The new potent and selective histamine H2 receptor agonist amthamine as a tool to study gastric secretion

Summary The new histamine H₂ receptor agonist amthamine, [2-amino-5-(2-aminoethyl)-4-methylthiazole], was tested for its activity on gastric acid secretion in different in vivo and in vitro experimental models. Amthamine induced a dose-related increase in acid secretion both in conscious cats with a gastric fistula (ED₅₀ = 0.069 mol/kg/h) and in anaesthetized rats with a lumen-perfused stomach (ED₅₀ = 11.69 gmol/kg i.v.). In this last preparation the efficacy of amthamine was significantly higher than that of histamine and dimaprit. Amthamine was an effective secretagogue also in the rat isolated gastric fundus, behaving as a full agonist (EC₅₀ = 18.9 mol/l). In all the experimental models amthamine was more potent than dimaprit (from 3 to 10 fold) and approximately equipotent with histamine, and its effect was competitively antagonized by the histamine H₂ receptor antagonists famotidine or ranitidine. Experiments with H₁ and H₃ receptor antagonists indicated that Amhamine is devoid of stimulatory activity at H₁ and H₃ receptors. The present data indicate that amthamine is a full agonist at histamine H₂ receptors and, being more effective and selective than the other compounds of the family, it may represent a good alternative to the other available histamine H₂ receptor agonists for the study of gastric acid secretion.Correspondence to G. Bertaccini at the above address     

Alpha2-adrenoceptor-mediated responses to so-called selective alpha1-adrenoceptor agonists after partial blockade of alpha1-adrenoceptors

Summary In dog saphenous vein — a tissue possessing both postsynaptic ₁- and ₂-adrenoceptors — the effects of two selective ₁-adrenoceptor agonists (phenylephrine and methoxamine) were compared with that of the selective ₂-adrenoceptor agonist, UK-14,304, before and after phenoxybenzamine. Furthermore, the influence exerted by prazosin, yohimbine and verapamil on the effects of these agonists was also studied before and after phenoxybenzamine. In the absence of phenoxybenzamine, prazosin (56 nmol/l) caused a parallel shift of the concentration-response curves of both phenylephrine and methoxamine to the right (by 0.94 and 1.1 log units, respectively) and had no effect on the concentration-response curve of UK-14,304, while 20 nmol/l yohimbine caused a marked parallel shift of the concentration-response curve of UK-14,304 to the right (by 1.18 log units) and caused only minor displacements of those of phenylephrine and methoxamine (by 0.2 and 0.33 log units, respectively). After exposure of the strips to 30 nmol/l phenoxybenzamine, prazosin (56 nmol/l) caused small shifts of the concentration-response curves of both phenylephrine (by 0.36 log units) and methoxamine (by 0.31 log units) and did not change that of UK-14,304, while yohimbine (20 nmol/l) caused pronounced parallel shifts of the concentration-response curves (to the right) of all the agonists: phenylephrine (by 1.0 log units), methoxamine (by 0.93 log units) and UK-14,304 (by 1.28 tog units). When UK-14,304 was added to the bath during a sub-maximal contraction to phenylephrine it caused a further contraction almost up to the maximum; if this procedure was repeated after phenoxybenzamine (30 nmol/1), there was no further contraction to UK-14,304.In the absence of phenoxybenzamine, verapamil (5 mol/l) caused a parallel shift of the concentration-response curve of phenylephrine (or methoxamine) to the right and a non-parallel shift (with marked depression of the maximal effect) of that of UK-14,304. However, after phenoxybenzamine (30 nmol/l), the same concentration of verapamil caused non-parallel shifts of the concentration-response curves of the three agonists to the right with about equal depression of the maximal effects. We conclude that, after removal of ₁-adrenoceptor reserve by phenoxybenzamine, the responses to selective ₁-adrenoceptor agonists are predominantly ₂-adrenoceptor-mediated. This may explain why under these conditions, the selective ₁-and ₂-adrenoceptor agonists are equally antagonized by calcium entry blockers.This work was supported by a grant from the University of Porto (Subsidio para acção de investigação no. 36/85) Send offprint requests to S. Guimarães at the above address     

