Vasoconstriction mediated by postsynaptic alpha 2-adrenoceptor stimulation

Development and validation of a chiral LC-MS method for the enantiomeric resolution of (+) and (-)-medetomidine in equine plasma by using polysaccharide-based chiral stationary phases

The detection and separation of medetomidine enantiomers from the complex biological matrices poses a great analytical challenge, especially in the field of forensic toxicology and pharmacology. Couple of researchers reported resolution of medetomidine using protein-based chiral columns, but the reported method is quiet challenging and tedious to be employed for routine analysis. This research paper reported a method that enables the enantio-separation of medetomidine by using polysaccharide cellulose chiral column. The use of chiralcel OJ-3R column was found to have the highest potential for successful chiral resolution. Ammonium hydrogen carbonate was the ideal buffer salt for chiral liquid chromatography (LC) with electrospray ionization (ESI)+ mass spectrometry (MS) detection for the successful separation and detection of racemic compound. The method was linear over the range of 0 to 20 ng/mL in equine plasma and the inter-day precisions of levomedetomidine, dexmedetomidine were 1.36% and 1.89%, respectively. The accuracy of levomedetomidine was in the range of 99.25% to 101.57% and that for dexmedetomidine was 99.17% to 100.99%. The limits of quantification for both isomers were 0.2 ng/mL. Recovery and matrix effect on the analytes were also evaluated. Under the optimized conditions, the validated method can be adapted for the identification and resolution of the medetomidine enantiomers in different matrices used for drug testing and analysis.

Pharmacokinetics and pharmacodynamics of dexmedetomidine-induced vasoconstriction in healthy volunteers

AIMS

Alpha-2 agonists are direct peripheral vasoconstrictors, which achieve these effects by activating vascular smooth muscle alpha-2 adrenoceptors. The impact of this response during dexmedetomidine infusion remains poorly quantified. Our goal was to investigate the pharmacokinetic (PK) and pharmacodynamic (PD, vasoconstriction) effects of a computer-controlled dexmedetomidine infusion in healthy volunteers.

METHODS

After local ethics committee approval, we studied 10 healthy volunteers. To study the peripheral vasoconstrictive effect of dexmedetomidine without concurrent sympatholytic effects, sympathetic fibres were blocked with a brachial plexus block. Volunteers received a dexmedetomidine target-controlled infusion for 15 min, to a target concentration of 0.3 ng ml-1 . Arterial blood samples were collected during and for 60 min after dexmedetomidine infusion for PK analysis. Peripheral vasoconstriction (PD) was assessed using finger photoelectric plethysmography. PK/PD analysis was carried out using nonlinear mixed-effect models.

RESULTS

We found that the computer-controlled infusion pump delivered mean concentrations greater than 0.3 ng ml-1 over the 15-min infusion duration. The peripheral vasoconstrictive effect correlated with dexmedetomidine plasma concentrations during and after the infusion. A three-compartment model provided a better fit to the data than a two-compartment model.

CONCLUSIONS

We found that dexmedetomidine-induced vasoconstriction is concentration dependent over time. Dexmedetomidine PK were best estimated by a three-compartment model with allometric scaling. Our results may contribute to future modelling of dexmedetomidine-induced haemodynamic effects.

Pharmacological profile of the clonidine-induced inhibition of vasodepressor sensory outflow in pithed rats: correlation with alpha(2A/2C)-adrenoceptors

BACKGROUND AND PURPOSE

Resistance blood vessels are innervated by sympathetic and primary sensory nerves, which modulate vascular tone through the release of noradrenaline and calcitonin gene-related peptide (CGRP), respectively. Moreover, electrical stimulation of the perivascular sensory outflow in pithed rats results in vasodepressor responses which are mainly mediated by CGRP release. The present study has investigated the role of alpha(2)-adrenoceptors in the inhibition of these vasodepressor responses.

EXPERIMENTAL APPROACH

144 pithed male Wistar rats were pretreated with hexamethonium (2 mg kg(-1) min(-1)) followed by i.v. continuous infusions of either methoxamine (15 and 30 microg kg(-1) min(-1)) or clonidine (3, 10 and 30 microg kg(-1) min(-1)). Under these conditions, electrical stimulation (0.56-5.6 Hz; 50 V and 2 ms) of the spinal cord (T(9)-T(12)) resulted in frequency-dependent decreases in diastolic blood pressure.

KEY RESULTS

The infusion of clonidine (10 microg kg(-1) min(-1)), as compared to those of methoxamine (15 or 30 microg kg(-1) min(-1)), inhibited the vasodepressor responses to electrical stimulation without affecting those to i.v. bolus injections of alpha-CGRP (0.1-1 microg kg(-1)). This inhibition by clonidine was: (i) antagonized by 300 microg kg(-1) rauwolscine (alpha(2A/2B/2C)), 300 and 1000 microg kg(-1) BRL44408 (alpha(2A)), or 10 and 30 microg kg(-1) MK912 (alpha(2C)); and (ii) unaffected by 1 ml kg(-1) saline, 100 microg kg(-1) BRL44408, 3000 and 10,000 microg kg(-1) imiloxan (alpha(2B)) or 3 microg kg(-1) MK912.

CONCLUSIONS AND IMPLICATIONS

The inhibition produced by 10 microg kg(-1) min(-1) clonidine on the vasodepressor (perivascular) sensory outflow in rats may be mainly mediated by prejunctional alpha(2A)/alpha(2C)-adrenoceptors.

The role of NO-cGMP pathway and potassium channels on the relaxation induced by clonidine in the rat mesenteric arterial bed

The mechanisms involved in the vasodilation action of clonidine have not yet been completely elucidated. We investigated the potential mechanisms that seem to be involved in the clonidine vasodilator effect using rat isolated mesenteric arterial bed (MAB). In precontracted MAB, clonidine (10-300 pmol) induced a dose-dependent relaxation, that was inhibited by endothelium removal (deoxycholic acid - 2.5 mM) and reduced by the alpha(2) adrenoceptor inhibitors yohimbine (1-3 microM) and rauwolscine (1 microM). The endothelium-dependent vasodilation induced by clonidine was reduced by the nitric oxide (NO) synthase inhibitor L-NAME (0.3 mM) and guanylyl cyclase inhibitor ODQ (10 microM) but was not affected by indomethacin (3-10 microM) alone. High K+ (25 mM) solution reduced the vasodilator effect of clonidine that was further attenuated by L-NAME. In the presence of high K+ plus L-NAME, the residual vasodilator effect of clonidine was further reduced by indomethacin (3 microM). The Ca(2+)-dependent K+ channel (K+(Ca2+)) inhibitors, charybdotoxin (ChTx; 0.1 microM) plus apamin (0.1 microM), also reduced the vasodilation induced by clonidine, however this response was not further reduced in the presence of L-NAME as observed with acetylcholine (10 pmol). In the presence of ATP-dependent K+ channel (K+(ATP)) blocker, glibenclamide (10 microM), the inhibitory effect of ChTx plus apamin plus L-NAME was increased. In contrast, the vasodilation induced by clonidine was not affected by voltage-dependent K+ channels (K(V)) blocker, 4-aminopyridine (4-AP, 1 mM). In conclusion, our results demonstrate that clonidine activates alpha(2)-adrenoceptors in rat MAB and that the endothelium-dependent vasodilation is mediated by activation of NO-cGMP pathway, hyperpolarization due to activation of K+(Ca) and K+(ATP) channels. Prostaglandins might participate in the vasodilator effect of clonidine when NO and EDHF mechanisms are blunted.

