Multiple effects of putative α-adrenoceptor agonists on the electrical and mechanical activity of guinea-pig papillary muscle

Summary The effects of the putative -adrenoceptor agonists phenylephrine, methoxamine and clonidine on force of contraction and on calcium-dependent action potentials were studied in guinea-pig papillary muscles.Phenylephrine increased the force of contraction by stimulating -adrenoceptors as well as -adrenoceptors. It increased the amplitude and duration of slow action potentials, but this effect was exclusively due to stimulation of -adrenoceptors. The positive inotropic effect mediated by -adrenoceptors can presumably not be explained by an increase in calcium influx during the action potential via the slow inward current.Methoxamine had no effect on the force of contraction at 10^–5 and 10^–4 mol/l, but at 10^–4 mol/l it slightly decreased amplitude and duration of slow action potentials.Clonidine produced a large increase in force of contraction and in amplitude and duration of slow action potentials. These effects were due to stimulation of H₂-histamine receptors.It is concluded that in guinea-pig papillary muscle the tested putative -adrenoceptor agonists do not share a common -adrenoceptor effect, but produce prominent effects which are mediated through either -adrenoceptors (phenylcphrine), or H₂-histamine-receptors (clonidine) or are non-specific (methoxamine) in nature.     

Evidence in favour of a selective α1-adrenoceptor blocking action of WB 4101 in vivo

Summary The -adrenoceptor blocking potency of WB 4101 at ₁- and ₂-adrenoceptors has been investigated in pithed rats.WB 4101 was approximately 97 times more potent at antagonizing the vasopressor responses produced by the selective ₁-adrenoceptor agonist phenylephrine, than those produced by the selective ₂-adrenoceptor agonist M-7.A dose of WB 4101 (3 mg/kg) that caused extensive blockade of vascular ₁-adrenoceptors, but little or no blockade of vascular ₂-adrenoceptors, exerted no significant blockade of the presynaptic ₂-adrenoceptors in the rat heart.The results support the view that WB 4101 is a highly selective antagonist at ₁-adrenoceptors in vivo.     

A comparison of the effects of TMB-8 and nifedipine on pressor responses to α1- and α2-adrenoceptor agonists in pithed rats

TMB-8 has been characterized as an inhibitor of the release of Ca⁺ from intracellular pools. We have studied the modification of the pressor responses to selective _l-adrenoceptor agonists (methoxamine and phenylephrine), and to selective ₂-adrenoceptor agonists (B-HT 920 and B-HT 933) in pithed rats, produced by TMB-8. We have compared this modification with that produced by the calcium antagonist nifedipine. Nifedipine (100 g/kg, 300 g/kg, and 1000 g/kg) inhibited in a dose-dependent manner the pressor responses to the ₁- and ₂-adrenoceptor agonists, the dose-response curves to the ₂-adrenoceptor agonists being shifted further to the right. TMB-8 at a dose of 3000 g/kg did not modify the pressor effects of the _l-adrenoceptor agonists, and neither did it reinforce the inhibition of such responses produced by nifedipine. By contrast, TMB-8 pretreatment (0.03 g/kg, 0.3 g/kg, 3 g/kg, 30 g/kg, 300 g/kg and 3000 g/kg) inhibited the responses to both ₂-adrenoceptor agonists, the inhibition being more pronounced with B-HT 920. A similar effect was obtained with 0.03 g/kg TMB-8 and 0.3 g/kg TMB-8, particularly in the case of B-HT 920. It was stronger with higher doses, but similar for all doses over 3 g/kg. The inhibition of the pressor responses mediated by the stimulation of ₂-adrenoceptors by TMB-8 was less in rats treated with the Ca²⁺ entry promoter BAY K 8644 (300 g/kg), and could also be reduced by the continuous infusion of CaCl₂ (0.25 g/min). These results suggest that in pithed rats TMB-8 may also behave as an inhibitor of the Ca⁺ influx into vascular smooth muscle.     

α1-adrenoceptor subtype affinities of drugs for the treatment of prostatic hypertrophy

Summary We have used radioligand binding and inositol phosphate accumulation studies to determine the affinity at mixed _1A- and _1B-adrenoceptors (rat cerebral cortex and kidney), _1A-adrenoceptors (rat cerebral cortex and kidney following inactivation of _1B-adrenoceptors by chloroethylclonidine treatment) and _1B-adrenoceptors (rat spleen) for drugs currently under investigation for the treatment of benign prostatic hypertrophy, alfuzosin, naftopidil and (–)- and (+)-tamsulosin. Alfuzosin and naftopidil had similar affinities in all model systems (approximately 10 nM and 130 nM, respectively) and lacked relevant selectivity for ₁-adrenoceptor subtypes. Their potency to inhibit noradrenaline-stimulated inositol phosphate formation in cerebral cortex matched their affinities as determined in the binding studies. Tamsulosin had higher affinity at _1A- than at _1B-adrenoceptors, and was slightly more potent than alfuzosin and naftopidil at _1B- and considerably more potent at _1A-adrenoceptors. However, the interaction of the tamsulosin isomers with chloroethylclonidine-insensitive (_1A-like) adrenoceptors was complex. A detailed analysis of the tamsulosin data and those obtained with other drugs, most notably noradrenaline and oxymetazoline, suggested that chloroethylclonidine-insensitive ₁-adrenoceptors may be heterogenous and that this heterogeneity may differ between cerebral cortex and kidney of the rat.     

