The effects of alpha adrenergic agents on human platelet aggregation.

The effects of alpha adrenergic agonists and antagonists on human in vitro platelet aggregation were studied to characterize further the platelet alpha adrenergic receptor. Aggregation induced by ADP and U46619; a stable prostaglandin endoperoxide analog, was potentiated by alpha adrenergic agonists, an effect which was completely blocked by the alpha adrenergic antagonist phentolamine (1 X 10(-6) M) but not by prazosin (1 X 10(-6) M). The order of potency for the alpha adrenergic agonists in potentiating ADP-induced aggregation was clonidine greater than or equal to epinephrine greater than alpha-methylnorepinephrine greater than norepinephrine greater than phenylephrine greater than methoxamine. Epinephrine-induced platelet aggregation was blocked by phentolamine, yohimbine, dihydroergotamine, clonidine and lofexidine but not by phenoxybenzamine (1 X 10(-5) M). These findings suggest that: 1)clonidine and lofexidine are partial agonists and 2) that the alpha adrenergic receptor of the platelet is different from the classical postsynaptic alpha adrenergic receptor and more closely resembles presynaptic alpha adrenergic receptors.     

Postjunctional alpha-1 and alpha-2 adrenoceptors in human skin arteries. An in vitro study

Experiments were performed to characterize the postjunctional alpha adrenoceptors that mediate adrenergic constriction in human skin arteries. Abdominal s.c. arteries were obtained from patients who died 3 to 12 hr before, and vascular segments 2 mm in length and 600 to 1050 microns in external diameter were prepared for isometric tension recording. On application of norepinephrine, phenylephrine (alpha-1 adrenergic agonist) or clonidine (alpha-2 adrenergic agonist) the arteries contracted in a dose-dependent manner and, in terms of the mean EC50 values, the order of potencies was clonidine greater than norepinephrine greater than phenylephrine. With regard to their ability to induce maximal contraction, the order was norepinephrine = phenylephrine greater than clonidine. In the presence of phentolamine (nonspecific alpha adrenergic antagonist) or yohimbine (selective alpha-2 adrenergic antagonist) the control curve for norepinephrine was displaced to the right in a parallel way. Prazosin (selective alpha-1 adrenergic antagonist) depressed both the slope and maximal response of the control curve for norepinephrine but the shift was not significant. Prazosin and yohimbine produced a parallel rightward shift in the control curve for phenylephrine and clonidine, respectively. These results suggest that skin arteries of humans have a mixed population of postjunctional alpha-1 and alpha-2 adrenoceptors and that alpha-2 adrenoceptors are more prominent. They also suggest that the alpha-2 adrenergic component of the response to norepinephrine is a low-maximum effect compared to the alpha-1 adrenergic component. This could be of significance in regulating skin blood flow and thermoregulatory function.     

Pharmacological characterization of alpha adrenergic receptors in the young and old female rabbit urethra.

The pharmacological characteristics of alpha-1 and alpha-2 adrenergic receptors in young (6 month) and old (4.5-5 year) female rabbit urethra were studied using isolated muscle bath techniques. Norepinephrine, phenylephrine, clonidine, oxymetazoline and UK 14,304 produced concentration-dependent contractions in both age groups. The maximum contractile responses (Emax) to norepinephrine, phenylephrine, oxymetazoline and UK 14,304 were of similar magnitude and were significantly greater than the contractile responses to clonidine. The rank order of the ED50 values for these drugs was: oxymetazoline less than UK 14,304 much less than clonidine = norepinephrine = phenylephrine. Prazosin (10(-8) M) shifted the concentration-response curves to phenylephrine and UK 14,304 to the right, but did not shift the concentration-response curves to clonidine and oxymetazoline. Yohimbine (10(-7) M) shifted the concentration-response curves to clonidine, oxymetazoline and UK 14,304 to the right, but did not shift the concentration-response curve to phenylephrine. The ED50 values for phenylephrine and clonidine were smaller in the older than in the younger age group. There were no other age-dependent differences in the response to agonists. Pretreatment with chlorethylclonidine, which selectively alkylates the alpha-1B subtype, did not affect the Emax value of phenylephrine-induced contractions, but significantly shifted the curve to the right. The ratios of the Emax values in Ca++ free buffer to that in normal Ca++ buffer for phenylephrine, UK 14,304, clonidine, oxymetazoline and KCl were 0.30, 0.38, 0.08, 0.07 and 0, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence against alpha-adrenoceptors mediating relaxation in rat thoracic aortae: alpha-agonists relaxation depends on interaction with alpha-adrenoceptors

