Heterogeneity of smooth muscle alpha adrenoceptors in rat tail artery in vitro

Vascular smooth muscle alpha adrenoceptors in the proximal end of the rat isolated tail artery have been classified by determining pA2 values and -log KB values for the antagonists prazosin, corynanthine and idazoxan (RX 781094, a new synthetic alpha-2 adrenoceptor antagonist) against norepinephrine. The effects of the antagonists on responses to intramural sympathetic nerve stimulation were also assessed. Artery segments were perfused and superfused with Krebs' solution containing cocaine (4 microM) and propranolol (1 microM). Maximum responses to KCl, norepinephrine and nerve stimulation were not significantly different from one another. Corynanthine (0.1-100 microM), prazosin (10 nM-1 microM) and idazoxan (1-100 microM) caused competitive antagonism of norepinephrine responses with pA2 values consistent with the presence of alpha-1 adrenoceptors. However, idazoxan (10-100 nM) also caused parallel shifts in the concentration-response curves to norepinephrine with -log KB values higher than those consistent with the presence of alpha-1 adrenoceptors. The results have been interpreted to suggest a predominance of smooth muscle alpha-1 adrenoceptors in addition to a subpopulation of smooth muscle alpha-2 adrenoceptors which also contribute to vasoconstrictor responses to norepinephrine. In contrast to the results obtained with exogenous norepinephrine, responses to electrical stimulation were exquisitely sensitive to blockade by prazosin but resistant to idazoxan , suggesting an involvement of an alpha-1 adrenoceptor in these responses. It is concluded that idazoxan may be used to distinguish alpha-1 and alpha-2 adrenoceptors on vascular smooth muscle in vitro and that the results favor the existence of alpha-1 and alpha-2 adrenoceptors in terms of the existing subclassification scheme for alpha adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis of the presence of postjunctional alpha-2 adrenoceptors in the rat anococcygeus muscle

When the tone is raised by guanethidine, rat anococcygeus muscle produces inhibitory responses to field stimulation, whose mechanism is not understood properly. The present study is an attempt to investigate the role of alpha adrenoceptors in the field stimulation-induced relaxations in isolated rat anococcygeus muscle. When the tissues are contracted with clonidine, UK-14,304 and low doses of oxymetazoline, field stimulation produced relaxations at lower frequencies, but not in the tissues precontracted with phenylephrine and norepinephrine. Relaxations induced by low frequencies were blocked by idazoxan, but not by phentolamine, prazosin, indomethacin, N-methyl-hydroxylamine, ouabain or 3,4-diminopyridine. When the tone of the muscle is raised by norepinephrine, prazosin reversed the field stimulation-induced contractions to relaxation responses. The data of the present study suggested the possible involvement of alpha-2 adrenoceptors during the field stimulation-induced relaxations of the rat anococcygeus muscle. To analyze and quantitate the alpha-2 adrenoceptor antagonism in the rat anococgygeus muscle, Schild analyses of clonidine-induced contractions against idazoxan were conducted either for idazoxan alone or after partially alkylating the alpha-1 adrenoceptors with phenoxybenzamine and by pharmacologic resultant analysis by blocking the alpha-1 adrenoceptors with prazosin. The Schild regression for idazoxan and pharmacologic resultant analysis suggested that the rat anococcygeus muscle responds to alpha-2 agonists with alpha-1-mediated contractions and idazoxan competes with alpha-1 antagonists for the same site, i.e., alpha-1 adrenoceptor site. However, the atypical Schild regression of idazoxan after partial alkylation with phenoxybenzamine indicated the existence of a second alpha adrenoceptor site in the rat anococcygeus muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduced function of the stimulatory GTP-binding protein in beta adrenoceptor-adenylate cyclase system of femoral arteries isolated from spontaneously hypertensive rats

