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K Starke

Publications and source records attributed to K Starke.

At least 109 records · Page 6Linked to original sources
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↗
PubMed1994

Evidence for P2-purinoceptor-mediated inhibition of noradrenaline release in rat brain cortex.

1. Some postganglionic sympathetic axons possess P2Y-like P2-purinoceptors which, when activated, decrease the release of noradrenaline. We examined the question of whether such receptors also occur at the noradrenergic axons in the rat brain cortex. Slices of the brain cortex were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine (1 microM) and stimulated electrically, in most experiments by trains of 4 pulses/100 Hz. 2. The selective adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; 0.03-3 microM) as well as the non-subtype-selective agonist 5'-N-ethylcarboxamido-adenosine (NECA; 0.3-3 microM) reduced the evoked overflow of tritium, whereas the adenosine A2a-receptor agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.003-30 microM) and the adenosine A3-receptor agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA; 0.03-3 microM) caused no change. Of the nucleotides tested, ATP (30-300 microM), adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; 30-300 microM), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S; 30-300 microM), P1,P4-di(adenosine-5')-tetraphosphate (Ap4A; 30-300 microM) and the preferential P2Y-purinoceptor agonist, 2-methylthio-ATP (300 microM) decreased the evoked overflow of tritium. The P2X-purinoceptor agonist, alpha,beta-methylene-ATP (3-300 microM) caused no change. 3. The A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM) attenuated the effects of the nucleosides CPA (apparent pKB value 9.8) and NECA as well as of the nucleotides ATP (apparent pKB 9.3), ATP gamma S (apparent pKB 9.2) and ADP beta S (apparent pKB 8.7). CGS-21680 and APNEA were ineffective also in the presence of DPCPX. The A2-selective antagonist 1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KF-17837) reduced the effects of CPA, NECA and ATP gamma S only when given at a concentration of 300 nM but not at 1O nM.4. The P2-purinoceptor antagonists, suramin (300 micro M), reactive blue 2 (30 micro M) and cibacron blue 3GA(30 micro M) did not change the effect of CPA. Suramin and cibacron blue 3GA shifted the concentration response curve of ATP gamma S to the right (apparent pKB values 3.7 and 5.0, respectively). Reactive blue 2 also attenuated the effect of ATPyS, and cibacron blue 3GA attenuated the effect of ATP, but in these cases the agonist concentration-response curves were not shifted to the right. There was no antagonistic effect of suramin against ATP and ADP beta S.5. The results indicate that rat cerebrocortical noradrenergic axons possess, in addition to the knownadenosine Al-receptor, a separate purinoceptor for nucleotides (P2) which, in contrast to the Al-receptor,is blocked by suramin, reactive blue 2 and cibacron blue 3GA. Nucleotides such as ATP and ATP gamma S activate both receptors. Inconsistencies in antagonist effects against nucleotides are probably due to this activation of two receptors. The presynaptic P2-purinoceptor is P2Y-like, as it is in the peripheral sympathetic nervous system.

Adenosine↗
PubMed1994

Is the sympathoinhibitory effect of rilmenidine mediated by alpha-2 adrenoceptors or imidazoline receptors?

The involvement of alpha-2 adrenoceptors or imidazoline-1 receptors in the sympathoinhibitory effect of rilmenidine was investigated in conscious rabbits. Dose-response curves were determined for the effects of rilmenidine as well as of the alpha-2 selective agonist UK 14304 [5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline] on blood pressure, heart rate, renal postganglionic sympathetic nerve activity and the plasma norepinephrine concentration. The interaction with the alpha-2 selective antagonist SK&F 86466 [6-chloro-N-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine] also was studied. Rilmenidine (30-1000 micrograms kg-1 i.v.) and UK 14304 (1-30 micrograms kg-1 i.v.) elicited the same pattern of changes: transient hypertension followed by prolonged hypotension, bradycardia, a depression of sympathetic nerve firing and a fall in the plasma norepinephrine concentration. The ED50 values for these effects increased in the same order for both drugs: plasma norepinephrine fall < hypotension < depression of sympathetic nerve firing << bradycardia. Pretreatment with SK&F 86466 (3 mg kg-1 i.v. followed by an infusion of 1 mg kg-1 hr-1) altered the dose-response curves of rilmenidine and UK 14304 in a qualitatively and quantitatively almost identical manner; a rightward shift of the hypotensive, sympathoinhibitory and plasma norepinephrine level dose-response curves but no change of the dose-response curve for heart rate. The similarity of the effects of rilmenidine and UK 14304, and the almost identical antagonism of SK&F 86466 against either drug, argue against different sites of action of rilmenidine and UK 14304.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Agonists↗
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

