Direct and indirect actions of dopamine on tracheal smooth muscle

Summary The effects of dopamine (DA) were studied on guinea-pig isolated tracheal chains. At a low concentration (10^–6M) DA occasionally produced a small contraction; this was followed by a dose-dependent relaxation 3×10^–6–3×10^–3 M).On a molar basis, DA was about 40 times less potent than noradrenaline (NA) in relaxing tracheal chains, and about 2700 times less potent than isoprenaline (ISO). The maximum degree of relaxation obtained with each drug was the same.Pretreatment of the guinea-pig with reserpine (5 mg/kg) resulted in a 3-fold shift of the DA curve to the right without concomitantly affecting the ISO doseresponse curve. Reserpine completely abolished the relaxant effects of tyramine, but a small contractile response remained.Desipramine (DMI), at a concentration of 10^–5 M, caused a 4-fold shift of the DA curve to th right. The same concentration of DMI resulted in a shift to the left of the NA dose-response curve by about 8-fold. Benztropine (10^–5 M) and haloperidol (10^–5 M and 3×10^–5 M) did not affect the DA dose-response curve.The DA-induced relaxation was inhibited by propranolol (10^–8–10^–6 M) in a dose-dependent manner. The higher concentrations of propranolol (10^–7 and 10^–6 M) unmasked the contractile effect of DA. In the presence of propranolol, phentolamine (10^–5 M) abolished the contractile effect of DA.It is concluded that DA has both direct and indirect actions on guinea-pig isolated tracheal smooth muscle. The relaxant effects of DA are predominantly due to a direct action on smooth muscle -adrenoceptors, with a component due to release of NA from adrenergic nerves. The contractile effects, which under normal conditions are masked by the relaxant effect of DA, are mediated by functional -adrenoceptors. There is no evidence for either specific dopaminergic nerves, uptake mechanisms or receptors in guinea-pig trachealis muscle.     

A postsynaptic effect of tyramine in ventricular muscle

Summary Tachyphylaxis and susceptibility to pretreatment with reserpine demonstrate that tyramine increases contractility of the guinea-pig papillary muscle in concentrations up to 10^–4 M by means of an indirect sympathomimetic effect.In higher concentrations (up to 3×10^–3 M) tyramine causes an increase in force of contraction which is characterized by a slowing of relaxation of the isometric contraction curve. Pretreatment with reserpine reveals that this positive inotropic effect is composed of an indirect sympathomimetic and a direct post-synaptic effect; the latter is not subject to tachyphylaxis and produces by itself (at 3×10^–3 M) an increase in force of contraction which amounts to about 50% of the maximum of the indirectly produced inotropic effect.The postsynaptically-induced positive inotropic effect of tyramine is not antagonized by propranolol (5×10^–6 M). It is unlikely, therefore, that it is brought about by stimulation of adrenergic -receptors.The height of the isometric contraction curve in reserpine-pretreated muscles is increased by tyramine as a result of an increase in the rate of force development despite a marked concentration-dependent increase in relaxation time (t ₂).The direct inotropic effect of tyramine is correlated with a prolongation of the duration of the action potential (AP) which amounts to 70% at 3×10^–3 M. In addition to its influence on the duration of the AP, tyramine slows the rate of depolarization and decreases the membrane resting potential.     

Dopamine-induced inhibition of synaptic transmission in lumbar paravertebral ganglia of the dog

Summary Dopamine, injected into the lumbar aorta in doses which produce a neurogenic vasodilatation in the isolated perfused hindleg of the dog (0.5–64×10^–8 moles), provokes a reversible inhibitory of the synaptic transmission in the paravertebral lumbar ganglia. This inhibitory effect is mimicked by epinine (1–64×10^–8 moles) and apomorphine (0.6–38.4×10^–8 moles) and is preferentially blocked by haloperidol (0.26×10^–6 moles), pimozide (2.2×10^–6 moles) and aceperone (2.5×10^–6 moles). (-)-Noradrenaline (0.6–19.2×10^–8 moles) is equipotent with dopamine in inhibiting ganglionic transmission; this noradrenaline inhibitory effect is preferentially blocked by phentolamine (8×10^–6 moles).The possibility that the dopamine-induced ganglionic inhibition is mediated via specific dopamine receptors is discussed.This work was aided by grants from the Coordination Committee for Scientific Research (C.C.A.S.) and from the Fund for Interdisciplinary Research, Belgium.     