Inhibition of presynaptic alpha-2-adrenoceptor and opioid receptor agonist responses in the rat vas deferens by chronic imipramine treatment

Summary The effects of chronic imipramine administration on agonist responses in rat isolated smooth muscle preparations were investigated. The administration of 20 mg/kg imipramine two times a day for 4 and 11 days resulted in an equivalent subsensitivity (approximately 8-fold) of clonidine-induced inhibition of electrically evoked contractions in the rat vas deferens (presynaptic ₂-adrenoceptor response). Imipramine (4 days) resulted in a marked inhibition of the ability of [d-Ala², d-Leu⁵] enkephalin to decrease electrically evoked contractions of the vas deferens (presynaptic opioid receptor response) but did not significantly affect the carbachol-induced increase in electrically evoked contractions (muscarinic receptor response). In the absence of cocaine the contractile effects of norepinephrine and tyramine in the vas deferens were, respectively, potentiated and inhibited, following imipramine (4 days), suggesting a decrease in the activity of the neuronal uptake mechanism. When determined in the presence of cocaine, the potency of the postsynaptic effects of norepinephrine in the vas deferens (₁-adrenoceptor response) was not significantly altered by imipramine (4 days). With regard to other postsynaptic receptors, imipramine (4 days) decreased slightly the potency of phenylephrine in the aorta (₁-adrenoceptor response) and increased slightly the potency of carbachol in the trachea (muscarinic receptor response) and the potency of serotonin in the rat aorta (5HT₂-receptor response). Thus, chronic imipramine administration decreased the potency of presynaptic ₂- and opioid agonist responses in the vas deferens but caused very little or no changes in the potencies of agonists at postsynaptic sites. Send offprint requests to R. A. Keith at the above address     

Subclassification of presynaptic α2-adrenoceptors: α2D-autoreceptors and α2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of _2A, _2B, _2C and _2D the presynaptic ₂-autoreceptors, as well as the ₂-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with ³H-noradrenaline or ³H-choline and then superfused and stimulated electrically.The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; ³H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to ₂-autoinhibition or inhibition of ³H-acetylcholine release by endogenous noradrenaline. The ₂-selective agonist 5-bromo6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both ³H-noradrenaline and ³H-choline experiments. Most (³H-noradrenaline) or all (³H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pK_d values of the antagonists were calculated from the shifts. pK_d values from ³H-noradrenaline experiments correlated with pK_d values from ³H-choline experiments (r = 0.981).It is concluded that ₂-autoreceptors and ₂-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native ₂ binding sites, binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that both are _2D. The results are consonant with the hypothesis that at least the majority of ₂-autoreceptors belong to the _2A/D branch of the ₂-adrenoceptor tree, across mammalian or at least across rodent and lagomorph species. The same may hold true for ₂-adrenoceptors on non-noradrenergic neurones.     

Evidence that histamine H3 receptors are involved in the control of gastric acid secretion in the conscious cat

Summary In an attempt to assess the role of histamine H₃ receptors in the control of gastric acid secretion, the effects of the selective histamine H3 receptor agonist, (R)-methylhistamine and antagonist, thioperamide were evaluated in the conscious gastric fistula cat under basal conditions and against different stimuli. (R)-methylhistamine (0.05–0.2 mol/kg/h) was ineffective against spontaneous and dimaprit-induced acid secretion; it also did not reduce significantly pentagastr-ininduced acid output, but caused a dose-dependent (0.05–0.1 mol/kg/h) and significant inhibition of the acid response to 2-deoxy-d-glucose. Thioperamide (0.02–0.04 mol/kg/h) did not modify spontaneous acid secretion, whereas it evoked a significant enhancement of the acid response to submaximal doses (50 mg/kg i. v.) of 2-deoxy-d-glucose. Thioperamide completely reversed the inhibitory effect of (R)-methylhistamine against 2-deoxy-d-glucose-induced secretion, while leaving unaffected the inhibition induced by somatostatin. These data suggest that histamine H3 receptors may be involved in the control of acid secretion stimulated by indirectly acting secretagogues. Send offprint requests to G. Bertaccini at the above address     