In rat aorta, the presence of functional alpha(2)-adrenoceptors (alpha(2)-AR) was investigated in ring preparations preconstricted with alpha(1)-adrenergic and non- alpha(1)-adrenergic agonists. Particularly, the hypothetical interference of alpha(2)-AR agonists with alpha(1)-AR-mediated vasoconstriction was evaluated. Relaxant and contractile responses to alpha(2)-AR agonists were obtained. In endothelium-intact and endothelium-denuded aortic rings preconstricted with phenylephrine (1 x 10(-6) m), the imidazoline derivatives, clonidine and UK14304, induced relaxations with similar order of potencies (-log EC(50)) and maxima relaxant effects respectively. Pretreatment with the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME) had no effect on the relaxant responses to clonidine and UK14304. In phenylephrine-constricted rings with endothelium, relaxations to clonidine and UK 14304 were not antagonized by the selective alpha(2)-AR antagonist, rauwolscine (< or =1 x 10(-6) m). Clonidine and UK 14304 induced only contractions on endothelium-intact and endothelium-denuded aortic rings contracted with prostaglandin F(2alpha) (3 x 10(-7) m). Moreover, clonidine and UK 14304-induced relaxation of endothelium-denuded arteries precontracted with methoxamine but not with serotonin. Finally, the concentration-contraction curves to clonidine and UK 14304 in endothelium-denuded aortic rings were significantly shifted to the right by the alpha(1D)-AR selective antagonist, BMY 7378, and rauwolscine. The pA(2) and pK(B) values for BMY 7378 and rauwolscine, respectively, against endothelium-independent actions of clonidine and UK 14304 were characteristic of an effect on the alpha(1D)-AR. The other selective alpha(2)-AR agonist tested BHT 933 (an azepine derivative), lacks considerable relaxant and contractile effects in rat aorta. The results provide no evidence for the presence of functional alpha(2)-AR in rat aorta. Respectively, the relaxant and contractile effects of the imidazoline derivatives, clonidine and UK 14304, may be due to an adjustable (in relation to the agonist-dependent active state of the alpha(1)-AR), inhibitory and excitatory, interaction with alpha(1)-ARs.     

Ablation of the myenteric plexus impairs alpha but not beta adrenergic receptor-mediated mechanical responses of rat jejunal longitudinal muscle

The mechanical responses produced by alpha and beta adrenergic receptor agonists were evaluated in control and myenteric neuron-ablated rat jejunal longitudinal muscle. The myenteric plexus of the jejunum was destroyed by serosal application of benzalkonium chloride (BAC). The beta adrenergic receptor agonists isoproterenol and sulfonterol produced a concentration-dependent relaxation of both control and BAC-treated jejunum. Dose-response curves obtained in control and BAC-treated jejunum were nearly superimposable regardless of the beta agonist used. Isoproterenol-induced relaxation was antagonized by the beta receptor antagonists propranolol and practolol but not by butoxamine. The alpha-1 selective agonists phenylephrine and methoxamine were more potent and efficacious in producing relaxation of control than BAC-treated jejunum. The relaxant responses of methoxamine and phenylephrine in control jejunum were blocked by prazosin but not by yohimbine. The supposed alpha-2 selective agonist clonidine also produced a concentration-dependent, prazosin-sensitive, yohimbine-resistant relaxation which was markedly greater in control than BAC-treated jejunum, consistent with alpha-1 receptor stimulation. Clonidine tested in the presence of prazosin and the alpha-2 selective receptor agonists UK-14,304, M-7 and B-HT 920 all produced a concentration-dependent contraction of control but not BAC-treated jejunum. The contractile response produced by UK-14,304 was antagonized by yohimbine but not by atropine. Our results suggest that in rat jejunal longitudinal muscle: beta adrenergic receptors mediate relaxation and are located on the smooth muscle; alpha-1 adrenergic receptors mediate relaxation and are located on both the smooth muscle and myenteric plexus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of adrenergic and angiotensinergic receptors in the paraventricular nucleus in the angiotensin II-induced vasopressin release.