Die indirekt sympathicomimetische Wirkung von Heptaminol auf den Herzmuskel

Summary On the isolated guinea-pig papillary muscle heptaminol causes half maximal inotropic effect in a concentration of 4×10^–4 M. A variation of the extracellular potassium concentration between 2.4 and 9.6 mM as well as a reduction of the extracellular sodium concentration from 140 mM to 70 mM have no influence on the action of heptaminol; this would have been characteristic for a digitalis-like action. The positive inotropic effect is suppressed by 5×10^–4 M propranolol and by pretreatment of the animal with 5 mg/kg reserpine. The action of heptaminol, therefore, is clearly that of an indirectly acting sympathomimetic amine.Tachyphylaxis can be demonstrated only with concentrations higher than the ED₅₀ (4×10^–4 M). A concentration of 5.5×10^–4 M has no inotropic effect after the muscle has been treated with 10^–2 M heptaminol.The inotropic effect of norepinephrine is increased twentyfold by 5.5×10^–4 M heptaminol after pretreatment with 3.5×10^–2 M heptaminol.Heptaminol does not influence the dose-response curves of dihydro-ouabain and of calcium.

Ich danke Frl. Monika Klein für ihre kompetente Mitarbeit.     

Modulatory role of catecholamines on tyrosine hydroxylase induction

Summary The possible modulatory role of cytoplasmic catecholamines on tyrosine hydroxylase induction was studied. Rat superior cervical ganglia were kept in organ culture and after 48 h tyrosine hydroxylase activity was determined. Exposure to 10^–4 M carbachol during 4 h almost doubled the control activity. Incubation with 10^–5 M noradrenaline or 10^–5 M dopamine impaired the carbachol-mediated induction of the enzyme. This effect was not blocked by 10^–7 M propranolol, 2.4×10^–6 M haloperidol or 3.1×10^–6 M phentolamine. Inhibition of monoamine oxidase activity by 5.1×10^–4 M pargyline inhibited the effect of carbachol. When the pool of endogenous catecholamines was decreased by -methyl-p-tyrosine, carbachol induced tyrosine hydroxylase to the same extent as in non-depleted ganglia. It is suggested that the long-term regulation of tyrosine hydroxylase is modulated by a strategic cytoplasmic pool of catecholamines.     

Field stimulation-induced responses of circular smooth muscle from guinea-pig stomach

Summary Field stimulation of circular smooth muscle of guinea-pig stomach from the regions of the cardia and fundus caused contraction responses at low stimulation frequencies (0.25–1 Hz) with relaxation at higher frequencies (1–10 Hz), whilst tissues of the body and antrum responded with contraction throughout the frequency range. Atropine (10^–9–10^–8 M) antagonised the contraction responses of all tissues, with relaxation developing at higher concentrations (except for antral tissue). In contrast, metoclopramide (10^–8–10^–6 M) caused modest (cardia, fundus) or marked (body, antrum) enhancement of contractions to field stimulation, whilst domperidone (10^–8–10^–7 M), haloperidol (10^–8–10^–6 M), prazosin, propranolol and methysergide (10^–8–10^–6 M) failed to modify the contraction responses. However, whilst yohimbine and guanethidine failed to modify the contractions of the cardia, fundus and body tissues, those of the antral preparations were antagonised by nanomolar concentrations of yohimbine and by guanethidine (10^–6–5×10^–5 M). To optimise the relaxation responses for study, atropine was included in the physiological solution. Relaxation to field stimulation of preparations from the body and cardia, but not the fundus, was antagonised by reserpine pretreatment (5 mg/kg i.p., 24h), addition of guanethidine (10^–5–10^–4 M), phentolamine, prazosin or propranolol (10^–7–10^–6 M) (the effects of prazosin and propranolol being additive). Higher concentrations of haloperidol and domperidone antagonised the relaxation responses of the body preparations only. Metoclopramide, yohimbine and methysergide (10^–8–10^–6 M) were ineffective. Thus, it is concluded that the contractile effects of the 4 stomach areas to field stimulation reflects a major cholinergic involvement, with an additional ₂-adrenoceptor contractile component in antral tissue. Relaxation responses of cardia and body tissue involve ₂- and -adrenoceptors plus a further, unidentified, non-adrenergic component; the latter represents the total relaxation response of the fundic preparation.     

