Specific bradycardic agents. 1. Chemistry, pharmacology, and structure-activity relationships of substituted benzazepinones, a-new class of compounds exerting antiischemic properties

Structural modification of the calcium-antagonist verapamil (1) by replacement of the lipophilic alpha-isopropylacetonitrile moiety by various heterocyclic ring systems has led to a new class of cardiovascular compounds which are characterized by a specific bradycardic activity. These agents reduce heart rate without binding to classical calcium channels or beta-adrenoceptors, interacting instead specifically with structures at the sino atrial node. Therefore they have also been termed sinus node inhibitors. The prototype falipamil (2) has been submitted to further optimization mainly by manipulation of the phthalmidine moiety. This has resulted in a second generation of specific bradycardic agents with increased potency and selectively and prolonged duration of action represented by the benzazepinone-derivative UL-FS 49 (4). Structure-activity relationships within this novel class of compounds have revealed a marked dependence of activity on the substitution pattern of the aromatic rings, the nature of the central nitrogen atom, and the length of the connecting alkyl chains. The crucial role of the benzazepinone ring for bradycardic activity can be best explained by its special impact on the overall molecular conformation.

Effects of alinidine on survival and infarct size in rats with coronary artery occlusion

Ligation of the left anterior descending coronary artery was performed in open-chest anaesthetized rats. One group had coronary occlusion for 3 h while ligation lasted for 30 min in a second group and was followed by a 150-min reperfusion period. The area at risk and area of infarction were determined immediately after premature death or 3 h after the ligature was set, by means of Evans blue and triphenyltetrazoliumchloride staining and subsequent photometric quantification. Saline or alinidine (5 mg/kg i.v.) was administered 15 min prior to ligation. The alinidine groups received a further 0.5 mg/kg i.v. 1 and 2 h after ligation. A large number of animals in the control groups died during the first 30 min. The animals that survived 3 h had a smaller area at risk than those dying prematurely and about 100% of the area at risk became infarcted. All animals in the two alinidine groups survived the first 30 min. All these animals survived with a larger area at risk than the control groups. The area of infarction in relation to the area at risk was significantly smaller than in the control groups. The cardioprotective effects of alinidine may be explained by a reduction in heart rate and a slight reduction in blood pressure.

The dopamine autoreceptor agonist, B-HT 920, preferentially reduces brain dopamine release in vivo: biochemical indices of brain dopamine, noradrenaline and serotonin in ventriculocisternal perfusates in the cat

B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine), a candidate for selective dopamine (DA) autoreceptor agonist activity, was tested for its interactions with biochemical parameters of brain dopaminergic, noradrenergic and serotoninergic systems as measured in ventriculocisternal perfusates of chloralose-anaesthetized cats. DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), noradrenaline (NA) and 5-hydroxyindolic acid (5-HIAA) were measured in samples of 30 min collection periods by high-pressure liquid chromatography with electrochemical detection. B-HT 920, in the dose range of 0.03-1 mg/kg i.v., promptly inhibited the efflux of DA and DOPAC in a dose-dependent manner. The 1 mg/kg dose of B-HT 920 reduced the DA levels below 25% of control levels for the whole length of the experiments. The HVA levels were reduced less and in a protracted manner. Only the highest dose of B-HT 920 tested (1 mg/kg) had a significant effect on the level of NA (marked, prompt reduction) and 5-HIAA (delayed, moderate reduction), reflecting its well known alpha 2-adrenoceptor agonist property. The effects of B-HT 920 on the dopaminergic indices were DA receptor-mediated as they were reversed by a low dose (0.05 mg/kg i.v.) of haloperidol. In contrast, the alpha 2-adrenoceptor blocking drug, idazoxan, 4 mg/kg i.v., while it reversed the NA and 5-HIAA reductions did not modify the effect of B-HT 920 on DA, DOPAC and HVA. Thus B-HT 920, in the dose range between 0.03-0.1 mg/kg, selectively affected brain dopaminergic parameters. Our experiments demonstrated that B-HT 920 causes an effective, long lasting and selective suppression of extracellular brain DA levels in vivo. B-HT 920 represents a promising compound for clinical use in pathological conditions known to be ameliorated by a reduction of brain DA activity, such as Huntington's disease, mania and schizophrenia.

