Cardiovascular effects of B-HT 958, an alpha-adrenoceptor agonist with a high pre/postsynaptic activity ratio

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]az epine), chemically related to clonidine-like drugs of the azepine type, was described previously as a partial alpha 2-adrenoceptor agonist which acted presynaptically mainly as agonist and postsynaptically as antagonist. Following i.v. infusion in anaesthetized cats, 3 mg/kg of B-HT 958 lowered blood pressure, heart rate, cardiac output and total peripheral vascular resistance. A small central nervous component was indicated, since 100 micrograms/kg injected into the vertebral artery was equipotent to 300 micrograms/kg i.v. in lowering blood pressure and heart rate. The drug (1 mg/kg i.v.) decreased the discharge rate of the preganglionic sympathetic splanchnic nerve, but in contrast to the effect of clonidine this could not be demonstrated in decerebrate cats. As blood pressure and heart rate were decreased by B-HT 958 in decerebrate cats, the main site of action was assumed to be peripheral. Also in contrast to clonidine, B-HT 958 did not induce vagal baroreflex bradycardia in anaesthetized dogs with blocked beta-adrenoceptors following intracisternal (30 micrograms/kg as well as 3 mg/kg) injection. In anaesthetized rats the decrease in blood pressure and heart rate caused by 1 mg/kg B-HT 958 i.v. was antagonized by 0.5 mg/kg piperoxan i.v. It is suggested that the cardiovascular effects of B-HT 958 depend on its high selectivity for alpha 2-adrenoceptors and are due to its agonist action presynaptically on peripheral adrenergic nerve terminals.     

An investigation into the selectivity of a novel series of benzoquinolizines for alpha 2-adrenoceptors in vivo.

The potencies and selectivities of a novel series of benzoquinolizines for the alpha 2-adrenoceptor have been investigated in the rat in comparison with yohimbine and indoramin. Peripheral postjunctional alpha 2- and alpha 1-adrenoceptor blockade was measured as the reversal of B-HT 933 and methoxamine-induced pressor responses, respectively, in the pithed rat. Peripheral prejunctional alpha 2-adrenoceptor blockade was measured as the reversal of B-HT 933-induced inhibition of an electrically evoked tachycardia in the pithed rat. Central alpha 2-adrenoceptor blockade was measured as a reversal of the hypotension induced in anaesthetized rats by central (i.c.v.) administration of clonidine. Wy 25309, Wy 26392, Wy 26703 and yohimbine (0.3-3 mg kg-1 i.v.) evoked dose-dependent shifts to the right of the dose-response curves to B-HT 933 whilst having minimal effects on the methoxamine dose-response curve. The selectivity for alpha 2-adrenoceptors increased with the dose of antagonist administered. In general, the order of selectivity was Wy 25309 greater than Wy 26392 greater than Wy 26703 greater than yohimbine. Indoramin (1 mg kg-1 i.v.) shifted the methoxamine pressor dose-response curve to the right without affecting the B-HT 933 dose-response curves, confirming its selective alpha 1-antagonist activity. Peripheral administration of all three benzoquinolizines (1-100 micrograms kg-1 i.v.) led to a dose-dependent reversal of the hypotension evoked by central administration of clonidine (500 ng i.c.v.). The reversal was incomplete, higher doses causing a further decrease in blood pressure. (ABSTRACT TRUNCATED AT 250 WORDS)     

Antagonism by E. coli endotoxin of some cardiovascular effects induced in the rat by two alpha 2-adrenoceptor agonists

E. coli endotoxin (0.01, 0.1 and 1 microgram/kg i.v.) 1 h before alpha 2-adrenoceptor agonists B-HT 933 and clonidine (i.v.) antagonized their bradycardiac and hypotensive effects in intact rats. This antagonism seems not to depend on the presence of the adrenal glands since similar results were obtained in adrenalectomized rats. Endotoxin at higher doses (1 and 10 micrograms/kg) suppressed the hypotensive and reduced the bradycardiac effect of clonidine injected i.c.v. (5 micrograms/kg). In contrast, endotoxin (up to 100 micrograms/kg) did neither increase arterial pressure nor heart rate in the pithed rat. This suggests the participation of a central site of action for endotoxin. However, E. coli endotoxin (1, 10 or 100 micrograms/kg) did not decrease the inhibition by clonidine of the tachycardia induced by stimulation of the cardioacceleratory nerve. This excludes peripheral presynaptic alpha 2-adrenoceptor blockade by endotoxin. Only 100 micrograms/kg decreased the pressor response to clonidine in the pithed rat. These results show that E. coli endotoxin is a potent modificator of the cardiovascular regulation in the rat. It antagonized central alpha 2-adrenoceptor mediated cardiovascular effects at doses lower than those acting on postsynaptic peripheral alpha-adrenoceptors.     

