Acute dilation to α2-adrenoceptor antagonists uncovers dual constriction and dilation mediated by arterial α2-adrenoceptors

Background and purpose:

In mouse tail arteries, selective α₂-adrenoceptor antagonism with rauwolscine caused powerful dilation during constriction to the α₁-adrenoceptor agonist phenylephrine. This study therefore assessed phenylephrine''s selectivity at vascular α-adrenoceptors and the mechanism(s) underlying dilation to rauwolscine.

Experimental approach:

Mouse isolated tail arteries were assessed using a pressure myograph.

Key results:

The α₂-adrenoceptor agonist UK14,304 caused low-maximum constriction that was inhibited by rauwolscine (3 × 10⁻⁸ M) but not by the selective α₁-adrenoceptor antagonist prazosin (10⁻⁷ M). Concentration–effect curves to phenylephrine, cirazoline or noradrenaline were unaffected by rauwolscine but were inhibited by prazosin, which was more effective at high compared with low levels of constriction. In the presence of prazosin, rauwolscine inhibited the curves and was more effective at low compared with high levels of constriction. Although rauwolscine alone did not affect concentration–effect curves to phenylephrine, noradrenaline or cirazoline, it caused marked transient dilation when administered during constriction to these agonists. Dilation was mimicked by another α₂-adrenoceptor antagonist (RX821002, 3 × 10⁻⁸ M), was dependent on agonist selectivity, and did not occur during adrenoceptor-independent constriction (U46619). During constriction to UK14,304 plus U46619, rauwolscine or rapid removal of UK14,304 caused transient dilation that virtually abolished the combined constriction. Endothelial denudation reduced these dilator responses.

Conclusions and implications:

Inhibition of α₂-adrenoceptors caused transient dilation that was substantially greater than the contribution of α₂-adrenoceptors to the constriction. This reflects a slowly reversing α₂-adrenoceptor-mediated endothelium-dependent dilation and provides a rapid, sensitive test of α₂-adrenoceptor activity. This approach also clearly emphasizes the poor selectivity of phenylephrine at vascular α-adrenoceptors.     

Clonidine inhibits salivary secretion by activation of postsynaptic α2-receptors

Summary The effects of clonidine on the submaxillary gland of the rat were studied. Doses ranging between 100 to 3.000 g/kg produced a sustained secretory response which was blocked by 0.1 mg/kg of prazosin but not by 1 mg/kg of yohimbine. Clonidine 10 g/kg markedly inhibited the salivation induced by noradrenaline, methacholine and substance P but not that induced by isoproterenol. The inhibition caused by the ₂-agonist was greater for noradrenaline than for either methacholine or substance P. Blockade of ₂ adrenoceptors with yohimbine (0.3–1 mg/kg) prevented the inhibition by clonidine of noradrenaline, methacholine and substance P induced salivation. On the other hand, prazosin 0.1 mg/kg did not modify the inhibition by clonidine of methacholine induced secretion. The results obtained indicate that clonidine exerts a dual effect on salivary secretion: at high doses it elicits salivation through activation of ₁-adrenoreceptors; at the dose of 10 g/kg clonidine activates ₂-adrenoreceptors which inhibit the secretory response evoked through either muscarine, substance P and ₁-adrenorecptors agonists.Partially supported by grants No 3111 j/82, CONICET and 20241/82-9, SUBCYT     

N-ethylmaleimide (NEM) diminishes α2-adrenoceptor mediated effects on noradrenaline release

Summary The effect of N-ethylmaleimide (NEM), which has been shown to abolish rather selectively inhibition of adenylate cyclase, on the ₂-adrenoceptor modulation of noradrenaline release was studied. Slices of the rabbit hippocampus were loaded with ³H-noradrenaline, superfused continuously and stimulated twice electrically.NEM (30 mol/l) applied for 30 min enhanced both basal and stimulation-evoked tritium overflow significantly. Occupation of the receptor by the ₂-adrenoceptor agonist clonidine prior to and during NEM treatment did not protect the ₂-adrenoceptor-mediated autoinhibitory feedback system from being affected by NEM. Preincubation of the hippocampal slices with NEM was without any influence on ³H-noradrenaline uptake. The inhibitory effect of clonidine on ³H-noradrenaline release was attenuated in a non-competitive manner. In addition, the facilitatory effect of the ₂-adrenoceptor antagonist yohimbine on the stimulusevoked tritium overflow was reduced. The facilitation of the evoked noradrenaline release by yohimbine or yohimbine and NEM converged with increasing concentrations of yohimbine, suggesting that yohimbine and NEM were acting at the same signal-transduction system.These results are compatible with the idea that NEM, by alkylating the N_i-unit of a presynaptically located adenylate cyclase, prevents the ₂-adrenoceptor-mediated modulation of noradrenaline release.Abbreviations NEM N-ethylmaleimide - IAP islet-activating protein     