Pharmacologic differentiation between pre- and postjunctional α2-adrenoceptors by SK & F 104078

Summary Most ₂-adrenoceptor antagonists do not discriminate between pre- and postjunctional ₂-adrenoceptors, and this has led to the commonly held belief that pre- and postjunctional ₂-adrenoceptors may represent one homogeneous population of receptors. SK&F 104078 has been shown to be a potent antagonist at postjunctional ₂-adrenoceptors at concentrations that do not block prejunctional ₂-adrenoceptors. Thus, SK & F 104078 is a competitive postjunctional ₂-adrenoceptor antagonist in canine and rabbit saphenous veins, canine saphenous artery and human platelet with a dissociation constant of approximately 100 nmol/l. Conversely, SK & F 104078 is inactive as a prejunctional ₂-adrenoceptor antagonist in atria from dog, guinea pig, rabbit and rat, and in guinea-pig ileum at concentrations up to 10,000 nmol/l. Likewise, SK & F 104078 has the ability to block postjunctional arterial ₂-adrenoceptors in vivo in the pithed rat at doses that do not inhibit prejunctional ₂-adrenoceptors in the same model. The results suggest that pre- and postjunctional ₂-adrenoceptors may not represent one homogeneous class, but rather are discrete subtypes of the ₂-adrenoceptor that may be differentiated by SK & F 104078. Send offprint requests to R. R. Ruffolo, Jr.     

Electrophysiologic and inotropic effects of α-adrenoceptor stimulation in human isolated atrial heart muscle

Summary The effects of -adrenoceptor stimulation on force of contraction were investigated in human atrial heart muscle and compared with those of -adrenoceptor stimulation. The maximal positive inotropic effect produced by stimulation of -adrenoceptors with phenylephrine (in the presence of atenolol 10 mol/l) was significantly smaller than that seen in response to -adrenoceptor stimulation with isoprenaline. The maximal effect of phenylephrine (25% of the maximal effect of isoprenaline) required far higher concentrations (1 mmol/l) than isoprenaline (100 nmol/l); the EC₅₀ values amounted to 33.1 mol/l and 3.3 nmol/l, respectively. In the presence of the -adrenoceptor blocking agent phentolamine (1 mol/l), the concentration-response curve of phenylephrine was displaced to higher concentrations of the agonist; under these conditions, the EC₅₀ value amounted to 52.5 mol/l, The effects of the catecholamines noradrenaline and adrenaline on force of contraction remained unchanged in the presence of phentolamine (1 mol/l), or prazosin (1 mol/l), The positive inotropic effect of phenylephrine (1 mmol/l) was associated with a slight decrease in action potential duration; the effects on action potential were completely blocked in the presence of phentolamine (1 mol/l) These findings support the view that selective stimulation of -adrenoceptors may mediate a small but detectable positive inotropic effect in human atrial tissue under in vitro conditions. The requirement of high concentrations of -adrenoceptor agonists and the lack of effects of the endogenous catecholamines adrenaline and noradrenaline on -adrenoceptors (in concentrations which fully elicit the -adrenoceptors-mediated response) do not provide a basis for a functional role of -adrenoceptor-mediated effects under in vivo conditions. It is more likely that adrenaline- or noradrenaline-mediated changes in the force of contraction in the human atrium are virtually exclusively due to the stimulation of -adrenoceptors. Send offprint requests to H. Nawrath at the above address     

Postjunctional α-adrenoceptors

Summary The postsynaptic -adrenoceptors in rat aorta and in pithed rat were investigated according to their sensitivity to nine -adrenergic agonists and to the selective antagonists yohimbine (₂) and prazosin (₁) and the non-selective one, phentolamine. In addition, in radioligand binding studies, the affinity and selectivity of the drugs were determined on rat cerebral cortex using [³H] yohimbine and [³H] prazosin.On rat aorta, prazosin is 1,000 times more potent than yohimbine against each -adrenoceptor agonist, whether ₁- or ₂-selective. Rat aorta probably contains only ₁-adrenoceptors.Pressor effects in pithed rats are mediated by post-junctional ₁- and ₂-adrenoceptors. The dose-response curve for -methylnorepinephrine in the presence of prazosin, using Hofstee's plots, revealed ₁- and ₂-adrenoceptors, respective proportions being 80.5 and 19.5%     