Postsynaptic alpha 1- and alpha 2-adrenoceptors in the circulatory system of the pithed rat: selective stimulation of the alpha 2-type by B-HT 933

The antagonism by yohimbine (1 mg/kg, i.v.) of vasopressor responses in pithed rats was most pronounced towards B-HT 933 (dose ratio 18.3) and moderate towards clonidine (dose ratio 3.7) and especially L-phenylephrine (dose ratio 2.5). Prazosin (0.1 mg/kg, i.v.) had no effect on the pressor responses to B-HT 933, moderately affected those to clonidine (dose ratio 3.9), but strongly diminished those to L-phenylephrine (dose ratio 53). Phentolamine (1 mg/kg, i.v.) was devoid of a differential antagonism. The results obtained suggest a subclassification of postsynaptic alpha-adrenoceptors into alpha 1- and alpha 2-subtypes mediating pressor effects. B-HT 933 is a selective agonist and yohimbine an antagonist of postsynaptic alpha 2-adrenoceptors. L-Phenylephrine preferably stimulates and prazosin preferentially occupies the alpha 1-adrenoceptors. Clonidine is a potent agonist of both types and phentolamine behaves as a non-selective antagonist.     

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.     

Existence of spare alpha 1-adrenoreceptors, but not alpha 2-adrenoreceptors, for respective vasopressor effects of cirazoline and B-HT 933 in the pithed rat

The existence of a receptor reserve (spare receptors) was investigated for postsynaptic vascular alpha1- and alpha2-adrenoceptors in the pithed rat by evaluating the effects of progressive inactivation of alpha1- and alpha2-adrenoceptor pools by the irreversible antagonist phenoxybenzamine on the pressor responses of cirazoline and B-HT 933. Dose-response curves for the alpha1-adrenoceptor-mediated vasoconstrictor effects of cirazoline were shifted in a rightward direction with no depression of the maximum response by lower does of phenoxybenzamine (0.1-0.2 mg/kg, i.v.). Progressively higher doses of phenoxybenzamine (greater than 1 mg/kg, i.v.) produced further rightward shifts in the dose-response curves of cirazoline, but also depressed the maximum response. In contrast, all doses of phenoxybenzamine that inhibited the alpha2-adrenoceptor-mediated pressor effects of B-HT 933 produced a reduction in the maximum response. These results are highly suggestive of the existence of a receptor reserve in the pithed rat for the postsynaptic vascular alpha1-adrenoceptor-mediated effects of cirazoline, but not for the postsynaptic vascular alpha2-adrenoceptor-mediated effects of B-HT 933. Confirmation of the existence of a receptor reserve for only the postsynaptic vascular alpha1-adrenoceptor-mediated effects of cirazoline came from further analysis of the antagonism, by phenoxybenzamine, of cirazoline and B-HT 933 dose-response curves. The maximum pressor response that could be elicited by the alpha1-adrenoceptor agonist cirazoline was a hyperbolic function on the size of the alpha1-adrenoceptor pool, the latter being progressively decreased by phenoxybenzamine treatment. Such a hyperbolic relationship is indicative of a receptor reserve. In marked contrast, the maximum pressor response that could be evoked by the alpha2-adrenoceptor agonist B-HT 933 was a linear function of the size of the intact alpha2-adrenoceptor pool, characteristic of a lack of spare receptors. Further analysis showed that the occupancy-response relationship is fivefold more favorable for the alpha1-adrenoceptor-mediated pressor effects of cirazoline than for the alpha2-adrenoceptor-mediated pressor effects of B-HT 933, indicating that any given maximum pressor response in the pithed rat may be obtained with one-fifth as many alpha1-adrenoceptors being activated by cirazoline than alpha2-adrenoceptors being stimulated by B-HT 933. (ABSTRACT TRUNCATED AT 250 WORDS)     

Clonidine-induced hypoactivity and mydriasis in mice are respectively mediated via pre- and postsynaptic alpha 2-adrenoceptors in the brain