Pharmacological evidence that alpha2A- and alpha2C-adrenoceptors mediate the inhibition of cardioaccelerator sympathetic outflow in pithed rats

It has been suggested that the alpha(2)-adrenoceptors mediating cardiac sympatho-inhibition in pithed rats closely resemble the pharmacological profile of the alpha(2A)-adrenoceptor subtype. However, several lines of evidence suggest that more than one subtype may be involved. Thus, the present study has pharmacologically re-evaluated the receptor subtype(s) involved in the inhibitory effect of the alpha(2)-adrenoceptor agonist, B-HT 933, on the tachycardic responses elicited by selective cardiac sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) in desipramine-pretreated pithed rats. I.v. continuous infusions of B-HT 933 (30 microg/kg min), which failed to modify the tachycardic responses to exogenous noradrenaline, inhibited those induced by preganglionic (C(7)-T(1)) stimulation of the cardiac sympathetic outflow at all frequencies of stimulation (0.03-3 Hz). This cardiac sympatho-inhibitory response to B-HT 933 was: (1) unaltered by saline (1 ml/kg) or the antagonists BRL44408 (100 microg/kg; alpha(2A)) or imiloxan (3000 and 10,000 microg/kg; alpha(2B)); (2) partially antagonized by BRL44408 (300 microg/kg) or MK912 (10 microg/kg; alpha(2C)) given separately; and (3) completely antagonized by rauwolscine (300 microg/kg; alpha(2)), MK912 (30 microg/kg) or the combination of BRL44408 (300 microg/kg) plus MK912 (10 microg/kg). Moreover, the above doses of antagonists, which are high enough to block their respective receptors, failed to block per se the tachycardic responses to sympathetic stimulation. These results suggest that the cardiac sympatho-inhibition induced by B-HT 933 in pithed rats is mainly mediated by stimulation of alpha(2A)- and alpha(2C)-adrenoceptors.

Structure-activity relationship at alpha-adrenergic receptors within a series of imidazoline analogues of cirazoline

Several analogues of cirazoline (2), a selective alpha1-adrenoreceptor agonist, were prepared and their pharmacological profiles studied. Although at the alpha1-adrenoreceptor all the compounds displayed a significant agonist activity, at the alpha2-adrenoreceptor they showed either agonist or antagonist activity depending on the nature of the phenyl substituent. The qualitative structure-activity relationship led us to the conclusion that the oxygen atom in the side-chain is essential for alpha1-agonist activity, while the cyclopropyl ring is not, and may be replaced by several groups. Of the groups studied, isopropoxy appears to be the best. Instead, the same substitution (i.e., isopropoxy for the cyclopropyl ring) at alpha2-adrenoreceptors causes a reversal of activity. On the other hand, the cyclopropyl ring seems to be important for alpha1-selectivity. Compound 20 is the most potent alpha1-agonist of the series, being equiactive with cirazoline on rat vas deferens and in pithed rat.

Sedative and analgesic effects of medetomidine in beagle dogs infected and uninfected with heartworm

The sedative and analgesic effects of medetomidine were evaluated in heartworm-infected (HW+) and uninfected (HW-) beagle dogs by intravenous (IV) and intramuscular (IM) administration of 30 micrograms/kg and 40 micrograms/kg doses, respectively. Posture, response to noise and the pedal reflex were monitored. A procedure for mock radiographic positioning was performed to evaluate its overall clinical use. Observation times were 0, 15, 30, 60, 90, 120 and 180 min. In addition, the times from injection until the dog could not stand on its feet (down time), from lateral to sternal recumbency (sternal recumbency time), and from sternal recumbency to rising again (rising time) were also noted. Medetomidine produced rapid sedation and analgesia by both routes. Down times for the IM and IV routes were similar, which verified the manufacturer's recommended doses. The HW+ dogs had shorter down times, probably owing to increased blood flow to the brain caused by adrenergic alpha-2 activity. Sternal recumbency and rising times did not differ between the groups, suggesting a similar metabolism. Sedation and analgesia were adequate for performing the procedure in all dogs. HW- dogs showed less resistance to handling during the procedure than HW+ dogs. Overall, medetomidine seems to be a suitable agent for short-term chemical restraint in dogs, even with subclinical heartworm infestation.

Characterization of alpha-adrenoceptor subtype(s) mediating vasoconstriction in the perfused rabbit ovarian vascular bed

1. alpha 1-Adrenoceptor agonists, noradrenaline, phenylephrine, methoxamine, oxymetazoline and SDZ NVI 085 but not alpha 2-adrenoceptor agonists, UK 14304, tizanidine or clonidine evoked dose-dependent vasoconstriction of the isolated perfused rabbit ovarian vascular bed. The rank order of agonist potency was noradenaline > oxymetazoline > phenylephrine > SDZ NVI 085 > methoxamine. 2. Prazosin (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/pKB values ranged between 7.27 and 7.66 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085 and were not significantly different from each other. Prazosin was however significantly less potent against oxymetazoline (pA2 6.38). Yohimbine (10(-6) M - 10(-5) M) was not very effective against any of the agonists. 3. WB 4101 (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/ pKB values ranged between 7.08 and 7.93 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085. WB 4101 was significantly less potent against oxymetazoline (pKB 6.85). 4. SZL-49 (5 x 10(-6) M) but not chloroethylclonidine (3 x 10(-5) M) significantly reduced vasoconstrictor responses to all the agonists. 5. Electrical field stimulation of the ovarian bed produced frequency-dependent vasoconstrictor effects which were abolished by 6-OHDA. The responses were also antagonized in a concentration-dependent by prazosin (10(-7) M - 10(-5) M) and WB 4101 (3 x 10(-8) M - 3 x 10(-7) M). Yohimbine reduced the response to electrical stimulation by 20% at 10(-5) M. The vasoconstrictor effect was also inhibited by SZL-49 but not by chloroethylclonidine. 6. These results would suggest that the vasoconstrictor responses of the ovarian vascular bed to adrenergic agonists and to electrical stimulation are mediated via the alpha 1A-adrenoceptor subtype.