Preferential antagonism by diltiazem of α2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats

Summary Vasoconstrictor responses mediated by the ₂-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the ₁-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (>300 nmol/l) diltiazem became an effective antagonist of all -adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent againts vasoconstrictor responses to TL99 than in SHR. The blockade of ₁-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries.The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/l–3 mol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow SHR-tail arteries after labelling the tissue with³H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations.The tail artery preparation of SHR contains a population of postsynaptic ₂-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular ₂-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.     

Modulation of motor activity by α1 and α2 stimulation in mice

Summary The influence of two -adrenoceptor agonists, clonidine and B-HT 920, on motor activity was tested in mice. Both, clonidine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine) in the dose range 30–300 g/kg s.c. equieffectively inhibited exploratory activity. On the other hand only clonidine, which stimulates ₂- and ₂-adrenoceptors increased locomotor activity in mice treated with reserpine (5 mg/kg) and apomorphine (3 mg/kg) in the doses of 0.3 and 1 mg/kg i.p. The highly selective ₂-agonist B-HT 920 was ineffective under these conditions up to 30 mg/kg i.p. It is concluded, that in mice sedative -adrenoceptors are of the ₂- and excitatory of the ₁-type.     

Vasopressor nerve responses in the pithed rat, previously identified as α2-adrenoceptor mediated, may be α1D-adrenoceptor mediated

Responses to pressor nerve stimulation in the pithed rat have been variously described as mediated, at least in part, by α(2)-adrenoceptors and by α(1A) and α(1D)-adrenoceptors. We have examined the subtypes of α-adrenoceptor involved in rises in diastolic blood pressure in the pithed rat preparation produced by vasopressor nerve stimulation with 10 pulses at 1 Hz or 20 pulses at 5 Hz. Vasopressor nerve responses to 1 Hz stimulation were markedly inhibited by the α(1A)-adrenoceptor antagonist RS 100329 (0.1mg/kg) and by the α(1D-)adrenoceptor antagonist BMY 7378 (0.1mg/kg). The α(2)-adrenoceptor antagonist yohimbine (0.1mg/kg) significantly increased pressor nerve responses to 1 Hz stimulation, but yohimbine (1mg/kg) significantly reduced pressor nerve responses. However, following BMY 7378 (0.1mg/kg), yohimbine (1mg/kg) did not produce any further inhibition of pressor nerve responses to 1 Hz stimulation. The α(2A)-adrenoceptor antagonist BRL 44408 (1mg/kg) did not reduce pressor responses to 1 Hz stimulation. BMY 7378 produced much less inhibition of pressor nerve responses to 5 Hz stimulation, whereas RS 100329 produced similar inhibition of 1 Hz and 5 Hz responses. Yohimbine (0.1 and 1mg/kg) did not significantly affect pressor nerve responses to 5 Hz stimulation. In conclusion, pressor nerve responses in the pithed rat involve both α(1A) and α(1D)-adrenoceptor, but there is no clear evidence for the involvement of α(2)-adrenoceptors.     