Specific bradycardic agents--a novel pharmacological class?

Data have been reviewed and presented which suggest that substances from two different chemical groups, congeners of alinidine and falipamil, respectively, can be described as representatives of a novel and distinct pharmacological class: specific bradycardic agents (SBAs). They are characterized by a slowing of the sinus rate within physiological limits as the prominent cardiovascular effect. Involvement of alpha-adrenoceptors, beta-adrenoceptors and cholinergic receptors as mediators of the bradycardic effects have been excluded. Experiments in isolated atrial preparations suggested that drugs of the same type as alinidine or as falipamil have a similar mode of rate lowering action which is different from that of Ca-channel blockers: low external Ca2+ increased and low Na+ as well as high K+ decreased the bradycardic effect of SBA; verapamil behaved in the opposite way. Combination of different SBAs did not result in excessive additive rate-lowering effects; in contrast, addition of verapamil to maximally acting concentrations of SBAs resulted in a further significant reduction in rate. SBAs were much more potent in reducing spontaneous sinus rate than in reducing BaCl2 induced automaticity, whereas Ca-channel blockers behaved in the opposite way. Differences in the cardiovascular profile against other drugs with rate lowering effects have been pointed out: beta-adrenoceptor blocking agents more markedly decrease contractility and Ca-channel blocking agents more markedly decrease contractility, slow down AV conduction and have vasorelaxing properties.(ABSTRACT TRUNCATED AT 250 WORDS)

The dopamine autoreceptor agonist B-HT 920 inhibits in vivo dopamine release into the cerebroventricular system of cats

In anesthetized cats the dopamine autoreceptor agonist B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), 1 mg/kg i.v., greatly decreased the amount of dopamine in cerebroventricular perfusates. This effect was antagonized by a low dose (50 micrograms/kg i.v.) of haloperidol, but not by the alpha 2-adrenoceptor blocker idazoxan. Our observations provide evidence that B-HT 920 inhibits brain dopamine release in vivo and may be therapeutically valuable in diseases presumed to be accompanied by a predominance of brain dopamine activity, such as Huntington's disease, mania and schizophrenia.

Investigations differentiating the mechanism of specific bradycardic agents from that of calcium channel blockers

The effects of two "specific bradycardic agents", falipamil (AQ-A 39) and the alinidine-congener STH 2148 (2-[N-(cyclopropylmethyl)-N-(2,6-dibromophenyl)amino]-2-imidazolin e), on the spontaneous electrical discharge rate of intact guinea-pig sinus node preparations were investigated in comparison to that of the "calcium channel blocker" verapamil. Addition of falipamil (10 micrograms/ml) to a maximally rate lowering concentration of STH 2148 (30 micrograms/ml) exerted no further bradycardic effect. In contrast, verapamil (0.1 microgram/ml) added to either STH 2148 (30 micrograms/ml) or a maximally effective concentration of falipamil (30 micrograms/ml) resulted in a further, significant reduction of sinus rate. The results are compatible with the idea of a common mechanism of the two specific bradycardic agents, different from that of calcium channel blockers.

The dopamine autoreceptor agonist B-HT 920 stimulates denervated postsynaptic brain dopamine receptors in rodent and primate models of Parkinson's disease: a novel approach to treatment

