Interactions of three inotropic agents,ASL-7022, dobutamine and dopamine,with α- and β-adrenoceptors in vitro

Summary Three inotropic agents, ASL-7022, dobutamine and dopamine, were evaluated for their -and -adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. All compounds were ₁-adrenoreceptor agonists in rat and guinea pig aortae, but the rank orders of potency were exactly opposite in these two tissues. Only the rank potency order of dobutamine>ASL-7022>dopamine obtained in rat aorta was consistent with the results obtained in radioligand binding studies to ₁-adrenoreceptors in rat cerebral cortex and to previous results obtained in vivo in the pithed rat. The results obtained in guinea pig aorta did not parallel the radioligand binding studies in rat brain or our previous results in pithed rat, and suggests that species differences exist between postsynaptic vascular ₁-adrenoreceptors in rat and guinea pig aorta, consistent with previous conclusions. ASL-7022 was found to be a potent ₂-adrenoreceptor agonist in field-stimulated guinea pig ileum, and was approximately 10-fold more potent than dobutamine in this respects, which was also confirmed by radioligand binding studies to ₂-adrenoreceptors in rat cerebral cortex. The ₁-adrenoreceptor mediated effects of these compounds were evaluated in guinea pig atria, where the rank order of potency was dobutamine>ASL-7022>dopamine. An identical rank order of affinity was established for these compounds by displacement of ³H-dihydroalprenolol from ₁-adrenoreceptors in rat cerebral cortex. The ₁-adrenoreceptor mediated effects of dobutamine and ASL-7022 in guinea pig atria were completely direct in nature and not secondary to the release of endogenous catecholamines. In contrast, a major component of the ₁-adrenoreceptor mediated tachycardia produced by dopamine in guinea pig atria was indirect in nature as evidenced by the marked attenuation in potency that occurred following catecholamine depletion with reserpine. All three compounds elicited ₂-adrenoreceptor mediated inhibition of tone in rat uterus, with the rank order of potency being ASL-7022>dobutamine>dopamine. Again, this rank order of ₂-adrenoreceptor potency was also reflected in ₂-adrenoreceptor affinity as assessed by displacement of ³H-dihydroalprenolol from ₂-adrenoreceptors in rat cerebellum. Based on these results, it may be concluded that for -adrenoceptors, dobutamine is a selective ₂-adrenoreceptor agonist, ASL-7022 is a selective ₂-adrenoreceptor agonist, and dopamine is a nonselective -adrenoceptor agonist. For -adrenoceptor mediated effect, ASL-7022 is a selective ₂ agonist, while dobutamine and dopamine are nonselective -adrenoceptor agonists. It is likely that the complex inotropic and hemodynamic activities of ASL-7022, dobutamine and dopamine result from the sum of their individual effects at the -and -adrenoceptor subtypes.     

Effects of the Aconitum alkaloid mesaconitine in rat hippocampal slices and the involvement of α- and β-adrenoceptors

1. The effects of mesaconitine, the main alkaloid contained in Aconiti tuber, were investigated by use of extracellular recordings of stimulus-evoked population spikes and field excitatory postsynaptic potentials (e.p.s.ps) in the CA1 region of rat hippocampal slices.
2. At a concentration of 10 nM, mesaconitine evoked excitations, which were manifested as an increase in the amplitude of the orthodromic spike and the appearance of multiple spikes following the first postsynaptic spike, without affecting the magnitude of paired-pulse facilitation. The increase in spike amplitude was persistent and was not reversed by up to 90 min of washout. At concentrations of 30 and 100 nM, the alkaloid produced a biphasic effect, that is an excitation followed by an inhibition without having any effect upon the field e.p.s.p. At concentrations above 100 nM, mesaconitine suppressed the orthodromic population spike and the field e.p.s.p.
3. The excitatory effect was also observed when electrical stimulation was stopped completely during the application of mesaconitine (10 nM) and during the first 15 min of washout.
4. The enhancement of the population spike and the appearance of multiple spikes induced by mesaconitine (10–100 nM) were blocked by pretreatment with the β-adrenoceptor antagonists propranolol (1 μM) and timolol (1 μM), whereas the inhibitory effect was blocked by the α-adrenoceptor antagonists yohimbine (1 μM) and phentolamine (10 μM). However, when the β-adrenoceptor antagonist timolol was added 10 min after the application of mesaconitine, it failed to block the long-lasting enhancement of the spike amplitude and the appearance of multiple population spikes.
5. Application of the selective β-adrenoceptor agonist isoprenaline (500 nM) to the hippocampal slices induced an increase in the amplitude of the orthodromic population spike and elicited 2–3 additional spikes. Mesaconitine (10 nM) did not further potentiate this enhancement of the spike amplitude when added after a 15 min pretreatment with isoprenaline.
6. Perfusion of forskolin, which directly activates adenylate cyclase, enhanced the population spike. Mesaconitine had no additional effect when applied after pretreatment with forskolin.
7. It is concluded that the excitatory effects evoked by lower concentrations of the plant alkaloid mesaconitine are mediated by stimulation of β-adrenoceptors and the consequent activation of intracellular processes which lead to the long-lasting changes in excitability.