The role of hypothalamic paraventricular adrenoceptors and angiotensin II (ANG II)-AT 1 receptors in mediating the vasopressin (AVP) release into the plasma in response to i.c.v. and local paraventricular ANG II injections was investigated in conscious chronically instrumented rats. Noradrenaline (NA) administered bilaterally into the paraventricular nucleus (PVN) dose-dependently stimulated AVP release. Bilateral PVN microinjections of the alpha 1 adrenoceptor agonists methoxamine and phenylephrine, or of the alpha2 adrenoceptor agonist clonidine, did not affect plasma AVP when given alone, but increased plasma AVP when methoxamine and clonidine were given in combination. In contrast, PVN microinjections of both the beta 1 adrenoceptor agonist dobutamine and the beta 2 adrenoceptor agonist salbutamol significantly reduced basal plasma AVP. Bilateral PVN pretreatment with the alpha 1 and alpha 2 adrenergic antagonists prazosin, idazoxan and rauwolscine, but not of the beta 1 and beta 2 adrenoceptor antagonists atenolol and ICI 118 551, significantly attenuated the i.c.v. ANG II-induced AVP release. ANG II injected bilaterally into the PVN dose-dependently increased plasma AVP. Bilateral PVN pretreatment with the specific ANG II-AT 1 receptor antagonist losartan partially inhibited the i.c.v. ANG II-induced AVP release. We conclude: 1) Beta 1 and beta 2 adrenoceptors in the PVN exert an inhibitory action on basal AVP secretion. 2) ANG II can release AVP by directly stimulating its ANG II-AT 1 receptors in the PVN. 3) PVN mediated AVP release in response to periventricular ANG II-AT 1 receptor stimulation is at least partially effected through ANG II-AT 1 receptors in the PVN impinging on alpha adrenergic terminals.     

Clonidine evokes vasodepressor responses via alpha2-adrenergic receptors in gigantocellular reticular formation

The gigantocellular depressor area (GiDA) is a functionally defined subdivision of the medullary gigantocellular reticular formation where vasodepressor responses are evoked by glutamate nanoinjections. The GiDA also contains reticulospinal neurons that contain the alpha2A-adrenergic receptor (alpha2A-AR). In the present study, we sought to determine whether nanoinjections of the alpha2-AR agonist clonidine into the GiDA evoke cardiovascular responses and whether these responses can be attributed to the alpha2-AR. We found that nanoinjections of clonidine into the GiDA evoke dose-dependent decreases in arterial pressure and heart rate. These responses were equivalent in magnitude to responses produced by clonidine nanoinjections into the sympathoexcitatory region of the rostral ventrolateral medulla. Furthermore, the vasodepressor and bradycardic responses produced by clonidine injections into the GiDA were blocked in a dose-dependent fashion by the highly selective alpha2-AR antagonist 2-methoxyidazoxan, but not by prazosin, which is an antagonist at both the alpha1-AR and the 2B subtype of the alpha-AR. The antagonism by 2-methoxyidazoxan was site specific because injections of the antagonist into the rostral ventrolateral medulla failed to block the responses evoked by clonidine injections into the GiDA. These findings support the notion that clonidine produces sympathoinhibition through multiple sites within the medullary reticular formation, which is consistent with the wide distribution of the alpha2A-AR in reticulospinal neurons. These data also suggest that clonidine may have multiple mechanisms of action because it evokes a cardiovascular depressive response from regions containing neurons that have been determined to be both sympathoinhibitory and sympathoexcitatory.     

Peripheral P2X receptors and nociception: interactions with biogenic amine systems