Down-regulation of α2-adrenoceptor subtypes

To characterize further the α₂-adrenoceptor subtypes in terms of their regulation, monolayers of cells expressing either the α_2A (CHO-A2AR cells) or α_2C (OK cells) subtype were preincubated with norepinephrine for various times and the extent of receptor down-regulation was assessed. Exposure to 30 μM norepinephrine caused a similar time course and extent of down-regulation (approximately 50%) in both cells lines. The extent of down-regulation caused by 0.3 μM norepinephrine in OK cells was similar to that with 30 μM norepinephrine in CHO-A2AR cells, although the time course was somewhat slower. Reversal of the down-regulation of the α₂-adrenoceptor caused by 30 μM norepinephrine was more rapid in the CHO-A2AR than in the OK cell. With 0.3 μM norepinephrine, reversal of down-regulation of the α₂-adrenoceptor in the OK cell was slightly faster than that of the CHO-A2AR cell with 30 μM norepinephrine. These data indicate that although norepinephrine is more potent in causing down-regulation of the α_2C (OK cells) as compared to the α_2A subtype (CHO-A2AR cells), the time courses for down-regulation and its reversal are similar for the two subtypes.     

Calcium entry blockers inhibit vasoconstrictor responses to sympathetic nerve stimulation mediated by α1-adrenoceptors

Summary Stimulation of the sympathetic outflow (spinal cord segments T 7–9) in pithed rats resulted in an increase in mean arterial pressure, heart rate, total peripheral vascular resistance and cardiac output. The increase in blood pressure and peripheral resistance was markedly depressed by prazosin and to a lesser extent by yohimbine, suggesting that these responses were mediated primarily by postjunctional ₁-adrenoceptors. The calcium entry blockers nifedipine, tiapamil and verapamil also depressed pressor responses and the increase in total peripheral resistance to electrical stimulation of the sympathetic outflow in these rats. This depression resulted primarily from an effect on peripheral vascular resistance components, as cardiac output remained unaffected by the calcium entry blockers. This conclusion was supported by studies on isolated, perfused rat renal arteries. Vasoconstrictor responses of this in vitro preparation to perivascular nerve stimulation were depressed by 1,000-fold lower concentrations of prazosin than rauwolscine, demonstrating the predominantly ₁-adrenoceptor nature of these effects. Likewise, these vasoconstrictor responses were depressed by nifedipine, tiapamil and verapamil in a concentration-dependent manner. The results of this study suggest that vasoconstrictor responses of rat resistance vessels to sympathetic nerve stimulation are mediated primarily by postjunctional ₁-adrenoceptors and can be inhibited by calcium entry blockers. This implies that contractile responses of these resistance vessels to ₁-adrenoceptor stimulation are not independent of the availability of extracellular calcium.     

Islet-activating protein (pertussis toxin) diminishes α2-adrenoceptor mediated effects on noradrenaline release

Summary The effect of islet-activating protein (IAP) on ₂-adrenoceptor mediated modulation of noradrenaline release in the rabbit hippocampus was studied. Slices of the hippocampus were incubated for 6 h with IAP, subsequently loaded with³H-noradrenaline and superfused continuously. IAP-pretreatment significantly enhanced the electrically evoked transmitter release and diminished the facilitatory effect of the ₂-adrenoceptor antagonist yohimbine. In addition, the inhibitory effect of the ₂-adrenoceptor agonist clonidine was reduced. These results provide circumstantial evidence that an inhibitory guanine-nucleotide-binding protein, most probably N₁ of a presynaptically located adenylate cyclase, is involved in the ₂-autoreceptor mediated modulation of noradrenaline release.Abbreviations IAP islet-activating protein - LPA leukocytosis-promoting activity     

Effects of antidepressant drugs and electroconvulsive shock on pre- and postsynaptic α2-adrenoceptor function in the brain: rapid down-regulation by sibutramine hydrochloride