     

Contribution of both α- and β-adrenoceptors to the inotropic effects of catecholamines in the rabbit heart

Summary The functional role of -adrenoceptors was investigated in different parts of the rabbit heart. Phenylephrine (PE) caused a marked increase in force of contraction (F_c) and a prolongation of the action potential (AP) in preparations from the left atrium and the right ventricle. The response was less pronounced in the right atrium and in the left ventricle, whereas APs of spontaneously beating sinoatrial preparations remained completely unchanged. Phentolamine as well as the diesters phorbol 12,13 dibutyrate (PDBu) or 12-O-tetradecanoylphorbol-13-acetate (TPA) eliminated the effects of PE. The contribution of a-adrenoceptors to the effects of adrenaline (Adr) and noradrenaline (NA) on F_c was determined in preparations from the right ventricle. Phentolamine and the phorbol diesters reduced the effects of Adr and NA by about 30 to 60%; the remaining response was abolished by propranolol. It can be derived from our experiments that, in some parts of the rabbit heart, a considerable amount of the effects of Adr and NA is due to the stimulation of a-adrenoceptors. The present findings therefore support the view that, in the rabbit heart, the maximally effective drive of the heart requires the stimulation of both - and -adrenoceptors. The inhibitory effects of phorbol diesters on the -adrenoceptor-mediated response indicate that the activation of protein kinase C (PKC) specifically uncouples -adrenoceptors from the effector system, whereas the response to -adrenoceptor stimulation remains unchanged.Correspondence to: H. Nawrath at the above address     

Role of α1- and β3-adrenoceptors in the modulation by SR59230A of the effects of MDMA on body temperature in the mouse

Background and purpose:

We have investigated the ability of the β₃-adrenoceptor antagonist 1-(2-ethylphenoxy)-3-[[(1S)-1,2,3,4,-tetrahydro-1-naphthalenyl]amino]-(2S)-2-propanol hydrochloride (SR59230A) to affect the hyperthermia produced by methylenedioxymethamphetamine (MDMA) in conscious mice and whether α₁-adrenoceptor antagonist actions are involved.

Experimental approach:

Mice were implanted with temperature probes under anaesthesia, and allowed 2 week recovery. MDMA (20 mg·kg⁻¹) was administered subcutaneously 30 min after vehicle or test antagonist and effects on body temperature monitored by telemetry.

Key results:

Following vehicle, MDMA produced a slowly developing hyperthermia, reaching a maximum increase of 1.8°C at 130 min post injection. A low concentration of SR59230A (0.5 mg·kg⁻¹) produced a small but significant attenuation of the slowly developing hyperthermia to MDMA. A high concentration of SR59230A (5 mg·kg⁻¹) revealed a significant and marked early hypothermic reaction to MDMA, an effect that was mimicked by the α₁-adrenoceptor antagonist prazosin. Functional and ligand binding studies revealed actions of SR59230A at α₁-adrenoceptors.

Conclusions and implications:

1-(2-ethylphenoxy)-3-[[(1S)-1,2,3,4,-tetrahydro-1-naphthalenyl]amino]-(2S)-2-propanol hydrochloride in high concentrations modulates the hyperthermic actions of MDMA in mice in two ways: by blocking an early α₁-adrenoceptor-mediated component to reveal a hypothermia, and by a small attenuation of the later hyperthermic component which may possibly be β₃-adrenoceptor-mediated (this seen with the low concentration of SR59230A). Hence, the major actions of SR59230A in modulating the actions of MDMA on temperature involve α₁-adrenoceptor antagonism.     