ATP is implicated in peripheral nociception following activation of P2X, and particularly P2X(3) receptors. The present study examined interactions between alphabeta-methylene-ATP (a P2X(3) receptor ligand) and 5-hydroxytryptamine (5-HT), noradrenaline (NA) and histamine, following local administration into the hindpaw, on spontaneous pain behaviors and thermal hyperalgesia in Sprague-Dawley rats. The interaction with NA was further explored using systemic 6-hydroxydopamine (6-OHDA) and locally administered indomethacin. alphabeta-methylene-ATP produced no spontaneous pain behaviors. Coadministration of 5-HT with alphabeta-methylene-ATP mildly augmented flinching behaviors, while histamine had no such effect. Coadministration of NA with alphabeta-methylene-ATP produced a pronounced expression of flinching and biting/licking behaviors. alphabeta-Methylene-ATP, given alone, produced thermal hyperalgesia, and this was markedly augmented by both 5-HT and NA, but not histamine. 6-OHDA (neurotoxin for sympathetic neurons) and indomethacin (cyclooxygenase inhibitor) reduced the augmenting effect of NA on alphabeta-methylene-ATP-induced thermal hyperalgesia, but had no effect on spontaneous pain behaviors produced by the alphabeta-methylene-ATP/NA combination. Effects of alphabeta-methylene-ATP, NA and their combination were also examined in Long Evans and Wistar rats. In both strains, alphabeta-methylene-ATP and NA both individually led to significant intrinsic flinching behaviors, and the effect of their combination was even more pronounced than in Sprague-Dawley rats. These results provide evidence for: (a) a strong enhancement by NA and 5-HT of nociception produced by peripheral P2X receptors in Sprague-Dawley rats, (b) an indirect action of NA, via sympathetic efferents and prostanoids, with thermal hyperalgesia, and (c) a greater expression of spontaneous pain behaviors with alphabeta-methylene-ATP and NA alone, and with their combination, in Wistar and Long Evans rats compared to Sprague-Dawley rats.     

Systemic injections of alpha-1 adrenergic agonists produce antinociception in the formalin test.

The role of alpha 1 receptors in antinociception was investigated in the formalin test, a well established test of tonic pain. The effect of systemic injections of selective alpha 1-adrenergic agonists (phenylephrine and methoxamine), a mixed alpha agonist selective for alpha 2 receptors (ST-91), and 2 adrenergic antagonists (prazosin and idazoxan) was measured in groups of Long-Evans rats. All agonists tested produced significant antinociception in this test. Dose-response curves for each agonist were statistically parallel and equally efficacious (100% antinociception). Prior injection of 0.15 mg/kg prazosin (an alpha 1 antagonist) completely antagonized the antinociception produced by either an ED50 or a maximally effective dose of each agonist tested. Idazoxan (0.5 mg/kg), an alpha 2 antagonist, was without effect on the antinociception produced by phenylephrine or methoxamine. ST-91 produced significant antinociception in the presence of idazoxan although the response was different from that obtained with ST-91 alone. The observed antinociception in the formalin test was not due to drug-induced changes in peripheral inflammation as measured using plethysmometry. Moreover, none of the drugs tested produced significant changes in coordinated motor behavior (accelerated rotarod test) at doses that produced significant analgesia (ED50). We conclude that alpha 1 receptors contribute significantly to adrenergic analgesia in the formalin test by an undefined action on sensory processing mechanisms.     

Electrophysiological analysis of adrenergic neurotransmission and its modulation by chronic denervation in canine saphenous veins

The present experiments were undertaken to investigate the electrophysiological responses of the canine saphenous vein evoked by perivascular nerve stimulation, norepinephrine or selective alpha adrenergic agonists before and after chronic sympathetic denervation. Unilateral sympathectomy was performed from T12 to L9 in adult female dogs. After 3 to 5 weeks, the denervated saphenous veins were removed. Innervated saphenous veins were obtained from unoperated dogs. In innervated but not in denervated veins, electrical stimulation generated excitatory junction potentials and a slow depolarization. The slow depolarization was inhibited by rauwolscine or phentolamine, but not by prazosin, whereas excitatory junction potentials were not inhibited by alpha adrenergic blockers. Exogenously applied norepinephrine caused a depolarization of the membrane that was inhibited by rauwolscine but not by prazosin. The selective alpha-1 adrenergic agonist, phenylephrine, and the selective alpha-2 adrenergic agonist, UK 14,304, caused depolarization. In denervated veins, the threshold concentrations of norepinephrine or UK 14,304 required to depolarize the smooth muscle cell membrane were reduced. Responses to phenylephrine were not affected by denervation. These results indicate that in the canine saphenous vein norepinephrine, whether added exogenously or released from sympathetic nerves, causes predominant depolarization by activating alpha-2 adrenergic receptors. Denervation augments selectively the electrical response to alpha-2 adrenergic stimulation.     