B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), an agonist at alpha 2-adrenoceptors and at dopamine autoreceptors, was tested with respect to stimulation of postsynaptic brain dopamine receptors in mice, rats and rhesus monkeys. In mice B-HT 920 (0.2-20 mg/kg s.c.) injected 4 h after reserpine did not stimulate locomotor activity; this was in contrast to apomorphine (0.1-10 mg/kg s.c.) which elicited locomotor activity in a dose-dependent manner. However, B-HT 920 was effective in inducing locomotor activity when injected 12, 24 and 48 h after reserpine. This effect was dose-dependent and increased with the duration of reserpine pretreatment. In naive rats, B-HT 920 (0.02-2.0 mg/kg s.c.) only decreased exploratory activity and did not elicit stereotyped activity in doses up to 4 mg/kg s.c. This was in contrast to the stereotypy-inducing effect of apomorphine (2.0 and 4.0 mg/kg s.c.). In rats with unilateral striatal ibotenic acid lesion, B-HT 920 (0.2-2.0 mg/kg s.c.) was ineffective in producing significant ipsilateral rotation, whereas apomorphine (0.5-10.0 mg/kg s.c.) was very potent in this model. In rats with unilateral 6-OH-dopamine lesions of the medial forebrain bundle B-HT 920 elicited strong contralateral rotation in a dose-dependent manner (0.02-1.0 mg/kg s.c.). In this model B-HT 920 was equi-effective but long acting when compared with apomorphine. The contralateral rotation produced by B-HT 920 was antagonized by the D2-antagonist sulpiride but not by the D1-antagonist SCH 23390. In rhesus monkeys with severe parkinson-like symptoms induced by MPTP, B-HT 920 in doses of 10 micrograms/kg i.m. and higher restored normal behavior, resulting in complete relief of parkinson symptoms in all animals with 100 micrograms/kg i.m. It is concluded that the property of B-HT 920 to stimulate the 'denervated' supersensitive (reserpine, 6-OH-dopamine, MPTP) but not the normosensitive postsynaptic dopamine receptor in the striatum may represent a novel principle for a specific approach to dopamine substitution treatment of Parkinson's disease.

Investigations into the bradycardic effects of UL-FS 49 (1,3,4,5-tetrahydro-7,8-dimethoxy-3-[3-[[2-(3,4-dimethoxyphenyl)ethyl] methylimino]propyl]-2H-3-benzazepin-2-on-hydrochloride) in isolated guinea pig atria

The new bradycardic agent UL-FS 49 (1,3,4,5-tetrahydro-7,8-dimethoxy-3-[3-[[2-(3,4-dimethoxyphenyl]ethyl] methylimino]propyl]-2H-3-benzazepin-2-on-hydrochloride) was investigated in isolated guinea pig atria. In spontaneously beating preparations UL-FS 49, (0.03 and 0.1 microgram/ml) reduced the rate of contraction and decreased the maximal effect of isoprenaline added thereafter. The cumulative concentration-response curve of isoprenaline was antagonized, but not in a competitive manner, excluding an interaction at the beta-adrenoceptor. The rate of spontaneous electrical activity in sinoatrial node preparations was increased by superfusion with isoprenaline (0.1 microgram/ml). Addition of UL-FS 49 (0.1 microgram/ml) as well as propranolol (0.3 microgram/ml) reduced rate to control values. In electrically driven (1 Hz) left atria UL-FS 49 (1 microgram/ml) did not reduce contractile force and did not antagonize the positive inotropic effect of isoprenaline added cumulatively thereafter. When contractile force was first elevated by isoprenaline (0.1 microgram/ml), addition of UL-FS 49 (0.1 microgram/ml) did not affect contractility, whereas propranolol (0.3 microgram/ml) abolished the positive inotropic effect of isoprenaline. The experiments, therefore, demonstrate the specificity of UL-FS 49 to decrease heart rate but not contractility during beta-adrenoceptor stimulation. In contrast to propranolol (0.3 microgram/ml) UL-FS 49 (0.1 microgram/ml) also reduced sinoatrial rate elevated by histamine (1 microgram/ml) or theophylline (300 micrograms/ml), thus indicating a possible use as an antitachycardic drug at tachycardias of various origins. In sinus node preparations depolarized by high external K+ concentrations (10.8 mM), the bradycardic effect of UL-FS 49 (0.1 microgram/ml) was diminished.(ABSTRACT TRUNCATED AT 250 WORDS)