Age-related alterations in α1- and β-adrenoceptors mediated responsiveness of rat aorta

Summary We have examined the responsiveness of - and -adrenoceptors in aortae from 1.5, 3, 6 and 24 month old rats. The isometric contraction to phenylephrine was antagonised competitively by prazosin with a pA₂ value of 9.45, suggesting that the receptor is an ₁-adrenoceptor. The potency of phenylephrine was significantly reduced in 24 months old as compared with all younger rats combined. The maximum contraction to phenylephrine was unaltered in 24 month old rats. The maximum contraction to potassium chloride was significantly less than that to phenylephrine only in 1.5 months old rats. In tissues contracted by potassium chloride, isoprenaline produced a marked relaxation in 1.5 months old animals, but there was a progressive loss with increasing age of the -adrenoceptor-mediated relaxation which was markedly reduced by 6 months and abolished in 24 months old. It is concluded that, in the rat aorta, there is a decrease in ₁-adrenoceptor responsiveness in senescence, and a loss of -adrenoceptor-mediated responses in maturation. Send offprint requests to J. R. Docherty at the above address     

Actions of tizanidine on α1- and α2-adrenoceptors in the peripheral tissues

• 1.1. Tizanidine behaved as the partial agonist on the α₁-adrenoceptor in high doses (10⁻⁶−10⁻⁴ M) and as the α₂-adrenoceptor agonist in low doses (3 × 10⁻⁹−10⁻⁶ M).
• 2.2. Tizanidine is about one-third as potent as clonidine in α₂-agonistic effects.

     

Regional distribution of β1- and β2-adrenoceptors in the failing and nonfailing human heart

Summary Total -adrenoceptor density and ₁- and ₂-subtype distribution in right and left atria and in different ventricular regions from 14 failing and seven nonfailing human hearts have been compared. End-stage heart failure was due to idiopathic dilated cardiomyopathy (n=8) or ischaemic cardiomyopathy (n=6).In nonfailing hearts the total -adrenoceptor density was similar in the right and left atria and in all the ventricular regions studied (about 70 to 80 fmol/mg protein). The ₁:₂-adrenoceptor ratio in both nonfailing atria was similar (about 70:30%) and was significantly smaller than in the different regions of both ventricles (about 80:20%). The ₁-subtype density was similar in nonfailing atria and ventricles (about 55 fmol/mg protein). The ₂-subtype density was significantly higher in the right and left atrium (about 25 fmol/mg protein) than in both ventricles (about 15 fmol/mg protein).In patients with end-stage heart failure due to idiopathic dilated cardiomyopathy or ischaemic cardiomyopathy the total -adrenoceptor density was reduced by 50–60% in all regions. On the other hand, the ₁- and ₂-subtype distribution differed with the cause of heart failure. In patients with idiopathic dilated cardiomyopathy, the ₁-adrenoceptor density was lower in all regions, but the ₂-adrenoceptor density was not significantly reduced. In patients with ischaemic cardiomyopathy both ₁- and ₂-adrenoceptors were reduced in all regions.It is concluded that downregulation of -adrenoceptors in patients with end-stage idiopathic dilated cardiomyopathy or ischaemic cardiomyopathy occurs uniformly throughout the heart. The results support the hypothesis that changes in -adrenoceptor subtypes may be related to the cause of heart failure.     

Preponderance of β- over α-adrenoceptors in mediating the positive inotropic effect of phenylephrine in the ferret ventricular myocardium

Summary [³H]prazosin bound to the membrane fraction derived from the ferret ventricular muscle with high affinity in a saturable manner (K _d = 0.25 nmol/l and B _max = 27 fmol/mg protein in the right ventricle). [³H]CGP-12177, a -adrenoceptor ligand, bound to the membrane fraction with a _{K d} value of 0.29 nmol/l and a B _max of 42 fmol/mg protein. In the isolated ferret papillary muscle driven at 1 Hz at 37°C, phenylephrine elicited a concentration-dependent positive intropic effect. The maximal effect of phenylephrine was comparable to that of isoprenaline. Prazosin (0.3 ol/l) shifted the concentration-response curve for phenylephrine slightly but significantly to the right, the maximal response being unaffected. In contrast, bupranolol (0.3 gmol/l) shifted the curve for phenylephrine markedly downwards: the maximal response was depressed significantly to 40% and the curve became less steep. In the presence of prazosin and bupranolol the curve was shifted to the right, being essentially parallel to the control curve. These results indicate that in the ferret ventricular myocardium both - and -adrenoceptors mediate the positive inotropic effect of phenylephrine. The extent of contribution of the two classes of adrenoceptor is quite different from that in other mammalian species. In the ferret heart, -adrenoceptors predominate over -adrenoceptors in mediating the positive inotropic effect of phenylephrine, although the number of -adrenoceptors is not especially high when compared with other species. Send offprint requests to M. Endoh at the above address     

