Presynaptic dopamine DA2-receptors in rabbit jejunal arteries

Summary Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mol/l, SKF 38393 10 mol/l, dopamine 10 ol/l and clonidine 0.1 mol/l depressed all e j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mol/l, domperidone 1 mol/l, SCH 23390 1 mol/l and rauwolscine 1 mol/l did not change, or even depressed the first e j.ps. Of these compounds only S- and R-sulpiride, 10 mol/l and rauwolscine 1 mol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 ol/l had the largest effect. S- and R-sulpiride, 10 mol/l, as well as domperidone 1 ol/l antagonized the action of LY 171555 1 mol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mol/l and rauwolscine 1 mol/l blunted the effect of SKF 38393 10 mol/l. Rauwolscine 1 mol/l slightly reduced the inhibition by dopamine 10 mol/l; S-sulpiride 10 mol/l was antagonistic only in the presence of rauwolscine 1 mol/l. When rauwolscine 1 mol/l, prazosin 0.1 mol/l, propranolol 1 mol/l and cocaine 10 mol/l was added to the medium, dopamine 10 mol/l continued to produce the same depression of e j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mol/l also counteracted dopamine 10 gmol/l. Rauwolscine 1 mol/l prevented the effect of clonidine 0.1 mol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA₂- and postsynaptic DA₁-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e j.ps. Dopamine interferes with neuroeffector transmission due to ₂-adrenoceptor agonist properties; its DA₂-effect is unmasked only after ₂-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine. Send offprint requests to P. Illes at the above address     

Effect of the phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62711) on gastric secretion and gastric mucosal cyclic AMP

Summary The effect of the phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62711) on gastric secretion and the cyclic AMP system of the gastric mucosa was studied in rats and guinea pigs. In rats, 0.03–0.3 moles/kg ZK 62711 i.v. stimulated acid and pepsin secretion in a dose-dependent manner and 0.03 moles/kg i.v. enhanced the effect of histamine. In guinea pigs no reproducible stimulation was found after intravenous injections of ZK 62711. The stimulation of gastric secretion in the rat by 0.3 moles/kg ZK 62711 i.v. was not affected by vagotomy but was totally inhibited by 10 moles/kg cimetidine i.v. The structurally related phosphodiesterase inhibitor, 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidine (Ro 20-1724), at the dose of 3.3 moles/kg i.v. stimulated gastric secretion in anaesthetised rats to a similar extent as 0.3 moles/kg ZK 62711 i.v. The content of cyclic AMP in the rat gastric mucosa in vivo was slightly increased by 10 moles/kg ZK 62711 i.v, whereas lower doses did not change it. Accumulation of cyclic AMP in the rat gastric mucosa by 50 moles/kg histamine i.v. was enhanced by simultaneous injections of 3.3 moles/kg ZK 62711 i.v. In rat gastric tissue slices in vitro 1–50 M ZK 62711 increased the level of cyclic AMP but 100 M histamine had no effect in the absence or presence of ZK 62711. In gastric mucosal slices of the guinea pig 10 and 50 M ZK 62711 increased the cyclic AMP content which effect was inhibited by 100 M cimetidine and enhanced the stimulatory effect of 100 M histamine on cyclic AMP. The activity of soluble cyclic AMP phosphodiesterase was inhibited by ZK 62711 in the rat (IC₅₀=18 M) and guinea pig gastric mucosa (IC₅₀=1.5 M). Adenylate cyclase was not affected in the homogenate of the guinea pig gastric mucosa. The results indicate that the phosphodiesterase inhibitor ZK 62711 which increases cyclic AMP levels in the gastric mucosa in vivo and in vitro, is a potent stimulator of gastric acid secretion.     

Influence of papaverine derivatives on phosphodiesterase activity,cyclic 3′,5′-AMP levels and relaxing effect on rabbit ileum

Summary The correlations between the relaxing effect of papaverine derivatives, inhibition of low K_m-phosphodiesterase (cAMP-PDE=EC 3.1.4.17) activity and cyclic 3,5-AMP (cAMP) levels in isolated rabbit ileum were investigated. There was a strong correlation between the relaxing effect, inhibition of PDE activity and cAMP content for eupaverine, ethylpapaverine and papaverine. Eupaverine was the most effective relaxing agent (I₅₀=7.5 M) and the most potent inhibitor of PDE activity (K_i=0.6 M), followed by ethylpapaverine (I₅₀=10 M); K_i=0.8 M) and papaverine (I₅₀=20 M; K_i=2 M). In contrast, there was a strong relaxing effect (I₅₀=6 M) but only slight inhibition of PDE activity (K_i=350 M) by tetrahydropapaveroline (THP). The adenylate cyclase stimulating effect of THP which was shown by others is most likely the reason for comparatively higher cAMP levels, which were found to be elevated about seven times over basal levels of 0.35 nmoles/g wet weight, and effective relaxation. Relaxation could be induced by exogenously added cAMP (I₅₀=45 M) and dibutyryl-cAMP (I₅₀=450 M). Our results support the assumption that smooth muscle relaxation in rabbit ileum is mediated by cAMP. Some of these observations have been published in abstract form (Schulz and Berndt, 1972).     