Arterial relaxant responses to beta adrenoceptor agonists are decreased in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY). To determine which component of the beta adrenoceptor-adenylate cyclase (AC) system is involved in the decreased beta adrenoceptor responses, effects of two activators of AC-forskolin and cholera toxin and of dibutyryl cyclic AMP (DBcAMP) were compared between the strips of femoral arteries isolated from 13-week-old SHR and age-matched WKY. Arterial relaxant responses to either forskolin, an activator of AC or DBcAMP were not significantly different between the SHR and WKY, whereas the relaxant responses to norepinephrine (NE) via beta adrenoceptors were significantly weaker in the SHR than in the WKY. In the absence of timolol, a beta adrenoceptor antagonist, contractile responses to NE were significantly greater in the SHR than in the WKY. Timolol augmented the contractile responses to NE to a greater extent in the WKY than in the SHR. After the blockade by timolol of beta adrenoceptors, contractile responses to alpha adrenoceptor stimulation with NE were not significantly different between the two strains. The pretreatment of the strips with cholera toxin, an activator of stimulatory GTP-binding protein (Gs), antagonized the alpha adrenoceptor-mediated contractions much greater in the WKY than in the SHR. The alpha adrenoceptor-mediated contractions after the pretreatment with cholera toxin were comparable to the contractile responses to NE determined in the absence of timolol in either the SHR or the WKY. Forskolin and DBcAMP also antagonized the alpha adrenoceptor-mediated contractions. However, these antagonisms were not significantly different between the two strains. The cellular cAMP content in arterial strips after the stimulation with NE was significantly less in the SHR than in the WKY, whereas the cAMP contents were similar in arterial strips from both strains which were stimulated with forskolin. These results suggest that the reduced function of Gs is involved in the abnormality of beta adrenoceptor-AC system in the SHR femoral artery.     

Pharmacological characterization of alpha-2 adrenergic receptor subtype involved in the release of insulin from isolated rat pancreatic islets

Alpha-2 adrenoceptors are involved in the inhibition of insulin release induced by sympathetic nerve stimulation. To test the possibility that one of the postulated subtypes of alpha-2 adrenoceptors is differentially implicated in the inhibition of insulin release, we compared the effects of several agonists and antagonists with preferential selectivity for the alpha-2 adrenoceptor subtypes on the release of insulin induced by glucose in rat isolated islets. Similar to the inhibition of glucose-evoked release of insulin by the alpha-2 agonist (nonsubtype selective) UK 14.304, the alpha-2A preferential agonist oxymetazoline, concentration-dependently inhibited the release of insulin. Glucose-evoked insulin release was similarly inhibited by other alpha-2 adrenoceptor agonists such as clonidine, p-aminoclonidine, epinephrine and norepinephrine. However, neither the alpha-1 selective agonist cirazoline, nor the beta adrenoceptor agonist isoproterenol affected glucose-evoked insulin release, thus suggesting that this inhibitory effect is mediated by alpha-2 adrenoceptors, possibly of the alpha-2A subtype. The inhibition of glucose-evoked insulin release induced by the alpha-2 adrenoceptor agonists was concentration-dependently inhibited by the alpha-2 antagonists yohimbine, phentolamine, rauwolscine and idazoxan. However, neither the alpha-1 selective antagonist prazosin, nor the beta selective antagonist propranolol attenuated the inhibition of insulin release induced by alpha-2 adrenoceptor agonists. Furthermore, the inhibition of insulin release induced by UK 14.304 was concentration-dependently antagonized by the alpha-2A preferential antagonist WB-4101.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acute and chronic administration of idazoxan on blood pressure and plasma catecholamine concentrations of rats

To test the hypothesis that alpha-2 adrenoceptor antagonists can modulate sympathetic nerve release of norepinephrine in vivo through blockade of peripheral prejunctional alpha-2 adrenoceptors, acute and chronic effects of the alpha-2 adrenoceptor antagonist idazoxan on mean arterial pressure (MAP), heart rate and plasma catecholamine concentrations have been investigated in conscious and anesthetized rats. In normotensive rats, a single i.v. dose of idazoxan (300 micrograms kg-1) caused an immediate 2-fold increase in plasma concentration of norepinephrine and epinephrine, a transient increase in heart rate but no significant change in MAP. Plasma norepinephrine concentration of conscious normotensive rats increased significantly during a 4-hr i.v. infusion of idazoxan (300 micrograms kg-1 hr-1) with no concomitant changes in MAP or heart rate. In anesthetized spontaneously hypertensive rats, the increases in plasma norepinephrine concentration and heart rate caused by i.v. idazoxan (300 micrograms kg-1) were accompanied by a significant decrease in MAP. The increase in plasma norepinephrine after idazoxan in spontaneously hypertensive rats was much greater than that produced by an equihypotensive dose of the vasodilator hydralazine. Normotensive rats treated continuously for 7 days with s.c. idazoxan (7.5 mg kg-1 day-1) had similar blood pressures and plasma catecholamine concentrations to vehicle-treated rats. These results suggest that idazoxan causes a greater increase in plasma norepinephrine concentration than that which can be attributed to baroreceptor stimulation. Blockade of prejunctional alpha-2 adrenoceptors by idazoxan may, therefore, increase release of norepinephrine from peripheral sympathetic nerves of anesthetized and conscious rats. This effect is short-lived and does not influence blood pressure of normotensive rats.     