Relaxant effect of dopamine on the isolated rat uterus

Summary The effect of dopamine was studied on the isolated uterus of diethylstilboestrol-treated rats. Dopamine, at concentrations (10⁷–10^–4 M) produced a concentration-dependent relaxation in the K⁺-depolarized rat uterus. On a molar basis, dopamine was about 500 times less potent than adrenaline in relaxing the uterus, the maximum degree of relaxation obtained with both drugs was the same. Pretreatment of the rats with reserpine (5 mg/kg) did not produce any modification of the dose-response curve to dopamine. Similarly, cocaine (3 × 10^–6 M) failed to modify the relaxant effect of dopamine. The dopamine induced relaxation was inhibited by propranolol (10^–9–10^–7 M) in a dose-dependent manner. Prazosin (10^–7 M), SCH 23390 (10^–7 M) and sulpiride (10^–7 M) did not affect the dopamine dose-response curve. In the isolated rat uterus which was not preconstricted by KCl neither dopamine nor adrenaline produced any effect when added to the organ bath. This lack of response to both catecholamines was present even in tissues pretreated with propranolol or sulpiride. It is concluded that dopamine produced a concentration-dependent relaxation of the uterus from diethylstilboestrol-treated rats by direct activation of beta-adrenoceptors. There was no evidence for indirect action (catecholamine release and neuronal uptake mechanisms) and specific dopamine receptor mediated relaxation and alpha-adrenoceptor mediated contractions have not been found in this preparation. Send offprint requests to F. J. Morales-Olivas at the above address     

Cardiac electrophysiology of four neuroleptics: Melperone,haloperidol, thioridazine and chlorpromazine

Summary Effects of dopamine receptor agonists and antagonists on the release of dopamine were studied in the caudate nucleus of the rabbit. The nucleus contained 6.7 g/g of dopamine, but negligible levels of noradrenaline and dopamine--hydroxylase. No formation of ³H-noradrenaline was detected in caudate slices preincubated with ³H-dopamine, and more than 95% of the tritium content of the tissue consisted of ³H-dopamine.When caudate slices were preincubated with ³H-dopamine and then superfused with amine-free medium, there was a basal outflow of tritium that was not or only slightly changed by tetrodotoxin (10^–7 and 10^–6 M), apomorphine (up to 10^–5 M), bromocriptine (up to 10^–6 M), chlorpromazine (up to 10^–6 M), haloperidol (up to 10^–7 M), or omission of calcium. Electrical stimulation (3 Hz, 24 mA, 2 ms pulse duration, 2-min periods) greatly increased the outflow of tritium. The stimulation-evoked overflow was abolished by tetrodotoxin (10^–7 and 10^–6 M) and in calcium-free medium. Apomorphine (10^–8–10^–5 M) and bromocriptine (10^–8–10^–6 M) reduced, whereas chlorpromazine (10^–7 and 10^–6 M) and haloperidol (10^–8 and 10^–7 M) enhanced the evoked overflow. The inhibitory effect of apomorphine and bromocriptine was antagonized by chlorpromazine and haloperidol, but not by phentolamine.Silicone tubings that had been in contact with ³H-haloperidol retained tritiated material that was slowly eluted during perfusion with water or physiological salt solution. The material was identified as ³H-haloperidol. When silicone tubings pretreated with unlabelled haloperidol were used in subsequent dopamine release experiments, the inhibitory effect of apomorphine was not reproduced.It is concluded that, in the caudate nucleus of the rabbit, apomorphine and bromocriptine depress, whereas chlorpromazine and haloperidol facilitate action potential-evoked release of dopamine. The effects are mediated by specific receptors which may be located on the dopaminergic nerve terminals. The receptors appear to be normally activated by released dopamine itself, which thus inhibits its own further release. Part of the discrepancies in the literature concerning dopaminergic modulation of dopamine release may be due to retention of neuroleptic drugs in superfusion assemblies, followed by slow elution and interference with subsequent experiments.     