Since brain alpha 2-adrenoceptors occur both pre- and postsynaptically, experiments were carried out to determine the synaptic locations of those receptors mediating clonidine-induced hypoactivity and mydriasis. Intraperitoneal (i.p.) injection of clonidine (1-3000 micrograms/kg) to mice dose dependently induced these two responses and also decreased brain concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG). The ED50 values were: 120 micrograms/kg for hypoactivity (95% confidence limits 103-140 micrograms/kg), 54 micrograms/kg for mydriasis (95% confidence limits 40-74 micrograms/kg) and 18 micrograms/kg for MHPG reduction (95% confidence limits 8-36 micrograms/kg) suggesting that these responses could all be presynaptically mediated. However, methamphetamine which increases noradrenaline turnover was found to dose dependently produce mydriasis, but not hypoactivity, after peripheral (0.1-5 mg/kg i.p.) or central (0.5-10 micrograms i.c.v.) injection. The mydriasis produced by methamphetamine (0.5 mg/kg i.p.) was abolished by i.c.v. injection of 1 micrograms idazoxan or yohimbine, but not 2.5 micrograms prazosin or pindolol, showing this effect was mediated by central alpha 2-adrenoceptors. Methamphetamine (1-10 micrograms i.c.v.) potentiated the mydriasis induced by clonidine (50 micrograms/kg i.p.) suggesting this was a postsynaptic alpha 2-adrenoceptor response. By contrast, methamphetamine (1-10 micrograms i.c.v.) dose dependently reversed clonidine (100 micrograms/kg i.p.) hypoactivity indicating this response was mediated by presynaptic alpha 2-adrenoceptors. These hypotheses were confirmed by destruction of noradrenergic neurones using DSP-4 (100 mg/kg i.p. x 2). This treatment prevented the mydriasis response to methamphetamine (0.5 mg/kg i.p.), but not clonidine (100 micrograms/kg i.p.) and markedly attenuated clonidine (100 micrograms/kg i.p.) hypoactivity.     

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine.

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.     

B-HT 958 lowers blood pressure and heart rate in the rat through stimulation of dopamine receptors.

To investigate whether the hypotensive and bradycardiac effects of B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo-(5,4-d) azepine) are due to the stimulation of peripheral prejunctional alpha2-adrenoceptors, the selective alpha2-adrenoceptor antagonist idazoxan was given either intravenously (i.v.) or intracerebroventricularly (i.c.v.) to anaesthetized rats before the administration of i.v. B-HT 958. Plasma noradrenaline was used as an approximate index of peripheral sympathetic nervous activity. B-HT 958 350 micrograms kg-1 i.v. caused significant falls in blood pressure and heart rate which were maximal 5 min after dosing (-29.25 +/- 3.2 mmHg and - 52 +/- 6.8 beats min-1 respectively, mean of all control animals). The hypotension and bradycardia were accompanied by significant falls in plasma noradrenaline concentration of 30-40%. Idazoxan 300 micrograms kg-1 i.v. caused a marked, but transient tachycardia and a large sustained rise in plasma noradrenaline concentration. Idazoxan 300 micrograms kg-1 and 1000 micrograms kg-1 i.v. did not prevent B-HT 958-induced falls in mean arterial pressure, heart rate and plasma noradrenaline concentration. Responses to B-HT 958 were unaffected by idazoxan 20 micrograms i.c.v. B-HT 958-induced falls in mean arterial pressure, heart rate and plasma noradrenaline concentration were significantly attenuated by i.v. administration of the dopamine receptor antagonist, sulpiride 300 micrograms kg-1. Sulpiride 10 micrograms and 50 micrograms i.c.v. caused inhibition of B-HT 958 hypotension and bradycardia similar to that of intravenous sulpiride. After i.c.v. sulpiride, B-HT 958 did not cause a significant fall in plasma noradrenaline concentration. A combination of idazoxan 1000 micrograms kg-1 i.v. and sulpiride 300 micrograms kg-1 i.v. did not cause further significant inhibition of B-HT 958 hypotension and bradycardia compared with sulpiride 300 micrograms kg-1 i.v. alone. This combination however had a significantly greater effect in inhibiting B-HT 958 hypotension than had idazoxan 1000 micrograms kg-1 alone, and almost completely blocked the B-HT 958-induced fall in plasma noradrenaline concentration. These results suggest that in the anaesthetized rat the cardiovascular effects of B-HT 958 are due to stimulation of dopamine receptors, probably located within the central nervous system, and not to stimulation of peripheral prejunctional alpha2-adrenoceptors.     