Pre- and postsynaptic effects of yohimbine stereoisomers on noradrenergic transmission in the pulmonary artery of the rabbit

Summary Effects on noradrenergic neurotransmission of five stereoisomers of yohimbine and of the closely related compound yohimbol were studied in strips of the pulmonary artery of the rabbit. In some experiments the tissue was preincubated with ³H-noradrenaline. Three effects were observed. Firstly, antagonism to the contractile effect of noradrenaline and of sympathetic nerve stimulation; the antagonism reflected competitive blockade of postsynaptic -adrenoceptors. Secondly, an increase in the stimulation-evoked overflow of total tritium and ³H-noradrenaline; the increase appeared to be due to blockade of presynaptic -adrenoceptors. Thirdly, an increase in the basal outflow of ³H-3,4-dihydrophenylglycol, presumably by impairment of the vesicular storage of ³H-noradrenaline. According to their relative potencies in eliciting these effects, the drugs could be divided into three groups. Rauwolscine, -yohimbine and yohimbol preferentially blocked the presynaptic -adrenoceptor; rauwolscine and -yohimbine, like yohimbine, at low concentrations increased the contractile response to sympathetic nerve stimulation. Corynanthine preferentially blocked the postsynaptic -adrenoceptor. Pseudoyohimbine and 3-epi--yohimbine were very weak antagonists at either receptor; they mainly accelerated the basal outflow of ³H-3,4-dihydroxyphenylglycol.From these results and those of a previous study it is concluded that, in a series of twelve -adrenolytic drugs, rauwolscine shows the greatest preference for presynaptic and corynanthine the greatest preference for postsynaptic -adrenoceptors. In view of the chemical similarity of the two compounds these opposite properties are striking. Corynanthine and rauwolscine might be useful tools for the subclassification of -adrenoceptors.     

Cardiovascular effects of alpha-adrenergic drugs: Differences between clonidine and guanabenz

Summary Guanabenz induced a pressor effect in pithed rats through postsynaptic ₂-adrenoceptors whereas clonidine activated both vascular ₁ and ₂-adrenoceptors. Previous treatment with prazosin, and ₁-antagonist, or depletion of the noradrenergic stores by reserpine produced supersensitivity to the pressor response to clondine only, probably through postsynaptic ₁-adrenoceptors.The hypotension and bradycardia developed in normotensive rats after intravenous guanabenz administration were abolished by prazosin, whereas the central effects of clonidine were antagonized by both prazosin and yohimbine.Selective destruction of central noradrenergic neurons by [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] (DSP 4) or reserpine plus blockade of catecholamine synthesis by -methyl-p-tyrosine abolished the hypotension and bradycardia produced by guanabenz but merely reduced the bradycardia from clonidine.The present results suggest that, in rats, guanabenz is a selective stimulant of central -autoadrenoceptors antagonized by prazosin whereas at a vascular level guanabenz preferentially activates -adrenoceptors antagonized by yohimbine. The differences observed between the mechanisms by which guanabenz and clonidine produce their central cardiovascular responses might be attributed to their acting on different nuclei.     

Complete blockade by phenoxybenzamine of α1- but not of α2-vascular receptors in dogs and the effects of propranolol

Summary In pithed dogs pressor responses to phenylephrine were completely inhibited 1 h after phenoxybenzamine 20 mg/kg i.v., but those to norepinephrine were only partially inhibited. The pressor effects of norepinephrine in phenoxybenzamine-treated animals were inhibited by yohimbine, 2.0 mg/kg i.v., but not by prazosin, 0.5 mg/kg i.v. In animals treated with phenoxybenzamine, 20 mg/kg i.v., plus propranolol, 5.0 mg/ kg i.v., the partially restored pressor response to epinephrine, and the responses to norepinephrine, were completely inhibited by yohimbine, 2.0 mg/kg i.v., partially inhibited by corynanthine, 5.0 mg/kg i.v., but not affected by prazosin, 0.5 mg/kg i.v. In additional animals treated with phenoxybenzamine plus propranolol, yohimbine, 10, 50, 200 and 500 g/kg i.v., caused dose-related inhibition of both the partially restored pressor response to epinephrine, and the pressor responses to norepinephrine. It is concluded that: 1) phenoxybenzamine completely blocks ₁, but not ₂ vascular receptors; 2) the pressor effect of norepinephrine in phenoxybenzamine-treated animals, and the partially restored pressor effect of epinephrine in phenoxybenzamine-propranolol-treated animals, are both mediated by ₂ vascular receptors which are resistant to blockade by phenoxybenzamine.     

Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H₃ receptor selective agonists (R)--methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC₅₀ values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)--methyl-histamine to the right (pK_B value 8.38). The inhibitory effect of 1 mol/l (R)--methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC₅₀: 65 nmol/l) and dimaprit (IC₅₀: 8.6 mol/l); however, the effect of (R)--methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mol/l) and not significantly affected by other H₃ receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 mol/l). In conclusion, H₃ receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H₃ receptors may exist. The data suggest that histamine H₃ receptors modulating 5-HT release in pig small intestine do not belong to either H_3A or H_3B receptors as defined in rat tissue. Correspondence to: K. Racke at the above address     

Peripheral presynaptic and central effects of clonidine,yohimbine and rauwolscine on the sympathetic nervous system in rabbits

Summary The function of presynaptic ₂-autoreceptors at postganglionic sympathetic neurones under conditions of normal, ongoing sympathetic impulse traffic was studied in anaesthetized rabbits (alfadolone + alfaxalone). Clonidine was used as an ₂-adrenoceptor agonist, and yohimbine and rauwolscine were used as antagonists. Mean arterial pressure, postganglionic renal sympathetic firing rate, arterial plasma noradrenaline concentration and heart rate were measured before (basal values) and at the end of 3-min infusions of sodium nitroprusside and phenylephrine, which were given to modulate efferent activity in the sympathetic nervous system through the baroreflex.The nitroprusside- and phenylephrine-induced changes of mean arterial pressure produced the expected changes in sympathetic nerve activity, plasma noradrenaline and heart rate. Clonidine (5 µg kg^–1 + 0.5 µg kg^–1 min^–1) reduced the basal mean arterial pressure, sympathetic nerve activity and heart rate. It also reduced the nitroprusside-induced increase in the plasma noradrenaline level without changing the nitroprusside-induced increase in sympathetic firing. These results, as well as the mean arterial pressure-sympathetic nerve activity and the sympathetic nerve activity-plasma noradrenaline function curves indicate that clonidine inhibited both sympathetic tone centrally and the average release of noradrenaline per action potential peripherally. Yohimbine (1 mg kg^–1 + 0.1 mg kg^–1 h^–1) and rauwolscine (0.5 mg kg^–1 + 0.1 mg kg^–1 h^–1) increased the basal plasma noradrenaline level without any increase of renal sympathetic nerve activity. They also enhanced the nitroprusside-induced increase in plasma noradrenaline without any enhancement of the nitroprusside-induced increase in sympathetic firing. The hypotensive response to nitroprusside was attenuated, whereas the heart rate response was augmented. These results, as well as the mean arterial pressure-sympathetic nerve activity and the sympathetic nerve activity-plasma noradrenaline function curves indicate that the main effect of yohimbine and rauwolscine was to increase the average release of noradrenaline per action potential.The simultaneous measurement of postganglionic sympathetic nerve activity and the arterial plasma noradrenaline concentration proved suitable to differentiate central (or ganglionic; this distinction was not possible) effects of ₂-adrenoceptor ligands from peripheral presynaptic effects. The results show that endogenous presynaptic, ₂-adrenergic autoinhibition of noradrenaline release from postganglionic sympathetic neurones operates physiologically in anaesthetized rabbits with ongoing, uninterrupted sympathetic nerve activity. The results also indicate that blockade of ₂-autoreceptors enhances the sympathetic reflex compensatory response to a hypotensive stimulus. Send offprint requests to B. Szabo at the above address     

