Differences between full and partial α-adrenoceptor agonists in eliciting phasic and tonic types of responses in the longitudinal smooth muscle of the rat portal vein

Summary The aim of the present investigation was to study, taking into account both quantitative and qualitative differences, the influence of full and partial -adrenoceptor agonists on spontaneous myogenic activity in the rat portal vein.We found that the -adrenoceptor agonists cirazoline, adrenaline, noradrenaline, phenylephrine, St 587, Sgd 101/75, B-HT 920 and UK-14,304 could increase the amplitude of the phasic myogenic contractions in the rat portal vein with apparent differences in EC₅₀ and E_max values. In addition to an increase in phasic myogenic activity, the -adrenoceptor agonists cirazoline, adrenaline, noradrenaline, and phenylephrine were also able (in higher concentrations) to increase the basal tone of the rat portal vein preparation, again with apparent differences in EC₅₀ and E_max values. Changing the extracellular Ca²⁺ concentration from 0.9 mmol/l to 2.5 mmol/1 had no influence on the phasic character and the concentration range in which St 587 and UK-14,304 increased spontaneous myogenic activity, although changes in amplitude and frequency of the spontaneous myogenic contractions were less pronounced at a higher extracellular Ca²⁺ concentration (2.5 mmol/1). By the use of Schild analysis with the competitive a-adrenoceptor antagonists prazosin (pA₂ = 8.74) and 5-methyl-urapidil (pA₂ = 8.37), it was established that the contractile responses to St 587 were mediated by the same ₁-adrenoceptor subtype as the phasic and tonic type of contraction elicited by phenylephrine as described in a previous study. The concentration-response curve of UK-14,304 was significantly shifted to the right by low concentrations of prazosin (3 nmol/1–30 nmol/1), indicating stimulation of ₁-adrenoceptors by UK-14,304 in the rat portal vein. The -adrenoceptor antagonists phenoxybenzamine and chloroethylclonidine irreversibly blocked the contractile responses to St 587. Based on the method of receptor alkylation with phenoxybenzamine an affinity constant was calculated for St 587 (pK_a = 5.91). Phenoxybenzamine was approximately 1000-fold more potent in inactivating ₁-adrenoceptors than chloroethylclonidine.In conclusion there appeared to be a divergence in the excitation-contraction coupling of ₁-adrenoceptors in the rat portal vein, which is reflected by two types of contraction (phasic versus tonic). The extent to which both the phasic and tonic types of contraction are stimulated by agonists depends on the affinity and intrinsic efficacy for each of the receptor-coupled effector pathways. Thus, partial and full agonism can only meaningfully be discussed if confined to one particular effector pathway. Send offprint requests to H. R. Schwietert at the above address     

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     

Subclassification of presynaptic α2-adrenoceptors: α2D-autoreceptors in mouse brain

The study was devised to classify, by means of antagonist affinities, the presynaptic ₂-autoreceptors in mouse cerebral cortex in terms of _2A, _2B, _2C and _2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Slices of the cortex were preincubated with ³H-noradrenaline and then superfused and stimulated electrically.The stimulation periods used (4 pulses, 100 Hz) did not lead to ₂-autoinhibition as shown by the lack of an increase by rauwolscine of the evoked overflow of tritium. The ₂-selective agonists 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) and -methylnoradrenaline reduced the evoked overflow. All 10 antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. Rauwolscine also shifted the concentration-inhibition curve of -methylnoradrenaline to the right. pK_d values of the antagonists were calculated from the shifts. The pK_d values of rauwolscine against UK 14,304 and -methylnoradrenaline were very similar (8.0 and 7.9, respectively).Comparison with antagonist affinities for prototypic native ₂ binding sites, ₂ binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that the ₂-autoreceptors in mouse brain cortex are _2D. This is the first subtype determination of ₂-autoreceptors in the mouse. It supports the hypothesis that at least the majority of ₂-autoreceptors belong to the _2A/D branch of the ₂-adrenoceptor tree.     

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.     