Dopamine-induced neurogenic vasodilatation in isolated perfused muscle preparation of the dog

Summary Dopamine, injected into the lumbar aorta of the dog in doses which produce a reversible inhibition of synaptic transmission in the lumbar paravertebral ganglia (0.5–64×10^–8 moles), produces a neurogenic vasodilatation in the isolated perfused hindleg or gracilis muscle. This was abolished by acute preganglionic decentralization and by administration into the perfused preparation of -adrenoceptor blocking agents, but not of atropine or diphenhydramine. After decentralization, preganglionic electrical stimulation restored the dopamine-induced indirect vasodilatation. The neurogenic vasodilatation was also seen with intra-aortic injections of epinine (2–32×10^–8 moles) and apomorphine (1.2–19.2×10^–8 moles) and was preferentially blocked by haloperidol (0.26×10^–6 moles). (-)-Noradrenaline, injected into the lumbar aorta in baroreceptor-denervated dogs, was found to be equipotent with dopamine in eliciting the neurogenic vasodilatation; this (-)-noradrenaline-induced effect was preferentially blocked by phentolamine (8×10^–6 moles).The possibility that the neurogenic vasodilatation, which occurs upon intra-aortic injection of dopamine in the dog, is due to its ganglionic-inhibitory effect is discussed.This work was aided by grants from the Coordination Committee for Scientific Research (C.C.A.S.) and from the Fund for Interdisciplinary Research, Belgium.     

“Supramaximal” enhancement of the inotropic effect of noradrenaline by tyramine

Summary The positive inotropic effect of noradrenaline on the guinea-pig papillary muscle is potentiated in the presence of 1×10^–5 M tyramine, the concentration of noradrenaline that is necessary to produce a half maximal increase in force of contraction being reduced to about one third. There is no alteration of the maximal inotropic effect since the concentration-effect curve of noradrenaline is simply shifted to the left.In the presence of 3×10^–3 M tyramine, which by itself increases contractility by a dual mechanism (an indirect sympathomimetic and a direct postsynaptic one which is not induced by stimulation of adrenergic -receptors), noradrenaline (1×10^–5 M) produces an additional inotropic effect leading to a force of contraction which surmounts the maximum of the normal concentration-effect curve of noradrenaline by about 30%.The supramaximal isometric contraction curve of the papillary muscle produced by the combined effects of 3×10^–3 M tyramine and 1×10^–5 M noradrenaline differs from the contraction curve in the presence of 1×10^–5 M noradrenaline alone in having a steeper ascending slope and a slower relaxation phase. The mean velocity of force development (S ₁) exceeds the maximum value of the normal concentration-effect curve of noradrenaline by about 50%. There is no increase in the maximum of the mean velocity of relaxation (S ₂).The relaxation time of the supramaximal contraction curve as well as the duration of its action potential are the result of the opposing influences of the two substances, noradrenaline shortening and tyramine prolonging both action potential and relaxation time.     

Characterization of adrenoceptors mediating positive inotropic responses in the ventricular myocardium of the dog

1 The pharmacological characteristics of adrenoceptors mediating the positive inotropic action in the dog heart were assessed by the use of blood-perfused papillary muscles and isolated strips of ventricular myocardium.

2 On the blood-perfused papillary muscle driven at 2 Hz and in sinus node preparations, phenylephrine induced positive inotropic and chronotropic responses in the same dose range and was much less potent than isoprenaline. The dose-response curve for the chronotropic action of phenylephrine was parallel to that of isoprenaline, whilst the dose-response curve for the inotropic action of phenylephrine was less steep than that of isoprenaline.