Antifibrillatory properties of alinidine after coronary artery occlusion in rats

Ligation of the left anterior descending coronary artery was performed in open-chest anaesthetized rats and mortality as well as changes in ECG were evaluated for 30 min thereafter. Saline or drugs were administered 15 min prior to ligation. In the control group, following a 4 min lag period ventricular arrhythmias as single ectopic beats, ventricular tachycardia and ventricular fibrillation (VF) appeared, reaching a maximum between 10 and 20 min and disappearing after 30 min. Mortality (40% in the control group) coincided with the period of maximal arrhythmias, with VF more common in animals that died than in those surviving. Alinidine, a drug which reduces sino-atrial rate specifically but has no conventional antiarrhythmic properties, reduced mortality and VF. By means of order statistics the quantity 'risk of death' was used for evaluation of drug effects, considering incidence of death and VF as well as duration of VF. This quantity was reduced in correlation with the dose of alinidine (1-6 mg/kg i.v.) and in correlation with the reduction of heart rate. Mexiletine, an antiarrhythmic drug with membrane-depressant properties, also reduced the 'risk of death' dose dependently (1-10 mg/kg i.v.), but there was no correlation with a decrease in heart rate. It is suggested that alinidine reduced 'risk of death' by means of a reduced oxygen demand due to a decrease in heart rate.

Possible function of alpha 1-adrenoceptors in the CNS in anaesthetized and conscious animals

The influence of St 587 (2-(2-chloro-5-trifluoromethylphenylimino)imidazolidine), a selective alpha 1-adrenoceptor agonist which easily penetrates the blood-brain barrier, was tested on behavior and cardiovascular functions, respectively. The substance (up to 10 mg/kg subcutaneously (s.c.)) did not increase the exploratory activity of naive mice. The hexobarbitone 'sleeping' time in mice was reduced in a dose-dependent manner (St 587 ED50 = 14.4 mg/kg s.c.). Haloperidol 10 mg/kg s.c. induced catalepsy which was antagonized by St 587 in a dose-dependent manner (ED50 = 2.7 mg/kg i.p.). Conversely, the alpha 1-adrenoceptor-blocking agents prazosin and corynanthine elicited catalepsy in mice which had been treated with a subthreshold dose (2 mg/kg s.c.) of haloperidol; the ED50 values of the antagonists were 0.26 and 4.7 mg/kg i.p., respectively. In anaesthetized cats blood pressure and heart rate were not affected by 100 micrograms/kg St 587 injected into the left vertebral artery. In conscious dogs with beta-adrenoceptors blocked, the drug was without effect (100 micrograms/kg intracisternally) on vagally mediated reflex bradycardia, as evoked by intravenous noradrenaline injection. As a positive control the alpha 2-adrenoceptor agonist B-HT 920 which is equipotent to St 587 with respect to peripheral vasopressor effects in rats was injected with 10 micrograms/kg intracisternally and facilitated the reflex bradycardia. It is concluded that alpha 1-adrenoceptors within the brain mediate behavioral activation in states of CNS depression but remain without effect on cardiovascular centers.

Decrease in bradycardic effect of AQ-A 39 and alinidine in guinea-pig sinoatrial node depolarized by high external K+-concentration

The effects of the two "specific bradycardic agents" AQ-A 39 and alinidine on the spontaneous electrical discharge rate of intact guinea-pig sinus node preparations were investigated. At high external K+-concentrations (10.8 and 16.2 mmol/l) the bradycardic effect of the two drugs was diminished or abolished. In contrast, the negative chronotropic effect of the reference compound verapamil ("Ca2+-antagonist") was enhanced. These results show that the bradycardic effects of AQ-A 39 and alinidine are diminished in depolarized preparations, which makes it unlikely that in intact sinus node preparations the mechanism of action is the same as that of "Ca2+-antagonists".