Clonidine (0.1 mg/kg IP)-induced hypoactivity and mydriasis responses were respectively used as functional indices of pre- and postsynaptic ₂-adrenoceptors in mouse brain. A single injection of various antidepressant drugs had no effect on either response when measured 24 h later. However, 14 days' treatment with sibutramine HCl (3 mg/kg IP), dothiepin (50 mg/kg IP), amitriptyline (10 mg/kg IP), desipramine (10 mg/kg IP) or tranylcypromine (10 mg/kg IP) markedly attenuated both clonidine-induced hypoactivity and mydriasis. Repeated administration of zimeldine (10 mg/kg IP), mianserin (10 mg/kg IP) or clenbuterol (5 mg/kg IP) had no effect on either response. Subchronic treatment with sibutramine HCl (3 mg/kg IP; 3 days) also attenuated pre- and postsynaptic ₂-adrenoceptor function. Five ECS (200 V, 2 s) spread over 10 days, but not a single shock, reduced the hypoactivity and mydriasis responses to clonidine. Together, the results indicate that pre- and postsynaptic ₂-adrenoceptor function is attenuated by repeated treatment with those antidepressants which acutely increase synaptic levels of noradrenaline. These adrenergic receptor populations are also desensitized by ECS, although this effect is probably mediated via a different mechanism. Finally, the rapid down-regulation observed with sibutramine HCl is not confined to-adrenoceptors alone, because pre- and postsynaptic ₂-adrenoceptor function is also attenuated by 3 days of treatment with this novel antidepressant drug.     

The effect of selective α1- and α2-adrenoceptor stimulation on intraocular pressure in the conscious rabbit

Summary The influence of topically applied selective ₁- and ₂-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the ₁-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the ₂-subtype caused a marked and dose-dependent ocular hypotensive response, which was blocked by the selective ₂-adrenoceptor antagonist yohimbine. ₂-Agonists induced neither macroscopic ocular side effects, nor an effect on the pupil size. Possibly, the subclass of ₂-adrenoceptor stimulating drugs represent a group of new antiglaucomatous agents.     

Effect of converting enzyme inhibition and angiotensin receptor blockade on the vasoconstriction mediated by α1 and α2 stimulation in pithed normotensive rats

Summary The effect of functional impairment of the renin-angiotensin system on the vasoconstriction mediated by postsynaptic ₁ and ₂-adrenoceptors in pithed normotensive rats was studied. Selective ₁-adrenoceptor stimulation was induced by intravenously administered cirazoline, whereas B-HT 920 was used as a selective agonists at ₂-adrenoceptors. The angiotensin converting enzyme was inhibited by intravenous treatment of the pithed rats with captopril, teprotide or enalapril. Blockade of angiotensin receptors was produced by intravenously applied [Sar¹ Ala⁸]angiotensin II (saralasin). Pretreatment with angiotensin converting enzyme inhibitors or with saralasin in doses which produced a maximal reduction in basal diastolic blood pressure, only slightly attenuated the hypertensive response to cirazoline. In contrast, these drugs provoked a most significant reduction of the ₂-adrenoceptor mediated vasoconstriction. Restoration of the basal diastolic blood pressure by intravenous infusion with angiotensin II or with vasopressin completely reversed the inhibitory effect of captopril on the vasopressor response to B-HT 920. One hour after bilateral nephrectomy, captopril still reduced the ₂-adrenoceptor mediated vasoconstriction. However, 18–24 h after bilateral nephrectomy, captopril had no additional inhibitory effect on the vasopressor response to selective ₂-adrenoceptor stimulation. It is concluded that in pithed normotensive rats the pressor response to ₂-adrenoceptor stimulation is significantly potentiated by endogenous angiotensin II, even at low circulating levels of the octapeptide. The modulatory action of angiotensin II on the -adrenoceptor mediated vasoconstriction probably represents an effect on the basal arteriolar muscular tone rather than a specific interaction.     

Nicotinic excitatory postsynaptic potentials in hippocampal CA1 interneurons are predominantly mediated by nicotinic receptors that contain α4 and β2 subunits

In the hippocampus, activation of nicotinic receptors that include α4 and β2 subunits (α4β2*) facilitates memory formation. α4β2* receptors may also play a role in nicotine withdrawal, and their loss may contribute to cognitive decline in aging and Alzheimer’s disease (AD). However, little is known about their cellular function in the hippocampus. Therefore, using optogenetics, whole cell patch clamping and voltage-sensitive dye (VSD) imaging, we measured nicotinic excitatory postsynaptic potentials (EPSPs) in hippocampal CA1. In a subpopulation of inhibitory interneurons, release of ACh resulted in slow depolarizations (rise time constant 33.2 ± 6.5 ms, decay time constant 138.6 ± 27.2 ms) mediated by the activation of α4β2* nicotinic receptors. These interneurons had somata and dendrites located in the stratum oriens (SO) and stratum lacunosum-moleculare (SLM). Furthermore, α4β2* nicotinic EPSPs were largest in the SLM. Thus, our data suggest that nicotinic EPSPs in hippocampal CA1 interneurons are predominantly mediated by α4β2* nicotinic receptors and their activation may preferentially affect extrahippocampal inputs in SLM of hippocampal CA1.     