Classification of phenoxybenzamine/prazosin-resistant contractions of rat spleen to norepinephrine by Schild analysis: similarities and differences to postsynaptic alpha-2 adrenoceptors

The striking resistance of norepinephrine contractions of rat splenic strips to antagonism by the selective alpha-1 adrenoceptor antagonist prazosin was examined by Schild analysis. Prazosin was a simple competitive antagonist of contractions to phenylephrine indicating that this tissue possesses alpha-1 adrenoceptors. In contrast, the Schild regression for prazosin, with norepinephrine as the agonist, was nonlinear and had an overall slope of 0.24. These data indicated that norepinephrine activated a prazosin-resistant adrenoceptor in this tissue. As a working hypothesis, it was assumed that the prazosin-resistant receptor was an alpha-2 adrenoceptor; the concomitant addition of yohimbine, in concentrations below those required to block alpha-1 adrenoceptors, converted the atypical Schild regression for prazosin (norepinephrine as agonist) to a linear regression identical with that found for antagonism of phenylephrine responses. Selective alkylation of alpha-1 adrenoceptors with phenoxybenzamine (POB) eliminated responses to phenylephrine but not those to norepinephrine. After POB-alkylation and in the presence of a concentration of prazosin that was sufficient to produce a profound blockade of alpha-1 adrenoceptors, a response to norepinephrine remained. It was determined that the POB/prazosin-resistant response most likely was mediated by a homogeneous population of receptors by the finding that the Schild regressions for both yohimbine and idazoxan were identical with respect to slope and elevation when either norepinephrine or cobefrin were utilized as agonists, i.e., a difference in the regressions for these antagonists would be expected if the two agonists activated a heterogeneous receptor population.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alpha adrenoceptor subtypes involved in the emetic action in dogs.

In order to assess the involvement of alpha-1 and alpha-2 adrenoceptors in emesis, the emetic effect of eight alpha agonists was studied in dogs. The i.m. administration of each agonist elicited dose-dependent emesis. The order of potency in inducing emesis was: clonidine greater than oxymetazoline greater than tramazoline greater than naphazoline greater than xylazine greater than epinephrine greater than methoxamine = phenylephrine. The clonidine-induced emesis was antagonized by adrenoceptor antagonists showing alpha-2 blocking activity, yohimbine, tolazoline and phentolamine. Among these antagonists, yohimbine was the most effective. The alpha-1 and beta adrenergic, cholinergic, dopaminergic, histaminergic, serotonergic and opioid receptor antagonists did not prevent the clonidine-induced emesis. The emesis induced by oxymetazoline, tramazoline, xylazine, naphazoline and epinephrine was also antagonized by a selective alpha-2 adrenoceptor antagonist, yohimbine, but not by a selective alpha-1 adrenoceptor antagonist, prazosin. In contrast, methoxamine and phenylephrine-induced emesis was antagonized by prazosin, but not by yohimbine. Neither yohimbine nor prazosin prevented the morphine- and histamine-induced emesis. These results indicate that alpha-2 adrenoceptors are involved in the mediation of emetic action, and that the alpha adrenoceptor-mediated emesis does not involve beta adrenergic, cholinergic, dopaminergic, histaminergic, serotonergic and opioid receptors in the emetic pathway. This study further suggests that alpha adrenoceptors involved in the emesis are mainly of the alpha-2 type, although the involvement of alpha-1 adrenoceptors cannot be ruled out.     

Characterization of brain imidazoline receptors in normotensive and hypertensive rats: differential regulation by chronic imidazoline drug treatment.

