 1 mM), suramin (up to 1 mM) and reactive blue 2 (up to 320 microM) reduced and eventually abolished contractions elicited by the P2x-purinoceptor-selective agonist alpha,beta-methylene ATP 3 microM with IC50 values of 2.1, 10.1 and 27.0 microM, respectively. In contrast, iso-PPADS and suramin caused only a partial inhibition of contractions elicited by ATP 1 mM, maximal reduction by about 40%, IC50 values 1.3 and 5.0 microM, respectively; reactive blue 2 did not change ATP-induced contractions. In tissues exposed to iso-PPADS 320 microM throughout, increasing concentrations of reactive blue 2 or suramin decreased contractions elicited by ATP 1 mM, IC50 values 2.6 and 14.5 microM, respectively. In tissues exposed to suramin 320 microM throughout, increasing concentrations of iso-PPADS decreased contractions elicited by ATP 1 mM, IC50 37.9 microM, whereas reactive blue 2 slightly enhanced these contractions. In tissues exposed to reactive blue 2 100 microM throughout, increasing concentrations of iso-PPADS reduced contractions elicited by ATP 1 mM, IC50 26.6 microM, whereas suramin caused no change. Pre-exposure to alpha,beta-methylene ATP 1 microM to desensitize P2x-purinoceptors reduced the response to ATP 1 mM by 91% in otherwise untreated tissues, but did not reduce the response to ATP 1 mM in tissues exposed throughout to iso-PPADS 320 microM, suramin 320 microM or reactive blue 2 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1994

Presynaptic modulation of the release of the co-transmitters noradrenaline and ATP.

The release of both sympathetic co-transmitters, noradrenaline and ATP, is modulated via presynaptic receptors. However, the degree of the modulation may differ indicating that the ratio of the released co-transmitters changes upon presynaptic receptor activation. For example, alpha 2-autoinhibition affects the release of noradrenaline more markedly than the release of ATP. Some sympathetic axon terminals possess presynaptic P2-purinoceptors which are activated by endogenous ATP. These receptors are a novel kind of auto-receptor: they mediate a presynaptic negative feedback mechanism in which released ATP inhibits subsequent co-transmitter release.

Adenosine Triphosphate↗
PubMed1994

Blockade by 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) of P2X-purinoceptors in rat vas deferens.

1. The possibility of an antagonist effect of 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) at P2X-purinoceptors was studied in rat vas deferens. 2. DIDS reduced contractions elicited by alpha,beta-methylene ATP 3 microM, IC50 1.6 microM, but did not change contractions elicited by K+ 35 mM. DIDS 3.2 microM slightly shifted the concentration-response curve of alpha,beta-methylene ATP to the right and reduced the maximum. DIDS 10 microM markedly decreased and DIDS 32 microM abolished contractions over the entire range of the alpha, beta-methylene ATP concentration-response curve. DIDS 32 microM also abolished contractions elicited by ATP but did not change contractions elicited by noradrenaline. The antagonist effect of DIDS was only slowly reversible. 3. The presence of either suramin 320 microM or alpha,beta-methylene ATP 10 microM during the exposure to DIDS protected the tissue from the long-lasting blocking effect of DIDS. 4. 4,4'-Diisothiocyanatodihydrostilbene-2,2'-disulphonate (H2DIDS) was equipotent with DIDS whereas several analogues in which one or both of the isothiocyanate residues were replaced were less effective or without effect against alpha,beta-methylene ATP. 5. DIDS attenuated the purinergic component of neurogenic contractions elicited by electrical field stimulation, IC50 3.9 microM, but did not change the adrenergic component. 6. It is concluded that DIDS causes a selective, long-lasting, non-equilibrium blockade of P2X-purinoceptors in rat vas deferens. Due to this effect it also selectively blocks the purinergic component of neurogenic contractions.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1994

Alpha 1-adrenoceptors and calcium sources in adrenergic neurogenic contractions of rat vas deferens.