3 The infusion of pindolol, a β-adrenoceptor blocking agent, at a rate of 1 μg/min, shifted the isoprenaline dose-response curves to the right, and to the same extent, in both papillary muscle and sinus node preparations. In contrast to isoprenaline, the antagonism of phenylephrine by pindolol was noncompetitive. Phentolamine did not affect the positive inotropic and chronotropic actions of phenylephrine.

4 On isolated ventricular strips α-adrenoceptor blockade by 10^-6 M phentolamine did not affect dose-response curves to phenylephrine or dopamine. Pindolol shifted the dopamine dose-response curves to the right in a competitive manner and those of phenylephrine in a noncompetitive manner.

5 On ventricular strips from reserpine-pretreated dogs phenylephrine and tyramine dose-response curves were shifted markedly to the right and downwards. Desipramine (10^-5 M) which enhanced the action of noradrenaline considerably reduced the myocardial responses of phenylephrine.

6 Papaverine (10^-5 M) decreased the threshold concentration of phenylephrine required to stimulate the myocardium and shifted phenylephrine dose-response curves to the left.

7 Raising the temperature from 32°C to 37°C shifted phenylephrine dose-response curves to the right; when the temperature was raised from 37°C to 42°C the affinity of the drug was not changed.

8 Other α-adrenoceptor stimulants, methoxamine and clonidine, decreased the active tension of ventricular strips. The responses to noradrenaline and adrenaline (in the presence of pindolol; 3 × 10^-8 M) were not affected by phentolamine (10^-6 M).

9 The results indicate that adrenoceptors mediating positive inotropic responses in the dog ventricle are of the β-type and that post-synaptic α-adrenoceptors are not involved. Phenylephrine acts mainly by releasing noradrenaline from adrenergic nerve endings and partly by a weak direct action on β-adrenoceptors.

     

Nitric oxide induces acetylcholine-mediated contractions in the guinea-pig small intestine

In longitudinal muscle/myenteric plexus preparations of the guinea-pig ileum, exogenous nitric oxide (NO) induced concentration-dependent relaxations. In tissues at basal tone, NO (3 × 10^–6 M) induced a moderate relaxation followed by a pronounced contraction, consisting of a quick and sustained component. Tetrodotoxin (5 × 10^–7 M) abolished both phases of the contraction. Atropine (5 × 10^–7 M) abolished the quick component and reduced the sustained component of the contraction; the latter was further suppressed by the selective NK₁ receptor antagonist CP 96,345. Hexamethonium (5 × 10^–5 M) failed to affect the contractile response to NO. It is concluded that administration of exogenous NO in the guinea-pig ileum can lead to activation cholinergic and to a lesser degree tachykininergic neurones. Correspondence to: L. Barthó at the above address     

Evidence for two types of 5-hydroxytryptamine receptor on secretomotor neurons of the guinea-pig ileum

Summary Flat sheet preparations of the mucosa plus submucosa from the guinea-pig ileum were placed in Ussing chambers so that short circuit currrent (I _sc), an index of electrolyte movement across the mucosa, could be measured. In these preparations, 5-hydroxytryptamine (5-HT) increasesI _sc indirectly by stimulating both cholinergic and non-cholinergic secretomotor neurons. The 5-HT₃ receptor antagonist, ICS 205–930 (10^–13–10^–5 M), substantially depressed the secretory response due to 5-HT (10^–6 M), but not that produced by direct activation of muscarinic receptors on the mucosal epithelium with carbachol (10^–6 M), or by stimulation of secretomotor neurons with substance P (10^–8 M) or 1,1-dimethyl-4-phenylpiperazinium (10^–5 M). The residual response to 5-HT, after the addition of a maximally effective concentration of ICS 205–930 (10^–6 M), was further reduced by hyoscine (10^–7M). When that part of the 5-HT response attributable to the release of acetylcholine was blocked by hyoscine (10^–7M), ICS 205–930 did not further modify the response to 5-HT. The hyoscine-resistant component was, however, sustantially depressed by tetrodotoxin (3.5 × 10^–7 M). The response remaining after ICS 205–930 and hyoscine was not affected by methysergide (2 × 10^{– 5} M) or cyproheptadine (10^–7 M). We conclude that there are ICS 205–930 sensitive 5-HT receptors on cholinergic secretomotor neurons, and ICS 205–930, methysergide, and cyproheptadine insensitive 5-HT receptors on non-cholinergic secretomotor neurons.     