Cardiovascular changes associated with anaesthesia induced by medetomidine combined with ketamine in cats

Fifteen cats had anaesthesia induced by intramuscular injection of medetomidine combined with ketamine. By five minutes after drug administration, heart rate had decreased by 31 per cent, respiratory rate had decreased by 70 per cent and systolic blood pressure had increased by 69 per cent. Atipamezole administration was associated with a decrease in systolic blood pressure and an increase in heart and respiratory rates. Time to first head lift was eight minutes and to sternal recumbency 12 minutes after atipamezole administration. Postoperative analgesia was provided by methadone, administered when the cats adopted sternal recumbency.

Mechanisms of signal transduction during alpha 2-adrenergic receptor-mediated contraction of vascular smooth muscle

Little is known about the signaling pathways involved in alpha 2-adrenergic receptor-mediated contraction of vascular smooth muscle. In the present study, we measured intracellular Ca2+ ([Ca2+]i), myosin light chain (MLC) phosphorylation, and myofilament Ca2+ sensitivity during stimulation with the relatively selective alpha 2-agonist UK 14304. These effects were compared and contrasted with corresponding changes during depolarization by elevation of the [K+] in the bathing medium. These studies were performed using spiral strips of the rabbit saphenous vein, a tissue with a relatively high density of postsynaptic alpha 2-receptors. UK 14304 (10(-5) M) caused parallel changes in [Ca2+]i, MLC phosphorylation, and force consisting of an initial phasic, followed by a sustained steady-state response. The steady-state increase in [Ca2+]i, MLC phosphorylation, and force caused by UK 14304 in the presence of 2.5 mM extracellular Ca2+ were indistinguishable from those during 51 mM K+ depolarization. However, when extracellular Ca2+ was removed in the presence of UK 14304, [Ca2+]i and MLC phosphorylation fell to resting levels, but force remained significantly elevated above basal levels. UK 14304 caused no change in the steady-state [Ca2+]i-MLC phosphorylation relation. Thus, the [Ca2+]i sensitization of force was not caused by a sensitization of MLC phosphorylation. These results indicate that in a 2.5-mM Ca2+ bathing medium, the dominant mechanism by which alpha 2-adrenergic receptor stimulation causes an increase in vascular tone is through a relatively large increase in [Ca2+]i and MLC phosphorylation. However, in Ca(2+)-free bathing medium, a second mechanism is unmasked which appears to involve an increased Ca2+ sensitivity and is independent of myosin phosphorylation.

The inhibition of alpha 1-adrenoceptor-mediated contractions of rabbit pulmonary artery by Ca2+-withdrawal, pertussis toxin and N-ethylmaleimide is dependent on agonist intrinsic efficacy

Contractions were induced in rings of rabbit pulmonary artery with the preferential alpha 1-adrenoceptor agonists, phenylephrine, methoxamine and St 587 [2-(2-chloro-trifluoromethyl-phenylimino)imidazolidine and the preferential alpha 2-adrenoceptor agonists, clonidine and B-HT 920 [6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine] [corrected]. Phenylephrine and methoxamine acted as full agonists whereas St 587, clonidine and B-HT 920 were partial agonists (intrinsic activities 0.62, 0.38 and 0.42, respectively). Experiments with alpha 1- and alpha 2-adrenoceptor antagonists indicated that the receptors involved are of the alpha 1 type only. Removal of extracellular Ca2+ inhibited maximal contractions to phenylephrine and methoxamine by 30% and 49%, respectively. The remaining contraction components of the full agonists were abolished by the "intracellular Ca2+ antagonist" TMB-8 [8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate]. Contractions to St 587, clonidine and B-HT 920 were virtually abolished in Ca2+-free medium. Pretreatment of the donor rabbits with pertussis toxin (2.5 micrograms/kg i.v., 5-6 days before sacrifice) attenuated the efficacies of the full agonists, phenylephrine and methoxamine by only 24% and 17%, respectively, whereas maximal contractions to the partial agonists, St 587, clonidine and B-HT 920, were inhibited by 46%, 61% and 75%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery

1. The effects of the alpha2-adrenoceptor agonists clonidine, rilmenidine, TL99 and UK14304 on the vasoconstrictor response to sympathetic nerve stimulation and on the concentration-response curves to noradrenaline and phenylephrine were compared in two isolated, perfused vascular tissues: the rat tail artery (which has both postjunctional alpha 1- and alpha 2-adrenoceptors), and the rabbit ear artery (in which only alpha 1-adrenoceptors are present postjunctionally). 2. In the rabbit ear artery, the first observable effect of alpha 2-adrenoceptor agonists was inhibition of vasoconstrictor responses to sympathetic nerve stimulation. This occurred with concentrations of the alpha 2-adrenoceptor agonists which were far below those producing vasoconstriction. Responses to noradrenaline were not affected. 3. In contrast, in the rat isolated perfused tail artery, alpha 2-adrenoceptor agonists, in concentrations that produced no other observable effects, enhanced the vasoconstrictor responses to sympathetic nerve stimulation and to noradrenaline. Much higher concentrations of alpha 2-adrenoceptor agonists produced vasoconstriction in most preparations and only then reduced the response to sympathetic nerve stimulation. The enhancing effect of alpha 2-adrenoceptor agonists was blocked by idazoxan, but not by prazosin. 4. Vasoconstrictor responses in the rat tail artery to the relatively selective alpha 1-adrenoceptor agonist phenylephrine were enhanced by alpha 2-adrenoceptor agonists. The enhancement of the response to phenylephrine was greater than that to the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline. 5. Vasoconstrictor responses in the rat tail artery to vasopressin, ATP and KCl, like those to alpha 1-adrenoceptor agonists, were enhanced by alpha 2-adrenoceptor agonists.2+owever, vasoconstrictor responses to

Influence of calcium-entry blockade on vasoconstrictor responses in feline mesenteric vascular bed