Both α1- and β1-adrenoceptors in the bed nucleus of the stria terminalis are involved in the expression of conditioned contextual fear

BACKGROUND AND PURPOSE

The bed nucleus of the stria terminalis (BNST) is a limbic structure that is involved in the expression of conditioned contextual fear. Among the numerous neural inputs to the BNST, noradrenergic synaptic terminals are prominent and some evidence suggests an activation of this noradrenergic neurotransmission in the BNST during aversive situations. Here, we have investigated the involvement of the BNST noradrenergic system in the modulation of behavioural and autonomic responses induced by conditioned contextual fear in rats.

EXPERIMENTAL APPROACH

Male Wistar rats with cannulae bilaterally implanted into the BNST were submitted to a 10 min conditioning session (6 footshocks, 1.5 ma/ 3 s). Twenty-four hours later freezing and autonomic responses (mean arterial pressure, heart rate and cutaneous temperature) to the conditioning box were measured for 10 min. The adrenoceptor antagonists were administered 10 min before the re-exposure to the aversive context.

KEY RESULTS

L-propranolol, a non-selective β-adrenoceptor antagonist, and phentolamine, a non-selective α-adrenoceptor antagonist, reduced both freezing and autonomic responses induced by aversive context. Similar results were observed with {"type":"entrez-protein","attrs":{"text":"CGP20712","term_id":"874704353"}}CGP20712, a selective β₁-adrenoceptor antagonist, and WB4101, a selective α₁-antagonist, but not with ICI118,551, a selective β₂-adrenoceptor antagonist or RX821002, a selective α₂-antagonist.

CONCLUSIONS AND IMPLICATIONS

These findings support the idea that noradrenergic neurotransmission in the BNST via α₁- and β₁-adrenoceptors is involved in the expression of conditioned contextual fear.     

Regulation of the expression of the proenkephalin gene in cultured meningeal fibroblasts: opposite effects of α1- and β2-adrenoceptors

Meningeal fibroblasts express the proenkephalin gene during embryonal development but terminate the expression shortly before birth. When brought into primary culture at postnatal day 1, the fibroblasts again express the gene. Activation of protein kinase A reduces this expression and thus may contribute to its prenatal termination. Since the noradrenergic innervation of the meninges begins around the time of birth, it was investigated in the present study, how adrenergic agonists affected the levels of proenkephalin mRNA in cultured fibroblasts. The ₂-adrenoceptor agonists salbutamol and procaterol increased the levels of endogenous cAMP and diminished the concentration of proenkephalin mRNA indicating that the cultured fibroblasts possessed this -subtype. In contrast, noradrenaline increased the level of proenkephalin mRNA in a concentration-dependent manner. This effect was independent of endogenous cAMP and was mediated by ₁-adrenoceptors. The data indicate that the noradrenergic innervation of the meninges at the time of birth is not responsible for the termination of the proenkephalin gene expression.     

Effects of long-term active immunization with the second extracellular loop of human β1- or β3-adrenoceptors in thoracic aorta and mesenteric arteries in Lewis rats