In vitro acetylcholine release from rat caudate nucleus as a new model for testing drugs with dopamine-receptor activity

Summary The effects of a number of dopamine-receptor agonists on depolarization-induced (26 mM K⁺) release of ³H-acetylcholine from slices of rat caudate nucleus were examined with a superfusion method. Apomorphine (10^–6 M) and N,N-dipropyliso-ADTN (10^–7 M) inhibited acetylcholine-release in vitro by about 50% and these inhibitory effects were antagonized by haloperidol. For N,N-dipropyl-iso-ADTN an EC₅₀ of approximately 3×10^–9 M was estimated from its dose-response curve. However, dopamine (10^–6 M) itself and bromocriptine (10^–6 M) inhibited acetylcholine-release less. Presumably: the weak effect of exogenous dopamine is due to its (partial) uptake in dopaminergic nerve terminals and to the fact that released endogenous dopamine already partially activates the receptors involved in the inhibition of acetylcholine-release.Pretreatment of young rats with 6-hydroxydopamine (+ desipramine) increased the inhibitory effects of dopamine-receptor agonists, including dopamine itself, on acetylcholine-release from caudate slices, indicating dopamine-receptor supersensitivity. This was corroborated by the finding that apomorphine-induced stereotyped behavior was significantly higher in rats lesioned with 6-hydroxydopamine than in controls.It is suggested that K⁺-induced release of radiolabelled acetylcholine from caudate nucleus slices provides a functional model to study the characteristics of post-synaptic dopamine-receptors in vitro. The concentrations of dopamine-receptor agonists needed to inhibit acetylcholine-release appear to be in the nanomolar range, in agreement with their affinities as determined in dopamine-receptor binding studies. In contrast, these concentrations are much lower than those required for stimulation of dopamine-sensitive adenylate cyclase activity.     

Die Haftung verschiedener Cardenolide am Papillarmuskel und einer mikrosomalen ATPase des Meerschweinchenherzens

Summary In experiments on isolated electrically stimulated guinea pig papillary muscles and on isolated cardiac Na⁺-K⁺-activated ATPase preparations the action and the reversibility of action of 3 different cardenolides-digitoxin, k-strophanthidin and strophanthidin-3-bromoacetate (SBA) (supposed to be an irreversible inhibitor of the transport ATPase)-were studied.The equieffective concentrations for maximum positive inotropic effects (around 90%) were 2×10^–6, 2×10^–5 and 4×10^–5 M, respectively. In washout experiments the positive inotropic action of all these substances was found to be completely reversible: the rates of decline of the positive inotropic effects were about 2.7%/min with digitoxin, 24%/min with strophanthidin and 22%/min respectivety 5.7%/0/min (two components) with SBA.The equieffective concentrations for maximum inhibition (90–95%) of the Na⁺-K⁺-activated ATPase by digitoxin, strophanthidin and SBA were 10^–4, 2×10^–4 and 10^–4 M respectively. In washout experiments (repeated centrifugations) different degrees of reversibility of these inhibitory effects were observed depending upon the experimental conditions. Preincubation of the enzyme with the cardenolides in theabsence of Na⁺, Mg²⁺ and ATP resulted in a persisting inhibition of the Na⁺-K⁺-ATPase of 14% with digitoxin, 10% with k-strophanthidin and- significantly higher (p < 0.05)-33% with SBA. Corresponding experiments with preincubation of the enzyme in thepresence of Na⁺, Mg²⁺ and ATP, however, demonstrated a full reversibility of the inhibitory action of all these substances.These results are in contrast, in certain respects, with those obtained in previous experiments on brain ATPase.It is concluded that SBA is able to inhibit irreversibly only the non-phosphorylated form of the cardiac Na⁺-K⁺-activated ATPase, whereas the phosphorylated intermediate of this enzyme seems to be protected against the irreversible inhibition by this substance. Assuming that the latter state of the enzyme is predominant in the intact heart muscle cell, a complete reversibility of the pharmacological action of SBA would be expected if the inotropic effect is mediated by an inhibition of the enzyme. Our results are compatible with this hypothesis.