The subtypes of postjunctional alpha-adrenoceptors activated by neuronally released and exogenous norepinephrine and the source of calcium used for vasoconstrictor responses were investigated in the feline mesenteric vascular bed. Under constant flow conditions, intra-arterial injections of phenylephrine and UK14304, alpha 1- and alpha 2-adrenoceptor agonists, increased mesenteric arterial perfusion pressure in a dose-related manner. Prazosin, an alpha 1-antagonist, reduced vasoconstrictor responses to phenylephrine without altering responses to UK14304. Yohimbine, an alpha 2-antagonist, reduced responses to UK14304 without altering responses to phenylephrine. The same pattern of blockade was observed in animals pretreated with 6-hydroxydopamine to destroy the integrity of adrenergic terminals. Responses to phenylephrine and UK14304 were reduced by nitrendipine, a calcium-entry blocking agent, and this agent decreased vasoconstrictor responses to sympathetic nerve stimulation, tyramine, and norepinephrine. Responses to sympathetic nerve stimulation were selectively blocked by prazosin, but responses to norepinephrine were selectively blocked by yohimbine. Vasoconstrictor responses to tyramine were reduced by both prazosin and yohimbine. Nitrendipine also reduced responses to angiotensin II, U46619, a prostaglandin endoperoxide analogue, Bay K 8644, and potassium chloride. These data suggest the presence of alpha 1- and postjunctional alpha 2-adrenoceptors and support the hypothesis that norepinephrine released by nerve excitation acts mainly on alpha 1-receptors but that exogenous norepinephrine acts primarily on alpha 2-receptors. However, norepinephrine released by tyramine acts on both receptor subtypes. Nitrendipine inhibited responses to the alpha 1- and alpha 2-adrenoceptor agonists as well as those to nerve released and exogenous norepinephrine, the calcium agonist, Bay K 8644, and to other vasoconstrictor agents. These data suggest that in the feline mesenteric vascular bed, an extracellular source of calcium ions is required for vasoconstriction induced by a variety of mechanisms including activation of alpha 1- and postjunctional alpha 2-adrenoceptors.

Postsynaptic effects of alpha agonists on adrenoceptors of the reserpinized rat vas deferens

The activities of the alpha-1 antagonist prazosin and alpha-2 antagonist yohimbine were evaluated against noradrenaline (NA), methoxamine (Me) and clonidine (Clo) on the reserpinized rat vas deferens. Prazosin antagonized competitively Me but not NA and Clo. On the other hand yohimbine showed a low and not competitive antagonism towards all the three agonists. Similar results were obtained when the antagonistic activities were tested in the presence of the alternative antagonist, in the attempt to isolate a single receptor population. We can conclude that the smooth musculature of the rat vas deferens contains prevalently alpha-1 adrenoceptors and a small population of NA activated receptors resistant to alpha-2 antagonists.

Dopaminergic and alpha 1-adrenergic properties of B-HT920 revealed in morphine-dependent rats

B-HT920 is known to be a selective alpha 2-adrenoceptor agonist, and has been used in a study on morphine-withdrawal in rats. In accordance with other alpha 2-agonists B-HT920 was found to potentiate "jumping" and to reduce "body shakes." However, B-HT920 did not suppress body weight loss. Furthermore, it induced strong salivation and prevented ptosis (described for the alpha 1-adrenergic agonist ST-587). Rearing and locomotor activity appeared to be enhanced, an effect shared by dopamine-agonist lisurid. The effects of B-HT920 have been specified using the alpha-adrenergic antagonists yohimbine and prazosin and the dopamine antagonist haloperidol. Yohimbine could not antagonize any of the actions of B-HT920. However the increase in rearing and locomotion was blocked by haloperidol. The induction of salivation was prevented by prazosin. Pretreatment with prazosin showed a decrease in the loss of body weight caused by B-HT920, while pretreatment with yohimbine showed that B-HT920 induced an increased loss in body weight. These data suggest that B-HT920 under certain conditions exerts dopamine-agonistic actions in stimulating locomotor activity and alpha 1-adrenergic actions in inducing salivation and enhanced loss of body weight.

Neurochemical evidence for two types of presynaptic alpha 2-adrenoceptors

Neurochemical and pharmacological evidence has been obtained that noradrenergic varicosities (in mouse and rat vas deferens) and cholinergic varicosities (in the Auerbach's plexus) contain heterogenous alpha 2-adrenoceptors through which the release of [3H]noradrenaline and [3H]acetylcholine can be modulated. The quantitative data also support the hypothesis that different noradrenaline and xylazine sensitive alpha 2-adrenoceptors are present prejunctionally in the vas deferens and Auerbach's plexus preparations. Prazosin, although it has a presynaptic inhibitory effect on alpha 2-adrenoceptors of noradrenergic axon terminals, has no effect on cholinergic axon terminals. These data suggest that there are two different types of alpha 2-adrenoceptors at the presynaptic axon terminals.

Prejunctional alpha 2-adrenoceptors modulate stimulation-evoked norepinephrine release in rabbit lateral saphenous vein

Segments of rabbit lateral saphenous vein prelabelled with [3H]noradrenaline were perfused and superfused with physiological salt solution. Tritium overflow evoked by transmural nerve stimulation (3 Hz for 2 min) was abolished by tetrodotoxin (1 microM). The selective alpha 2-adrenoceptor agonist UK 14,304 inhibited stimulation-evoked 3H-overflow in a concentration-dependent manner, with an IC50 of 71 nM. In contrast, the alpha 2-adrenoceptor agonist B-HT 933 had no effect on 3H-overflow in concentrations up to 10 microM. The selective alpha 2-adrenoceptor antagonists idazoxan and SKF 86466, as well as the non-selective alpha-antagonist phentolamine, facilitated the nerve stimulation evoked 3H-overflow, with an order of potency of idazoxan greater than or equal to phentolamine greater than SK&F 86466. Prazosin (100 nM) had little effect on 3H-overflow. These findings suggest that stimulation-evoked neurotransmitter release is modulated via prejunctional alpha 2-adrenoceptors.

Evidence for medetomidine as a selective and potent agonist at alpha 2-adrenoreceptors

The activity on alpha-adrenoreceptors of medetomidine ((+/-)-4-(alpha,2,3-trimethylbenzyl)imidazole), an alpha-methyl derivative of detomidine, has been characterized in vivo and in vitro using detomidine, MPV 207, MPV 295, azepexole, clonidine and xylazine for reference purposes. Medetomidine (1-100 micrograms/kg i.v.) was a hypotensive and bradycardic compound in anaesthetized rats. Furthermore, it induced vasopressor (PD50 1.7 microgram/kg) and sympatho-inhibitory (ID50 1.6 microgram/kg) actions in pithed rats, the effects being antagonized by idazoxan (0.3 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Medetomidine (30-300 micrograms/kg i.m.) had an alpha 2-adrenoreceptor mediated sedative effect on chicks. Medetomidine was, overall, more potent than detomidine, MPV 207, clonidine, xylazine, MPV 295 or azepexole in central (sedation in the chick) and peripheral (cardiac presynaptic in the pithed rat) actions on alpha 2-adrenoreceptors. Clonidine had, however, about an equal potency to medetomidine in the vascular smooth muscle of the pithed rat. Like detomidine and MPV 295, medetomidine had no agonistic activity in the rat aortic ring, but high concentrations antagonized methoxamine-induced contractions, giving a pA2 value of 5.68 for alpha 1-adrenoreceptor antagonism. The overall lipophilicity (log P') of medetomidine in the octanol/buffer (pH 7.4, 24-26 degrees C, HPLC technique) was 2.80. In summary, the experimental data suggest that medetomidine is a lipophilic compound with selective alpha 2-adrenoreceptor-stimulating properties and high potency. It may, therefore, prove to be a suitable pharmacologic tool for interventions in alpha 2-adrenoreceptor mediated effects in the autonomic nervous system.