Alpha adrenergic responses of blood vessels of rabbits after ovariectomy and administration of 17 beta-estradiol

Experiments were designed to determine the effect of estrogen pretreatment on alpha adrenergic responsiveness of blood vessels of the rabbit. Rabbits were ovariectomized and, after 8 days of recovery, treated with 17 beta-estradiol (100 micrograms i.m.; estrogen group) or solvent (control group) for 4 days. Rings of saphenous vein and femoral artery (both without endothelium) were mounted for isometric tension recording in organ chambers filled with modified Krebs-Ringer bicarbonate solution (37 degrees C), gassed with 95% O2-5% CO2. All experiments were performed in the presence of inhibitors of neuronal uptake, extraneuronal uptake and beta adrenoceptors. In the saphenous vein, the estrogen treatment did not significantly affect the concentration-effect curves evoked by norepinephrine (either under control conditions or after alpha-1 or alpha-2 adrenergic blockade), phenylephrine (an alpha-1 adrenergic agonist) or UK 14,304 (an alpha-2 adrenergic agonist). In the femoral artery, estrogen treatment depressed the contractile responses evoked by norepinephrine (under control conditions) but not those produced by phenylephrine; UK 14,304 did not evoke a contractile response. The depressant effect of estrogen treatment on the concentration-effect curve to norepinephrine in the femoral artery was prevented by the alpha-2 adrenergic antagonist, rauwolscine. The results in the femoral artery but not in the saphenous vein suggest that estrogens depress alpha-2 but not alpha-1 adrenergic responsiveness. In the femoral artery, alpha-2 adrenoceptor stimulation does not cause contraction per se but apparently can facilitate alpha-1 adrenergic responses. This probably results from a reduced density of alpha-2 adrenoceptors in this blood vessel.(ABSTRACT TRUNCATED AT 250 WORDS)     

Binding, partial agonism, and potentiation of alpha(1)-adrenergic receptor function by benzodiazepines: A potential site of allosteric modulation

Benzodiazepines, a class of drugs commonly used to induce anesthesia and sedation, can attenuate intracellular calcium oscillations evoked by alpha(1)-adrenergic receptor (alpha(1)-AR) stimulation in pulmonary artery smooth muscle cells. We postulated a direct action of benzodiazepines in modulating alpha(1)-AR function at the receptor level. Benzodiazepines bound to each of the cloned alpha(1)-AR subtypes (alpha(1a)-, alpha(1b)-, or alpha(1d)-AR) on COS-1 cell membranes transiently transfected to express a single population of alpha(1)-AR subtype. The ability of benzodiazepines to alter alpha(1)-AR signal transduction was investigated by measuring total inositol phosphate generation in rat-1 fibroblast cells, stably transfected to express a single alpha(1)-AR subtype. By themselves, benzodiazepines displayed partial agonism. At alpha(1b)-ARs and alpha(1d)-ARs, the maximal inositol phosphate response to phenylephrine was potentiated almost 2-fold by either midazolam or lorazepam (100 microM). At alpha(1a)-ARs, diazepam, lorazepam, and midazolam all increased the maximal response of the partial agonist clonidine at these receptors, whereas the response to the full agonist phenylephrine was unaltered or inhibited. The potentiating actions of midazolam and its partial agonism at alpha(1)-ARs was blocked by the addition of 1 microM prazosin, an alpha(1)-AR antagonist, and not by a gamma-aminobutyric acid(A)-receptor antagonist. These studies show that benzodiazepines modulate the function of alpha(1)-ARs in vitro, and this is the first report of a potential allosteric site on alpha(1)-ARs that may be therapeutically useful for drug design.     

Cooling and alpha adrenergic responses in the saphenous vein of the rabbit

Experiments were designed to determine the effects of cooling on alpha-1 and alpha-2 adrenergic responses in a cutaneous vein of the rabbit. Rings of saphenous vein were suspended in physiological salt solution for the recording of isometric force. Cooling (from 37-24 degrees C) caused no significant increase in force in quiescent rings. Similarly, the same degree of cooling had no significant effect on the response to exogenous norepinephrine (10(-9)-10(-5) M), whether under control conditions or in the presence of either the alpha-1 adrenergic antagonist prazosin (3 X 10(-7) M) or the alpha-2 adrenergic antagonist rauwolscine (10(-7) M). Contractions evoked by the alpha-1 adrenergic agonist phenylephrine were reduced, but those induced by the alpha-2 adrenergic agonist UK 14,304 (10(-9)-10(-5) M) were unaffected by the same degree of cooling. Cooling augmented the response elicited by electrical field stimulation of the sympathetic nerves, although only under conditions of alpha-1 or combined alpha-1 and alpha-2 adrenergic blockade. Data obtained with the sympathomimetic tyramine suggest that both alpha-1 and alpha-2 adrenoceptors are innervated in this blood vessel. Together, the present data suggest that the effects of acute cooling on the saphenous vein of the rabbit, unlike that of the dog, are not mediated by changes in the affinity of postjunctional alpha-2 adrenoceptors.     