The thermogenic actions of alpha 2-adrenoceptor agonists in reserpinized mice are mediated via a central postsynaptic alpha 2-adrenoceptor mechanism.

1. The dose-related effects of the selective alpha 2-adrenoceptor agonists clonidine, UK-14,304 and B-HT 933 on the body temperature of untreated and reserpine-treated mice were investigated. 2. In untreated mice all three agonists induced a dose-related hypothermia. The highest doses of UK-14,304 and B-HT 933, 3 and 100 mg kg-1 respectively, elicited a marked (10 degrees C) hypothermia, whereas the maximal hypothermic effect of clonidine (5.5 degrees C) was less pronounced and reached a plateau at a dose of 0.5 mg kg-1 i.p. 3. Reserpine (2.5 mg kg-1, s.c.) induced a marked hypothermia in the mouse; 18 h after injection body temperature had decreased to only slightly (0.5-1.5 degrees C) above ambient (19 degrees C). 4. All three alpha 2-agonists produced a partial dose-related reversal of reserpine-induced hypothermia; maximal thermogenic responses (9-10 degrees C increases in body temperature) were elicited by doses of 0.2, 0.5 and 16 mg kg-1 i.p. of clonidine, UK-14,304 and B-HT 933 respectively, and the log dose-response curves for all 3 agonists were bell-shaped. 5. Following intracerebroventricular administration to reserpine-treated mice, the thermogenic response to clonidine was more rapid in onset, and the agonist was 20 fold more potent than when injected i.p. 6. The selective alpha 2-adrenoceptor antagonists, idazoxan (0.05-0.5 mg kg-1), Wy 26392 (0.3-5.0 mg kg-1) and yohimbine (0.1-1.6 mg kg-1) given orally attenuated the thermogenic responses to all 3 agonists in reserpinized mice in a dose-related manner. Pretreatment with a single dose of idazoxan (0.3 mg kg-1, orally) elicited a 6 fold parallel shift to the right in the dose-response curve to clonidine. 7. The selective alpha 1-adrenoceptor antagonists, prazosin (10 mg kg-1) and indoramin (3-10 mg kg-1), and the beta-adrenoceptor antagonist, propranolol (10 mg kg-1), only partially attenuated the thermogenic responses to the alpha 2-agonists in reserpinized mice. These effects were variable and not clearly dose-related. 8. Pretreatment of reserpinized mice with the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, markedly attenuated (60-95%) the thermogenic response to the noradrenaline uptake inhibitor, desipramine (0.13-12.5 mg kg-1, i.p.), but only slightly reduced (10-35%) that to clonidine (0.032-0.5 mg kg-1, i.p.). 9. These results suggest that alpha2-adrenoceptor agonists reverse reserpine-induced hypothermia via a central mechanism involving activation of postsynaptic alpha 2-adrenoceptors.     

Differential effect of pertussis toxin on pre- and postjunctional alpha 2-adrenoceptors in the cardiovascular system of the pithed rat

The role of the pertussis toxin-sensitive guanine nucleotide binding regulatory protein in pre- and postjunctional alpha 2-adrenoceptor-mediated responses has been investigated in the pithed rat. Pertussis toxin (50 micrograms/kg i.v., administered 3 days prior to experimentation) markedly inhibited the alpha 2-adrenoceptor-mediated vasopressor response to B-HT 933, but had no effect on the alpha 2-adrenoceptor-mediated cardiac neuroinhibitory effect of B-HT 933. Thus, postjunctional alpha 2-adrenoceptor activation in vascular smooth muscle, but not prejunctional alpha 2-adrenoceptor activation on sympathetic neurons, utilizes a pertussis toxin-sensitive guanine nucleotide binding regulatory protein.     

