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     

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.     

Vasoconstriction mediated by postsynaptic α2-adrenoceptor stimulation

Summary The postsynaptic -adrenoceptors involved in vasoconstriction brought about by B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin) administered i.v. to pithed, normotensive rats were characterized. The rate of onset of the hypertensive response to i.v. B-HT 933 is slower than that induced by (–)-phenylephrine, an agonist of ₁-adrenoceptor. The antagonism of the -adrenoceptor blocking drugs rauwolscine, yohimbine and corynanthine was quantified towards B-HT 933-induced increases in diastolic pressure. Rauwolscine (pA₂=7.06) and yohimbine (pA₂=6.83) were effective antagonists, whereas corynanthine proved much less potent (pA₂=5.03). On the basis of the reported selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for ₁- and ₂-adrenoceptor, it is concluded that the postsynaptic -adrenoceptors triggered by B-HT 933 are of the ₂-type. B-HT 933 identifies a subclass of postsynaptic ₂-adrenoceptor in vascular smooth muscle distinct from postsynaptic ₁-adrenoceptor. Both types of -adrenoceptors are likely to be involved in the mediation of vasoconstriction.Preliminary data were presented at the 21th Spring Meeting of the German Pharmacological Soceity, Mainz, March 18–21, 1980 (Timmermans, 1980)     

The effects of some α-adrenoceptor antagonists on the responses of the canine saphenous vein to B-HT 933, UK-14304 and methoxamine

Summary The benzoquinolizines Wy 25309, Wy 26703 and Wy 27127, previously reported as potent antagonists at presynaptic ₂-adrenoceptors were also potent antagonists of B-HT 933 in isolated saphenous veins of the dog confirming their activity at post synaptic ₂-adrenoceptors. Yohimbine was a more potent antagonist of B-HT 933 in isolated saphenous vein than were the Wy compounds or idazoxan contrasting with the reported potencies of these compounds at presynaptic sites in rat vas deferens and raising the possibility of differences between pre- and postsynaptic ₂adrenoceptors. Contractions of the saphenous vein were observed with high concentrations of idazoxan. Send offprint requests to K. F. Rhodes at the above address     

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

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

H3 receptor-mediated inhibition of noradrenaline release: an investigation into the involvement of Ca2+ and K+ ions,G protein and adenylate cyclase

The effects of ₂-adrenoceptor agonists (dexmedetomidine, oxymetazoline), alone or in combination with various -adrenoceptor subtype-selective antagonists (CH-38083, idazoxan, WB4101, BRL44408, ARC-239, prazosin), on noradrenaline- and isoprenaline-induced lipolysis were investigated in human isolated abdominal subcutaneous fat cells. The rank order of potency of antagonists in preventing dexmedetomidine- and oxymetazoline-evoked suppression of isoprenaline-induced lipolysis was (pA₂-values): CH-38083 (7.69 and 7.48) idazoxan (7.5 and 7.41) > BRL 44408 (7.23 and 7.19) WB 4101 (7.13 and 7.12) > prazosin (5.18 and 5.17) > ARC-239 (4.72, 4.9). While CH-38083 and idazoxan, non-subtype selective ₂-adrenoceptor antagonists and BRL44408, a selective a_2A-adrenoceptor antagonist as well as WB4101 potentiated the lipolytic effect of noradrenaline, ARC-239, the selective _2B-adrenoceptor antagonist failed to affect it. In addition since the _2A-adrenoceptor selective agonist, oxymetazoline concentration dependently inhibited the lipolytic effect of isoprenaline, and WB4101 and BRL44408 (a_2A-adrenoceptor antagonists) antagonised the effect of oxymetazoline in a competitive manner, it is concluded that the a_2A-adrenoceptor subtype is involved in antilipolysis. In addition, functional evidence was obtained that there is an interaction between _2A- and -adrenoceptors located on the cell surface of adipocytes, through which locally released noradrenaline and/or circulating circulating adrenaline influence lipolysis.On leave from Institute of Medical Pharmacology, University of Pisa, Via Roma 55, I-5626 Pisa, Italy Correspondence to: E. S. Vizi at the above address     

SK&F 104078, a post-functionally selective α2-adrenoceptor antagonist in the human saphenous vein in vitro