The locomotor effect of clonidine and its interaction with α-flupenthixol or haloperidol in the developing rat

Summary The interaction of clonidine with -flupenthixol and haloperidol on the locomotion was investigated in the rat during postnatal development. A subcutaneous injection of clonidine 0.039–3.9 mol/kg produced a marked hypermotility in infant animals between day 1 and day 7 but hypomotility in animals older than 20 days. Pretreatment with -flupenthixol and haloperidol significantly reduced clonidine-hypermotility in infants. In adult rats, clonidine-hypomotility were increased by a preceding administration of -flupenthixol. It is suggested that intact function of both -adrenoceptors and dopamine receptors is involved in the control of locomotor activity in developing rats and that there is the complex interaction of noradrenergic and dopaminergic system in the control of locomotor activity.     

Contraction-mediating α2-adrenoceptors in the mouse vas deferens

Summary The question of the existence of postjunctional, contraction-mediating ₂-adrenoceptors, in addition to the known ₁-adrenoceptors, was studied in the mouse isolated vas deferens. Both the ₁-selective agonist phenylephrine and the ₂-selective agonist 5-bromo-6-(2imidazolin-2-ylamino)-quinoxaline (UK 14,304) caused contraction of the vas deferens. In the presence of the ₁-selective antagonist prazosin (added in order to prevent an ₁ component in the effect of high concentrations of UK 14,304), the ₂-selective antagonists yohimbine and idazoxan shifted the concentration—response curve of UK 14,304 to the right in a manner compatible with competitive antagonism and with dissociation constants K_B indicating the involvement of ₂-adrenoceptors. The maximal contraction elicited by UK 14,304 (in the presence of prazosin) was much lower than the maximal contraction elicited by phenylephrine. The effect of UK 14,304 was not changed by the P₂-purinoceptor agonist ,-methylene-ATP and was reduced by neuropeptideY, but was markedly enhanced by relatively low concentrations of phenylephrine. When the sympathetic fibres of the vas deferens were stimulated by trains of ten widely spaced (0.5 Hz) electric pulses, the tissue responded with ten separate twitches in which purinergic and adrenergic components were isolated by prazosin and suramin, respectively. Prazosin reduced the first adrenergic twitch in these trains at concentrations close to its K_B value at ₁-adrenoceptors, whereas yohimbine and idazoxan reduced the first adrenergic twitch at concentrations far lower than their K_B values at ₁-adrenoceptors. The results indicate that the smooth muscle of the mouse vas deferens possesses contraction-mediating ₂-adrenoceptors. They are activated by UK 14,304 and probably also by noradrenaline of neural origin. Responses mediated by the ₂-adrenoceptors are enhanced by simultaneous ₁-receptor activation, an interaction that may increase the contribution of the ₂-adrenoceptors to the adrenergic phase of neurogenic contractions. Send offprint requests to R. Bültmann at the above address     

The modulation of [3H]noradrenaline and [3H]serotonin release from rat brain synaptosomes is not mediated by the α2B-adrenoceptor subtype

Summary The present study aimed at relating the presynaptic ₂-adrenoceptors, known to modulate noradrenaline and serotonin release, with the recently described _2A- and _2B-adrenoceptor subtypes. The effects of the agonist oxymetazoline (selective for _2A subtype) and of three adrenoceptor antagonists (idazoxan, 1-(2-pyrimidinyl)piperazine (PmP) and prazosin, the last one known to be _2B selective) were evaluated on [³H]noradrenaline and [^H]serotonin release in superfused synaptosomes from rat brain cortex. These drugs were also tested in [³H]yohimbine binding to human platelet membranes (containing only _2A receptors) and to neonatal rat lung membranes (containing only _2B receptors).The affinity pattern of these compounds at _2A-adrenoceptors in binding studies was oxymetazoline > = idazoxan > PmP > prazosin; at _2B-adrenoceptors it was idazoxan > = prazosin > PmP = oxymetazoline. Oxymetazoline inhibited with high and similar potencies the K⁺-evoked [³H]noradrenaline and [³H]serotonin release, IC₅₀ 18 and 7 nM, respectively; in the same conditions, the IC₅₀ values of noradrenaline were 42 and 168 nM, respectively. The antagonist affinity pattern (antagonism against noradrenaline) was idazoxan > PmP > prazosin, either on [³H]serotonin release.These results indicate that presynaptic ₂ auto- or heteroreceptors do not belong to the _2B subtype and suggest that the modulation of noradrenaline and serotonin release may be mediated by the _2A-adrenoceptor subtype. Send offprint requests to M. Gobbi at the above address