Clonidine produces mydriasis in conscious mice by activating central alpha 2-adrenoceptors

Intraperitoneal (i.p.) injection of the alpha 2-adrenoceptor agonist clonidine (1-3000 micrograms/kg) produced dose-dependent pupil dilatation in conscious C57/Bl/6 mice with an ED50 of 54 micrograms/kg (95% confidence limits 40-74 micrograms/kg). This response was rapid in onset and of approximately 30 min duration. The alpha 2-adrenoceptor antagonists idazoxan (1 or 3 mg/kg i.p.) and yohimbine (1 or 3 mg/kg i.p.) both produced dose-related miosis, but the alpha 1- and beta-adrenoceptor antagonists prazosin (1 or 3 mg/kg i.p.) and pindolol (1 or 3 mg/kg i.p.) were without effect. These doses of idazoxan and yohimbine potently reversed the mydriasis induced by clonidine (100 micrograms/kg i.p.), while prazosin and pindolol were again ineffective. Clonidine-induced mydriasis was also unaltered by the 5-HT antagonists, methysergide (2.5 mg/kg i.p.) and ketanserin (0.1 mg/kg i.p.) or 0.1 mg/kg i.p. of the dopamine antagonists, haloperidol, SCH 23390 and BRL 34778. A dose of 0.25 microgram clonidine, which was ineffective when administered i.p., produced marked mydriasis after intracerebroventricular (i.c.v.) injection. In addition, the mydriasis produced by i.p. injection of clonidine (100 micrograms/kg) was abolished by i.c.v. dosing of 2.5 micrograms idazoxan or yohimbine, but again not by prazosin or pindolol. Together, these data provide strong evidence to indicate that clonidine-induced mydriasis is exclusively mediated via central alpha 2-adrenoceptors and that this response provides a useful model for studying the function of these receptors.     

Evidence for a peripheral component in the sympatholytic effect of clonidine in rats.

In an attempt to assess separately the peripheral and central effects of clonidine on cardiovascular parameters and plasma catecholamine levels, the selective alpha 2-adrenoceptor antagonist idazoxan (RX 781094) was given either intravenously (i.v.) or intracerebroventricularly (i.c.v.) to anaesthetized rats before administration of intravenous clonidine. Plasma noradrenaline and plasma growth hormone concentrations were used as indices of peripheral sympathetic nervous activity and central alpha-adrenoceptor stimulation, respectively. Peripheral and central administration of idazoxan antagonized the cardiovascular responses to i.v. clonidine, 5 micrograms kg-1. However, idazoxan was more effective against the hypotension than the bradycardia induced by clonidine. Idazoxan 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. prevented clonidine-induced falls in plasma noradrenaline and adrenaline. The results suggest that 50 micrograms idazoxan i.c.v. caused some blockade of peripheral as well as central alpha 2-adrenoceptors. Idazoxan, 10 micrograms i.c.v., caused similar inhibition of the hypotensive response to clonidine as 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. but did not significantly inhibit the clonidine-induced fall in plasma noradrenaline concentration. Animals pretreated with i.v. or i.c.v. idazoxan had significantly lower levels of plasma growth hormone than vehicle-treated rats. Idazoxan 10 micrograms and 50 micrograms i.c.v. suppressed growth hormone secretion to the same extent. These results suggest that stimulation of peripheral, prejunctional alpha 2-adrenoceptors in anaesthetized rats may contribute to the fall in plasma catecholamines produced by i.v. clonidine, and confirm that the hypotensive effect is centrally mediated.     

Interactions between the autonomic nervous system and the cardiovascular effects of ouabain in guinea-pigs

Anaesthetized guinea-pigs were intoxicated with an intravenous infusion of ouabain. This infusion induced a marked pressor response which was reduced in bilaterally adrenalectomized or pithed animals. Ouabain produced initial bradyarrhythmias in 60% of guinea-pigs. Bilateral vagotomy or pretreatment with atropine abolished the bradyarrhythmias and sensitized the animals to the arrhythmic effects of ouabain. Pithing or beta-adrenoceptor blockade reduced the potency of ouabain for producing arrhythmias, but bilateral adrenalectomy did not give protection. Preferential alpha 2-adrenoceptor stimulation with clonidine (10-300 micrograms . kg-1 i.v.) also reduced the arrhythmogenic effects of ouabain, whereas no protection was found with St 91, a clonidine related compound which does not cross the blood-brain barrier. The effect of clonidine was antagonized by piperoxan. Preferential alpha 2-adrenoceptor blockade with piperoxan (6 mg . kg-1 i.v.) did not change the pressor response to ouabain, but sensitized the animals to the arrhythmogenic effects of ouabain. In contrast, the preferential alpha 1-antagonistic agent AR-C 239 (0.3 mg . kg-1 i.v.) abolished the pressor response to ouabain and in addition increased the dose of ouabain required to produced ventricular premature beats and ventricular fibrillation. These experiments indicate: (i) that ouabain produces in guinea-pigs a pressor response which seems to be due to catecholamine release from the adrenal medulla probably by an action on the central nervous system; (ii) that the ventricular arrhythmias induced by ouabain are due in part to stimulation of the central nervous system leading to an increase in beta-adrenoceptor activity; (iii) that central alpha-adrenoceptors appear to be involved in the arrhythmogenic effects of ouabain, as clonidine reduced these effects. On the other hand piperoxan, a preferential alpha 2-adrenoceptor antagonist did not change the pressor response to ouabain and increased the arrhythmogenic effects whereas AR-C 239 had opposite effects.     