Summary The present study investigated the effects of SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxy]-3methyl-1H,2,3,4,-tetrahydro-3-benzazapine) at pre- and post functional ₂-adrenoceptors in the human isolated saphenous vein. Noradrenaline (0.001–100 mol/l) produced concentration-dependent contractions of the human saphenous vein which were competitively antagonised by the ₁-adrenoceptor antagonist prazosin (0.01–1.0 mol/l) and the ₂-adrenoceptor antagonist, rauwolscine (0.01–1.0 mol/l), indicating the presence of both post functional ₁- and ₂-adrenoceptors in this preparation. The selective ₂-adrenoceptor agonist, UK-14,304 (0.01–100 mol/l) also produced concentration-dependent contractions of the human saphenous vein which were antagonised by both rauwolscine (0.1 mol/l) and prazosin (0.1 mol/l). In the presence of angiotensin II (0.05 mol/l), which itself produced a transient contraction, rauwolscine (0.1 mol/l) produced a rightward shift of the UK-14,304 concentration-response curve while prazosin (0.1 mol/l) had no effect. SK&F 104078 (10.0 mol/l) under these conditions also produced a rightward shift of the concentration-response curve to UK-14,304, but was at least 100-fold less potent than rauwolscine. At pre functional ₂-adrenoceptors, exogenous noradrenaline (0.01 and 0.1 gmol/l) induced a concentration-dependent inhibition of stimulation-evoked [7-³H]-noradrenaline release from the human saphenous vein in vitro, which was antagonised by rauwolscine (0:1 mol/l) and tolazoline (10.0 mol/l) but not by SK&F 104078 (10.0 gmol/l).Rauwolscine (0.1 mol/l) produced a small increase in stimulation-evoked [7-³H]-noradrenaline release while both tolazoline and SK&F 104078 failed to produce any enhancement in release in the absence of exogenous agonist atconcentrationsupto10 gmol/l.Insummary, noradrenaline and UK-14,304 contracted the human isolated saphenous vein by an action at both postfunctional ₁- and ₂-adrenoceptors. These data demonstrate that SK&F 104078 discriminates between post- and pre-junctional ₂-adrenoceptors in the human isolated saphenous vein. Send offprint requests to M. V. Sennitt at the above address     

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.     

Interactions between the putative calcium entry promotor Bay k 8644 and pressor responses produced by α-1 and α2-adrenocepter agonists in the pithed normotensive rat

Summary Interactions between the putative calcium entry promotor Bay k 8644 and both -₁ and ₁-adrenocepter mediated increases in diastolic pressure were studied in the pithed normotensive rat. The ₁-adrenocepter mediated pressor responses elicited by B-HT920, TL-99, DP-6,7-ADTN and B-HT958 were potentiated by Bay k 8644, reflected by a leftward shift and an increase in the maximum of the log dose-pressor respinse curves. The -₁-adrenocepter effects elicited by cirazoline, methoxamine, (–)-amidephrine, St 587, (–)-phenylephrine and Sgd 101/75 were less enhanced by Bay k 8644. Only a leftward shift of the dose-response curves was observed, which was most pronounced for (–)-phenylephrine and Sgd 101/75. The -₁ and ₂-adrenocepter pressor components of (–)-noradrenaline were similarly distinguished by Bay k 8644 as observed for the selective -₁ or ₂-adrenocepter agonists.Effects of Bay k 8644 on the increase in diastolic pressure mediated by B-HT 920, St 587 and cirazoline were also studied after pretreatment with the calcium entry blocker nifedipine. After additional pretreatment with nifedipine the potentiation by Bay k 8644 observed for B-HT 920 and St 587 was more pronounced. The presence of nifedipine had no effect on the interaction between Bay k 8644 and cirazoline.It is concluded that Bay k 8644 behaves as a mirror image of nifedipine. The observation that Bay k 8644 enhances ₂-adrenocepter mediated pressor effects more effectively than ₁-adrenocepter increases in diastolic pressure is in accordance with the hypothesis of the more pronounced calcium dependency of ₂-adrenocepter mediated pressor responses. The data obtained for ceptor mediated pressor responses. The data obtained for St 587 and (–)-phenylephrine are in apparent contradiction to the finding that the pressor responses to the former drug are more markedly inhibited by calcium entry blockade than those of the latter. It is suggested that St 587 employs calcium channels which are already maximally modulated and that (–)-phenylephrine makes use of calcium channels which are in a rather inactive state. The hypothesis is put forward that the intrinsic activity of ₂-adrenocepter agonists reflects their ability to bring calcium channels in an active state.     