Pertussis toxin does not attenuate α2-adrenoceptor mediated inhibition of noradrenaline release in mouse atria

Summary 1. The ₂-adrenoceptor agonist clonidine (0.03 and 0.1 ol/l) significantly inhibited stimulation-induced overflow of radioactivity from mouse isolated atria pre-incubated with [³H]-noradrenaline. This effect of clonidine was blocked by idazoxan (0.3 gmol/l) but not prazosin (0.3 ol/l), indicating that an ₂-adrenoceptor was involved. 2. In some experiments mice were injected with pertussis toxin (1.5 g/mouse) 4 days before their atria were removed and subsequently incubated with [³H]-noradrenaline. Alternatively, isolated atria from untreated mice were suspended in Krebs-Henseleit solution, incubated for 16 h with pertussis toxin (1.0 and 4.0 g/ml) or vehicle and subsequently incubated with [³H]-noradrenaline. The effectiveness of pertussis toxin pretreatment was assessed indirectly using carbachol. Carbachol caused a dose dependent fall in both the rate and force of contraction of isolated, spontaneously beating atria from mice pretreated with vehicle in vivo or in vitro. This effect of carbachol was not seen in atria from mice pretreated with pertussis toxin in vivo or in vitro, suggesting that active toxin penetrated the myocardium. 3. Pertussis toxin pretreatment, either in vivo or in vitro did not alter the inhibitory effect of clonidine (0.03 and 0.1 gmol/l), or the facilitatory effect of the -adrenoceptor antagonist phentolamine (1.0 mol/l), on the stimulation-induced overflow of radioactivity. These results suggest that ₂-adrenoceptor modulation of noradrenaline release from sympathetic nerve terminals is not dependent on an inhibitory guanine-nucleotide-binding protein. Send offprint requests to I. Musgrave     

Rb efflux mediated by α4β2*-nicotinic acetylcholine receptors with high and low-sensitivity to stimulation by acetylcholine display similar agonist-induced desensitization

The nicotinic acetylcholine receptors (nAChR) assembled from α4 and β2 subunits are the most densely expressed subtype in the brain. Concentration-effect curves for agonist activation of α4β2*-nAChR are biphasic. This biphasic agonist sensitivity is ascribed to differences in subunit stoichiometry. The studies described here evaluated desensitization elicited by low concentrations of epibatidine, nicotine, cytisine or methylcarbachol of brain α4β2-nAChR function measured with acetylcholine-stimulated ⁸⁶Rb⁺ efflux from mouse thalamic synaptosomes. Each agonist elicited concentration-dependent desensitization. The agonists differed in potency. However, IC₅₀ values for each agonist for desensitization of ⁸⁶Rb⁺ efflux both with high (EC₅₀ ≈ 3 μM) and low (EC₅₀ ≈ 150 μM) acetylcholine sensitivity were not significantly different. Concentrations required to elicit desensitization were higher that their respective K_D values for receptor binding. Even though the two components of α4β2*-nAChR-mediated ⁸⁶Rb⁺ efflux from mouse brain differ markedly in EC₅₀ values for agonist activation, they are equally sensitive to desensitization by exposure to low agonist concentrations. Mice were also chronically treated with nicotine by continuous infusion of 0, 0.5 or 4.0 mg/kg/h and desensitization induced by nicotine was evaluated. Consistent with previous results, chronic nicotine treatment increased the density of epibatidine binding sites. Acute exposure to nicotine also elicited concentration-dependent desensitization of both high-sensitivity and low-sensitivity acetylcholine-stimulated ⁸⁶Rb⁺ efflux from cortical and thalamic synaptosomes. Although chronic nicotine treatment reduced maximal ⁸⁶Rb⁺ efflux from thalamus, IC₅₀ values in both brain regions were unaffected by chronic nicotine treatment.     