Characterization of alpha-adrenoceptors mediating sympathetic vasoconstriction in rat autoperfused hindlimb: effects of SK&F 104078

In the autoperfused hindlimb of pithed rats, vasoconstrictor responses to intra-arterial infusions of the selective alpha 2-adrenoceptor agonist, B-HT 933, were antagonized by the alpha 2-adrenoceptor antagonist, rauwolscine (1 mg/kg i.v.), and by the selective postjunctional alpha 2-adrenoceptor antagonist, SK&F 104078 (1 mg/kg), but not by the selective alpha 1-adrenoceptor antagonist, prazosin (0.1 mg/kg). In contrast, responses to the selective alpha 1-adrenoceptor agonist, methoxamine, were antagonized by prazosin, but not by rauwolscine or SK&F 104078. Vasopressor responses to stimulation of sympathetic nerves were inhibited by prazosin, increased by rauwolscine, and not affected by SK&F 104078. The results indicate that vascular neuroeffector transmission in rat hindlimb is mediated by postjunctional alpha 1-adrenoceptors, and that SK&F 104078 is a selective antagonist of postjunctional alpha 2-adrenoceptor, and lacks the prejunctional alpha 2-adrenoceptor antagonist action of rauwolscine.     

Centrally mediated cardiovascular effects of B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine dihydrochloride), a hypotensive agent of the "clonidine type"

Intravenous injection of 30 micrograms/kg of B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine dihydrochloride) into cats lead initially to an increase in blood pressure and then to a long-lasting decrease in blood pressure and heart rate. The central site of the hypotensive and bradycardiac action was demonstrated by the significantly greater effect after intracisternal (i.ci.) than after intravenous injection of B-HT 920, 3 micrograms/kg. The drug decreased the rate of spontaneous discharges in preganglionic splanchnic nerve fibers of normal and noradrenaline-depleted cats (3 micrograms/kg, i.ci.). Vagally mediated reflex bradycardia elicited by angiotensin injection in beta-adrenoceptor-blocked dogs was facilitated by intracisternal injection of 10 micrograms/kg B-HT 920. Both sympathoinhibition and vagal reflex facilitation were antagonized by the alpha-adrenoceptor-blocking agent piperoxan (50 micrograms/kg, i.ci). Therefore, B-HT 920 can be classified as an agent of the clonidine type, despite its different chemical structure. Quantitative differences between B-HT 920 and clonidine are discussed with respect to the greater alpha 2/alpha 1 adrenoceptor activity ratio of the former drug, as reported previously.     

Anticonvulsant effects of clonidine mediated through central alpha2-adrenoceptors

The effects of centrally (0.3-30 ng/kg) and peripherally (0.01 microgram/kg-1.0 mg/kg) administered clonidine on PTZ-induced seizures was studied in rats. At low doses, dose-dependent decreases in the duration of seizures was observed but at higher doses the duration returned to control levels. Anticonvulsant activity was antagonized by yohimbine (100 microgram/kg i.p.) indicating alpha2-adrenoceptor involvement, whereas the second phase of the response was antagonized by prazosin (10 ng/kg i.c.v.) indicating that it involved alpha1-adrenoceptors.     

Contribution of alpha 2-adrenoceptors to the central cardiovascular effects of clonidine and S 8350 in anaesthetized rats.