α-肾上腺素受体在脊髓损伤大鼠血管高反应性中的作用

目的研究高位脊髓横切（SCT）慢性期大鼠腹主动脉离体血管环对α-肾上腺紊受体（α—AR）激动剂的反应性及腹主动脉α1-AR、α2-AR各亚型mRNA的表达量，探讨其与腹主动脉高反应性的关系。方法取SCT4周后大鼠的腹主动脉，利用离体血管环张力测定技术，以血管环对梯度浓度去氧肾上腺素（Phe）和可乐定的收缩力，反映血管环对α1-AR、α2-AR激动剂的反应性；通过实时定量聚合酶链反应（PCR）测定腹主动脉α1A—AR、α1B-AR、α1D—AR、α2A—AR、α2B—AR和α2c—AR的mRNA表达量改变，并与假手术组比较。结果与假手术组比较，SCT4周组大鼠腹主动脉对可乐定反应性上升（P〈0．05或P〈0．01），对Phe反应性无明显改变；SCT组腹主动脉α1A—AR和α1D—AR的mRNA表达上调（P〈0．05或P〈0．01），而α1B—AR mRNA表达未见明显改变（P〉0．05），α2A—AR、α2B—AR和α2c—AR的mRNA表达均上调（P〈0．05或P〈0．01）。结论在SCT慢性期，腹主动脉高反应性的机制可能为血管对α2-AR激动剂敏感性上升，α2-AR mRNA表达上调。虽然α1-AR RNA表达上调，但离体血管对Phe收缩力无明显改变，α1—AR可能并不是引起SCT慢性期血管高反应性的原因。     

Characterization of alpha-2 adrenergic receptors in the OK cell, an opossum kidney cell line

We have characterized alpha-2 adrenergic receptors in OK cells, an opossum kidney-derived cell line. In membrane saturation binding experiments, [3H]rauwolscine (Kd = 74 pM) was 3-fold more potent than [3H]yohimbine (Kd = 230 pM). Each labeled a single class of binding sites with densities of 135 and 124 fmol/mg of protein for [3H]rauwolscine and [3H]yohimbine, respectively. Inhibition of [3H]rauwolscine and [3H]yohimbine binding by several alpha adrenergic agonists and antagonists demonstrated the radioligands labeled an alpha-2 type adrenergic receptor with a pharmacological profile similar to the alpha-2B receptor subtype. The rank order of potency for antagonist inhibition of binding was yohimbine greater than prazosin = phentolamine greater than chlorpromazine = corynanthine, whereas the rank order of agonist potency was oxymetazoline = clonidine greater than or equal to UK-14,304 greater than or equal to (-)-epinephrine greater than (-)-norepinephrine. The oxymetazoline, clonidine and antagonist inhibition curves were routinely monophasic and modeled best as a single class of binding sites. For the other agonists, inhibition binding curves were biphasic with approximately 35% of the binding sites existing in a high affinity state. These curves were shifted to the right in the presence of 0.1 mM GTP, and in general modeled as a single class of binding sites. UK-14,304, (-)-epinephrine, (-)-norepinephrine and oxymetazoline attenuated parathyroid hormone-stimulated cyclic AMP production by up to 70% in whole cell monolayers in a dose-dependent manner via a pertussis toxin-sensitive mechanism. With the exception of oxymetazoline, this inhibition could be reversed with alpha adrenergic antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological characterization of alpha adrenoceptors in the rat gastric fundus