Calcium channel modulation by dihydropyridines modifies sufentanil-induced antinociception in acute and tolerant conditions

Summary The study was aimed at elucidating the possible participation of l-type Ca²⁺ channel in the acute analgesic effect of an opiate and the development of tolerance to this action. Sufentanil, a selective p agonist, and two dihydropyridines, the Ca²⁺ antagonist nimodipine and the Ca²⁺ agonist Bay K 8644, were selected. The tail-flick test was used to assess the nociceptive threshold. In naive rats, nimodipine (200 g/kg) potentiated the analgesic effect of sufentanil reducing the ED₅₀ from 0.26 to 0.08 g/kg. Similar results were observed with its (–)-enantiomer Bay N 5248, while the (+) enantiomer Bay N 5247 was ineffective. Tolerance to the opiate was induced by chronic subcutaneous administration of sufentanil with minipumps (2 g/h, 7 days). In these conditions the dose-response curve to sufentanil was displaced to the right and the ED₅₀ was increased to 1.49 g/kg. In tolerant rats, nimodipine preserved its potentiating ability and prevented the displacement to the right of the sufentanil dose response-curve (ED₅₀ = 0.48 g/kg). When nimodipine was pumped (1 g/h, 7 days) concurrently with sufentanil, the development of tolerance to the opioid was not disturbed. However, the expression of tolerance was abolished and even the effect of acutely administered sufentanil was markedly potentiated (ED₅₀ = 0.03 g/kg). Similar experiments were performed with Bay K 8644. In naive rats, Bay K 8644 at a low dose (20 g/kg) that behaves as a calcium agonist, antagonized the analgesic effect of sufentanil (ED₅₀ = 0.58 g/kg), whereas at a high dose (200 g/kg) it potentiated this action (ED₅₀ = 0.15 g/kg). In tolerant rats, Bay K 8644 (20 g/kg) preserved its antagonizing ability inducing a displacement to the right of the sufentanildose-response curve (ED₅₀ = 4.2 g/kg). When Bay K 8644 was pumped (1 g/h, 7 days) concurrently with sufentanil, it enhanced the expression of tolerance to the opiate (ED₅₀ = 3.8 g/kg). These results suggest that the calcium fluxes through the l-type channel in neurones are functionally linked to the activation of the opiate receptor: the blockade of the channel increased the potency of sufentanil, whereas its activation reduced the potency of the opiate. In chronic experiments, DHPs concurrently administered with sufentanil did not affect the development of tolerance to the opiate. However, nimodipine prevented the expression of this phenomenon. Even more, the animals became hypersensitive to the opiate suggesting that the adaptative mechanisms induced by chronic opiate could be affected by chronic nimodipine.This work was supported by grants from Universidad de Cantabria-Caja Cantabria (1988) and Bayer AG, Wuppertal, FRGPredoctoral Fellow: Fondo de Investigaciones Sanitarias de la Seguridad Social.Send offprint requests to: M. A. Hurlé at the above address     

The involvement of central cholinergic system in the pressor effect of intracerebroventricularly injected U-46619, a thromboxane A2 analog, in conscious normotensive rats