Sympathoinhibition by rilmenidine in conscious rabbits: involvement of alpha 2-adrenoceptors.

Cardiovascular and sympathetic nervous system effects of the mixed alpha 2-adrenoceptor and imidazoline receptor agonist rilmenidine were studied in conscious rabbits chronically instrumented for the recording of the firing rate of renal sympathetic fibers. Separate experiments were carried out on pithed rabbits with electrically stimulated (2 Hz) sympathetic outflow. Drugs were administered intravenously in a cumulative manner. In conscious rabbits, rilmenidine 0.1, 0.3 and 1.0 mg kg-1 dose-dependently lowered blood pressure, renal sympathetic nerve activity, heart rate and the plasma concentration of noradrenaline and adrenaline. The effect on blood pressure and plasma catecholamines was maximal after 0.3 mg kg-1 whereas heart rate and renal sympathetic nerve activity decreased further after rilmenidine 1.0 mg kg-1. Yohimbine 0.1 and 0.5 mg kg-1, when injected subsequently, attenuated and at the higher dose abolished all effects of rilmenidine. The effects of rilmenidine were also antagonized by the alpha 2-adrenoceptor antagonist 2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1H-imid azole HCl (RX821002; 0.1 and 0.5 mg kg-1). Yohimbine 0.1 and 0.5 mg kg-1 did not attenuate or attenuated only slightly the decrease of heart rate and renal sympathetic nerve activity produced by infusion of vasopressin. In pithed rabbits with electrically-stimulated sympathetic outflow, yohimbine 0.1 submaximally and yohimbine 0.5 mg kg-1 maximally increased the plasma noradrenaline concentration. The experiments show by direct measurement of sympathetic nerve firing and plasma catecholamines that rilmenidine causes sympathoinhibition in conscious rabbits, presumably through central sites of action.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-2 Receptor Antagonists↗
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

Presynaptic alpha 2-autoreceptors in brain cortex: alpha 2D in the rat and alpha 2A in the rabbit.

Presynaptic alpha 2-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [3H]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz. The alpha 2-adrenoceptor agonist bromoxidine (UK 14304) reduced the electrically evoked overflow of tritium with EC50 values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)aze pine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant KD. The KD values correlated only weakly between the rat and the rabbit. Dissociation constants KA of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the alpha 2-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The KA value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit. The results confirm the species difference between rat and rabbit brain presynaptic alpha 2-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the alpha 2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the alpha 2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of alpha 2-RG20, the putative rat alpha 2D-adrenoceptor gene (r = 0.97; P < 0.01), but not with their binding affinities for the gene product of alpha 2-C10, the putative human alpha 2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the alpha 2-C10 (r = 0.98; P < 0.001) but not with the alpha 2-RG20 gene product. Since presynaptic alpha 2-autoreceptors are also alpha 2D in rat submaxillary gland and perhaps vas deferens and alpha 2A in rabbit pulmonary artery, the possibility arises that the majority of alpha 2-autoreceptors generally are alpha 2D in the rat and alpha 2A in the rabbit. Moreover, receptors of the alpha 2A/D group generally may be the main mammalian alpha 2-autoreceptors.

Animals↗
PubMed1993

Inhibition of uptake 1 by (+)-oxaprotiline reveals a differential central regulation of noradrenaline and adrenaline release.