1. The involvement of alpha 1-adrenoceptor subtypes in adrenergic neurogenic contractions of different type was studied in epididymal and prostatic portions of the rat vas deferens. 2. The adrenergic component of neurogenic contractions was isolated by suramin (300 microM). Twitch-like and tonic contractions were elicited by appropriate pulse patterns of electrical field stimulation, and contractions relying on intracellular calcium mobilization and calcium entry were isolated by means of nifedipine (10 microM) and ryanodine (20 microM), respectively. Increasing concentrations of 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)- amino)-propyl)benzeneacetonitrile (HV 723), prazosin and 5-methylurapidil progressively, monophasically and with potency decreasing in that order reduced and finally abolished all types of contraction, with one exception: concentration-effect curves of 5-methylurapidil in epididymal segments in the presence of ryanodine levelled off at about 75% inhibition. In the presence of both nifedipine (10 microM) and ryanodine (20 microM), contractions were abolished. 3. Contractions elicited by exogenous noradrenaline were also studied in the presence of either nifedipine 10 microM (prostatic segments) or ryanodine 20 microM (epididymal segments). Increasing concentrations of tamsulosin, WB 4101, benoxathian, HV 723, prazosin, 5-methylurapidil and urapidil progressively, monophasically and with potency decreasing in that order reduced and eventually abolished both kinds of contraction, with two exceptions: in epididymal segments in the presence of ryanodine, the concentration-effect curve of 5-methylurapidil was biphasic and the curve of urapidil levelled off at only partial inhibition. 4. In slices prepared from the prostatic end and preincubated with [3H]-noradrenaline, WB 4101, HV 723, prazosin and 5-methylurapidil, at the highest concentrations tested against neurogenic contractions, increased only slightly the overflow of tritium elicited by trains of 50 pulses at 5 Hz. 5. It is concluded that two alpha l-adrenoceptor subtypes mediate adrenergic neurogenic contractions of rat vas deferens. The main one, pharmacologically alpha 1A, activates both calcium mobilization and entry. In addition there is a second receptor, not previously detected in the vas deferens and not corresponding to any named alpha l subtype, characterized by high and similar affinity for tamsulosin, WB 4101, benoxathian,HV 723 and prazosin and very low affinity for 5-methylurapidil and urapidil, and linked exclusively to calcium entry. Both subtypes and their respective transduction pathways also contribute to contractions elicited by exogenous noradrenaline. An alpha 1B-adrenoceptor-mediated contraction was not found under any experimental conditions.

Adrenergic alpha-Agonists↗
PubMed1994

Blockade of P2X-purinoceptors by trypan blue in rat vas deferens.

1. The possibility of an antagonist effect of trypan blue at P2X-purinoceptors was studied in rat vas deferens. 2. Trypan blue (3.2-320 microM) shifted the concentration-contraction response curve of alpha,beta-methylene ATP (alpha,beta-MeATP) to the right and simultaneously increased the maximum of the curve by up to 40%. The Schild plot had a slope not significantly different from unity and yielded a pA2 value of 5.3 (KB 4.9 microM). 3. Suramin (32 microM) also shifted the concentration-response curve of alpha,beta-MeATP to the right, KB 2.6 microM, and increased the maximum by 31%. In the presence of suramin (32 microM), trypan blue (32 microM) did not change the concentration-response curve of alpha,beta-MeATP. 4. 1-Amino-8-naphthol-3, 6-disulphonate (H-acid) 10 mM, the sulphonic acid-carrying moiety of trypan blue, shifted the concentration-response curve of alpha,beta-MeATP to the right, KB 1.4 mM, and increased the maximum by 33%. 5. Trypan blue did not change contractions elicited by high K+ and noradrenaline. 6. Trypan blue attenuated the purinergic component of neurogenic contractions, IC50 44.9 microM, but did not change the adrenergic component. 7. It is concluded that trypan blue blocks P2X-purinoceptors in rat vas deferens. The increase of the maximum of the alpha,beta-MeATP concentration-response curve is similar in mechanism to the increase produced by suramin.

Adenosine Triphosphate↗
PubMed1993

Inhibition by ethanol of contractions of rat vas deferens: no evidence for selective blockade of P2X-purinoceptors.

Some ligand-gated ion channels are important sites of action of ethanol. The aim of the study was to find out whether the P2X-purinoceptors mediating contraction of the rat isolated vas deferens also are selectively sensitive to ethanol. Contractions were elicited by ATP (1 mmol/l), alpha, beta-methylene ATP (0.3 mumol/l), noradrenaline (3 mumol/l), high K+ (20 mmol/l) or electrical (neural) stimulation by pairs of pulses 3 s apart. In electrical stimulation experiments, purinergic and adrenergic response components were isolated by prazosin and suramin, respectively. Concentration-effect curves were determined for ethanol and, for comparison, nifedipine. Tritium outflow from tissues preincubated with 3H-noradrenaline was also examined. Ethanol at relatively low concentrations reduced contractions elicited by high K+ (IC30 145 mmol/l), ATP (IC30 211 mmol/l) and alpha, beta-methylene ATP (IC30 215 mmol/l) as well the purinergic component of neurogenic twitches (IC30 110-126 mmol/l; a significant effect at 10-32 mmol/l) and the adrenergic component of twitch 2 of the twitch pairs (IC30 63 mmol/l). These contractions also were very sensitive to nifedipine. Higher concentrations of ethanol were needed to reduce contractions elicited by noradrenaline (IC30 365 mmol/l) and the adrenergic component of twitch 1 of the twitch pairs (IC30 382 mmol/l), contractions that also were less sensitive to nifedipine. Ethanol 1 mol/l abolished all contractions. In contrast, concentration-effect curves for the inhibition by nifedipine of contractions evoked by ATP, alpha,beta-methylene ATP and noradrenaline (rapid phase) levelled off at 60-70% inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1993

Evans blue blocks P2X-purinoceptors in rat vas deferens.