The aim of this study was to determine the involvement of the central cholinergic system in the rise in blood pressure evoked by the thromboxane A2 (TxA2) analog, U-46619, given centrally. Intracerebroventricular (i.c.v.) injections of U-46619 (0.5, 1.0 and 2.0 g) caused dose- and time-related increases in blood pressure and decreased heart rate in awake rats. U-46619 (1 g; i.c.v.) also produced an approximately 65% increase in posterior hypothalamic extracellular acetylcholine and choline levels. Pretreatment with SQ-29548 (8 g; i.c.v.), selective TxA2 receptor antagonist, completely inhibited both the cardiovascular responses and the increase in acetylcholine and choline levels to subsequent injection of U-46619 (1 g; i.c.v.). Atropine (10 g; i.c.v.), nonselective muscarinic receptor antagonist, pretreatment did not affect the cardiovascular responses observed after U-46619 (1 g; i.c.v.). Pretreatment with the nonselective nicotinic receptor antagonist, mecamylamine (50 g; i.c.v.) attenuated the pressor effect of U-46619 (1 g; i.c.v.). Higher doses of mecamylamine (75 and 100 g; i.c.v.) pretreatments did not change the magnitude of the blockade of pressor response to U-46619; however, they abolished the bradycardic effect of U-46619 dose-dependently. Interestingly, pretreatment of rats with methyllycaconitine (10 g; i.c.v.) or -bungarotoxin (10 g; i.c.v.), selective antagonists of 7 subtype of nicotinic acetylcholine receptors (7nAChRs), partially abolished the pressor response to i.c.v. injection of U-46619 (1 g). Similar to the mecamylamine data, the use of higher doses of methyllycaconitine (25 and 50 g; i.c.v.) produced the same magnitude of blockade that was observed after the 10 g methyllycaconitine pretreatment, but it completely abolished the bradycardic effect of U-46619 (1 g; i.c.v.) at the dose of 25 g. The present results show that central administration of U-46619 produces pressor and bradycardic effect and increase in hypothalamic acetylcholine and choline levels by activating central TxA2 receptors. The activation of central nicotinic receptors, predominantly 7nAChRs, partially mediates the cardiovascular responses to i.c.v. injection of U-46619.     

The effect of some antidepressants on prejunctional muscarinic receptors on the sympathetic nerves of the isolated rabbit ear artery

Summary The antimuscarinic activity of amitriptyline, desipramine, iprindole, mianserin and viloxazine on prejunctional sympathetic nerve endings were compared in the isolated rabbit ear artery. In the presence of cocaine (10 M) and yohimbine (1 M), amitriptyline (0.5–1 M), desipramine (1–3 M) and iprindole (5–10 M), desipramine (1–3 M) and iprindole (5–10 M) produced parallel rightward shifts of the concentration-response curve for the inhibitory effect of carbachol (CCh) on responses to electrical stimulation of the preparation at 3 Hz. Mianserin (3 M) produced some inhibition but altered the slope of the concentration-responses curve to CCh while viloxazine (10 M) produced no inhibition.The depression of tritium efflux by CCh from arteries preincubated in ³H-noradrenaline was inhibited significantly (P<0.05) by amitriptyline (0.1 M) and desipramine (1 M) and not by iprindole (17 M), mianserin (3 M) or viloxazine (10 M). Amitriptyline was 10-fold more active than desipramine and at least 30-fold more active than the other antidepressants as a muscarine receptor blocking drug in this preparation.Thus, mianserin, viloxazine and iprindole exhibit much weaker antimuscarinic activity relative to amitriptyline on prejunctional muscarine receptors on sympathetic nerve endings compared with that observed by others for excitatory muscarine receptors in sympathetic ganglia. The findings support an earlier suggestion that these receptors differ.     

P2-purinoceptor-mediated autoinhibition of sympathetic transmitter release in mouse and rat vas deferens

Effects of drugs acting at P₂-purinoceptors on the release of newly taken up [³H]-noradrenaline were studied in slices of mouse and rat vas deferens. The slices were superfused and stimulated electrically, in most experiments by trains of 60 pulses/8 Hz.In mouse vas deferens, the P₂-purinoceptor antagonists reactive blue 2 (1.8–100 M) and brilliant blue G (10–300 M) increased the stimulation-evoked overflow of tritium in a concentration-dependent manner as shown previously for suramin. Reactive blue 2, which preferentially blocks the P_2Y-subtype, was the most potent compound and the compound with highest maximal effect, an increase by 104%. Pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS), in contrast, caused a small increase only at a single concentration (30 M). The effects of reactive blue 2, brilliant blue G and suramin were not additive. The P₂ agonist adenosine 5-O-(3-thio)-triphosphate (ATPS) reduced the evoked overflow of tritium. As shown previously for suramin, reactive blue 2 30 M and brilliant blue G 100 M antagonized the effect of ATPS. From the shift of the ATPS concentration-response curve to the right, an apparent pK_B value of 5.3 was estimated for reactive blue 2 and an apparent pKB of 4.5 for brilliant blue G. In rat vas deferens, reactive blue 2 (3–30 M), brilliant blue G (10 M) and suramin (30–300 M) also increased the evoked overflow of tritium. As in the mouse, reactive blue 2 was the most potent compound and the compound with highest maximal effect, an increase by 9001o. As previously demonstrated in the mouse, suramin (300 M) increased the evoked overflow of tritium only when rat vas deferens slices were stimulated by trains of 60 pulses at 1 or 8 Hz, but not when they were stimulated by trains of 6 pulses/100 Hz.The results confirm the operation of a P₂-purinoceptor-mediated prejunctional negative feedback controlling the release of noradrenaline in mouse vas deferens and demonstrate the same mechanism in rat vas deferens. The prejunctional P₂-purinoceptors are P_2Y-like in both species. They are a novel kind of autoreceptors, operating in parallel to prejunctional ₂-autoreceptors. Correspondence to: I. von Kügelgen at the above address     