Wir danken der Deutschen Forschungsgemeinschaft für die Unterstützung durch Sachbeihilfen und der Volkswagenstiftung für die Geräteausstattung.     

Role of alpha1A adrenoceptor subtype in production of the positive inotropic effect mediated via myocardial alpha,adrenoceptors in the rabbit papillary muscle: influence of selective alpha1A subtype antagonists WB 4101 and 5-methylurapidil

Summary In order to elucidate the contribution of alpha_1A subtype to the positive inotropic effect mediated by myocardial alpha, adrenoceptors, the influence of the alpha_1A selective antagonists WB 4101 and 5-methylurapidil on the alpha,-mediated positive inotropic effect (induced by phenylephrine in the presence of a beta adrenoceptor blocking agent bupranolol) was assessed in the isolated rabbit papillary muscle. WB 4101 (10^–9-10^–7mol/l) shifted the concentration-response curve of the alpha,-mediated positive inotropic effect to the right in parallel, but the slope of Schild plot did not meet the competitive antagonism: WB 4101 shifted the curve by log one unit at 10^–9 mol/1, whereas it did not cause further shift at higher concentrations of 10^–8 and 10^–7 mol/l. WB 4101 did not affect the beta adrenoceptor-mediated positive inotropic effect. 5-Methylurapidil (10^–9 to 10^–7 mol/l) shifted the curve of alpha₁-mediated positive inotropic effect to the right and downwards in a concentration-dependent manner; the slope of Schild plot calculated at the level of 20% of the maximum response to phenylephrine was close to unity. 5-Methylurapidil at 3 × 10^–7 mol/1 abolished the alpha₁-mediated positive inotropic effect. In addition, 5-methylurapidil inhibited the beta adrenoceptor-mediated positive inotropic effect in the same concentration range as it antagonized the alpha₁-mediated positive inotropic effect, indicating that 5-methylurapidil is not selective for myocardial alpha, adrenoceptors. In the membrane fraction derived from the rabbit ventricular muscle, 5-methylurapidil displaced the specific binding of [³H]CGP-12177 with high affinity, whereas WB 4101 did not affect the [³H]CGP-12177 binding in the concentration range that it antagonized the alpha,-mediated positive inotropic effect. The present results indicate that alpha_1A adrenoceptor subtype plays a role in production of the positive inotropic effect mediated by myocardial alpha, adrenoceptors, but the extent is less than that mediated by alpha_1B subtype in the rabbit ventricular myocardium. Send offprint requests to M. Endoh at the above address     

Effect of phentolamine on noradrenaline uptake and release

Summary The influence of phentolamine on the uptake of exogenous noradrenaline infused into the aortic cannula and on the overflow of endogenous noradrenaline caused by sympathetic nerve stimulation was investigated in the isolated perfused rabbit heart. 10^–6 M phentolamine doubled the overflow of endogenous noradrenaline, but did not change noradrenaline uptake. 10^–5 M phentolamine increased the stimulation-induced overflow of noradrenaline 4-fold and inhibited amine uptake by about 50%. 10^–4 M phentolamine elevated the overflow of noradrenaline less than 10^–5 and 3×10^–5 M did. The augmentation of transmitter overflow was only partly reversed by 13 min perfusion with drug-free medium.Pretreatment of hearts with 1.5×10^–5 M cocaine or with 10^–7 or 10^–6 M desipramine did not change the effect of phentolamine on the overflow of noradrenaline evoked by nerve impulses. Pretreatment of hearts with 10^–5 M, but not with 10^–6 M, phentolamine prevented the increase of transmitter overflow by cocaine.It is concluded that low concentrations of phentolamine potentiate the overflow of noradrenaline during nerve stimulation by a mechanism different from that of cocaine, i.e. different from inhibition of neuronal re-uptake. The nature of this mechanism is discussed.This work was supported by the Deutsche Forsehungsgemeinschaft. We have the pleasure to thank Mrs. Ch. Arts, Miss B. Piel and Mr. E. Hagelskamp for skilful technical assistance.     