In rat vas deferens, Evans blue 100 microM increased contractions elicited by high K+ and by noradrenaline but markedly reduced contractions elicited by the P2X-purinoceptor-selective agonist alpha,beta-methylene ATP (3 microM). The concentration-response curve of alpha,beta-methylene ATP was shifted to the right by Evans blue 30 microM and the maximal contraction was increased. In tissues incubated with nifedipine 10 microM, Evans blue 100 microM tended to increase the residual contraction elicited by noradrenaline and abolished the residual response to alpha, beta-methylene ATP (3 microM). The concentration-response curve of alpha,beta-methylene ATP was progressively shifted to the right by increasing concentrations of Evans blue in the presence of nifedipine; maximal contractions were increased by Evans blue 10 and 30 but not 100 microM. From the shifts to the right caused by Evans blue 30 microM, apparent pKB values of 5.9 (no nifedipine) and 6.0 (nifedipine present) were calculated. It is concluded that Evans blue blocks P2X-purinoceptors in rat vas deferens and in addition causes a non-receptor-specific enhancement of contractions.

Adenosine Triphosphate↗
PubMed1993

Chloroethylclonidine: an irreversible agonist at prejunctional alpha 2-adrenoceptors in rat vas deferens.

1. The possibility that chloroethylclonidine (CEC) activates prejunctional alpha 2-adrenoceptors was studied in the isolated vas deferens of the rat. Tissues were stimulated electrically and both the stimulation-evoked overflow of tritium (after preincubation with [3H]-noradrenaline) and the purinergic contraction component (isolated by prazosin 0.3 microM) were measured. 2. CEC (0.1-3 microM) concentration-dependently reduced the overflow of tritium evoked by trains of 6 pulses/100 Hz. The inhibition by CEC was not altered by prazosin (0.3 microM) but was prevented by pre-exposure to rauwolscine (0.3 microM). The inhibition, once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 3. CEC (0.1-3 microM) concentration-dependently reduced the purinergic component of contractions elicited by single pulses. The inhibition, again, was prevented by pre-exposure to rauwolscine (0.3 microM) and once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 4. CEC (3 microM) reduced the overflow of tritium evoked by 20 pulses/10 Hz, did not alter the overflow evoked by 100 pulses/10 Hz and increased the overflow evoked by 500 pulses/10 Hz. 5. CEC (3 microM) reduced the early peak, but increased the late plateau phase, of purinergic contractions elicited by 100 pulses/10 Hz. 6. It is concluded that CEC reduces the release of noradrenaline and a purinergic co-transmitter by irreversible activation of prejunctional alpha 2-adrenoceptors. CEC seems to be a partial alpha 2-agonist with an efficacy lower than that of noradrenaline. The prejunctional inhibitory effect limits the suitability of CEC for the characterization of postjunctional alpha 1-adrenoceptors mediating responses to sympathetic nerve stimulation.

Adrenergic alpha-Agonists↗
PubMed1993

Effect of opioid receptor subtype-selective agonists on purinergic and adrenergic components of neurogenic contractions of mouse vas deferens.

1. Effects of opioid agonists on the purinergic and adrenergic components of neurogenic contractions and in some experiments on transmitter overflow were studied in the mouse isolated vas deferens. 2. When the vas deferens was stimulated every 2 min by pairs of pulses 2 s apart in the presence of prazosin 0.3 microM (to isolate the purinergic component) or alpha,beta-methylene-ATP 3 microM (to isolate the adrenergic component), each pulse elicited a separate twitch. The opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, mu-receptor-selective), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-selective) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide (U-50488, kappa-selective) concentration-dependently reduced both purinergic and adrenergic contractions. For each agonist, maximal effects and concentrations causing half-maximal effects were very similar for inhibition of the purinergic component on the one hand and for inhibition of the adrenergic component on the other hand, although the adrenergic component was inhibited with a slight preference. Moreover, effects on contractions elicited by the first and the second pulse of the pairs were very similar. 3. When vasa deferentia preincubated with [3H]-noradrenaline were stimulated with trains of 100 pulses delivered at 20 Hz, morphine 10 microM reduced significantly both evoked tritium overflow and evoked contractions. Its effect was antagonized by naloxone. 4. It is concluded that, in contrast to drugs acting at some other presynaptic receptors, opioid mu-, delta- and kappa-agonists inhibit purinergic and adrenergic neurogenic contractions of the mouse vas deferens in a similar manner. In contrast to a previous report, no enhancement by morphine of the release of noradrenaline elicited by high frequency pulse trains was observed.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1993

Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse-to-pulse change in Ca2+ sources.

1. The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 microM suramin. 2. In Mg(2+)-free medium, nifedipine (0.01-10 microM) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 microM rauwolscine (when, due to interruption of prejunctional alpha 2-adrenoceptor-mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 mM Mg2+. 3. Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine-sensitive component decreased with increasing current strength. 4. When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5. In contrast to nifedipine, 20 microM ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval.6. In vasa deferentia preincubated with [3H]-noradrenaline, I microM nifedipine and 20 microM ryanodine did not change the electrically evoked overflow of tritium, whereas 10 microM nifedipine increased it.7. It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse(or the first pulse of a pair) in Mg2+-free medium, both Ca2+ mobilization inside the smooth muscle cells and Ca2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca2+ sources occurs: intracellular Ca2+mobilization is decreased during twitch 2, whereas Ca2+ entry is increased.

Animals↗