B-HT 958--an antagonist at alpha 2-adrenoceptors and an agonist at dopamine autoreceptors in the brain

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]az epine), a compound chemically related to clonidine-like drugs of the azepine type, was described previously as a mixed agonist-antagonist at peripheral alpha 2-adrenoceptor sites. In the present experiments the actions of B-HT 958 on brain noradrenergic and dopaminergic mechanisms were examined using behavioural, pharmacological and biochemical methods. (i) In the dog, intracisternally injected B-HT 958 (300 micrograms/kg) abolished the reflex bradycardia facilitated by the alpha 2-agonist B-HT 920 (10 micrograms/kg i.ci.). (ii) In the whole mouse brain as well as in the rat hypothalamus and neocortex, but not in the hippocampus, amygdala and nucleus accumbens B-HT 958 (5-20 mg/kg s.c.) accelerated the alpha-methyltyrosine (alpha-MT)-induced disappearance of noradrenaline. (iii) B-HT 958 decreased in a dose-dependent manner the exploratory activity of mice (ED50: 6.3 mg/kg s.c.) and slightly enhanced the motor activity of reserpine-treated mice only in high doses. (iv) B-HT 958 (20 mg/kg s.c.) lowered the level of homovanillic acid in the striatum and nucleus accumbens and dose dependently (1-20 mg/kg) slowed the alpha-MT-induced disappearance of dopamine in these 2 brain regions of the rat as well as in the whole mouse brain. (v) The gamma-butyrolactone-induced increase in DOPA levels was effectively antagonized by B-HT 958 (1 and 5 mg/kg s.c.) in the rat striatum and nucleus accumbens. These behavioural, pharmacological and biochemical observations indicate that B-HT 958 possesses central alpha-adrenoceptor blocking activity and exerts a strong agonistic effect on brain dopamine autoreceptors.

Cardiovascular characterization of UL-FS 49, 1,3,4,5-tetrahydro-7,8-dimethoxy-3-[3-][2-(3,4-dimethoxyphenyl)ethyl] methylimino]propyl]-2H-3-benzazepin-2-on hydrochloride, a new "specific bradycardic agent"

UL-FS 49, a chemical congener of AQ-A 39 with structural similarities to verapamil, decreased the rate of spontaneously beating guinea-pig atria at much lower concentrations (effective concentration 30%, EC30 = 0.030 microgram/ml) than it decreased the contractility (2.5 Hz; EC30 = 108 micrograms/ml) and maximal driving frequency (EC30 = 11 micrograms/ml) in electrically driven atria. In comparable experiments AQ-A 39 was much less effective, the EC30 for the negative chronotropic effect being 0.61 microgram/ml. In rabbit aortic strips in the presence of 43 mM K+ and 1.8 mM Ca2+, UL-FS 49 relaxed contraction by 30% at 15 micrograms/ml. In contrast to UL-FS 49, several "Ca2+-antagonists" elicited aortic relaxation at lower concentrations than bradycardia. In anaesthetized cats (n = 6) 0.3 mg/kg i.v., UL-FS 49 increased the cardiac cycle length by 56 +/- 3.5% (S.E.), there were slight or no changes in blood pressure and ECG intervals PQ and QRS. ST and the effective refractory period (ERP), as determined by R-triggered extrastimuli in the right ventricle, were prolonged by 28 +/- 3.1% and 24 +/- 2.5% respectively. At comparable increases in cycle length AQ-A 39 prolonged ST and ERP significantly more than UL-FS 49. In isolated guinea-pig atria UL-FS 49 antagonized the carbachol-induced bradycardia; a 10-fold shift of the dose-response curve (CA10) was achieved with 11.3 micrograms/ml and the CA10 for AQ-A 39 was 1.7 micrograms/ml. In conscious dogs UL-FS 49, 1 mg/kg i.v., decreased the heart rate without changes in blood pressure. This was observed in dogs with both genuine sinus rate and heart rate elevated by either atropine or hydralazine. The bradycardic effect was positively correlated with the control heart rate. In conclusion, sinus bradycardia was the most prominent action of UL-FS 49 in isolated preparations as well as in intact animals. In comparison to its congener AQ-A 39, UL-FS 49 was more potent in lowering heart rate but less effective in prolonging repolarization time and in anticholinergic activity. It thus represents a new specific bradycardic agent.