Comparative behavioral characterization of the neuroactive steroids 3α-OH,5α-pregnan-20-one and 3α-OH,5β-pregnan-20-one in rodents

Pregnan steroids have been shown to possess anesthetic, hypnotic, anticonvulsant and anxiolytic properties. In this study, two endogenous neuroactive steroid isomers, 3-hydroxy-5-pregnan-20-one (3,5-P) and 3-hydroxy-5-pregnan-20-one 3,5-P), were studied for differences in their pharmacological properties using behavioral assays. 3,5-P and 3,5-P were similar in their potencies and efficacies in blocking pentylenetetrazol-induced seizures in mice (ED₅₀: 3,5-P=2.8 mg/kg and 3,5-P=3.0 mg/kg). Similarly, both neuroactive steroids produced roto-rod deficits within the same range of potency (TD₅₀:3,5-P=18.8 mg/kg and 3,5-P=21.2 mg/kg). However, in animal models of anxiety, subtle differences were observed between the two isomers. In both the light/dark transition test and elevated plus-maze, 3,5-P was more efficacious than 3,5-P, though both compounds had similar potencies. In the Geller-Seifter test, 3,5-P was more potent and efficacious than 3,5-P. Neither compound had significant effects on unpunished responding within the dose range tested. Both compounds produced similar biphasic curves in the locomotor test. All together, the data indicate that 3,5-P and 3,5-P have similar anticonvulsant activity, but the 5-isomer possesses more potent and efficacious anxiolytic properties than the 5-isomer.     

Smooth muscle of rabbit aorta contains α1- but not α2-Adrenoceptors

Summary We have investigated the residual contractile response to noradrenaline remaining after phenoxybenzamine (3×10^–7 mol/l) in rabbit aorta, since it has been reported that phenoxybenzamine at low doses completely and irreversibly blocks ₁- but not ₂-adrenoceptors. The contraction elicited by noradrenaline slowly recovered with time after it had been almost abolished by phenoxybenzamine. This residual response was blocked by the ₁-selective antagonist prazosin (3×10^–8 mol/l) but not by the ₂-selective antagonist rauwolscine (3×10^–7 mol/l). The results confirm that the smooth muscle of rabbit aorta contains ₁- not ₂-adrenoceptors.     

α-Adrenergic agents. 3. Behavioral effects of 2-aminotetralins

Studies with 5-substituted-8-methoxy-2-aminotetralin compounds suggest that some are ₁-adrenoceptor agonists, which readily penetrate the blood-brain barrier. They potentiate the locomotor activity that is induced by apomorphine (AP) in reserpinized mice, an effect that has been suggested to result from activation of central -receptors. This effect is selectively blocked by the preferential ₁-antagonist phenoxybenzamine, but not by drugs that block other types of receptors. The effect is also produced by the centrally administered ₁-agonists phenylephrine and methoxamine, but not by various types of standard CNS stimulants. When administered in high doses, some of the aminotetralin compounds induce locomotor activity in reserpinized mice without AP, an effect also found with high doses of centrally administered phenylephrine and methoxamine. This effect is blocked by a series of drugs at doses that correspond to their ₁-antagonist potencies.     

Benzamide action at α2-adrenoceptors modifies catecholamine-induced contraction and relaxation of circular smooth muscle from guinea-pig stomach

Summary Dopamine was shown to act on the circular smooth muscle of the stomach body to cause contraction at a yohimbine-sensitive site (₂) and a relaxation at a prazosin-sensitive site (₁). Metoclopramide and tiapride failed to modify either response, failed to antagonise a relaxation to phenylephrine at 1(₁ sites in the same tissue, and failed to modify the contractions caused by dopamine and phenylephrine at an ₂-adrenoceptor site in the pyloric sphincter. However, (+)- and (–)-sultopride and (+)-sulpiride antagonised the dopamine-induced contractions of the stomach body indicating an ₂-antagonist action. An ability to attenuate the relaxation of this tissue may reflect a displacement of the contraction curve to the right rather than an ₂-antagonist action since the response to phenylephrine was not antagonised either in this tissue or in the pyloric sphincter. Within the central nervous system the (–)-enantiomers of sultopride and sulpiride have a highly selective dopamine receptor blocking action. This cotrasts with the present findings in the stomach musculature of a non-stereospecific antagonism at ₂-type adrenoceptors.     

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.     