Chlorimipramine,electroconvulsive shock and combination thereof: Differential effects of chronic treatment on apomorphine-induced behaviours and on striatal and mesocortical dopamine turnover

Summary We studied the influence of different pretreatment regimens (Chlorimipramine-Cmi, electroconvulsive shock-ECS, and Cmi+ECS all regimens being applied for either 2 or 15 days) on the open field behaviour, on the striatal and on the prefrontal dopamine-PFC DA turnover in rats injected with either apomorphine-AP 25 g/kg (stimulating presynaptic DA receptors), AP 200 g/kg (stimulating postsynaptic DA receptors), or vehicle (control).In the controls, AP 25 g/kg reduced the locomotor activity and the striatal, but not the PFC DA turnover. AP 200 g/kg increased the locomotor activity and reduced the striatal but not the PFC DA turnover.Short-term pretreatment: ECS and Cmi+ECS prevented the decrease of striatal DA turnover after AP 25 g/kg. No other influence of any pretreatment on behaviour or DA-turnover became significant.Long-term pretreatment: Chronic Cmi: marginally increased the open field behaviour and marginally decreased the PFC DA turnover; significantly increased the effect of AP (200 g/kg) on striatal DA turnover and the effect of AP (25 and 200 g/kg) on PFC DA turnover. Repeated ECS: decreased locomotion and rearing and increased PFC DA turnover; increased the effect of AP (200 g/kg) on locomotion and on striatal DA turnover; decreased the effect of AP (25 and 200 g/kg) on PFC DA turnover.Chronic Cmi+ECS: decreased locomotion and rearing and marginally decreased PFC DA turnover; increased the effect of AP on hyperlocomotion and on striatal DA turnover. No other influence of any chronic pretreatment on behaviour or on DA turnover became significant.The data support the view that chronic AD therapies increase DAergic functions related to postsynaptic rather than to presynaptic structures. It is suggested that the different effects of chronic Cmi and repeated ECS on AP-evoked PFC DA turnover help to understand the different influences exerted by both treatments on rats' behaviour.     

The influence of taurocholate and dehydrocholate choleresis on plasma disappearance and biliary excretion of indocyanine green in the rat

Summary The pharmacokinetics of indocyanine green (ICG; 3.9 moles/kg and 12.9 moles/kg) were investigated in rats given infusion of either saline, taurocholate (106 moles/h) or dehydrocholate (106 or 268 moles/h). During the infusion of saline and taurocholate the plasma concentration of ICG decreased in a mono-exponential manner. However, with dehydrocholate the clearance of ICG from plasma showed two phases with different half lives. The half life of the rapid component (2.2 min) was about the same as the one found in the control experiments.After injection of 12.9 moles/kg ICG the biliary excretion of the dye increased by 138% during taurocholate administration, while an equimolar dehydrocholate infusion resulted in a mean increament of 55%. Under these circumstances the bile flow was stimulated by 195% and 297% resp.With the lower dose of ICG (3.9 moles/kg) however, there was no stimulation of the biliary ICG excretion with taurocholate. At this dose level an infusion of dehydrocholate (106 mol/h) enchanced the biliary output of ICG by approximately 54%, while administration of 268 mol/h resulted in a slight but significant decrease of 31%.These observations can be explained by assuming interaction of the bile acids with the hepatic transport of ICG at different sites. The appearance of the second component of the plasma curve during dehydrocholate infusion is possibly related to a diminished hepatic storage capacity for ICG and is not due to an effect on the primary hepatic uptake or biliary output of the dye.     

Pharmacokinetic Drug–Drug Interaction of the Novel Anticancer Agent E7070 and Acenocoumarol