The binding of [3H]idazoxan in the presence of l-epinephrine was used to characterize and quantitate imidazoline receptors in the brain of spontaneously hypertensive (SHR), normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats before and after chronic imidazoline drug treatment. In the cerebral cortex of WKY and SHR rats, the rank order of potency of imidazoli(di)ne drugs (cirazoline greater than idazoxan greater than naphazoline greater than clonidine much greater than RX821002) competing with [3H]idazoxan showed the specificity for an imidazoline receptor which also appeared heterogeneous in nature. In SHR rats, the density of imidazoline receptors (hypothalamus greater than medulla oblongata greater than cerebral cortex) and proportion of high- and low-affinity sites for the receptor were not different from those in WKY and SD rats, suggesting that the receptor itself is not altered in hypertension. However, chronic treatment with idazoxan and cirazoline (10 and 1 mg/kg, i.p., every 12 h for 7 days) consistently increased (about 35%) the density of imidazoline receptors in the brain of WKY and SD, but not in SHR rats. A similar treatment with RX821002, the 2-methoxy analog of idazoxan, which is a highly selective alpha-2 adrenoceptor antagonist, did not increase the density of brain imidazoline receptors. Moreover, the up-regulation of these receptors induced by cirazoline was still present after alkylation of the alpha-2 adrenoceptors with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. The lack of regulation by idazoxan and cirazoline of the density of imidazoline receptors in the brain of SHR rats suggests the existence of a relevant abnormality in the adaptive process of these receptors in this genetic model of hypertension.     

Selective antagonism of the antinociceptive effect of intrathecally applied alpha adrenergic agonists by intrathecal prazosin and intrathecal yohimbine

We have attempted to define the alpha adrenoceptor subtype(s) on which intrathecally applied alpha adrenergic agonists act to produce their antinociceptive effect. The potencies of intrathecal (i.t.) prazosin or i.t. yohimbine to antagonize the elevations of thermal nociceptive threshold induced by i.t. 2-[2,6-diethylphenylamino]-2-imidazoline (ST-91), methoxamine or norepinephrine (NE) were determined in the rat. Tail-flick and hot plate tests were used to determine thermal nociceptive threshold. At the ID50 level, the alpha-2 selective antagonist yohimbine was significantly more potent than the alpha-1 selective antagonist prazosin at blocking the analgesia produced by the alpha-2 selective agonist ST-91, whereas prazosin was significantly more potent than yohimbine at antagonizing the analgesia produced by the alpha-1 selective agonist methoxamine or by the nonselective alpha agonist NE. Analgesic doses of methoxamine evoked a readily observable disturbance of motor and autonomic function, whereas such effects were not observed after analgesic doses of ST-91 or NE. Both i.t. methoxamine and i.t. ST-91 elevated thermal nociceptive threshold in rats depleted of lumbar spinal cord NE by pretreatment 7 days before with i.t. 6-hydroxydopamine. Our results suggest that stimulation of either one of two separate populations of postsynaptic spinal alpha adrenoceptors will inhibit spinal nociceptive transmission. One of these populations appears to be composed of alpha-2 adrenoceptors. The subtype classification of the alpha adrenoceptors composing the other population remains unclear.     

Involvement of alpha-2 adrenergic receptor subtypes in hyperglycemia

Alpha-2 adrenoceptor stimulation induces in the mouse a hyperglycemic response which is accompanied by a concomitant inhibition of insulin secretion. To test the possibility that one of the postulated subtypes of alpha-2 adrenoceptors is preferentially implicated in this response, we compared the interaction of several drugs with known selectivity toward alpha-2A or alpha-2B adrenoceptor subtypes in our model. The alpha-2A preferential agonist oxymetazoline induced in the mouse a hyperglycemic response similar to that of the nonselective alpha-2 adrenoceptor agonist UK 14.304. This hyperglycemic response to oxymetazoline was accompanied by a concomitant inhibition of insulin release. Both the effect on glycemic level and the inhibition of insulin release by oxymetazoline were antagonized by the alpha-2 adrenoceptor antagonist idazoxan. The alpha-2B preferential antagonists ARC-239, prazosin or chlorpromazine failed to block the modifications in both glycemic and insulin levels induced by alpha-2 adrenoceptor stimulation. The nonselective antagonists rauwolscine, yohimbine, WY 26703, phentolamine and corynanthine, as well as the receptor antagonists with alpha-2A selectivity like WB 4101, idazoxan and tolazoline, dose-dependently antagonized both the glycemic and the insulin responses to UK 14.304. A positive correlation was obtained between the potencies of these drugs in antagonizing the hyperglycemic response to UK 14.304 and their affinities for alpha-2A adrenergic receptors (r = 0.918, P less than .001) but no correlation was obtained with their affinities for alpha-2B adrenergic receptors (r = 0.048, P = N.S.)(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effect of alpha-1 adrenoceptor stimulation on cardiac sympathetic neurotransmission in pithed normotensive rats