The effects of noradrenaline, B-HT 920, methoxamine, angiotensin II and vasopressin on mean circulatory filling pressure in conscious rats

The effects of vasoactive substances on mean circulatory filling pressure (MCFP), an index of total body venous tone, were determined in conscious rats. Cumulative doses of saline (0.9% w/v NaCl solution), methoxamine (alpha 1-adrenoceptor agonist), B-HT920 (alpha 2-adrenoceptor agonist) noradrenaline and vasopressin, and individual doses of angiotensin II (AII), were infused into the rats. Mean arterial pressure (MAP), MCFP and heart rate (HR) were determined before and during the plateau responses to infusions of the vasoactive substances. The infusions of all the agonists caused a dose-dependent increase in MAP and a decrease in HR. The infusion of saline affected neither MAP nor HR. The infusions of saline and methoxamine did not affect MCFP while the infusions of B-HT 920, noradrenaline and AII increased MCFP. MCFP was slightly increased during the infusion of high doses of vasopressin. It was concluded that receptors for the alpha 2-adrenoceptor agonist and AII are involved in the control of venous tone. Receptors for the alpha 1-adrenoceptor agonist and vasopressin are not important for the control of venous tone.

Evidence for and against heterogeneity of alpha 1-adrenoceptors

Recent experimental evidence has suggested that the alpha 1 adrenoceptor may need to be further subdivided. It can no longer be stated categorically that alpha 1-adrenoceptors are present only at postjunctional sites, in view of several reports of alpha 1-mediated modulation of adrenergic and cholinergic neurotransmission. Furthermore, comparison of the pharmacologic characteristics of the alpha 1-adrenoceptor in different species and/or tissues can show clear differences in sensitivity to selective agonists and antagonists, and differences in the degree of dependence on extracellular calcium. However, in other cases, alpha 1-adrenoceptors at diverse sites have been found to have identical characteristics. Furthermore, the subcategories identified by the various selective agents do not fall into the same discrete groups, in contrast to division of alpha-adrenoceptors into alpha 1 and alpha 2-adrenoceptors. Therefore, at this time it seems premature to subdivide the alpha 1-adrenoceptor further.

Further investigations on the alpha 1-adrenoceptor blocking properties of AR-C 239 in rats

AR-C 239, a new alpha-adrenoceptor blocking drug, appears to act selectively on alpha 1 sites in rats. At peripheral sites, this drug did not change the tachycardia induced by spinal sympathetic outflow stimulation in pithed rats, and did not antagonize the inhibitory effects of clonidine on this preparation. In addition, AR-C 239 showed predominant alpha 1-adrenoceptor blocking properties in the bisected rat vas deferens preparation. AR-C 239 did not prevent or reverse the centrally mediated hypotensive and bradycardic actions induced by clonidine, in intact animals. In conclusion, AR-C 239 seems to be a very useful tool for the characterization of peripheral and central alpha 1-adrenoceptors, in this animal species.

Calcium entry blockers inhibit vasoconstrictor responses to sympathetic nerve stimulation mediated by alpha 1-adrenoceptors

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

Effect of prazosin and yohimbine on systolic blood pressure and on renal norepinephrine content in DOCA-salt rats

We studied the effect of alpha-1 and alpha-2 blockers (prazosin and yohimbine) on systolic blood pressure (SBP) and on renal norepinephrine (NE) content in Sprague-Dawley normotensive and DOCA-salt rats. The administration of desoxycorticosterone acetate (DOCA) to these rats for 6 weeks increased their SBP from 137 to 183 mmHg (p less than .001). This increase was prevented by simultaneous administration of prazosin (p less than .001), yohimbine (p less than .01), or prazosin + yohimbine (p less than .001). DOCA rats on saline and on yohimbine had lower renal NE content (p less than .05 and p less than .001, respectively) than normotensive rats. Renal NE content of DOCA rats on yohimbine decreased with respect to those treated with prazosin (p less than .001) or prazosin + yohimbine (p less than .05). Besides, renal NE content of DOCA rats on prazosin increased when compared to control DOCA rats (p less than .05). However, these drugs showed no effect on SBP and on renal NE content in normotensive rats. These findings further confirm that the alpha adrenoceptor blockade can prevent the hypertension of DOCA-salt rats in such a way that their blood pressure stabilizes at similar levels to those observed in normotensive treated animals.

Influence of extracellular calcium and nifedipine on alpha 1- and alpha 2-adrenoceptor-mediated contractile responses in isolated rat and cat cerebral and mesenteric arteries

The influence of extracellular Ca2+ and nifedipine on contractile responses to 10 microM noradrenaline (NA) was investigated in isolated rat and cat middle cerebral (RCA, CCA) and mesenteric (RMA, CMA) arteries. In the CCA (containing predominantly alpha 2-adrenoceptors), the NA-induced contractions developed considerably more slowly than in the RCA, RMA (containing mainly alpha 1-adrenoceptors) and CMA (sensitive to both alpha 1- and alpha 2-adrenoceptor selective antagonists). The tonic component of the NA-induced contraction in the four types of artery was substantially suppressed after only short periods in Ca2+-free solution. In each type of artery, excluding the CCA, the contractile response to 124 mM K+ was more sensitive to Ca2+ deprivation than that to NA. This suggests that NA, besides mobilizing extracellular Ca2+, can also release Ca2+ from an intracellular pool in the RCA, RMA and CMA, but not in the CCA. Thus, alpha 1-adrenoceptor-mediated contractions in the RCA and RMA seem to depend on both Ca2+ influx and intracellular Ca2+ release, whereas alpha 2-adrenoceptor-mediated contractile responses in the CCA appear to rely almost entirely on Ca2+ influx. Both the maximum response and the tonic component of the NA-induced contraction were significantly more sensitive to nifedipine in the CCA than in the RCA. In comparison with the NA-induced contractions in these arteries, those in the RMA and CMA were relatively resistant to nifedipine. In the CCA exposed to NA in Ca2+-free medium, nifedipine almost abolished the contraction induced by re-addition of Ca2+, whereas in the other types of artery, Ca2+ re-application evoked a significant contraction also in the presence of the drug. The differential effects of nifedipine presumably reflect differences between the arteries, not only in the relative contribution of Ca2+ influx and intracellular Ca2+ release to the contractile activation, but also in the nifedipine sensitivity of the Ca2+ entry pathways utilized by NA. It is concluded that the mechanisms through which NA induces contraction seem to be related both to the subtype of alpha-adrenoceptor stimulated by NA and to the type of vessel studied.