Modulation by antidepressant drugs of CNS postsynaptic α2-adrenoceptors mediating mydriasis in the rat

Summary The modulation of central postsynaptic ₂-adrenoceptors mediating mydriasis in the pentobarbitoneanaesthetized rat was studied after the acute and short/longterm administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline > protriptyline imipramine > clomipramine > nortriptyline > desipramine maprotiline) which were attenuated (17–55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of ₂-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7–21 days) but not the short-term (1–4 days) administration of cyclic antidepressant drugs (2.5–10 mg/kg, i. p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED₅₀ increased 1.2–2.0-fold; E _max decreased by 9–29%) which indicated desensitization of postsynaptic ₂-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1–1 mg/kg, i.p.) or idazoxan (3–10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic ₂-adrenoceptors mediating mydriasis in the rat. Send offprint requests to A. Menargues at the above address     

B-HT 958, a new α-adrenoceptor agonist with a high pre/postsynaptic activity ratio

Summary B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo [5,4-d] azepine) was a potent agonist at presynaptic and a less potent agonist at postsynaptic -adrenoceptor sites. Presynaptically this was shown in pithed rats by the inhibition of tachycardia evoked by sympathetic nerve stimulation (0.2 Hz). The 50% inhibitory dose (ID 50) was 0.3 mg/kg i.v. Moreover in isolated perfused cat hearts, the drug inhibited the tachycardia and the outflow of noradrenaline induced by sympathetic nerve impulses. These effects of B-HT 958 were antagonized by phentolamine or yohimbine. At postsynaptic sites high doses of B-HT 958 increased the blood pressure of decentralized rats. The dose which increased pressure by 30 mm Hg (PD30) was 46.3 mg/kg i.v. This effect was antagonized by rauwolscine 5 mg/kg i.v. After pretreatment with reserpine (7.5 mg/kg i.p., 18 h) B-HT 958 proved much more potent (PD30=0.6 mg/kg i.v.), and its effect was strongly antagonized by yohimbine but hardly by prazosin. The dose of yohimbine which shifted the dose-response curve of B-HT 958 by the factor of 10 (D 10) to the right was 1.8 mg/kg i.v., the corresponding dose of prazosin was 1,900 mg/kg i.v. (extrapolated).B-HT 958 showed also -adrenoceptor blocking properties. This was demonstrated presynaptically in pithed rats by the drug-induced augmentation of tachycardia elicited by electrical stimulation at high frequency (6.4 Hz). At postsynaptic sites B-HT 958 antagonized the blood pressure increase caused by B-HT 920 (₂; D 10=1.1 mg/kg i.v.) but not that caused by methoxamine (₁).It is concluded that B-HT 958 is a partial agonist at peripheral ₂-adrenoceptors. In doses of about 1 mg/kg and with low frequency sympathetic stimulation (<6.4 Hz) it acts presynaptically as agonist; in this dose the drug acts postsynaptically mainly as antagonist.Preliminary results were presented at the 23rd spring meeting of the German Pharmacological Society, March 16–19, 1982, Mainz FRG     

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

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

Nucleus locus coeruleus: Evidence for α1-adrenoceptor mediated hypotension in the cat

Summary The Bezold-Jarisch reflex characterized by hypotension and bradycardia was elicited in anaesthetized artificially respired dogs (pretreated with a beta-adrenoceptor antagonist) using capsaicin 10 g/kg (i.v.). Intracisternal administration of the highly selective clonidine-like alpha₂ adrenoceptor agonists B-HT 920 (10 g/kg) or B-HT 933 (30 g/kg) significantly facilitated this reflex bradycardia. The involvement of central alpha₂-adrenoceptors is suggested as intracisternal administration of the alpha₂ adrenoceptor blocking drugs yohimbine (50 g/kg) and piperoxan (50 g/kg) antagonized this facilitation. B-HT 920 also facilitated the vagally mediated baroreceptor reflex to the hypertensive effect of intravenous noradrenaline (3 g/kg). Although the Bezold-Jarisch reflex and the baroreceptor reflex have different afferent pathways, both reflexes may either converge into a common pathway or have separate neuronal chains within the medulla; however, this study indicates that both have a similar central modulatory system stimulated by alpha₂ adrenoceptors.