In the present study we investigated the inhibitory effect of the selective alpha-1 adrenoceptor agonists cirazoline, amidephrine and St 587 on the cardiac sympathetic neurotransmission in pithed normotensive rats. Increases in heart rate were elicited by electrical stimulation of the cardiac sympathetic nerves or by i.v. administration of norepinephrine, isoproterenol or tyramine. Intravenous pretreatment of the animals with cirazoline, amidephrine or St 587 diminished the heart rate response to sympathetic stimulation significantly. However, the tachycardia produced by norepinephrine, isoproterenol or tyramine was also inhibited significantly by the selective alpha-1 adrenoceptor agonists. The selective alpha-1 antagonist prazosin blocked the sympathoinhibitory effect to alpha-1 adrenoceptor stimulation significantly. However, the inhibitory effect of cirazoline and St 587 was not suppressed completely by a maximally effective dose of prazosin. In contrast, the sympathoinhibitory action of amidephrine was antagonized completely by prazosin. However, the selective alpha-2 antagonist rauwolscine also produced a significant, albeit modest, attenuation of the sympathoinhibitory effect to amidephrine. The results of the present study indicate that alpha-1 adrenoceptor agonists, at relatively high doses, inhibit the sympathetic neurotransmission in rat heart. This sympathoinhibitory effect is mediated largely by alpha-1 adrenoceptors which are localized postjunctionally rather than prejunctionally.     

Localization and characterization of alpha-2 adrenoceptors in the isolated canine pulmonary vein

Alpha-1 and alpha-2 adrenoceptor-mediated contractile responses were studied in isolated canine pulmonary veins. Norepinephrine elicited concentration-dependent contractile responses which were antagonized in a competitive manner by the selective alpha-2 adrenergic antagonist, rauwolscine, with a dissociation constant of 15.7 nM, whereas the selective alpha-1 adrenoceptor antagonist, prazosin, produced nonparallel rightward shifts in the norepinephrine concentration-response curve with a marked depression in the maximal response, indicating noncompetitive antagonism. B-HT 933, a selective alpha-2 adrenoceptor agonist, also produced a concentration-dependent contraction in canine pulmonary vein, with the maximal contraction elicited by B-HT 933 being approximately 45% of that produced by norepinephrine. The response mediated by B-HT 933 was antagonized in a competitive manner by rauwolscine with a dissociation constant of 4.4 nM, whereas prazosin again behaved in a noncompetitive manner producing nonparallel rightward shifts in the B-HT 933 concentration-response curve with a marked depression in the maximal response. However, another alpha-1 adrenoceptor antagonist, corynanthine, weakly blocked the response produced by B-HT 933 with a dissociation constant of 1400 nM, and this low affinity for corynanthine is consistent with interaction at alpha-2 adrenoceptors. Cirazoline, a selective alpha-1 adrenoceptor agonist, also produced a concentration-dependent vasoconstrictor response in canine pulmonary veins which was antagonized competitively by both alpha-1 adrenoceptor antagonists, corynanthine and prazosin, with dissociation constants of 180 and 1.4 nM, respectively, indicative of an interaction with alpha-1 adrenoceptors. Rauwolscine (10 nM) did not significantly affect the response produced by cirazoline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological characterization of the hyperglycemia induced by alpha-2 adrenoceptor agonists