ObjectiveTo evaluate whether active immunization producing β₁- or β₃-antibodies (β₁-ABs and β₃-ABs) detected in sera of patients with dilated cardiomyopathies has deleterious effects on vascular reactivity in Lewis rat thoracic aorta (TA) and small mesenteric arteries (SMA).Design and methodLewis rats were immunized for 6 months with peptidic sequences corresponding to the second extracellular loop of β₁- and β₃-adrenoceptors (ARs). During the immunization, systolic blood pressure (SBP) was monitored using the tail cuff method. The vascular reactivity of immunized rats was assessed by ex vivo studies on SMA and TA using various β-AR agonists, phenylephrine and KCl.ResultsThe immunizations producing functional β₁-ABs and β₃-ABs did not affect the SBP. However, in TA from β₁-AR-immunized rats, the relaxations mediated by dobutamine and salbutamol were significantly impaired in comparison with adjuvant rats whereas nebivolol-induced relaxation was not modified. Moreover, phenylephrine and KCl-mediated contractions were enhanced in these rats. In contrast, immunization with β₃-AR peptide led to the increase of relaxations induced by dobutamine in TA but did not change those induced by salbutamol and nebivolol. Surprisingly, in SMA from both rats immunized with β₁- or β₃-peptides, relaxations induced by the various β-agonists were not changed whereas phenylephrine and KCl-mediated contractions were impaired.ConclusionsOur study shows that β₁- and β₃-ABs can affect vascular reactivity. β₁-ABs would have a pathogenic action whereas β₃-ABs would have a beneficial effect on aorta reactivity.     

Opposite interactions between α- and β-endorphin fragments with dopamine mediated responses on the rat rectum in vitro

Summary Dopamine causes a dose-dependent contraction of the rat rectum in vitro followed by a relaxation. This contraction can be inhibited by apomorphine and phenylephrine. This inhibition can be attenuated by the -endorphin (E) fragments 2–17 (des-Tyr¹--endorphin, DTE) and 6-17 (des-enkephalin--endorphin, DEE). E 6-17 seems to be the shortest sequence with full activity in this respect since a shorter fragment (E 10-17) was less effective. The atypical neuroleptics oxypertine, sulpiride, and clozapine, the classic neuroleptic haloperidol and metoclopramide have a similar action to DEE. The peptides and atypical neuroleptics do not affect the dopamine response per se while the classic neuroleptics haloperidol and metoclopramide enhance the dopamine response.The effects of the -type endorphins are opposite to those of the -type endorphins, since des-Tyr¹--endorphin (DTE, E 2-16) and des-enkephalin--endorphin (DEE, E 6-16) enhance the phenylephrine-induced decreased responsiveness to dopamine. Structure-activity studies revealed that the active moiety of the -endorphin fragments probably resides in the 6–9 region. In addition the -type endorphins directly inhibit the dopamine response.It is concluded that the rat rectum may be used to analyse neuroleptic-like action. In this model - and -endorphin fragments may directly or indirectly influence the interaction of dopamine with the rectum. Because of the strong similarities between the effects of -type endorphins and that of neuroleptics the results support the purported neurolepticlike action of -type endorphins. The influence of -type endorphins and -type endorphins on the apomorphine or phenylephrine induced decreased responsiveness to dopamine, although opposite, seems to be mediated by an influence on different dopamine sensitive systems.     

Different steric characteristics of β1- and β2-adrenoceptors

Summary -Adrenoceptors of lung (75% ₂) and heart (95% ₁) of calf were labelled with ³H-(–)-propranolol. The stereoisomers of 10 ligands were used to inhibit the binding of ³H-(–)-propranolol to membrane particles. The affinity ratio of sereoisomers is consistently greater for ₁-adrenoceptors than for ₂-adrenoceptors, regardless of whether the ligands are agonists, partial agonists or antagonists. The ₁-adrenoceptor appears to possess stricter steric requirements than the ₂-adrenoceptor. This property may prove helpful in differentiating the -adrenoceptor subtypes during receptor solubilization and purification.     

Association of genes coding for the α-4, α-5, β-2 and β-3 subunits of nicotinic receptors with cigarette smoking and nicotine dependence

We assessed whether smoking behavior was associated with nine polymorphisms in genes coding for the nicotinic receptor subunits α-4 (rs1044394, rs1044396, rs2236196 and rs2273504), α-5 (rs16969968), β-2 (rs2072661 and rs4845378) and β-3 (rs4953 and rs6474413). We conducted an Internet survey and collected saliva by mail for DNA and cotinine analyses, in Switzerland in 2003. We conducted DNA analyses for 277 participants and cotinine analyses for 141 current daily smokers. Cotinine levels were higher in carriers of the CC genotype of CHRNA4 rs1044396 (371 ng/ml) than in those with the CT or TT genotypes (275 ng/ml, p = 0.049), a difference of 0.53 standard deviation units. However, this difference was not robust to correction for multiple testing using Bonferroni adjustment. These 9 polymorphisms were not otherwise associated with smoking behavior and nicotine dependence. There were possible associations between the temperament trait novelty seeking and CHRNA4 rs1044396, CHRNA5 rs16969968 and CHRNB2 rs4845378, but these associations were not robust to correction for multiple testing. We conclude that the analysis of polymorphisms in genes coding for four nicotinic acetylcholine receptor subunits (CHRNA4, CHRNA5, CHRNB2 and CHRNB3) and several smoking-related phenotypes revealed no statistically significant association.     