E7070 is a novel sulfonamide anticancer agent that arrests cancer cells at the G1/S boundary of the cell cycle. Three patients receiving chronic therapy with the oral anticoagulant acenocoumarol experienced bleeding and/or a prolonged prothrombin time after treatment with E7070 at a dose of 700mg/m² given as a 1-h infusion. In vitro studies have shown that E7070 has the potential to inhibit several cytochrome P450 (CYP)-enzymes, including CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. The major enzyme involved in the metabolism of acenocoumarol in man is CYP2C9. This study was performed to investigate the interaction between E7070 and acenocoumarol.Blood samples were obtained from two patients receiving daily oral maintenance treatment with acenocoumarol both prior to and following treatment with E7070. In addition, we incubated acenocoumarol enantiomers with pooled human microsomes with and without E7070 and measured the in vitro plasma protein binding of acenocoumarol after incubation with E7070. Pharmacokinetic parameters of acenocoumarol were calculated by noncompartmental analysis and revealed that in both patients the area under the concentration–time curve up to 24h after the acenocoumarol administration was higher following E7070 (2.56 and 1.58h*mol/L) compared to the systemic exposure in the absence of E7070 (1.87 and 1.23h*mol/l). The formation of acenocoumarol metabolites was retarded by E7070 at already low concentrations (2.1M). The plasma protein binding of acenocoumarol was reduced at higher concentrations of E7070 (259M).These results indicate that E7070 may primarily interact with acenocoumarol by reducing its systemic clearance. Displacement of acenocoumarol's plasma protein binding by E7070 may also occur but to a minor extent. In the absence of careful monitoring this drug–drug interaction may result in hypoprothrombinemia and a hemorrhagic tendency.     

Effect of nicotine and tacrine on acetylcholine release from rat cerebral cortical slices

Summary The effect of nicotine (1–10 M) and tacrine (9-amino-1,2,3,4-tetrahydroacridine; THA) on stimulation evoked release of [³H]acetylcholine from the rat brain slice preparation preincubated with [³H]choline was investigated.In these preparations, nicotine enhanced while tacrine inhibited evoked [³H]acetylcholine release. These effects were blocked by (+)tubocurarine (1 M) and atropine (0.1 M) respectively. In the presence of idazoxan (0.3 M) plus atropine (0.1 M), nicotine (3 M) continued to enhance evoked [³H]acetylcholine release while the inhibitory effect of tacrine (1 M) on evoked [³H]acetylcholine release was reversed to an enhancement. Under these circumstances the effects of both nicotine and tacrine were blocked by (+)tubocurarine (1 M).These findings demonstrate that tacrine can both inhibit or enhance [³H]acetylcholine release, most likely through its activity as a cholinesterase inhibitor. Under normal circumstances following tacrine the predominant effect of the elevated levels of acetylcholine will be activation of inhibitory presynaptic muscarine receptors on cholinergic nerves and an inhibition of evoked [³H]acetylcholine release. Under conditions where both presynaptic inhibitory muscarine and ₂-adrenoceptors are blocked, the elevated levels of acetylcholine produced by tacrine will lead to the activation of facilitatory presynaptic nicotine cholinoceptors on cholinergic nerves and an enhancement of evoked [³H]acetylcholine release. Send offprint requests to R. Loiacono at the above address     

Positive coupling of cholinergic muscarinic receptors to adenylate cyclase activity in membranes of rat olfactory bulb

Summary In membranes of rat olfactory bulb acetylcholine stimulated adenylate cyclase activity in a concentration-dependent manner. The maximal stimulation corresponded to 53% increase of basal enzyme activity and was obtained with 100 M acetylcholine. The concentration of the cholinergic agonist eliciting a half-maximal effect was 0.4 M. The stimulatory effect of acetylcholine was antagonized by 0.1 M atropine but not by 10 M (+)-tubocurarine. Moreover, the addition of micromolar concentrations of GTP was absolutely required for the enzyme stimulation by acetylcholine. The results demonstrate the presence in rat olfactory bulb of muscarinic receptors coupled to stimulation of adenylate cyclase probably via a GTP regulatory protein and provide evidence for a novel signal transduction mechanism of central muscarinic receptors. Send offprint requests to P. Onali at the above address     

Evans blue blocks P2X-purinoceptors in rat vas deferens

Summary In rat vas deferens, Evans blue 100 M increased contractions elicited by high K⁺ and by noradrenaline but markedly reduced contractions elicited by the P_2X-purinoceptor-selective agonist ,-methylene ATP (3 M). The concentration-response curve of ,-methylene ATP was shifted to the right by Evans blue 30 M and the maximal contraction was increased. In tissues incubated with nifedipine 10 M, Evans blue 100 M tended to increase the residual contraction elicited by noradrenaline and abolished the residual response to ,-methylene ATP (3 M). The concentration-response curve of ,-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 M. From the shifts to the right caused by Evans blue 30 M, apparent pK_B values of 5.9 (no nifedipine) and 6.0 (nifedipine present) were calculated. It is concluded that Evans blue blocks P_2X-purinoceptors in rat vas deferens and in addition causes a non-receptor-specific enhancement of contractions.Correspondence to: R. Bültmann at the above address     

Two distinct adrenoceptor-biophases in the vasculature: One for α-and the other for β-agonists