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%     

Presynaptic α2A-adrenoceptors inhibit the release of endogenous dopamine in rabbit caudate nucleus slices

₂-Adrenoceptors modulating the release of dopamine were identified and characterized in slices of the head of the rabbit caudate nucleus. Release of endogenous dopamine was measured by fast cyclic voltammetry as the increase in the extracellular concentration of dopamine elicited by electrical stimulation. The electrochemical signal was identified as dopamine by means of the oxidation potential, the voltammogram and the fact that the signal was not changed by desipramine, which inhibits the high affinity uptake of noradrenaline, but was greatly increased by nomifensine, which in addition inhibits the high affinity uptake of dopamine.Stimulation by 6 pulses/100 Hz increased the extracellular concentration of dopamine by about 85 nM. The selective ₂-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced this release with an EC₅₀ of 173 nM and by maximally 75%. The ₂-adrenoceptor agonists clonidine and oxymetazoline only tended to cause a decrease. Six drugs, including oxymetazoline, were tested as antagonists against UK 14,304. Their order of antagonist potency (pK_D values in brackets) was rauwolscine (8.0) > oxymetazoline (7.5) > 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101; 7.3) > phentolamine (7.1) > corynanthine (5.1) prazosin (< 6). Given alone, the antagonists did not change the release of dopamine elicited by 6 pulses/100 Hz, and the same was true for the dopamine receptor antagonist sulpiride. When caudate slices were stimulated by 10 pulses/1 Hz, sulpiride increased the release of dopamine. Desipramine and rauwolscine, in contrast, again caused no change.It is concluded that dopaminergic axons in the rabbit caudate nucleus possess release-inhibiting ₂-adrenoceptors. The antagonist affinities indicate that they belong to the _2A subtype. In this, they agree with all presynaptic ₂-autoreceptors studied so far in rabbits as well as with the ₂-heteroreceptors modulating the release of serotonin in rabbit brain cortex, suggesting that at least the majority of presynaptic ₂-adrenoceptors in the rabbit are _2A. The agonist sensitivity of the caudate presynaptic ₂-adrenoceptors is low in comparison with cerebrocortical presynaptic ₂-autoreceptors, possibly due to absence of a receptor reserve. Correspondence to: N. Limberger at the above address     

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.     

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

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

Lack of relationship between intrinsic activity and susceptibility of pressor responses to blockade by nifedipine among the α2 agonists B-HT 920 and B-HT 958

Summary Following i.v. bolus injections into pithed normotensive rats, the maximal diastolic pressor responses to B-HT 920 and B-HT 958 amounted to 115 and 35 mm Hg, respectively. Prazosin (0.1 mg/kg, i.v.,-15 min) was without effect on the log dose-pressor effect curve of B-HT 958, whereas yohimbine (1 mg/kg, i.v.,-15 min) shifted this curve about 30-fold to the right, showing the exclusive participation of ₂-adrenoceptors in the vasoconstrictor response to B-HT 958. In doses of 10 and 30 mg/kg, B-HT 958 displaced the log dose-vasoconstrictor effect curve of B-HT 920 approximately 6- and 30-fold, respectively, to the right, illustrating the partial agonism of B-HT 958 at postjunctional vascular ₂-adrenoceptors. Despite the marked difference in intrinsic activity of B-HT 920 and B-HT 958, the calcium entry blocker nifedipine exhibited a comparable inhibitory action on the vasopressor responses to both agonists. This finding indicates that partial and full agonism at vascular ₂-adrenoceptors are not related to the susceptibility of the initiated pressor response to inhibition by calcium entry blockade.     

Effect of α2-adrenoceptor agonists on the expression of morphine-withdrawal in rats