The selective alpha-2 adrenoceptor agonist UK 14.304 induced in the mouse a dose-dependent hyperglycemic response which was accompanied by a concomitant inhibition of insulin secretion. Similar effects were observed with the preferential alpha-2 receptor agonists clonidine and guanabenz whereas less pronounced effects were found with (-)-epinephrine. No significant effects on blood glucose levels were observed with the alpha-1 adrenoceptor agonist methoxamine. Adrenalectomy or depletion of catecholamine stores by reserpine, alpha-methylparatyrosine or DSP4 failed to modify the hyperglycemic response to UK 14.304. However, streptozotocin diabetic mice did not respond to UK 14.304. The hyperglycemia induced by submaximal doses of UK 14.304 was antagonized by the centrally and peripherally acting alpha-2 adrenoceptor antagonists rauwolscine, yohimbine, idazoxan and phentolamine, by the peripheral antagonist benextramine but not by prazosin (alpha-1 selective) or propranolol (beta adrenergic). Thus, it may be suggested that the alpha agonist-induced hyperglycemia is mediated via postsynaptic alpha-2 adrenoceptors located on pancreatic beta cells and that it is mediated through the inhibition of insulin secretion.     

SL 84.0418: a novel, potent and selective alpha-2 adrenoceptor antagonist: in vitro pharmacological profile.

One novel, potent and selective alpha-2 adrenoceptor antagonist is 2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2- propylpyrrolo[3,2,1-hi]-indole hydrochloride (SL 84.0418). It inhibits with high affinity the radioligand binding to rat cortical alpha-2 adrenoceptors, as well as to human platelet alpha-2 adrenoceptors labeled with [3H]idazoxan (Ki = 7 nM). SL 84.0418 has low affinity for alpha-1 adrenoceptors labeled with [3H]prazosin (Ki = 3.3 microM). In vitro, SL 84.0418 has no alpha agonist properties, whereas it is a potent alpha-2 adrenoceptor antagonist at both pre- and postsynaptic alpha-2 adrenoceptors. In contrast, it possesses low potency as an antagonist at postsynaptic alpha-1 adrenoceptors demonstrating a more than 1000-fold selectivity toward alpha-2 compared with alpha-1 adrenoceptors. In the same tests, the alpha-2 adrenoceptor antagonist idazoxan had a selectivity ratio of 200. SL 84.0418 is the racemic mixture of two enantiomers, SL 86.0715 [(+) enantiomer] and SL 86.0714 [(-) enantiomer]. The alpha-2 adrenoceptor blocking activities reside with SL 86.0715. Similar to idazoxan, SL 84.0418 increases in a concentration-dependent manner the electrically evoked release of [3H]norepinephrine from rat hypothalamic slices through the blockade of the presynaptic inhibitory alpha-2 adrenoceptors. In isolated hamster adipocytes, SL 84.0418 potently antagonizes the inhibition of lipolysis induced by UK 14,304. In addition, SL 84.0418 inhibits epinephrine-induced aggregation of rabbit platelets, effects mediated by postsynaptic alpha-2 adrenoceptors. SL 84.0418 does not inhibit (IC50 > 1,000 nM) radioligand binding to other receptors or recognition sites, nor does it inhibit calcium, sodium or potassium channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antinociceptive effects of intrathecal adrenoceptor agonists in a rat model of visceral nociception

Intrathecal administration of alpha adrenoceptor agonists in rats produces a significant elevation of nociceptive thresholds as measured by hot-plate and tail-flick latencies. That these antinociceptive effects were antagonized by alpha adrenoceptor antagonists suggests that spinal adrenoceptors modulate the rostrad transmission of nociceptive information. The role of spinal adrenoceptors in the modulation of nociception in visceral pain tests has in the past been examined primarily in the writhing model of visceral pain. Recently, however, a new model of visceral pain has been developed which utilizes colorectal distension (CRD) as the noxious visceral stimulus in awake, unanesthetized animals. CRD produces a pressor response and contraction of the abdominal and hindlimb musculature (a visceromotor response), both of which are readily quantifiable. Inhibition of both of these pseudoaffective reflexes is indicative of antinociception. The effects of intrathecally administered adrenoceptor agonists on the responses to CRD were examined in conscious rats. The visceromotor and pressor responses to CRD were dose-dependently inhibited by the alpha-2 adrenoceptor agonists clonidine and ST-91 and the nonselective alpha adrenoceptor agonist norepinephrine (NE), but not by the alpha-1 adrenoceptor agonist methoxamine or the alpha adrenoceptor agonist isoproterenol. Tizanidine, an alpha-2 adrenoceptor agonist, dose-dependently inhibited the pressor response to CRD but failed to significantly elevate the visceromotor threshold in response to CRD. The effects produced by clonidine, NE and ST-91 were antagonized by pretreatment with yohimbine. The effects produced by tizanidine were antagonized by idazoxan.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies that question the existence of alpha-2 adrenoceptors in tail arteries of normotensive Sprague-Dawley rats