Subclassification of presynaptic α2-adrenoceptors: α2A-autoreceptors in rabbit atria and kidney

The study was devised to classify, by means of antagonist affinities, the presynaptic ₂-autoreceptors of rabbit atria and kidney in terms of _2A, _2B, _2C and _2D-A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the left atrium and slices of the kidney cortex were preincubated with ³H-Noadrenaline and then superfused and stimulated electrically.In one series of experiments, tissue pieces were stimulated by relatively long pulse trains (1 or 2 min) leading to ₂-autoinhibition. All 11 (atria) or 10 (kidney) antagonists increased the evoked overflow of tritium. pEC_30% values (concentrations causing 30% increase) were interpolated from concentration-response curves. In a second series of experiments, tissue pieces were stimulated by brief pulse trains (0.4 s) that did not lead to ₂-autoinhibition, and concentration-inhibition curves of the ₂-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. Most of the 11 (atria) or 8 (kidney) antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. pK_d values of the antagonists were calculated from the shifts. pEC_30% values correlated with pK_d values, both in atria (r = 0.728) and in the kidney (r = 0.930). pEC_30% values in atria correlated with pEC_30% values in the kidney (r = 0.988) and pK_d values in atria correlated with pK_d values in kidney (r = 0.923).It is concluded that the ₂-autoreceptors in atria and the kidney are the same. Comparison with antagonist affinities for prototypic native ₂ binding sites, ₂ binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that both autoreceptors are _2A. This conclusion is reached with either of the two independent estimates of autoreceptor affinity, pEC_30% and pK_d. The results are compatible 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 rodent and lagomorph species.     

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     

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.     

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%     

Exploring the pharmacology of the pro-convulsant effects of α2-adrenoceptor antagonists in mice

The effects of selective and specific ₂-adrenoceptor antagonists on electroshock seizure threshold in mice were investigated. Idazoxan, at low doses, efaroxan, RX811059 and RX821002 significantly lowered seizure threshold. The ₁-agonist St 587 and the -agonist isoprenaline were also pro-convulsant. On the other hand the ₂-agonists clonidine and UK 14,304 produced small increases in seizure threshold. Anticonvulsant effects were also produced by low doses of the noradrenaline uptake inhibitor desipramine. This compound increases levels of noradrenaline in the synaptic cleft which could subsequently act at post-synaptic ₂-adrenoceptors. The pro-convulsant action of ₂-adrenoceptor antagonists could be explained in terms of two mechanisms: a) blockade of endogenous noradrenaline which may normally exert a tonic anti-convulsant influence on seizure threshold, through post-synaptic ₂-receptors and/or b) increased activation of ₁- and -adrenoceptors by elevated synaptic noradrenaline levels following blockade of pre-synaptic ₂-adrenoceptors. Of the ₂-antagonists tested, idazoxan was unusual in that high doses were not pro-convulsant; this difference may be explained by ₁-adrenoceptor mediated actions or be related to its recently described affinity at a non-adrenoceptor site — a function for which is currently unknown.     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Comparison of cloned and pharmacologically defined rat tissue α1-adrenoceptor subtypes

Multiple ₁-adrenoceptor subtypes have been defined by pharmacological and receptor cloning techniques, but the precise alignment of cloned and pharmacologically-defined subtypes is still unclear. We have compared the affinities of 8 subtype-selective compounds at three cloned ₁-adrenoceptor subtypes (rat _1B, bovine _1C rat _1A/D) with those previously determined by the same methods in rat spleen, cerebral cortex, and kidney (Naunyn-Schmiedeberg's Arch. Pharmacol. 348: 385–395, 1993). Among all compounds tested to date at cloned ₁-adrenoceptor subtypes (+)-tamsulosin appears to be the most selective with a rank order of potency _1C > _{1A/D 1B}. Affinities for the _1A-selective 5-methyl-urapidil, methoxamine, oxymetazoline, phentolamine and (–)- and (+)-tamsulosin and for noradrenaline and SDZ NVI-085 at the splenic _1B-adrenoceptors and at their low affinity sites in cerebral cortex and kidney correlated best with those at the cloned _1B-adrenoceptor. Affinities of these drugs at their high affinity sites in cerebral cortex (pharmacologically-defined _1A-adrenoceptor) were matched best by those at the cloned _1C-adrenoceptor. Rat kidney appears to contain two chloroethylclonidine-resistant ₁-adrenoceptor subtypes one of which is similar to the cloned at _1C- and one to the cloned _1A/D-adrenoceptor. We conclude that the cloned _1B-adrenoceptor is the genetic correlate of the pharmacologically-defined _1B-adrenoceptor. An ₁-adrenoceptor subtype corresponding to the cloned _1A/D-adrenoceptor appears to exist in rat kidney. Among cloned ₁-adrenoceptor subtypes, the bovine _1C-adrenoceptor bears the closest resemblance to the pharmacologically-defined _1A-adrenoceptor in rat cortex and to one of the chloroethylclonidine-insensitive subtypes in rat kidney.     