Summary Previous studies using clonidine indicate that ₂-adrenoceptors are involved in suppressing opiate-withdrawal symptoms. However, clonidine may act as a partial agonist at ₂-adrenoceptors and it also possesses significant ₁-receptor agonist activity.The aim of this study was to determine the role of ₂-adrenoceptors in the expression of opiate withdrawal signs using morphine-dependent rats. A range of agonists were selected for study on the basis of their differential preferences for -adrenoceptors.Hooded Wistar rats were made physically dependent on morphine (s.c. injection of an emulsion releasing a total of 250 mg/kg of morphine base over 48 h). Test drugs were injected s.c. followed by naloxone (10 mg/kg i.p.) 20 min later. The incidence of 5 selected withdrawal signs was recorded during the following 20 min. The ₂-adrenoceptor agonists displayed different profiles of activity. Azepexole (1–10 mg/kg) reduced all signs. Clonidine (80–800 g/kg) reduced all signs except paw shakes while guanfacine (25–250 g/kg) reduced all except jumping and diarrhoea. Talipexole (0.1–1 mg/kg) reduced all signs except diarrhoea which was not affected and jumping which was markedly enhanced. UK 14,304 (80–800 g/kg) reduced jumps, potentiated paw shakes but did not affect body shakes, teeth chattering or diarrhoea. The results suggest that there are subpopulations of ₂-adrenoceptors that modulate the expression of opiate withdrawal signs and/or that some of the drugs used affect receptors other than ₂-adrenoceptors.     

Effect of K+ channel blockers on the α2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus

Summary The question whether presynaptic ₂-adrenoceptors regulating noradrenaline release in hippocampus directly couple to tetraethylammonium chloride (TEA) or -dendrotoxin (-DTX)-sensitive K⁺ channels was investigated. Hippocampal slices, prelabelled with [³H] noradrenaline, were superfused in the presence of (+)-oxaprotiline and electrically stimulated with 4 pulses delivered at 100 Hz, in order to avoid autoinhibition due to released noradrenaline.TEA enhanced the evoked [³H]noradrenaline release in rabbit hippocampus in a concentration-dependent manner, yielding an approximately 4-fold increase at 30 mmol/l, whereas the spontaneous outflow of tritium was only slightly affected at this concentration. The ₂-adrenoceptor agonist clonidine, at 10–100 nmol/l inhibited the evoked [³H]noradrenaline release between 77% and 96%. The inhibitory effect of the ₂-agonist was distinctly diminished in the presence of 30 mmol/l TEA but was restored in low Ca²⁺/high Mg²⁺ buffer. Therefore, the diminution of the ₂-agonist effect by TEA observed in experiments with normal Ca²⁺ can be explained by an increase of the Ca²⁺ availability for the release process due to the prolongation of action potentials. In rabbit hippocampus -DTX (10–200 nmol/l) did neither affect the evoked release of [³H]noradrenaline nor its ₂-agonist-induced modulation. However, in rat hippocampus -DTX significantly increased the evoked transmitter release and diminished the effect of clonidine.Taken together, the present data for the rabbit hippocampus exclude the possibility that activation of presynaptic ₂-adrenoceptors inhibits depolarization-evoked [³H]noradrenaline release by inducing an outward K⁺ current through TEA- or -DTX-sensitive K⁺ channels. However, there are species differences between the rabbit and the rat so that in the rat the ₂-adrenoceptors could actually be coupled to K⁺ channels — provided that the release-enhancing properties of -DTX do not account for the ₂-antagonism observed.Correspondence to C. Allgaier at the above address     

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

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

α1-Adrenoceptor subtype mediating contraction of the smooth muscle in the lower urinary tract and prostate of rabbits

Summary The -adrenoceptor subtype mediating contraction of the smooth muscle in the urinary bladder base (trigone), proximal urethra and prostate isolated from male rabbits was investigated by comparing the responsiveness to -adrenoceptor agonists and antagonists under condition where -adrenoceptors and neuronal and extraneuronal uptakes were inhibited. Noradrenaline (non-selective), phenylephrine (₁-selective) and clonidine (₂-selective) caused a dose-dependent contraction in the trigone, urethra and prostate. Phenylephrine acted as a full agonist whereas clonidine was a partial agonist. YM-12617 and prazosin (₁-selective), phentolamine (non-selective) and yohimbine (₂-selective) produced dose-dependent shifts to the right of the dose-response curves for noradrenaline, phenylephrine and clonidine in the all three tissues. YM-12617 (pA₂=9.77, 9.67 and 9.73 for trigone, urethra and prostate, respectively), prazosin (pA₂=8.26, 8.20 and 8.08), phentolamine (pA₂=7.67, 7.62 and 7.45) and yohimbine (pA₂=6.30, 6.30 and 5.94) showed constant pA₂ values irrespective of the agonists and tissues used, suggesting that only a single subclass of -adrenoceptors is present. The actual pA₂ values for these antagonists are comparable to those reported previously in tissues said to contain mainly ₁-adrenoceptors. Thus, we concluded that the postsynaptic -adrenoceptors of the rabbit trigone, urethra and prostate mediating contraction belong to the ₁-subtype.     