Postsynaptic alpha-adrenoreceptor populations in several vascular systems of the anaesthetized rat

The contribution of postsynaptic alpha 1- and alpha 2-adrenoreceptors to vasoconstrictor responses was investigated in several vascular systems of pentobarbital-anaesthetized rats pretreated with atropine and propranolol. In the intact circulatory system of the anaesthetized rat, pressor responses were obtained to noradrenaline and phenylephrine. The pressor responses to noradrenaline were only partially blocked by prazosin and the responses which remained after prazosin were significantly reduced further by the subsequent addition of yohimbine. However, the responses to phenylephrine were largely antagonized by prazosin alone. In the blood-perfused hindquarter of the anaesthetized rat, a differential blocking activity of prazosin against noradrenaline and phenylephrine was also demonstrated. Prazosin, as observed in the intact circulatory system of the anaesthetized rat, was a more potent antagonist against phenylephrine than against noradrenaline. In the blood-perfused mesentery of the anaesthetized rat, sympathetic nerve stimulation, noradrenaline and phenylephrine produced a marked vasoconstrictor response whilst B-HT 920 hardly induced a pressor response. The pressor responses to nerve stimulation, noradrenaline and phenylephrine were largely blocked by prazosin alone. However, only the responses to all frequencies of nerve stimulation were enhanced by yohimbine pretreatment. These results obtained from the intact animal and blood-perfused hindquarter indicate that the pressor responses to exogenous noradrenaline result from the activation of both postsynaptic alpha 1- and alpha 2- adrenoreceptors. However, the result obtained from the blood-perfused mesentery indicates that the vasoconstrictor responses to neuronally released noradrenaline are largely mediated by activation of postsynaptic alpha 1-adrenoreceptors. Consequently, these results suggest that, in rats, the postsynaptic alpha-adrenoreceptor population in the mesenteric resistance blood vessels differs from that in other tissues.

Invariable susceptibility to blockade by nifedipine of vasoconstriction to various alpha 2-adrenoceptor agonists in pithed rats

The sensitivity of the increase in diastolic pressure brought about by the selective agonists of alpha 2-adrenoceptors, B-HT 920, B-HT 933, xylazine, UK-14,304, M-7, TL-99 and DP-6, 7-ADTN in pithed normotensive rats to blockade by the calcium entry inhibitor nifedipine has been investigated. To exclude any participation of vascular alpha 1- and beta 2-adrenoceptors, as well as cardiac beta 1-adrenoceptors, in the pressor responses, the study was made after treatment of the pithed rats with prazosin (0.1 mg kg-1) and (-)-propranol (1 mg kg-1). Without exception, the preferential agonists of alpha 2-adrenoceptors elicited vasoconstrictor responses which were susceptible to inhibition by nifedipine (0.03-1 mg kg-1) in a dose-dependent manner regardless of the differences in intrinsic activity of the compounds. The pressor activity was almost completely abolished after 1 mg kg-1 of nifedipine. The results show that vasoconstriction induced in pithed rats by various selective stimulating agents of postjunctional vascular alpha 2-adrenoceptors is invariably and equally sensitive to attenuation by nifedipine. This susceptibility of alpha 2-adrenoceptor-mediated vasoconstriction to impairment by blockade of calcium entry is not dependent on the nature, the potency or the efficacy of the agonist.

Effect of alpha 1- and alpha 2-adrenoceptor agonists and antagonists on ACTH secretion in intact and in hypothalamic deafferentated rats

The effect of systemically injected alpha 1- and alpha 2-adrenoceptor agonists and antagonists on ACTH secretion was studied in rats. Epinephrine, norepinephrine, phenylephrine, clonidine, B-HT933, and B-HT920 caused a significant and dose-related increase of the ACTH concentration in the serum. The order of median effective dose (ED50) of these drugs on ACTH secretion was as follows: epinephrine not equal to norepinephrine less than B-HT920 less than clonidine less than phenylephrine much less than B-HT933. Isoproterenol, a beta-adrenoceptor agonist, had no effect on ACTH secretion. ACTH secretion induced by epinephrine or phenylephrine was significantly inhibited by alpha-adrenoceptor antagonists, phentolamine and phenoxybenzamine. However, propranolol, a beta-adrenoceptor antagonist, had no effect on ACTH secretion induced by epinephrine. Prazosin, an alpha 1-antagonist, and yohimbine, an alpha 2-antagonist, significantly blocked ACTH secretion induced by phenylephrine, an alpha 1-agonist, and B-HT933, an alpha 2-agonist, respectively. ACTH secretion induced by norepinephrine or a low dose of clonidine was inhibited by both prazosin and yohimbine. However, ACTH secretion induced by a high dose of clonidine was blocked only by prazosin. In rats with complete deafferentation of the medial basal hypothalamus (MBH), ACTH secretion induced by epinephrine, norepinephrine, and clonidine was significantly blocked, as compared with intact rats. These results suggest that both peripheral alpha 1- and alpha 2-adrenoceptors are involved in ACTH secretion induced by systemically injected adrenergic drugs in rats, and intact neural pathways entering the MBH are necessary for this ACTH releasing action.

Specific binding of [3H]prazosin to myocardial cells isolated from adult rats

The characteristics of alpha-adrenoceptors in rat myocardium were investigated by specific binding of [3H]prazosin to cells isolated from adult rat heart by perfusion with collagenase and hyaluronidase. The cells were incubated in Krebs-Ringer bicarbonate buffer gassed with 95% O2 and 5% CO2 at 31 degrees with the appropriate concentrations of the different ligands. Non-specific binding was defined by the addition of 10(-5) mole/l. phentolamine. The binding of [3H]prazosin was saturable and reached equilibrium within 15 min. Scatchard analysis showed a straight line giving an apparent dissociation constant, Kd, equal to 155.9 +/- 8.0 pmole/l. and a maximal number of binding sites equal to 76.7 +/- 11.1 fmole/mg protein. Inhibition of specific [3H]prazosin binding by different adrenergic blockers showed the order of potency characteristic of alpha 1-adrenoceptors: prazosin much greater than phentolamine greater than yohimbine much greater than propranolol. Inhibition by adrenergic agonists showed the order of potency: adrenaline greater than noradrenaline = phenylephrine greater than isoprenaline. The same orders of potency were observed in the presence of propranolol. However, propranolol slightly decreased the affinity for noradrenaline and phenylephrine. Hofstee analyses of the inhibition curves showed two binding components for all ordinary alpha-adrenoceptor blockers and agonists including unlabelled prazosin. In contrast, [3H]prazosin showed only one binding component. Both binding components were of the alpha 1-adrenoceptor subtype according to the order of potency of blockers. The different ligands had different affinity ratios for the two binding components giving them different profiles. Trifluoperazine, a phenothiazine compound, also had high affinity for the [3H]prazosin binding sites. This drug, however, apparently detected one class of binding sites only, as interpreted from the Hofstee analysis. Hill analyses of the inhibition data consistently yielded Hill constants, nH, in the range 0.75-0.85 except for [3H]prazosin, where nH = 1.02 and for trifluoperazine, where nH = 1.07. Although the two binding components may serve different functions, it seems impossible at present to relate the negative and the positive inotropic components, respectively, of the alpha-adrenergic inotropic response observed in functional studies only to one or the other binding component.

Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat. Characterization and role in relation to presynaptic alpha 2-adrenoceptors

Additional experimental evidence was obtained for an inhibitory function of prejunctional alpha 2-adrenoceptors and/or dopamine receptors located on noradrenergic neurons innervating the heart and resistance vessels of the pithed normotensive rat. Mixed alpha 2-adrenoceptor/dopamine receptor agonists, differing in selectivity towards either receptor type, i.e. N,N-di-n-propyldopamine (DPDA), 2-N, N-di-n-propylamino-6, 7-dihydroxy-1,2,3,4-tetrahydro-naphthalene (DP-6,7-ADTN), B-HT 920 and B-HT 933 (azepexole) were used. In pithed normotensive rats, DPDA (30 and 100 micrograms/kg/min) dose-dependently inhibited the electrical stimulation-induced increase in diastolic pressure, but did not significantly affect the stimulation-evoked increase in heart rate. The inhibition exerted by DPDA was blocked by haloperidol and sulpiride (0.3 mg/kg of each), but not by yohimbine (1 mg/kg), indicating the involvement of dopamine receptors. In this respect, sulpiride and haloperidol were found approximately equipotent. DP-6,7-ADTN (10 and 30 micrograms/kg/min) impaired both tachycardic and vasoconstrictor responses in a dose-dependent manner. Sulpiride (0.3 mg/kg) only partially restored the DP-6,7-ADTN-depressed stimulation-evoked increase in diastolic pressure, whereas yohimbine (1 mg/kg) alone was without effect. The combination of both antagonists completely prevented the inhibition caused by DP-6,7-ADTN. On the other hand, yohimbine (1 mg/kg), but not sulpiride (0.3 mg/kg), selectively antagonized the DP-6,7-ADTN-induced inhibition of stimulation-evoked tachycardia. B-HT 920 (1, 3 and 10 micrograms/kg/min) very effectively reduced the increase in diastolic pressure and heart rate caused by electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Postjunctional alpha-adrenoceptors. Alpha 1 and alpha 2 subtypes in rat vasculature in vitro and in vivo

The postsynaptic alpha-adrenoceptors in rat aorta and in pithed rat were investigated according to their sensitivity to nine alpha-adrenergic agonists and to the selective antagonists yohimbine (alpha 2) and prazosin (alpha 1) and the nonselective 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 alpha-adrenoceptor agonist, whether alpha 1- or alpha 2-selective. Rat aorta probably contains only alpha 1-adrenoceptors. Pressor effects in pithed rats are mediated by post-junctional alpha 1- and alpha 2-adrenoceptors. The dose-response curve for alpha-methylnorepinephrine in the presence of prazosin, using Hofstee's plots, revealed alpha 1- and alpha 2-adrenoceptors, respective proportions being 80.5 and 19.5%.

Identification of vascular postsynaptic alpha 1- and alpha 2-adrenoceptors in man

We studied postsynaptic alpha-adrenoceptors in human blood vessels by measuring the influence on forearm blood flow induced by intra-arterial infusions of selective alpha 1- and alpha 2-adrenoceptor agonists (methoxamine, B-HT 933, clonidine and guanfacine) and antagonists (doxazosin and yohimbine). The studies were done in healthy volunteers, and forearm blood flow was measured by plethysmography. All agonists produced a significant and dose-dependent vasoconstriction. The effect of B-HT 933 was completely abolished by the concomitant infusion of yohimbine, whereas it was hardly influenced by doxazosin. The effect of methoxamine was prevented by doxazosin and little influenced by yohimbine. The vasoconstriction by clonidine and guanfacine was partially prevented by both doxazosin and yohimbine. The single intra-arterial infusion of yohimbine, as well as doxazosin, resulted in vasodilation. These findings provide strong evidence for the existence of postsynaptic alpha 1- as well as alpha 2-adrenoceptors, both mediating vasoconstriction and contributing to basal vascular tone. The (patho-)physiological significance of this subdivision of alpha-adrenoceptors remains to be elucidated.

Different effects of phenylephrine and clonidine on 86Rb efflux and on contraction in the rat caudal artery

A large transitory efflux of 86Rb was induced by concentrations of phenylephrine (10(-6) and 10(-4)M) or clonidine (10(-4)M) which were able to produce phasic contraction. By contrast, a lower concentration of clonidine (10(-6) M), which could only induce a slower (tonic) contraction, only produced a weak but sustained 86Rb efflux. These results show qualitative and quantitative differences in the effects of the two alpha-agonists. They further support the view that phasic and tonic alpha-adrenergic responses may be related to different mechanisms.

Characterization of the alpha-adrenoceptors in the female rabbit urethra

A radioligand binding technique was used to evaluate the proportions of alpha 1- and alpha 2-adrenoceptors in crude membrane preparations obtained from the female rabbit bladder base and urethra. In addition, urethral rings were studied in vitro in an attempt to determine if alpha 1- and/or alpha 2-adrenoceptors are located postjunctionally in the urethral smooth muscle. Studies of the inhibition of [3H]-dihydroergocryptine binding by the selective alpha 1-adrenoceptor antagonist prazosin or the selective alpha 2-adrenoceptor antagonist rauwolscine revealed the alpha-adrenoceptor population to consist of approximately 25% alpha 1-adrenoceptors and 75% alpha 2-adrenoceptors. These proportions were confirmed in saturation studies with [3H]-prazosin and [3H]-rauwolscine. The sum of alpha 1- and alpha 2-adrenoceptors labelled by these selective alpha 1- and alpha 2-adrenoceptor antagonists was about equal to the number labelled by the non-selective alpha-adrenoceptor antagonist [3H]-dihydroergocryptine. Noradrenaline, as well as the selective alpha 1-adrenoceptor agonist phenylephrine and the selective alpha 2-adrenoceptor agonist clonidine, induced contractions of urethral ring preparations. Prazosin blocked contractions induced by phenylephrine to a greater extent than contractions induced by clonidine. The opposite was true for the inhibitory effect of rauwolscine. In addition to showing that both alpha 1- and alpha 2-adrenoceptor binding sites exist in membrane preparations of the rabbit bladder base and urethra, the results reveal the presence of both adrenoceptor subtypes postjunctionally in the rabbit urethra; and both mediate contraction of the smooth muscle.

"Pertussis toxin induces tachycardia and impairs the increase in blood pressure produced by alpha 2-adrenergic agonists"

Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested.