1. The alpha 2-adrenoceptor agonist clonidine elicits centrally mediated effects through an interaction with both alpha 2-adrenoceptors and imidazoline binding sites. 2. We selected a new oxazoline derivative, S 8350, which competes with [3H]-yohimbine for binding to cerebral alpha 2-adrenoceptors (IC50, 67 +/= 17 nmol/L) and displays a higher affinity (35-fold) for alpha 2- than for alpha 1-adrenoceptors. 3. As observed for clonidine, intravenous (i.v.) administration of S 8350 resulted in a brief pressor effect followed by a prolonged hypotension. When S 8350 was administered i.v. to spinally pithed rats, only a rise in blood pressure was observed. 4. In order to discriminate the cardiovascular effects related to the central imidazoline receptor or alpha 2-adrenoceptor activation, the effects of intracisternal (i.c.) administration of clonidine and S 8350 were investigated in the rat. 5. In the anaesthetized rat, both clonidine and S 8350 displayed a profound central (i.c. route) hypotensive effect associated with a bradycardia. 6. The cardiovascular effects of S 8350 were abolished by the central administration of the selective alpha 2-adrenoceptor antagonist rauwolscine. Conversely, rauwolscine completely prevented bradycardia but it induced only a partial reversion of the hypotension elicited by clonidine. 7. These results suggest that central alpha 2-adrenoceptors are responsible for hypotension and bradycardia while imidazoline binding sites do not apparently contribute to heart rate control.     

alpha 2-Adrenoceptor agonists induced mydriasis in the rat by an action within the central nervous system.

1 The effects of intravenous administration of the selective alpha 2-adrenoceptor agonists clonidine, UK 14,304 and guanoxabenz on rat pupil diameter were investigated. 2 In rats anaesthetized with pentobarbitone, each agonist produced a marked dose-related increase in pupil diameter; the rank order of potency was: clonidine greater than UK 14,304 greater than guanoxabenz. 3 Pretreatment with the selective alpha 2-adrenoceptor antagonist, RX 781094 (0.5 mg/kg, i.v.), produced a parallel 30-40 fold shift to the right of the dose-pupil dilator response curves for the three agonists. Yohimbine (1.5 mg/kg, i.v.) produced about a 10 fold rightward shift of the dose-response curve for guanoxabenz. In contrast, the alpha 1-selective antagonist, prazosin (0.5 mg/kg, i.v.), failed to affect the dose-response relation for guanoxabenz. 4 Several antagonists of varying selectivities towards alpha 1- and alpha 2-adrenoceptors were tested for their ability to reverse the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). The rank order of potency of the antagonists producing a 50% reversal of this effect was: RX 781094 greater than yohimbine greater than piperoxan = rauwolscine greater than mianserin greater than RS 21361. Neither corynanthine nor prazosin reversed the guanoxabenz-induced mydriasis. 5 Topical application of RX 781094 (0.1 to 3% w/v solutions) onto one eye produced a slow reversal of guanoxabenz-induced mydriasis; the time course and degree of reversal were virtually the same in both eyes. 6 Intracerebroventricular administration of RX 781094 (1.25-15 micrograms total dose) caused a rapid dose-related reversal of the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). 7 Guanoxabenz (0.3 and 1.0 mg/kg, i.v.) did not produce any dilation of the physostigmine-constricted undamaged pupil of the pithed rat. Intravenous adrenaline was found to produce a small mydriatic effect, while atropine completely antagonized the effects of physostigmine in this preparation. 8 These results indicate that alpha 2-adrenoceptor agonists induce mydriasis in the rat through a central alpha 2-adrenoceptor mechanism. However, the site of action within the central nervous system remains to be determined.     

Interaction between beta 2- and alpha 2-adrenoceptor responses in the vascular system: effect of clenbuterol

Subchronic treatment with the beta 2-adrenoceptor agonist, clenbuterol (0.3 mg X kg-1, twice daily for 14 days), significantly increased the median blood pressure in anaesthetized normotensive rats. The treatment produced a marked reduction in the vasodilator effect of isoproterenol. Acute clenbuterol administration (0.01 mg X kg-1 i.v.) reduced the contractile response induced by the alpha 2-adrenoceptor agonists, guanabenz or B-HT 920, and the alpha 1- and alpha 2-adrenoceptor agonist, clonidine, whereas it did not affect the vasoconstriction by the alpha 1-adrenoceptor agonist, methoxamine, in the pithed rat. Subchronic treatment with clenbuterol attenuated the effect of the beta 2-adrenoceptor agonist on the vascular alpha 2-adrenoceptor responses and enhanced the alpha 1-adrenoceptor response to phenylephrine in pithed rats. The effect of the beta 2-adrenoceptor agonist was reduced by 1- but not d-propranolol. These results suggest that subchronic treatment with clenbuterol produces a subsensitivity of the beta 2-adrenoceptors and reduced the interaction between beta 2- and alpha 2-adrenoceptors at the vascular wall.