Summary Strips of two canine vessels with different patterns of sympathetic innervation were used: the mesenteric artery which has an adventitio-medial plexus and the saphenous vein in which nerve terminals are distributed throughout the media.The pD₂ of (–)-adrenaline for -and -adrenoceptors was determined for both vessels (in the presence of 0.5 M propranolol and 7 M phentolamine, respectively; in the latter case the strips were contracted by 0.28 M prostaglandin F2) and found to be very similar: 6.96 and 7.01 in the saphenous vein and 6.77 and 6.91 in the mesenteric artery, respectively. The similarity of pD₂ for adrenaline acting on -and -adrenoceptors in both preparations allowed us to compare the effect of inhibition of neuronal uptake by 12 M cocaine with that of inhibition of COMT by 50 M dihydroxy-2-methyl propiophenone (U-0521) on: a) the potency of adrenaline (noradrenaline was used in the saphenous vein only) acting on -and -adrenoceptors and b) the time required by the strips to recover 50% in oil (t ₅₀) after contractions or relaxations caused by 0.23 M adrenaline. In the saphenous vein cocaine increased the potency of both adrenaline and noradrenaline for -effects more than for -effects (2.8 times vs. no increase for adrenaline; 7.1 vs. 1.9 times for noradrenaline) and U-0521 increased the potency of both adrenaline and noradrenaline for -effects more than for -effects (4.1 vs. 2.6 times for adrenaline; 1.8 times vs. no increase for noradrenaline); regarding the termination of action of adrenaline, cocaine prolonged the t ₅₀ 1.6 times after contraction and did not change it after relaxation, whereas U-0521 prolonged the t ₅₀ 6.9 times after relaxation and only 1.8 times after contraction.In the mesenteric artery only sensitivity experiments were done. Cocaine increased the potency of adrenaline by a factor of 2.1 for the -effects while there was no influence on its potency with regard to its -effects, and U-0521 increased the potency of adrenaline for the -effects more than for -effects (3.9 vs. 1.8 times, respectively).These results show that there are two different biophases for sympathomimetic agonists in the vasculature: one for -adrenoceptors which is more under the influence of the neuronal uptake and one for -adrenoceptors which is more under the influence of COMT activity. We conclude that -adrenoceptors are situated close to the nerve endings and -adrenoceptors close to COMT sites. Since these results do not differ qualitatively in two vessels with different patterns of innervation, we conclude that this asymmetry in the distribution of -and -adrenoceptors may be due to either an uneven distribution of cells with only one type of receptors each or due to an uneven distribution of receptors on the same cell.     

ATP and endogenous agonists inhibit evoked [3H]-noradrenaline release in rat iris via A1 and P2y-like purinoceptors

Summary Effects of ATP, adenosine and purinoceptor antagonists on field stimulation-evoked (3 Hz, 2 min) [³H]-noradrenaline overflow were investigated in the rat isolated iris.ATP and adenosine inhibited the evoked overflow of [³H]-noradrenaline. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) shifted the concentration-response curve of ATP to the right in a concentration-dependent manner, but with a potency (–log K_B = 7.88) much lower than expected for an A1 adenosine receptor. In the continuous presence of DPCPX, the ATP-induced prejunctional inhibition was unaffected by suramin (100 mol/l) and DIDS (4,4-diisothiocyanatostilbene-2,2-disulfonic acid, 50 mol/l) but was antagonized by the P_2Y-receptor antagonist cibacron blue ( = reactive blue 2;30 and 100 mol/l, –log K_B = 4.7)and ,-methylene-ATP (10 mol/l). Whereas the evoked [³H]-noradrenaline overflow was unaffected by suramin and DIDS, cibacron blue and ,-methylene-ATP caused a small and transient increase. Cibacron blue at 30 mol/l failed to antagonize the inhibition of evoked [³H]-noradrenaline overflow that adenosine produced in the absence of DPCPX. Basal [³H]-noradrenaline overflow was enhanced by cibacron blue, not changed by ,-methylene-ATP and DIDS, and decreased by suramin.The results show that exogenous ATP inhibits sympathetic neurotransmission in the rat iris via A₁ and P_2Y-like purinoceptors. The latter have a low apparent affinity for cibacron blue and probably are blocked by ,-methylene-ATP. Under the present conditions, endogenous purines exert a tonic inhibition not only via A₁- but also via these P_2Y-receptors. Correspondence to: H. Fuder at the above address     

Der Einfluß des Prednison- und Prednisolonbisguanylhydrazons auf die Na+ + K+-stimulierte Membran-ATPase des Meerschweinchenherzens

Zusammenfassung Prednison- und Prednisolonbisguanylhydrazon hemmen ebenso wie k-Strophanthin eine aus Meerschweinchenherzen gewonnene durch Na⁺ + K⁺ stimulierte Membran-ATPase. Eine 50%ige Hemmung erfolgt bei Konzentrationen von 3,8 M Prednisonbisguanylhydrazon, 0,28 M Prednisolonbisguanylhydrazon bzw. 1,3 M k-Strophanthin. Dieses Wirksamkeitsverhältnis der drei Verbindungen entspricht etwa der Hemmung des aktiven Ionentransportes an Meerschweinchenerythrocyten und dem positiv inotropen Effekt am isoliert durchströmten Meerschweinchenherzen.