Alpha receptors on the intramural cholinergic neurons and on the smooth muscle cells are involved in the inhibitory effect of catecholamines on rat gastric fundus motility. The pharmacological characteristics of these alpha receptors were assessed using longitudinal muscle strips of the rat gastric fundus, contracted to a similar degree by electrical stimulation and by methacholine. All alpha agonists studied (norepinephrine, phenylephrine, methoxamine, clonidine, UK-14,304 and B-HT 920) concentration-dependently inhibited the stimulation-induced contractions. Norepinephrine, phenylephrine, methoxamine and clonidine also inhibited the methacholine-induced contractions, but for the same concentration of agonist, the inhibition was less pronounced than during electrical stimulation-induced contractions; UK-14,304 and B-HT 920 inhibited the methacholine-induced contractions only in a concentration of 10(-4) M. The effect of clonidine and UK-14,304 on electrical stimulation-induced contractions was antagonized competitively by the alpha antagonists rauwolscine and yohimbine (slope in the Schild plot not different from 1). The effect of norepinephrine and phenylephrine on methacholine-induced contractions was antagonized by the alpha antagonists prazosin, corynanthine and yohimbine; against phenylephrine, the antagonism was competitive (slope in the Schild plot not different from 1). It is concluded that the muscular alpha receptors in the rat gastric fundus are of the alpha-1-type. On the postganglionic cholinergic neurons, alpha-2-like receptors are present; it is not yet clear whether the pronounced effect of alpha-1 agonists on the cholinergic neuron activity is due to interaction with these receptors.     

Intravenous clonidine produces hypoxemia by a peripheral alpha-2 adrenergic mechanism

Intravenous injection of the alpha-2 adrenergic agonists, clonidine and xylazine, have been previously shown to produce hypoxemia in sheep. To characterize this effect further, clonidine and ST-91, a clonidine analog that does not cross the blood-brain barrier, were injected i.v. in 10 conscious ewes. Although both clonidine (3-15 micrograms/kg) and ST-91 (3-30 micrograms/kg) produced arterial hypoxemia, clonidine was more effective (arterial PO2 was 91 +/- 4 mm Hg after saline, 30 +/- 3 mm Hg after 15 micrograms/kg of clonidine and 43 +/- 6 mm Hg after 30 micrograms/kg of ST-91; mean +/- S.E., P less than .0001). ST-91 increased mean arterial blood pressure in a dose-dependent manner (P less than .0001), whereas clonidine did not affect blood pressure. Clonidine-induced hypoxemia was inhibited in a dose-dependent manner by the alpha-2 adrenergic antagonist idazoxan (0.01-1 mg/kg, complete inhibition after 1 mg/kg; P less than .0001), the hydrophilic alpha-2 adrenergic antagonist DG-5128 (0.1-10 mg/kg, 62 +/- 7% inhibition after 10 mg/kg; P less than .0005) and by infusion of prostacyclin (0.15-0.5 microgram/kg/min, 57 +/- 7% inhibition after 0.5 micrograms/kg/min; P less than .05). Hypoxemia was not inhibited by the opiate antagonist naloxone (1 mg/kg), the alpha-1 adrenergic antagonist prazosin (1 mg/kg) or the prostaglandin synthetase inhibitor ibuprofen (12.5 mg/kg). To characterize pulmonary vascular effects, clonidine was injected i.v. in four anesthetized, mechanically ventilated ewes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alpha adrenergic receptors mediate angiotensin-induced prostaglandin production in the rabbit isolated vas deferens

The effect of alpha adrenergic receptor antagonists on concentration-dependent response to angiotensins II and III was examined in the electrically stimulated isolated rabbit vas deferens. The force generated by a nonadrenergic neural mechanism was reduced by both peptides whereas the force generated by adrenergic neural mechanisms was enhanced. Angiotensin III-induced inhibition of the nonadrenergic contraction was significantly greater than that of angiotensin II for all groups. Yohimbine (1 X 10(-4) M), an alpha-2 receptor antagonist, attenuated the depression of the nonadrenergic contraction produced by angiotensins II and III. Yohimbine (1 X 10(-5) and 1 X 10(-4) M) also significantly reduced angiotensin II-induced prostaglandin E (PGE) synthesis. Yohimbine only significantly altered the angiotensin III-induced PGE synthesis at an antagonist concentration of 1 X 10(-4) M. Rauwolscine (1 X 10(-8) and 1 X 10(-7) M) attenuated angiotensin II-induced PGE production and at a higher concentration (1 X 10(-6) M) reduced angiotensin III-induced PGE production. The alpha-1 antagonist, prazosin (1 X 10(-6) M), did not alter nonadrenergic contractile or PGE responses to either angiotensin. The alpha-2 agonist, clonidine, both inhibited the nonadrenergic neural contraction and enhanced PGE synthesis. We interpret these data to indicate that angiotensins II and III may act via separate mechanisms to induce PGE synthesis in the vas deferens, with angiotensin II effects being more dependent on norepinephrine release from adrenergic nerves.     