Many pharmacological studies have demonstrated two distinct types of alpha adrenoceptor in the vasculature; these receptors have been named alpha-1 and alpha-2. In the present study, using isolated perfused tail arteries from normotensive Sprague-Dawley rats, we have demonstrated two types of alpha adrenoceptor but neither of these could be classified as an alpha-2 adrenoceptor. Dose-dependent contraction of rat tail arteries was produced by the following alpha adrenoceptor agonists or agonist-antagonist combination: phenylephrine (PE alpha-1), clonidine (alpha-1 and alpha-2), clonidine in the presence of 10(-7) M prazosin (alpha-2) and BHT-920 (alpha-2). The ED50 values for PE and clonidine were four orders of magnitude lower than those for clonidine plus prazosin and BHT-920. In addition, the action of PE was faster in onset than that of BHT-920, reached a higher maximum (5-fold) and attenuated more rapidly than that of BHT-920. The specific alpha-2 adrenoceptor antagonist yohimbine, in concentrations as high as 10(-6) M, did not antagonize arterial responses to BHT-920. However, responses to BHT-920 were antagonized by the alpha-1 adrenoceptor antagonist prazosin, in concentrations as low as 10(-10) M, and by the serotonin/alpha-1 adrenoceptor antagonist ketanserin (10(-7) M). These results suggest that the two alpha-adrenoceptor types in isolated rat tail arteries are both of the alpha-1 type. We also found that whereas responses to PE were stable and reproducible between 2 and 5 hr of arterial perfusion, responses to BHT-920 increased progressively over 5 hr. The latter effect probably resulted from a gradual disappearance of the arterial endothelium.     

Alpha adrenoceptor-mediated vasoconstriction in rat hindlimb: innervated alpha-2 adrenoceptors in the saphenous arterial bed

The alpha adrenoceptor subtypes mediating vasoconstriction to exogenous agonists and to spinal sympathetic nerve stimulation have been characterized in the autoperfused (constant flow) femoral (predominantly skeletal musculature) and saphenous (predominantly cutaneous) vascular beds of the pithed rat. Intra-arterial infusion of the alpha-1 adrenoceptor agonist, methoxamine, increased perfusion pressure in both vascular beds over the same range of infusion rates, and the maximum responses were similar. The selective alpha-2 adrenoceptor agonist, B-HT 933, also increased perfusion pressure in both beds, although the maximum response to B-HT 933 in the saphenous bed was approximately twice that observed in the femoral bed. Responses to methoxamine were blocked by the alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg), but not the alpha-2 adrenoceptor antagonist, rauwolscine (1 mg/kg), or by the selective postjunctional alpha-2 adrenoceptor antagonist, SK&F 104078 (1 mg/kg). Conversely, responses to B-HT 933 were blocked by rauwolscine and by SK&F 104078, but not by prazosin. Vasopressor responses to B-HT 933 in both vascular beds of the rat hindlimb also were reduced markedly by the calcium channel blocker, nifedipine (1 mg/kg), whereas responses to methoxamine were relatively resistant to inhibition by nifedipine. In the femoral bed, as in the systemic arterial circulation, responses to sympathetic nerve stimulation were strongly inhibited by prazosin, were potentiated by rauwolscine and were unaffected by SK&F 104078. In contrast, in the saphenous arterial bed, the responses to sympathetic nerve stimulation were inhibited by all three antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Indications for vascular alpha- and beta-2 adrenoceptors in synapses of the muscarinic pathway in the pithed normotensive rat