The role of α1- and α2-adrenoceptors in the coronary vasoconstrictor responses to neuronally released and exogenous noradrenaline in the dog

Summary 1. Coronary vasoconstriction was examined in response to the neuronal release of noradrenaline produced by bilateral carotid occlusion and the infusion of tyramine (5 – 50 Erg/kg/min i. v.) in anaesthetized dogs which had been vagotomized and treated with the -adrenoceptor antagonist propranolol (1.0 mg/kg i. v.). These responses were compared to those produced by the infusion of noradrenaline (0.1 – 0.5 g/kg/min i. v.). 2. Similar increases in late diastolic coronary resistance were produced by bilateral carotid occlusion (0.70 ± 0.25 mmHg min/ml), and intravenous infusions of tyramine, 20 g/kg/min (0.70 ± 0.12 mm Hg min/ml) and noradrenaline, 0.5 gg/kg/min (0.59 ± 0.11 mm Hg min/ml). 3. Selective antagonism at ₁-adrenoceptors with prazosin (0.5 mg/kg i. v.) attenuated the coronary constrictor response to bilateral carotid occlusion (0.36 ± 0.09 mm Hg min/ml), tyramine (0.12 ± 0.06 mm Hg min/ml) and noradrenaline (0.18 ± 0.07 mm Hg min/ml). Antagonism at ₂-adrenoceptors with idazoxan (1 mg/kg i. v.) attenuated the coronary vasoconstriction produced by bilateral carotid occlusion (0.30 ± 0.06 mmHg min/ml), tyramine (0.17 ± 0.08 mmHg min/ml) and noradrenaline (0.12 ± 0.03 mm Hg min/ml). Combined antagonism at both ₁- and ₂-adrenoceptors with prazosin and idazoxan abolished the responses to bilateral carotid occlusion, tyramine and noradrenaline. 4. These results show that coronary vasoconstriction produced by either neuronally released or exogenous noradrenaline is mediated by both ₁ and ₁-adrenoceptors. It appears that in the coronary resistance vessels of the dog postjunctional ₁- and ₂-adrenoceptors are both innervated by sympathetic nerves.     

Involvement of μ-opioid receptors and α-adrenoceptors in the immunomodulatory effects of dihydroetorphine

The present study investigated the effects of acutely administered dihydroetorphine on mitogen-stimulated lymphocytes proliferation and lyrnphokine production in mice.These immune functions were significantly suppressed by dihydroetorphine at 24μg·kg~(-1) and 128μ·g-kg~(-1) in a dose-dependent fashion.This study further examined the involvement of μ-opioid receptors and     

Involvement of α1- and α2-adrenoceptors in the responses of proximal and distal segments of the canine saphenous vein to exogenous and endogenous noradrenaline