Effects of peripheral alpha 1- and alpha 2-adrenoceptor agonists on baroreceptor responsiveness in conscious dogs

Baroreceptor responsiveness was investigated in conscious dogs following increasing doses (i.v.) of the selective alpha-adrenoceptor agonists methoxamine (alpha 1) and oxymetazoline (alpha 2), in the presence and absence of beta-adrenoceptor blockade. The study was repeated in another group of dogs with background afferent baroreceptor nerve activity reduced by continuous infusion of sodium nitroprusside. Both agonists dose dependently increased mean arterial pressure and reflexly decreased heart rate. In dogs pretreated with a beta-adrenoceptor antagonist a correlation between increase in mean arterial pressure (increase up to 70 mmHg) and increase in heart period (baroreceptor responsiveness) indicated no difference in the regression lines between methoxamine and oxymetazoline for both the normotensive and the sodium nitroprusside groups. However, in the dogs not pretreated with a beta-adrenoceptor antagonist the slope of the regression line for oxymetazoline was steeper than that for methoxamine (P less than 0.01) in the normotensive group. In the sodium nitroprusside group the regression line for oxymetazoline was situated significantly to the left of the methoxamine line (P less than 0.05). It is suggested that this greater bradycardic response to the alpha 2-adrenoceptor agonist oxymetazoline was caused by suppression of the cardiac sympathetic component (presynaptic modulation of noradrenaline release) in addition to the vagal activation and the sympathetic withdrawal component of the reflex. This indicates that drugs with alpha 2-adrenoceptor agonistic activity can influence a reflex physiological situation under conditions of low sympathetic nerve activity.

Facilitation of the Bezold-Jarisch reflex by central stimulation of alpha 2 adrenoceptors in dogs

The Bezold-Jarisch reflex characterized by hypotension and bradycardia was elicited in anaesthetized artificially respired dogs (pretreated with a beta-adrenoceptor antagonist ) using capsaicin 10 micrograms/kg (i.v.). Intracisternal administration of the highly selective "clonidine-like" alpha 2 adrenoceptor agonists B-HT 920 (10 micrograms/kg) or B-HT 933 (30 micrograms/kg) significantly facilitated this reflex bradycardia. The involvement of central alpha 2-adrenoceptors is suggested as intracisternal administration of the alpha 2 adrenoceptor blocking drugs yohimbine (50 micrograms/kg) and piperoxan (50 micrograms/kg) antagonized this facilitation. B-HT 920 also facilitated the vagally mediated baroreceptor reflex to the hypertensive effect of intravenous noradrenaline (3 micrograms/kg). Although the Bezold-Jarisch reflex and the baroreceptor reflex have different afferent pathways, both reflexes may either converge into a common pathway or have separate neuronal chains within the medulla; however, this study indicates that both have a similar central modulatory system stimulated by alpha 2 adrenoceptors.

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.

Actions of alinidine and AQ-A 39 on rate and contractility of guinea pig atria during beta-adrenoceptor stimulation

We studied two new agents, alinidine (St 567, 2-[N-allyl-N-(2,6-dichlorophenyl)-amino]-2-imidazoline) and AQ-A 39 (5,6-dimethoxy-2-[3[[alpha-(3,4-dimethoxy)-phenylethyl] methylamino]propyl]phthalimidine), in isolated guinea pig atria with respect to their specificity to decrease heart rate but not contractility during beta-adrenoceptor stimulation. Spontaneous electrical activity in sinoatrial node preparations was increased by perfusion with isoprenaline (0.1 micrograms/ml); addition of alinidine (3 micrograms/ml), AQ-A 39 (3 micrograms/ml), or propranolol (0.03-0.3 micrograms/ml) reduced sinus rate to the control values. The same concentrations of these drugs were tested in electrically driven (1 Hz) left atria. The positive inotropic effect of isoprenaline was not affected by alinidine and AQ-A 39; however, it was markedly reduced or abolished by propranolol. The experiments, therefore, demonstrated the bradycardic specificity of alinidine and AQ-A 39 under conditions of beta-adrenoceptor stimulation. A cumulative concentration-response curve for the positive chronotropic effect of isoprenaline was established in spontaneously beating atria. AQ-A 39, 1 and 10 micrograms/ml, reduced the control sinus rate but did not affect the following concentration-response curve of isoprenaline. A similar result was published earlier for alinidine and excludes an interaction of both drugs with isoprenaline on a possible subgroup of selective beta-adrenoceptors in the sinoatrial node.