In the pithed normotensive rat the adrenoceptors involved in the hypertensive and tachycardic effects of the indirectly acting sympathomimetic agent tyramine and of electrical stimulation of the spinal cord (TH5-L4 or C7-Th1) were analyzed. The tools used for the identification of the adrenoceptors were the selective alpha-1 adrenoceptor blocking drug prazosin, the selective alpha-2 adrenoceptor antagonist rauwolscine, the beta-1 blocker atenolol and the selective beta-2 adrenoceptor blocking agent ICI 118,551. The participation of vascular alpha-2 adrenoceptors in the pressor response of tyramine was shown. The increase in blood pressure induced by electrical stimulation of the spinal cord (TH5-L4) was used to demonstrate that alpha-2 adrenoceptors were activated via ganglionic muscarinic receptors. The tachycardia evoked by electrical stimulation of the spinal cord (C7-Th1) was not influenced by beta-2 adrenoceptor blockade. It was enhanced, however, by the alpha-2 adrenoceptor antagonist rauwolscine. It is hypothesized that activation of ganglionic nicotinic receptors leads to stimulation of the nearest varicosities interfering with alpha-1 adrenoceptors. However, activation of ganglionic muscarinic receptors may lead to an additional release of neurotransmitter in the more distant varicosities endowed with alpha-2 or beta-2 adrenoceptors.     

Evidence for enhanced inhibitory modulation by cyclooxygenase products of noradrenergic neurotransmission in the mesenteric vasculature of young spontaneously hypertensive rats.

The effects of cyclooxygenase inhibition by indomethacin and meclofenamate on pre- and postjunctional aspects of noradrenergic neurotransmission were determined in mesenteric vascular preparations from 4- to 6-week-old spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto rats (WKYs). Perfusion pressure responses to periarterial nerve stimulation and to exogenous norepinephrine (NE) were significantly greater in SHR than in WKY preparations, whereas fractional NE overflow was equivalent between the two strains. Indomethacin and meclofenamate enhanced perfusion pressure responses to periarterial nerve stimulation in both strains. Fractional NE overflow was significantly enhanced by indomethacin but only at 14 Hz in SHR preparations, whereas it was unaffected in WKY preparations. The combination of indomethacin and cocaine resulted in a significant enhancement of perfusion pressure responses to periarterial nerve stimulation in both strains that was significantly greater than that produced by either drug alone. This effect was significantly greater in SHR than in WKY preparations. This combination also resulted in a significant enhancement of responses to exogenous NE in both strains. Fractional NE overflow was significantly increased in SHR preparations in the presence of the combination of cocaine and indomethacin, whereas it remained unaltered in WKY preparations. These findings suggest that a cyclooxygenase product exerts both pre- and postjunctional inhibitory effects on vascular noradrenergic neurotransmission that differ in these two strains of rats. The prejunctional inhibitory effect of this cyclooxygenase product was observed only in SHR preparations and was especially evident in the presence of cocaine.     

Evidence that two stereochemically different alpha-2 adrenoceptors modulate norepinephrine release in rat cerebral cortex.

Cerebral cortex slices from the rat were loaded with [3H]norepinephrine ([3H]NE) and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. The (-)-isomer and the (+)-isomer of CH-38083 ([7,8-(methylenedioxy)-14- alpha-hydroxyalloberbane HCl), a selective alpha-2-adrenoceptor antagonist with an alloberbane skeleton, increased the electrically induced release of [3H]NE in a concentration-dependent manner, and a similar effect was observed with racemic CH-38083 and idazoxan. The stereoisomers of CH-38083 applied in a concentration range of 10(-8) to 10(-6) mol/l were equipotent in facilitating stimulation-evoked [3H]NE release: concentrations needed to enhance tritium outflow by 50% were 1.3 X 10(-7) mol/l for (-)-CH-38083 and 1.4 X 10(-7) mol/l for (+)-CH-38083. Exogenous NE decreased the electrically stimulated release of [3H]NE, and the stereoisomers of CH-38083 antagonized this inhibition with different potencies: the dissociation constant (KB) values for (-)-isomer and for (+)-isomer of CH-38083 were 14.29 and 97.18 nmol/l. These data indicate that presynaptic alpha-2 adrenoceptors that are available for NE released from axon terminals do not show stereospecificity toward enantiomers of CH-38083, whereas those that are occupied by exogenous NE are much more sensitive toward (-)-CH-38083. The alpha-1 adrenoceptor antagonist prazosin also differentiated between the alpha-2 adrenoceptor subtypes: prazosin (10(-6) mol/l) did not alter the increase of electrically induced [3H]NE release evoked by (-)- and (+)-CH-38083; however, in its presence, the stereoisomers of CH-38083 failed to antagonize the inhibitory effect of exogenous NE on its own release.(ABSTRACT TRUNCATED AT 250 WORDS)