Summary In the present study the relationship between adrenergic nerve terminals and postjunctional -adrenoceptors mediating the responses to the endogenous transmitter was compared at proximal and distal levels of the canine saphenous vein.Concentration-response curves to noradrenaline and to tyramine as well as frequency-response curves to electrical stimulation were compared at both levels of the vessel, in the absence and presence of either prazosin (100nmol·l^–1) or yohimbine (100nmol·l^–1) The influence of inhibition of neuronal uptake by cocaine (12 µmol·l^–1) on the responses to noradrenaline in the presence of prazosin (56 nmol·l^–1) or yohimbine (20 nmol·l^–1) was compared at the proximal level. The results show that, at the proximal level, the maximal responses to electrical stimulation and tyramine reached 80.1±2.2 (n = 18) and 74.2±1.9 (n = 18)%, respectively, of the maximal responses to noradrenaline, and 70.3±0.8 (n = 15) and 53.1 ± 1.2 (n = 14) %, respectively, at the distal level. Furthermore, the proximal strips were more sensitive to electrical stimulation than the distal ones. Prazosin had a much greater inhibitory effect on the contractile responses to noradrenaline than on those to electrical stimulation, at both levels. At proximal level, the shifts (to the right) of the concentration (frequency)-response curves (at EC₅₀) amounted to 0.58±0.02 (n = 16) and 0.18±0.02 (n = 8) log units, respectively (P<0.05), but, at the distal level, to 1.12±0.03 (n = 16) and 0.28±0.08 (n = 8) log units, respectively (P< 0.05). At the proximal level, yohimbine antagonizes about equally the responses to noradrenaline and the responses to electrical stimulation. However, at the distal level, the shift of the concentration-response curve to noradrenaline was much larger than that of the frequency-response curve to electrical stimulation [1.12±0.07 and 0.80±0.10 (n = 6) log units at EC₅₀, respectively (P<0.05)]. The leftward shift of the concentration-response curve to noradrenaline caused by cocaine was more pronounced in the presence of prazosin than in the presence of yohimbine: 0.95±0.15 and 0.69±0.12 (n = 12) log units, respectively (P<0.05).We conclude that, in the canine saphenous vein: 1) noradrenaline released from the adrenergic nerve terminals by electrical stimulation and by tyramine preferentially activates ₂-adrenoceptors at both proximal and distal levels; 2) the effectiveness of ₂-adrenoceptor stimulation is greater at the proximal than at the distal level; 3) ₁-adrenoceptors at the distal level seem to be different from those at the proximal one. Send offprint requests to S. Guimarães at the above address     

Blockade of putative β4- and β1-adrenoceptors by carvedilol in ferret myocardium

The putative β₄-adrenoceptor mediates positive inotropic effects, action potential shortening and arrhythmias in ferret ventricle. Here we compared the affinity of carvedilol at the putative β₄-adrenoceptor and β₁-adrenoceptor, activated by (±)-CGP 12177 and (–)-isoprenaline, respectively. In paced right ventricular preparations, carvedilol (0.01–10 μmol/l) was a simple competitive antagonist of the positive inotropic effects of (±)-CGP 12177 (slope of Schild-plot = 1.02, pK _B = 6.8) and (–)-isoprenaline (slope of Schild-plot = 0.98, pK _B = 8.1). Carvedilol also blocked putative β₄- and β₁-adrenoceptors of left ventricle, left atrium and sino-atrial pacemaker. Carvedilol therefore interacts with the putative β₄-adrenoceptor according to the law of mass action and may provide a lead in the development of putative β₄-adrenoceptor-selective antagonists. Received: 25 January 1999 / Accepted: 2 March 1999     

Involvement of central β-adrenoceptors in the regulation of yawning responses

Summary A behavioral study was performed in an attempt to understand the role of central -adrenoceptors in yawning in rats. Yawning was evoked by apomorphine and piribedil, mixed dopamine D₁/D₂-receptor agonists, but not by SK&F 38393 [1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8diol], a dopamine Dr-receptor angonist. The apomorphine-induced yawning was increased by pindolol, propranolol, indenolol, alprenolol and bukumolol which block the central -adrenoceptors, but not by the peripheral -adrenoceptor antagonists, carteolol and atenolol. These -adrenoceptor antagonists given alone did not elicit yawning. Conversely, the yawning was inhibited by salbutamol, a -adrenoceptor agonist, without being affected by prazosin, an -adrenoceptor antagonist. The combined administration of SK&F 38393 and the -adrenoceptor antagonists did not induce yawning. The yawning elicited by either apomorphine or piribedil in combination with pindolol was suppressed by spiperone and YM-09151-2 [cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4-methylaminobenzamide], dopamine D₂-receptor antagonists, and scopolamine, a muscarinic receptor antagonist, but not by SCH 23390 [R(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol], a dopamine D₁-receptor antagonist. Physostigmine or pilocarpine induced yawning, which was also enhanced by pindolol and propranolol. This enhanced yawning was inhibited by scopolamine, but not by spiperone, YM-09151-2 and SCH 23390. The present results suggest that the \-adrenoceptor blockade facilitates the occurrence of yawning induced by dopaminergic and cholinergic agonists. Therefore, the central adrenergic neuron systems may participate in the regulation of yawning responses.Send offprint requests to K. Yamada at the above address