Indirect adrenergic effect of histamine in cat cerebral arteries

Summary Histamine (10(^–4 M) induced an increase in the tritium outflow from cat cerebral arteries preloaded with ³H-noradrenaline. Pretreatment with reserpine (3 mg/kg, i.p., total dose) or removal of both superior cervical ganglia two weeks before the experiment abolished that increase. The presence of cocaine or diphenhydramine also prevented the rise in tritium efflux induced by histamine.Histamine (10(^–8 M to 10(^–3 M) elicited dose-dependent contractions in the isolated posterior communicating artery of the cat which were reduced in the presence of diphenhydramine at all doses except the highest three. The addition of phentolamine to the bath decreased the contractile responses at the doses lower than 10(^–6 M. Pretreatment with reserpine or removal of both superior cervical ganglia also diminished the responses at doses of histamine below 10(^–6 M and 10(^–5 M, respectively. When cocaine was added to the bath there was a decrease in the contraction elicited at all doses except the last one.These results suggest the existence of an indirect adrenergic mechanism in the contractile response to histamine in cat cerebral arteries.     

Pharmacological comparison of the sigma recognition site labelled by [3H]haloperidol in human and rat cerebellum

Summary The radioligand binding characteristics of [³H]haloperidol (in the presence of spiperone, 25 nmolL^–1) were investigated in rat and human cerebellar membranes.In both rat and human cerebellar membrane preparations saturation studies with [³H]haloperidol (non-specific binding defined by pentazocine, 10 molL^–1) demonstrated high affinity saturable specific binding to a homogenous population of binding sites (rat, Bmax 6693 ± 1242 fmol mg^–1 protein, pK_D 8.33 ± 0.08; human, Bmax 2550 ± 437 fmol mg^–1 protein, pK_D 8.59 ± 0.11; mean ± SEM, n = 3–6). Competition studies employing a wide range of structurally diverse competing compounds displayed that the [³H]haloperidol binding site was pharmacologically similar in both preparations and comparable to sigma recognition sites previously identified in various tissues originating from different species. In addition, with reference to the potential subtypes of sigma recognition sites, the labelling of these sites by low nanomolar concentrations of [³H]haloperidol provides evidence that they belong to the sigma-1 recognition site subtype.The present findings suggest that the pharmacology of the rat and human cerebellar sigma recognition site are directly comparable and provides further supporting evidence towards the use of [³H]haloperidol radioligand binding studies in the rat to detect sigma receptor ligands with potential therapeutic activity. Send offprint requests to: N.M. Barnes at the above address     

Characterization of the effects of histamine on the transmembrane electrical activity of guinea-pig and rabbit SA- and AV-node cells

Summary The effects of histamine on the transmembrane electrical activity of cells of small preparations (0.5 × 0.5 mm) of guinea-pig and rabbit sinoatrial- and atrioventricular-nodes were studied. Histamine at concentrations above 10^–7 mol/l increased the firing rate, the rate of diastolic depolarization, the maximum diastolic potential, the amplitude and the maximum rate of depolarization of the action potential of pacemaker cells of rabbit and guineapig sinoatrial cells and rabbit atrioventricular cells. These effects were antagonized by the HZ-receptor blocker cimetidine (2.5 × 10^–6 mol/1) but they were not modified by the H₁-receptor blocker chlorphenamine (2.5 and 5×10^–6 mol/1). Small preparations of guinea-pig atrioventricular node did not exhibit spontaneous activity, but it was induced by histamine and blocked by cimetidine. Histamine increased the maximum upstroke velocity of propagated action potential of cells of the central part of complete atrioventricular node in both species studied. These effects were blocked by cimetidine, but not by chlorphenamine. It is concluded that the increase in automaticity induced by histamine in guinea-pig and rabbit sinoatrial and atrioventricular nodes was due to stimulation of H₂receptors. Histamine did not depress electrical activity of atrioventricular node cells, but rather increased it. This effect was due to H₂-receptor stimulation. Send offprint requests to: J. Sanchez-Chapula at the above address