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Summary Prednison- and Prednisolonbisguanylhydrazon inhibit like k-Strophanthin the Na⁺ + K⁺-stimulated ATPase from guinea-pig hearts. 50% inhibition was stated with concentrations of 3,8 M Prednisonbisguanylhydrazon, 0,28 M Prednisolonbisguanylhydrazon or 1,3 M k-Strophanthin. This difference in effectiveness of the compounds corresponds to the inhibition of the active ion-transport in erythrocytes of guinea-pigs and to the positive inotropic effect in isolated perfused guinea-pig hearts.

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Mit 1 TextabbildungDie Ergebnisse wurden auf der 5. Frühjahrstagung der Deutschen Pharmakologischen Gesellschaft am 28. April 1964 in Mainz vorgetragen. [Naunyn-Schmiedebergs Arch. exp. Path. Pharmak. 247, 341 (1964).]     

Dopamine D-2 receptors inhibit D-1 stimulated cyclic AMP accumulation in striatum but not limbic forebrain

Summary The effect of dopamine D-2 receptor activtion on dopamine D-1 stimulated cyclic AMP accumulation was investigated in slices of rat striatum and limbic forebrain (nucleus accumbens and tuberculum olfactorium). In striatal slices the dose-dependent increase in cyclic AMP accumulation due to dopamine (3–100 mol/1) was enhanced by selective D-2 receptor blockade using (–)-sulpiride (30 mol/1). In limbic slices the increase in cyclic AMP due to dopamine (3–50 mol/l) was unaffected by selective D-2 receptor blockade. The enhancement of cyclic AMP accumulation due to the selective D-1 agonist SKF 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; 1 gmol/1) in striatal slices was attenuated in the presence of the selective D-2 receptor agonist LY 171555 (quinpirole hydrochloride; 10 mol/l). This attenuation was in turn blocked by (–)-sulpiride (10 mol/1). In limbic slices LY 171555 (10 mol/l) had no effect on SKF 38393 (1 mol/l) stimulated cyclic AMP accumulation. Conversely muscarine receptor activation, using carbachol (10 mol/l), attenuated D-1 stimulated cyclic AMP accumulation in both striatum and limbic forebrain. Dopamine D-2 or muscarine receptor stimulation in either striatal or limbic slices did not attenuate cyclic AMP accumulation due to VIP (vasoactive intestinal polypeptide; 0.5 mol/l), isoprenaline (10 mol/l) or 2-chloroadenosine (100 mol/l). This suggests that in striatal slices, D-2 receptors mediate a selective inhibition of D-1 stimulated cyclic AMP accumulation, but that in the limbic forebrain D-2 receptors are unlikely to be coupled to D-1 receptor-linked adenylate cyclase. These data indicate a fundamental difference in the properties of D-2 receptor-effector coupling in these brain regions. Send offprint requests to S. R. Nahorski at the above address     

Evidence for a vasoconstriction-mediating receptor for UTP,distinct from the P2 purinoceptor,in rabbit ear artery

Summary 1. The mechanism of uridine 5-triphosphate-(UTP-)induced vasoconstriction was studied in the rabbit ear artery. The arteries were incubated and perfused at a constant rate of flow. Vasoconstriction was measured as an increase in perfusion pressure. 2. Noradrenaline, adenosine 5'-triphosphate (ATP) and UTP caused concentration-dependent vasoconstriction. ATP and UTP were approximately equipotent. 3. The vasoconstrictor effect of UTP 300 mol/l was enhanced by a mixture of atropine, diphenhydramine and methysergide (1 mol/l each) and not affected by indometacin 10 mol/l. 4. Prazosin (0.01 –1 mol/l) and phentolamine (1–10 mol/l) reduced the vasoconstrictor effect of UTP 300 mol/l by up to 34%. Prazosin 1 mol/l failed to diminish the vasoconstrictor effect of UTP 300 mol/l after the sympathetic nerves had been destroyed with 6-hydroxydopamine. 5. , -Methylene-ATP (10–50 ol/l) elicited transient vasoconstriction. Subsequently, vasoconstrictor responses to ATP 100 or 300 pmol/1 were reduced by 88%, whereas responses to UTP 100 gmol/1 were enhanced, responses to UTP 300 mol/l decreased by only 32% and responses to UTP 1000 gmol/1 reduced by 74%. After in vitro-denervation with 6-hydroxydopamine or in the presence of phentolamine 1 mol/l throughout, a, -methylene-ATP (10–50 mol/l) reduced the vasoconstrictor effect of UTP 300 mol/l by 44% and 43%, respectively. 6. We suggest that, in the rabbit ear artery, the non-adrenergic and , -methylene-ATP-resistant vasoconstrictor response to UTP is mediated by a separate receptor mechanism, distinct from the P₂ purinoceptor. Send offprint requests to K. Starke     