Cardiovascular effects of clonidine in an avian species,Larus argentatus

Summary Blood pressure and heart rate were recorded in the sea gull, Larus argentatus, under light pentobarbitone anaesthesia. Clonidine 10^–7 and 10^–8 mol·kg^–1 (27 and 2.7 g·kg^–1) i.v. produced a biphasic effect on blood pressure, a brief initial increase being followed by a prolonged hypotensive response. There was an immediate reduction in heart rate which persisted throughout the hypotensive phase. After spinal transection at the level of C 4, clonidine administration elicited hypertension and bradycardia.Bilateral vagotomy abolished the effect of clonidine on heart rate but did not alter the blood pressure response.Vagotomy in combination with spinal transection abolished the effect of clonidine on heart rate but the hypertensive response was accentuated.Yohimbine 10^–7 or 10^–6 mol·kg^–1 (0.039 or 0.39 mg·kg^–1) given 5 min after clonidine 10^–7 mol·kg^–1 (27 g·kg^–1) effectively antagonized the cardiovascular effects of clonidine, while prazosin 10^–7 or 10^–6 mol·kg^–1 (0.042 or 0.42 mg·kg^–1) had no such effect.We conclude that clonidine acts in the central nervous system of the sea gull to produce a lowering of blood pressure and heart rate. These effects are mediated by central inhibition of sympathetic activity and, in the case of the heart rate, mostly by central activation of vagal activity to the heart. This central action of clonidine involves activation of -adrenoceptors which are blocked by yohimbine but not by prazosin and therefore may belong to the ₂ subtype.     

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%     

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.     

Antagonists that differentiate between α2A- and α2D-adrenoceptors

Four antagonists were examined for their ability to differentiate _2A from the orthologous _2Dadrenoceptors. The antagonists were (2S,12bS) 1, 3-di-methylspiro(1, 3, 4, 5, 6, 6, 7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinolizine)-2,4-pyrimidin-2-one (MK 912), 2-[2-(methoxy-1, 4-benzodioxanyl)imidazoline (RX 821002), efaroxan and benoxathian. The ₂-autoreceptors in rabbit brain cortex were chosen as _2A- and the a₂-autoreceptors in guinea-pig brain cortex as _2D-adrenoceptors. Slices of the brain cortex were preincubated with ³H-noradrenaline and then superfused and stimulated electrically by brief pulse trains (4 pulses, 100 Hz) that led to little, if any, ₂-autoinhibition. 5-Bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK 14,304) was used as an ₂-adrenoceptor agonist.UK 14, 304 decreased the stimulation-evoked overflow of tritium. The antagonists shifted the concentration-inhibition curve of UK 14, 304 to the right in an apparently competitive manner. Dissociation constants of the antagonists were calculated from the shifts. MK 912, RX 821002 and efaroxan had markedly higher affinity for (guinea-pig) _2D-adrenoceptors (pK _d values 10.0, 9.7 and 9.1, respectively) than for (rabbit) _2A-adrenoceptors (pK _d 8.9, 8.2 and 7.6, respectively). Benoxathian had higher affinity for _2A- (pK _d 7.4) than for _2D-adrenoceptors (pK _d 6.9). Ratios calculated from the K _d values of the four compounds differentiated between _2A and _2D up to 100 fold. It is concluded that MK 912, RX 821002, efaroxan and benoxathian are antagonists with high power to differentiate _2A- from _2D-adrenoceptors.     

Activation of histamine H3 receptors produces presynaptic inhibition of neurally evoked cat nictitating membrane responses in vivo