Presynaptic alpha adrenoceptor modulation of neurally mediated cholinergic excitatory and nonadrenergic noncholinergic inhibitory responses in guinea pig trachea

Cholinergic and nonadrenergic noncholinergic (NANC) excitatory nerves in guinea pig trachea are subject to presynaptic alpha-2 adrenoceptor inhibitory control. Although the trachea is also innervated by NANC inhibitory nerves, little is known about their presynaptic regulation. The present study assessed the capacity of alpha-1 and alpha-2 adrenoceptor agonists to modulate NANC inhibitory nerves and for comparison, cholinergic excitatory nerves in guinea pig trachea. To eliminate effects of intrinsic sympathetic nerve stimulation and prostanoid production, tissues were pretreated with guanethidine, propranolol and indomethacin. The alpha-2 adrenoceptor agonist, clonidine (1 microM), induced a 12-fold rightward shift of the frequency-response curve for neurally mediated cholinergic contractions but had no effect on the concentration-response curve for exogenously administered acetylcholine. This action of clonidine was inhibited in a concentration-dependent manner by the alpha-2 adrenoceptor antagonist, yohimbine, and by the alpha-1 adrenoceptor antagonist, prazosin, NANC inhibitory responses were unaffected by clonidine (1 microM). The alpha-1 adrenoceptor agonist, phenylephrine (1 microM), failed to influence responses induced by cholinergic or NANC inhibitory nerve stimulation, acetylcholine or vasoactive intestinal peptide. Furthermore, in tissues treated with propranolol but not subjected to adrenergic neuronal blockade with guanethidine, neurally mediated cholinergic responses were not altered by yohimbine (0.3 microM) treatment. These results indicate that in guinea pig trachea: 1) cholinergic nerves are modulated by presynaptic, prazosin-sensitive inhibitory presynaptic alpha-2 adrenoceptors and 2) NANC inhibitory nerves do not possess presynaptic, modulatory alpha adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological characterization and cross talk of alpha1a- and alpha1b-adrenoceptors coexpressed in human embryonic kidney 293 cells

We established three human embryonic kidney (HEK) 293 cell lines stably expressing alpha(1)-adrenoceptor (AR) subtypes, one (alpha(1A), (1B)-AR) coexpressing both receptors and the other two (alpha(1A)-AR and alpha(1B)-AR) expressing each receptor in isolation. In the alpha(1A), (1B)-AR cells, both receptors were clearly distinguished by the alpha(1A)-selective ligands (-)-1(3-hydroxypropyl)-5-((2R)-2-([2-(2,2,2-trifluoroethyl]oxy]phenyl)oxy)ethyl]amino)propyl)-2,3-dihydro-1H-indole-7-carboxamide (KMD-3213) and methoxamine, but not by the subtype-nonselective ligands prazosin and phenylephrine. In all three cell lines, phenylephrine caused a concentration-dependent increase in inositol phosphates and an increase in extracellular signal-regulated kinase 1/2 (ERK1/2) activation. However, there was a 2-fold or greater maximal response to phenylephrine and a somewhat higher agonist potency in ERK1/2 activation in the alpha(1A,1B)-AR cells, compared with the responses of cells expressing either receptor individually (alpha(1A)-AR or alpha(1B)-AR). Furthermore, the antagonistic affinities of prazosin (pK(b) of 10.1) and KMD-3213 (9.4) for inhibiting the phenylephrine response were intermediate between the values for inhibition in alpha(1A)-AR cells (prazosin, 9.3; KMD-3213, 10.5) and alpha(1B)-AR cells (prazosin, 11.0; KMD-3213, 8.1). The inhibitor pK(b) values in alpha(1A), (1B)-AR also differed from their ligand binding affinities measured in alpha(1A)-AR and alpha(1B)-AR cells. In contrast, the alpha(1A)-selective agonist methoxamine, which did not activate alpha(1B)-AR cells, stimulated either alpha(1A,) (1B)-AR or alpha(1A)-AR cells with a comparable potency and maximum effectiveness. Our data indicate that when coexpressed in the same cell, the activation of common pathways by individual AR receptor subtypes by a nonselective agonist can exhibit enhanced responsiveness and a distinct antagonist affinity compared with the parameters for the same receptors, when expressed alone in the same cell background.