Inhibition of uptake 1 in the central nervous system leads to a decrease of sympathetic outflow to many tissues; central alpha 2-adrenoceptors are involved in this decrease. The aim of the present study was to compare the effects of the selective uptake 1 inhibitor (+)-oxaprotiline on the plasma kinetics of noradrenaline and adrenaline in anaesthetized and in conscious rabbits. [3H]Noradrenaline and [3H]adrenaline were infused i.v. The arterial plasma concentrations of endogenous and radiolabelled noradrenaline and adrenaline were measured, and the clearance from and spillover into the plasma of noradrenaline and adrenaline were calculated. Results obtained in conscious and anaesthetized rabbits were similar. (+)-Oxaprotiline 0.2, 0.6 and 1.8 mg kg-1 i.v. dose-dependently reduced the clearance of [3H]noradrenaline from the plasma. The clearance of [3H]adrenaline was reduced less. The spillover of endogenous noradrenaline was decreased by up to 35%. In contrast, the spillover of adrenaline tended to be enhanced. Prazosin 0.1 and 1 mg kg-1 was injected i.v. in a second part of each experiment. It lowered the blood pressure and caused a marked increase in noradrenaline spillover but no increase or even a decrease in adrenaline spillover. The results are compatible with the following hypothesis. The sympathetic outflow from the central nervous system is subject to a twofold alpha-adrenoceptor-mediated modulation: alpha 2-adrenoceptor-mediated inhibition and alpha 1-adrenoceptor-mediated excitation. In the control of the sympathetic outflow to many extra-adrenal tissues, the alpha 2-adrenergic inhibition prevails. Uptake 1 inhibitors depress sympathetic outflow to such tissues by enhancing the alpha 2-adrenergic inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗
PubMed1993

Neural ATP release and its alpha 2-adrenoceptor-mediated modulation in guinea-pig vas deferens.

Contractions, release of previously stored [3H]-noradrenaline (measured as overflow of total tritiated compounds) and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) were studied in the superfused vas deferens of the guinea pig. Prazosin and suramin were used to suppress non-neural ATP release, and effects of bromoxidine and rauwolscine on the neural release thus isolated were examined. Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. Both prazosin (0.03-3 microM) and suramin (30-300 microM) reduced contractions as well as the evoked overflow of ATP. No visible contraction remained in 21 of 28 tissues exposed to prazosin 0.3 microM combined with suramin 300 microM. The evoked overflow of ATP under these conditions was about 17% of that observed in the absence of drugs. In the presence of prazosin 0.3 microM and suramin 300 microM, bromoxidine (0.01-1 microM) decreased and rauwolscine (0.1-10 microM) increased the evoked overflow of both tritium and ATP. Rauwolscine increased the evoked overflow of tritium to a significantly greater extent than the overflow of ATP. It is concluded that the overflow of ATP elicited by electrical (neural) stimulation in the presence of prazosin 0.3 microM and suramin 300 microM reflects purely neural release of ATP. This release of ATP, like the release of noradrenaline, is modulated through prejunctional alpha 2-adrenoceptors. The alpha 2-adrenoceptor modulation of the release of noradrenaline seems to be more marked than the modulation of the release of ATP.

Adenosine Triphosphate↗
PubMed1993

Axon terminal P2-purinoceptors in feedback control of sympathetic transmitter release.

Extracellular ATP acts on P2-purinoceptors of peripheral effector cells, and this is the basis for its function as a (co-)transmitter in peripheral efferent neurons. ATP also acts on P2-receptors of neuronal cell bodies or dendrites, and this is the basis for its function as a fast excitatory transmitter at neuroneural synapses. A third site of action is axon terminals. In the vas deferens of the mouse, noradrenaline and ATP are postganglionic sympathetic co-transmitters, and exogenous ATP acts on P2-purinoceptors of the sympathetic terminals to inhibit release of noradrenaline. Here we show that two P2 antagonists, suramin and Reactive Blue 2, increase the release of noradrenaline in mouse vas deferens. The increase is only obtained when there has been preceding nerve activity and is largely independent of the postjunctional response. These findings indicate a physiological function for axon terminal P2-purinoceptors: they mediate a novel prejunctional negative feedback in which released ATP inhibits subsequent transmitter release.

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↗