Influence of respiratory acidosis or alkalosis on pressor responses mediated byα 1- andα 2-adrenoceptors in pithed normotensive rats

Summary The effect of respiratory acidosis and alkalosis on the vasoconstriction to ₁- and ₂-adrenoceptor stimulation was studied in pithed normotensive rats. The selective ₁-adrenoceptor agonists (-)amidephrine, cirazoline, (±)erythro methoxamine (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selective ₂-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selective-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respective ₁- and ₂-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by the ₁- as well as the ₂-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influence ₁-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguised: (1) Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively. (2) Sgd 101/75, St 587, noradrenaline- ₁, amidephrine and adrenaline- ₁, eliciting pressor responses which are strongly acid-sensitive and base-insensitive. (3) B-HT 920, B-HT 933, DP-6, 7-ADTN (lower doses), of which vasoconstriction is markedly inhibited by both acidosis and alkalosis. (4) Noradrenaline- ₂, UK 14,304, M-7 (lower doses), adrenaline- ₂ and high doses of the agonists of the former group. Pressor responses of these agonists were found to be not or slightly base-sensitive, but profoundly acid-sensitive. Xylazine does not fit into this classification. The present data are not in accordance with purported subdivisions of-adrenoceptor agonists by others. It is therefore concluded that the differential effect of acid-base status on-adrenoceptor mediated vasoconstriction in pithed rats is an exponent of the differential way of interaction of the agonists with the adrenoceptors involved.Preliminary data were communicated at the Joint Meeting of the French and German Pharmacological and Toxicological Societies, Feiburg i. Br., September 19–22, 1983 (De Jonge et al. 1983)     

Frequency- and train length-dependent variation in the roles of postjunctional α1- and α2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

Summary The present paper examines the roles of postjunctional ₁- and ₂-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1–100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the ₁- and ₂-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by ,-methylene ATP (10 M) or suramin (500 M), added to desensitize or block P₂-purinoceptors, respectively prazosin (0.1 M) or SK&amp;F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl-1H-2, 3, 4, 5-tetrohydro-3-benzazepine, 0.1 M), used to block postjunctional ₁- and ₂-adrenoceptors respectively, nifedipine (10 M), blocker of Ca²⁺ influx through L-type channels, and ryanodine (10 M), which blocks mobilization of Ca²⁺ from intracellular stores; it was moderately enhanced by yohimbine (0.1 M), blocker of pre- and postjunctional ₂-adrenoceptors, and strongly enhanced by cocaine (3 M) or desipramine (1 M), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the ₁- and ₂-adrenoceptor agonists, phenylephrine andxylazine, prazosin (0.1 M)and SK & F 104078 (0.1 M) could be used to selectively block ₁- and ₂-adrenoceptors respectively, while yohimbine (0.1 M) was less selective, strongly depressing ₂- and slightly depressing ₁-adrenoceptor-mediated responses. The ₁-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by ₂-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1–10 M). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 M yohimbine or SK & F 104078 than by 0.1 M prazosin and, hence, mediated mainly by ₂-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by ₁-adrenoceptors. Cocaine or desipramine, as well as ,-methylene ATP or suramin, amplified both components of the NA induced contractile response especially that mediated via a₁-adrenoceptors and caused by single pulses or short trains.The main conclusions are (i) that the small NA-induced contractile responses of this artery to single pulses, or pulses at low frequency, or in short trains at high frequency, are mediated mainly via ₂-, and the larger responses to longer trains at high frequency increasingly via ₁-adrenoceptors, (ii) that the ₁- and ₂-adrenoceptor-mediated components interact cooperatively, probably at least in part by utilizing two different pathways to increase the intracellular Ca²⁺, (iii) that neuronal reuptake of NA strongly restricts both components of the NA-induced contraction, especially the ₁-adrenoceptor-mediated response to single pulses or short trains, and (iv) that both components of the NA-induced contraction, especially that mediated by ₁-adrenoceptors, may be depressed by ATP released by field stimulation and acting via P_2x-purinoceptors on smooth muscle. Based on these results a novel working hypothesis is proposed, in which it is assumed that the geometry of NA-mediated neuromuscular transmission in this vessel varies with the frequency and number of impulses in a stimulus train.Correspondence to J.-X. Bao at the above address