Summary This study was undertaken in order to determine the potential role of prejunctional histamine H₃ receptors in an in vivo adrenergic model system. Frequency-dependent nictitating membrane responses were elicited by sympathetic nerve stimulation in anesthetized cats. Systemic administration of the selective histamine H₃ receptor agonist, (R)--methylhistamine (RMeHA) produced a dose-related depression of amplitude of the evoked nictitating membrane responses with a threshold of about 10 g/kg and maximal effect (50% depression at the lowest frequency; 0.5 Hz) seen at 100–300 gg/kg. Responses obtained with low frequency stimulation were more sensitive to depression by RaMeHA than were responses evoked with higher frequencies of stimulation. Larger doses of RaMeHA given to the same animals, failed to produce additional inhibition.RMeHA depressed the amplitude of nictitating membrane responses evoked by either pre- or postganglionic nerve stimulation to an equivalent degree. This depressant action of RMeHA was antagonized by pretreatment with the specific histamine H₃ antagonist, thioperamide (3 mg/kg), but not by combined pretreatment with histamine H₁ and H₂ blockers chlorpheniramine (300 Ftg/kg) and cimetidine (5 mg/kg). Intravenous administration of adrenaline (1–30 wg/kg) also produced graded nictitating membrane responses that were not altered by subsequent administration of RMeHA.These results suggest that histamine H₃ receptors are involved in the modulation of neurally evoked noradrenaline release in the cat nictitating membrane by an inhibitory presynaptic action. The most likely site of drug action appears to be at the neuroeffector junction as no appreciable ganglionic effect of RMeHA was observed in this in vivo model system. Send offprint requests to M. C. Koss at the above address     

Inhibition by α2-adrenoceptor agonists of the secretion of catecholamines from isolated adrenal medullary cells

Summary In adrenal medullary cells, carbachol evokes the secretion of catecholamines with simultaneous uptake of⁴⁵Ca. Highly selective agonists for ₂-adrenoceptors, clonidine, naphazoline, guanfacine, tramazoline and oxymetazoline inhibited carbachol evoked secretion of catecholamines in a dose-dependent manner. These ₂-agonists also inhibited the uptake of⁴⁵Ca evoked by carbachol with similar dose-response curve to inhibition of catecholamine secretion. On the contrary, highly selective agonists for ₁-adrenoceptors, phenylephrine and norfenefrine did not inhibit the secretion of catecholamines and cellular uptake of⁴⁵Ca. The inhibition by ₂-agonists was not restored either by the increase in carbachol, or Ca concentrations, suggesting that the mode of inhibition was distinct from competition at cholinergic receptors or Ca-channels. It is likely that ₂-agonists inhibited the secretion of catecholamines via the inhibition of Ca uptake which was probably caused through the activation of ₂-adrenoceptors.     

Loss of selectivity of so-called selective α1-adrenoceptor agonists after phenoxybenzamine

Summary This study examined the nature of -adrenoceptor subtype involved in pressor responses to so-called selective ₁-adrenoceptor agonists after treatment with phenoxybenzamine in vivo. The influence of prazosin (0.1 mg/kg) and of yohimbine (1 mg/kg) on the dose-response curves for cirazoline in the pithed rat, and for phenylephrine in the anaesthetized dog were compared, after various doses of phenoxybenzamine.In the pithed rat, after 0.05 mg/kg phenoxybenzamine, prazosin caused a displacement of the dose-response curve of cirazoline to the right which was much larger than that caused by yohimbine; after 0.3 mg/kg phenoxybenzamine, prazosin and yohimbine caused about equal displacements; after 1 mg/kg phenoxybenzamine, yohimbine caused a marked displacement, while prazosin was without effect.In the anaesthetized dog, after 1 mg/kg phenoxybenzamine, prazosin and yohimbine produced about equal rightward shifts of the dose-response curve for phenylephrine. However, after 3 mg/kg phenoxybenzamine the rightward shift of the dose-response curve for phenylephrine was much larger after yohimbine than after prazosin. In the anaesthetized dog, verapamil (1 mg/kg) caused a small and parallel rightward shift of the dose-response curve for phenylephrine before phenoxybenzamine and a large and nonparallel one after phenoxybenzamine (3 mg/kg); the effect of verapamil on responses to the selective ₂-adrenoceptor agonist UK-14,304 (before and after phenoxybenzamine) were similar to those on responses to phenylephrine after phenoxybenzamine.It is concluded that after 1 mg/kg phenoxybenzamine in the pithed rat or after 3 mg/kg phenoxybenzamine in the anaesthetized dog, the responses to the so-called selective ₁-adrenoceptor agonists cirazoline and phenylephrine, respectively, are predominantly or totally ₂-adrenoceptor-mediated. This explains why, after inactivation of ₁-adrenoceptors by phenoxybenzamine, the so-called selective ₁- and ₂-adrenoceptor agonists are equally antagonized by calcium entry blockers.This work was supported by grants from INIC (Instituto Nacional de Investigação Cientifica): FmP₁ and FMP₃ Send offprint requests to S. Guimarães at the above address     