Pharmacological characterization of alpha-adrenoreceptor subtypes in rat isolated thoracic aorta

The subtype of alpha-adrenoreceptor mediating contraction in rat isolated thoracic aorta was classified pharmacologically using preferential agonists and antagonists, and by utilizing mixed agonist and antagonist interactions. Noradrenaline was 8 to 10-times more potent at contracting the aorta than phenylephrine and both agonists were about 1000 and 10,000-fold respectively more potent than azepexole (a preferential alpha 2-agonist). Prazosin (a preferential alpha 1-antagonist) inhibited the dose-response curves to noradrenaline and phenylephrine 100 and 1000-times respectively more effectively than either phentolamine or rauwolscine (a preferential alpha 2-antagonist). Furthermore, prazosin (5 x 10(-9) M) completely abolished contractions elicited by a single concentration of azepexole (3 x 10(-4) M). In mixed antagonist studies, rauwolscine (5 x 10(-7) M) failed to shift the dose-response curves to noradrenaline and phenylephrine obtained in the presence of prazosin (5 x 10(-9) M). In mixed agonist experiments, azepexole (3 x 10(-4) M) acted as a partial antagonist toward phenylephrine-induced contractions. The results suggest that the alpha-adrenoreceptor of the rat thoracic aorta is predominantly, if not exclusively, of the alpha 1-subtype.

Peroxidase secretion from rat lacrimal gland cells in vitro. I. Alpha-adrenergic stimulation in the absence of alpha-adrenoceptors

The identification of alpha-adrenergic receptors and subdivision into alpha 1- or alpha 2-subtypes were studied by measuring the specific binding of the radioligands [3H]-prazosin as well as [3H]-clonidine to membranes prepared from homogenized rat lacrimal glands. The absence of high-affinity binding for [3H]-prazosin as well as for [3H]-clonidine indicates that rat lacrimal glands do not possess a substantial amount of alpha 1- and alpha 2-adrenoceptors. The binding data correspond with the characterization by pharmacological means. Monolayers of lacrimocytes were incubated with various selective alpha 1-, alpha 2-, beta-adrenergic agonists and antagonists or other substances, and peroxidase discharge was measured over a period of 1 hr. Among various substances only L-norepinephrine, L-phenylephrine, tyramine and ionophore A 23187 were stimulants of peroxidase secretion, whereas the adrenergic-stimulated secretory response was only suppressed by phentolamine.

Alpha adrenoceptors and autonomic mechanisms in perinephritis hypertension in the rabbit

Increased pressor responses to norepinephrine and other pressor agents have been reported to occur in human essential hypertension and in several animal models of experimental hypertension. These increased responses might be related to the development of hypertension or could be a secondary consequence of the elevation in blood pressure. We have examined pressor responses to alpha-adrenoceptor agonists and to angiotensin II in male New Zealand White rabbits with perinephritic hypertension. Increased pressor responses were observed for the alpha 1 adrenoceptor agonist phenylephrine and the mixed alpha 1/alpha 2-adrenoceptor agonist norepinephrine but not for the alpha 2 adrenoceptor selective agonist guanabenz or angiotensin II. The increase occurred within 7 days of surgery and in some animals was observed when mean arterial pressure was not significantly elevated. It could not readily be attributed to intimal thickening or hypertrophy of the arterial wall, altered basal levels of norepinephrine or epinephrine, changes in norepinephrine clearance, beta-adrenoceptor interactions, or decreased baroreceptor sensitivity. However, the possibility that vascular hypertrophy and decreased baroreflex sensitivity may contribute to the increase at later times cannot be excluded. In all tissues examined, specific prazosin binding was decreased in the older animals and specific clonidine binding was decreased in forebrain. However, these changes were observed in both hypertensive and sham-operated animals and were probably age-related. We believe the increased response to alpha 1-adrenoceptor agonists may be related to changes at a postreceptor site in the coupling of receptor activation to smooth muscle contraction.

Vascular alpha-adrenergic blocking properties of quinidine

The specificity of the alpha-adrenergic vascular blockade by quinidine was tested in the intact dog, in rabbit isolated aortic strips, and in rats under ganglionic blockade. Quinidine did not affect the pressor response of angiotensin II in dogs, the contractile response of histamine nor angiotensin II in aortic strips, nor the dose-pressor response curve of the alpha-agonist, B-HT 933 in the rat. However, the pressor effect of adrenaline and noradrenaline (NA) were significantly reduced in dogs, and the dose-response curves to NA in aortic strips and to the alpha-agonist, phenylephrine in rats was shifted to the right in a parallel manner by quinidine. In the rat, quinidine is at least 14 times more potent in antagonizing the vasopressor effect of an alpha 1-vs. and alpha 2-adrenoceptor agonist.

Selective alpha 2-adrenoceptor agonist activity of the novel inotropic agent, ASL-7022: comparison with dobutamine

ASL-7022 is a novel inotropic agent capable of increasing the force of myocardial contraction at doses which produce little effect on heart rate. The inotropic selectivity of ASL-7022, like that of dobutamine, has been proposed to result, in part, from agonist activity at alpha-adrenoceptors. Following beta-adrenoceptor blockade, ASL-7022 and dobutamine increase diastolic blood pressure in pithed rat, with both compounds being equal in potency. The pressor activity of ASL-7022 was selectively antagonized by yohimbine (1 mg/kg i.v.) and was unaffected by prazosin (0.1 mg/kg i.v.), whereas the converse was true for dobutamine. These results indicate that the pressor effects of ASL-7022 and dobutamine are mediated by different populations of postjunctional vascular alpha-adrenoceptors in pithed rat, with ASL-7022 selectively stimulating alpha 2-adrenoceptors and dobutamine selectively activating alpha 1-adrenoceptors.

Interaction of clonidine, its methylene-bridged analog, St 1913, and the benzylic hydroxyl-substituted derivative, St 1965, with alpha 1- and alpha 2-adrenoreceptors

The effects of benzylic hydroxyl substitution on the activity of a close structural analog of clonidine was assessed at alpha 1- and alpha 2-adrenoreceptors both in vitro and in vivo in order to uncover possible differences that this substitution may have on the effects of imidazolines and phenethylamines at adrenoreceptors. In all test systems, the presence of the benzylic hydroxyl group was associated with a consistent and marked decrease in activity. These findings are in agreement with our previous studies with imidazolines having different pharmacological profiles and different physicochemical properties than the clonidine derivatives reported herein. We conclude, therefore, that the deleterious effects of the benzylic hydroxyl group is ubiquitous among imidazolines and, more importantly, is in marked contrast to the 100-1000 fold enhancement in activity that the benzylic hydroxyl substituent (i.e. beta-hydroxyl group) produces for the phenethylamines. The results support the concept that imidazolines and phenethylamines may interact differently with alpha-adrenoreceptors.