Uptake inhibitors do not change the effect of imidazoline α2-adrenoceptor agonists on transmitter release evoked by single pulse stimulation in mouse vas deferens

Summary The present study was undertaken to compare the presynaptic interaction of neuronal noradrenaline uptake inhibitors with imidazoline and phenylethylamine ₂adrenoceptor agonists under two different conditions: at low and high noradrenaline concentrations in the biophase.Isolated mouse vasa deferentia were stimulated with trains of 7 pulses given at 0.2 Hz and the inhibition by the ₂-adrenoceptor agonists clonidine, -methylnoradrenaline, and UK-14,304 of twitch responses was measured in the absence and in the presence of either cocaine (12 mol/l) or desipramine (40 nmol/l). The effects were determined for the first (equivalent to single pulse stimulation) and the last stimulus of each train. Both uptake inhibitors antagonized the presynaptic inhibitory effects of imidazolines (clonidine and UK-14,304) on the last twitch; the effects on the first twitch remained unchanged. In contrast, the uptake inhibitors potentiated the inhibitory effect of the phenylethylamine (-methylnoradrenaline) on both the first and the last twitches.These results support the view that the concentration of noradrenaline in the biophase plays a decisive role in the inhibition by a₂-adrenoceptor agonists of the electrically evoked release of noradrenaline. Agonists that are not substrates of neuronal uptake (i.e., clonidine, UK-14,304) become less effective when noradrenaline is present in the biophase while substrates of neuronal uptake (i. e., -methylnoradrenaline) do not. The results argue against the hypothesis that uptake inhibitors interact directly with presynaptic ₂-adrenoceptors or act at some link between uptake and receptor sites. Send offprint requests to S. Guimarães at the above address     

All three α<Subscript>2</Subscript>-adrenoceptor types serve as autoreceptors in postganglionic sympathetic neurons

Postganglionic sympathetic neurons and brain noradrenergic neurons use _2A- and _2C-adrenoceptors as presynaptic autoreceptors. The present experiments were carried out in order to see whether they possess presynaptic _2B-autoreceptors as well. Pieces of atria, vasa deferentia, the occipito-parietal cortex and the hippocampus were prepared from either wildtype (WT) mice or mice in which both the _2A- and the _2C-adrenoceptor gene had been disrupted (_2ACKO). The pieces were incubated with ³H-noradrenaline and then superfused and stimulated electrically. In a first series of experiments, single pulses or brief, autoinhibition-poor pulse trains were used for stimulation. The ₂-adrenoceptor agonist UK 14,304 (brimonidine) reduced the evoked overflow of tritium in all four tissues from WT mice but did not change it in any tissue from _2ACKO mice. A different pattern was obtained with medetomidine as ₂ agonist. Like UK 14,304, medetomidine reduced the evoked overflow of tritium in all four tissues from WT mice and did not affect overflow in brain slices from _2ACKO mice; however, in contrast to UK 14,304, medetomidine reduced evoked overflow also in atrial and vas deferens pieces from _2ACKO mice, although with a lower maximum and potency than in WT preparations. The -adrenoceptor antagonists rauwolscine, phentolamine, prazosin, spiroxatrine and WB 4101 shifted the concentration-response curve of medetomidine in _2ACKO atria and vasa deferentia to the right. The pK_d values of the five antagonists against medetomidine in _2ACKO atria and vasa deferentia correlated with pK_d values at prototypical _2B radioligand binding sites but not at _2A or _2C binding sites. In a second series of experiments, autoinhibition-rich pulse trains were used for stimulation. Under these conditions, rauwolscine and phentolamine increased the evoked overflow of tritium from _2ACKO atrial and vas deferens pieces but not from _2ACKO brain slices. The increase was smaller (by 40% in atria and by 70% in the vas deferens) than previously observed in WT preparations (by 200–400%). In a last series of experiments, mRNA for the _2B-adrenoceptor was demonstrated by RT-PCR in thoracolumbar sympathetic ganglia from WT, _2AKO, _2CKO and _2ACKO mice but not from _2BKO mice. The results show that brain noradrenergic neurons express only _2A- and _2C-adrenoceptors as autoreceptors. Postganglionic sympathetic neurons, however, can express _2B-adrenoceptors as presynaptic autoreceptors as well. The _2B-autoreceptors are activated by medetomidine but not by UK 14,304. They are also activated by previously released noradrenaline. The two-₂-autoreceptor hypothesis has to be replaced by a three-autoreceptor hypothesis for postganglionic sympathetic neurons.     