The effects of the 2-amino-tetrahydronaphthalene derivative M7, a selective α2-adrenoceptor agonist in vitro

Summary M7 was originally reported to be a selective presynaptic ₂-adrenoceptor agonist in the pithed rat preparation. Subsequent work showed that M7 also stimulated postsynaptic ₂-adrenoceptors in this preparation, producing a pressor response. We have now investigated the selectivity of M7 for ₂- and ₁-adrenoceptors in vitro. Our results demonstrate that M7 is very potent in stimulating presynaptic ₂-adrenoceptors in the rat vas deferens and postsynaptic ₂-adrenoceptors in the dog saphenous vein. However, at higher doses M7 is also an ₁-adrenoceptor agonist, its ED₅₀ at ₁-adrenoceptors being approximately 60 fold greater than that at postsynaptic ₂-adrenoceptors. It is clear that the postsynaptic effects of M7 will depend upon the relative proportions of ₁- and ₂-adrenoceptors contained in the tissue under study.     

The opposite effects of amitriptyline on noradrenaline-and methoxamine-evoked venoconstriction in man

The effects of a single oral dose (100 mg) of amitriptyline on noradrenaline- and methoxamine-evoked venoconstriction were compared, using the dorsal hand vein compliance technique, in 8 healthy male volunteers. Both noradrenaline and methoxamine produced dose-dependent venoconstriction; the geometric mean ED₅₀ for noradrenaline was 4.15 ng min^–1 and for methoxamine was 1143.54 ng min^–1; the potency ratio (noradrenaline/methoxamine) was 277. Amitriptyline shifted the dose-response curve for noradrenaline to the left (ANOVA: P < 0.025; dose-ratio: 0.38) consistent with potentiation, and the dose-response curve for methoxamine to the right (ANOVA: P < 0.025; dose ratio: 2.72) consistent with antagonism. The potentiation is likely to be due to noradrenaline uptake blockade, whereas the antagonism is likely to reflect the blockade of post junctional ₁-adrenoceptors.     

Possible involvement of presynaptic α1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR

Summary The effects of several -adrenoceptor antagonists have been examined on tritium release elicited by electrical stimulation from isolated perfused SHR tail artery preparations prelabelled with ³H-noradrenaline (³H-NA). Phentolamine and yohimbine potently facilitated the stimulation evoked release of tritium at low frequencies of stimulation, but the ₂-adrenoceptor antagonist idazoxan was only weakly active at 1 mol/l, despite antagonising the clonidine-evoked inhibition of ³H-release at a lower concentration of 0.1 mol/l. The ₁-adrenoceptor antagonists prazosin and corynanthine also increased stimulation evoked tritium release in this preparation, suggesting the presence of prejunctional ₁-adrenoceptors. Furthermore, the ₁-adrenoceptor agonist methoxamine (3 mol/l) caused a significant inhibition of tritium-evoked release, an effect which was blocked by prazosin (10 nmol/l).When ₁-adrenoceptors were blocked in the presence of prazosin, idazoxan (0.1 mol/l) produced a significant facilitatory effect on the electrically-evoked release of ³H-transmitter. On the other hand, when ₂-adrenoceptors were blocked in the presence of yohimbine, exposure to idazoxan (0.1 mol/l) reduced significantly the stimulation-evoked release of tritium elicited by electrical stimulation.The results indicate that in the SHR tail arteries, idazoxan has a partial agonist inhibitory activity on transmitter release, which can mask the facilitatory effects due to blockade of presynaptic ₂-adrenoceptors. The inhibitory effects of idazoxan appear to involve presynaptic ₂-adrenoceptors, which when stimulated, reduce ³H-NA release in SHR tail arteries.