Effect of ω-conotoxin on cholinergic and tachykininergic excitatory neurotransmission to the circular muscle of the guinea-pig colon

The aim of this study was to compare the stimulus-response characteristics of the cholinergic and tachykininergic excitatory transmission to the circular muscle of the guinea-pig proximal colon and their susceptibility to inhibition by the N-type calcium channel blocker -conotoxin (CTX). All experiments were performed in the presence of guanethidine (3 M), indomethacin (10 M), L-nitroarginine (L-NOARG, 30 M) and apamin (0.1 M). In the presence of the tachykinin receptor antagonists, FK 888 (10 M) and GR 94800 (3 M), to block NK₁ and NK₂ receptors, respectively, electrical field stimulation (EFS) produced frequency-dependent atropine- (1 M) sensitive contractions. In the presence of atropine (1 M), EFS produced tachykininergic contractions which were abolished by the combined administration of FK 888 (10 M) and GR 94 800 (3 M). The maximal responses produced by cholinergic and tachykininergic neurotransmission ranged between 80 and 100% of the maximal contractile response to 80 mM KCI. The frequency of stimulation, pulse width and voltage required to produce 50% of the maximal cholinergic and tachykininergic contraction were not different from each other, although cholinergic transmission appeared more efficient in producing twitch contractions in response to single pulse EFS. Furthermore, cholinergic transmission was more efficient than tachykininergic transmission in producing contraction in response to short periods of EFS.CTX (0.1 M for 30 min) produced a large and comparable rightward shift of the cholinergic and tachykininergic frequency-response curve (19 and 17 fold increase in the frequency of stimulation producing 50% of the maximal response, respectively) and markedly depressed (51 and 43% inhibition, respectively) the maximal concentrations response. CTX failed to affect the contraction of the colon produced by submaximally effective concentrations of the muscarinic receptor agonist, methacholine (0.1–0.3 M) and those produced by the tachykinin NK₁ and NK₂ receptor selective agonists [Sar⁹] substance P sulfone and [\Ala⁸] neurokinin A (4–10) (1–3 nM).The present findings demonstrate that the cholinergic and tachykininergic components of the excitatory transmission to the circular muscle of the guinea-pig colon are activated at comparable intensities of nerve stimulation and are both inhibited, in a qualitatively and quantitatively comparable manner, by CTX at the prejunctional level. These findings are consistent with the idea that acetylcholine and tachykinins are co-released from the same population of enteric motoneurones which innervate the circular muscle of the colon. Correspondence to: C. A. Maggi at the above address     

Role of dopamine receptors in the modulation of acetylcholine release in the rabbit hippocampus

Summary Modulation of acetylcholine release was studied in slices of the rabbit hippocampus preincubated with ³H-choline and then continuously superfused with a medium containing 10 mol/l hemicholinium-3. Electrical field stimulation of the superfused slices elicited an increase in tritium outflow, which was tetrodotoxin-sensitive and largely calcium-dependent. Stimulus-evoked acetylcholine release in the rabbit hippocampal slices was modulated by presynaptic muscarinic autoreceptors, as has been shown previously for the rat hippocampus. Drugs with affinity for - and or -adrenoceptors did not affect the evoked overflow of tritium from rabbit hippocampal slices. In contrast, the dopamine receptor agonist apomorphine (0.1 or 1 mol/l) and exogenous dopamine (1 or 10 mol/l) significantly reduced the evoked outflow by about 10 or 20%, respectively. This effect was antagonized by haloperidol (0.01 mol/l) but not by phentolamine (1 mol/l). Attempts to enhance (using nomifensine 10 mol/l) or reduce (using haloperidol, up to 1 mol/l; or bretylium, 1 mmol/l for 5 min) endogenous dopaminergic transmission in the hippocampal slices did not affect stimulation evoked acetylcholine release. In conclusion, presynaptic dopamine receptors modulating acetylcholine release are present in the rabbit hippocampus, but they seem not to be of physiological significance.