Inhibitory α2-Adrenergic mechanism regulating gastric secretion in pylorus-ligated rats

Summary Several -adrenoceptor agonists given intracerebroventricularly or subcutaneously to rats were assessed for their effects on gastric secretion under condition of pylorus ligation. Intracerebroventricular injection of -adrenoceptor agonists reduced gastric secretion, in the following order (relative potency, clonidine = 1): oxymetazoline (100) > clonidine (1) > methoxamine (0.024) > phenylephrine (0.003). The antisecretory effects of oxymetazoline and clonidine given intracerebroventricularly were antagonized with yohimbine administered by the same route. Subcutaneous injection of -adrenoceptor agonists also reduced gastric secretion, in the following order (relative potency, clonidine = 1): clonidine (1) > oxymetazoline (0.3) phenylephrine (0.001) methoxamine (0.0006). Oxymetazoline, when given intracerebroventricularly, was most effective in decreasing the volume and titratable acidity of gastric secretion. Pretreatment with 6-hydroxydopamine intracerebroventricularly (250 g/rat × 2; 72 and 120 h before) reduced the antisecretory effect of clonidine given intracerebroventricularly. Thus, gastric secretion appears to be regulated in an inhibitory manner by ₂-, but not by ₁- in the central and peripheral nervous systems, in pylorus-ligated rats.     

Minimal α1- and α2-adrenoceptor-mediated coronary vasoconstriction in the anaesthetized swine

Summary -Adrenoceptor-mediated coronary vasoconstriction contributes to the initiation and aggravation of experimental and clinical myocardial ischaemia. However, the extent of ₁- and ₂-adrenoceptor-mediated constriction has not been characterized in the porcine coronary circulation despite the frequent use of this experimental model.Fifteen swine were anaesthetized with either -chloralose, enflurane or isoflurane to determine the amount of -adrenoceptor-mediated coronary constriction elicited by either the selective ₁-adrenoceptor agonist methoxamine or the selective ₂-adrenoceptor agonist azepexole. The left anterior descending coronary artery was cannulated and perfused by an external pump delivering constant blood flow from the carotid artery. Following bilateral cervical vagotomy and ß-adrenoceptor blockade with propranolol (2 mg kg^–1), graded dosages of either one of the -adrenoceptor agonists (9–45 g kg^–1 min^–1) were infused into the coronary perfusion line while coronary arterial pressure (CAP) was measured through a distal side arm of the cannula to detect changes in coronary vascular resistance. Infusion of the -adrenoceptor agonists was terminated when systemic arterial pressure increased. Sonomicrometers were used to measure anterior left ventricular wall thickening for the assessment of regional contractile function. During methoxamine infusion, no increase in vascular resistance was observed during -chloralose, enflurane or isoflurane anaesthesia, whereas the infusion of azepexole increased CAP from 103 ± 31 mmHg to 120 ± 35 mmHg (-chloralose), from 101 ± 16 mmHg to 122 ± 11 mmHg (enflurane) and from 84 ± 20 mmHg to 94 ± 19 mmHg (isoflurane), respectively. In four additional swine anaesthetized with enflurane, the intracoronary infusion of the full catecholamine agonist noradrenaline in the presence of propranolol (6 mg kg^–1) increased CAP from 98 ± 10 to 105 ± 10 mmHg prior to an increase in regional left ventricular function or systemic arterial pressure.These results indicate that there are no ₁- and relatively little ₂-adrenoceptor-mediated coronary constrictive effects in swine. Furthermore, neither -adrenoceptor agonist produced any detectable change in regional myocardial contractile function, regardless of the anaesthesia used.Supported by the German Research Foundation (He 1320/3-2). Dr. Guth is the recipient of a scholarship from the Alexander von Humboldt-Foundation. Send offprint requests to G. Heusch at the above address