alpha 1/alpha 2 Selectivity ratio in a series of agonists and their relation to pre/postsynaptic activity ratios

beta-Blocked pithed rats were used to obtained dose-response curves (blood-pressure increase) for 13 various alpha-adrenoceptor agonists of clinical and theoretical interest, in the absence and in the presence of the selective antagonists rauwolscine (alpha 2) and prazosin (alpha 1). Antagonist doses which shifted the dose-response curves for an agonist 10-fold to the right along the abscissa were then calculated (D10rauwolscine = D10R; D10prazosin = D10P). The ratio D10R/D10P is considered to be a quantitative estimation of an agonist's alpha 1/alpha 2 selectivity ratio. A correlation with post/presynaptic activity ratios is presented.

Comparison of the bradycardic effects of alinidine (St 567), AQ-A 39 and verapamil on guinea-pig sinoatrial node superfused with different Ca2+ and NaCl solutions

Two new substances, alinidine (St 567) and AQ-A 39, previously described as 'specific bradycardic agents' were investigated with respect to their effects on the rate of electrical discharges from sinus nodes of isolated right guinea-pig atria. Both substances decreased the discharge rate concentration dependently within a wide range (3-100 micrograms/ml). Low external Ca2+ (0.18 mM) increased and low external NaCl (62 mM) decreased the rate lowering effect of alinidine as well as of AQ-A 39. In contrast, the reference compound verapamil (an inhibitor of slow inward current) was less effective in low Ca2+ and more effective in low NaCl. The similar reactions of alinidine and AQ-A 39 are discussed with respect to their different chemical structure and different electrophysiological action patterns as described so far.

Presynaptic activity of the imidazolidine derivative ST 587, a highly selective alpha 1-adrenoceptor agonist

St 587 increased the blood pressure of pithed rats. When the antagonists prazosin (alpha 1) and rauwolscine (alpha 2) were used the drug was shown to be a more selective alpha 1-adrenoceptor agonist than methoxamine. St 587 inhibited the tachycardia elicited by sympathetic stimulation in pithed rats. This effect was presynaptic and due to stimulation of alpha 1-adrenoceptors, as revealed by the different antagonism exerted by prazosin and yohimbine (alpha 2), respectively. The existence of presynaptic alpha 1-adrenoceptors is discussed.

B-HT 958, a new 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] azepine) was a potent agonist at presynaptic and a less potent agonist at postsynaptic alpha-adrenoceptor sites. Presynaptically this was shown in pithed rats by the inhibition of tachycardia evoked by sympathetic nerve stimulation (0.2 Hz). The 50% inhibitory dose (ID50) was 0.3 mg/kg i.v. Moreover in isolated perfused cat hearts, the drug inhibited the tachycardia and the outflow of noradrenaline induced by sympathetic nerve impulses. These effects of B-HT 958 were antagonized by phentolamine or yohimbine. At postsynaptic sites high doses of B-HT 958 increased the blood pressure of decentralized rats. The dose which increased pressure by 30 mm Hg (PD 30) was 46.3 mg/kg i.v. This effect was antagonized by rauwolscine 5 mg/kg i.v. After pretreatment with reserpine (7.5 mg/kg i.p., 18 h) B-HT 958 proved much more potent (PD 30 = 0.6 mg/kg i.v.) and its effect was strongly antagonized by yohimbine but hardly by prazosin. The dose of yohimbine which shifted the dose-response curve of B-HT 958 by the factor of 10 (D10) to the right was 1.8 mg/kg iv., the corresponding dose of prazosin was 1,900 mg/kg i.v. (extrapolated). B-HT 958 showed also alpha-adrenoceptor blocking properties. This was demonstrated presynaptically in pithed rats by the drug-induced augmentation of tachycardia elicited by electrical stimulation at high frequency (6.4 Hz). At postsynaptic sites B-HT 958 antagonized the blood pressure increase-caused by B-HT 920 (alpha 2; D10 = 1.1 mg/kg i.v.) but not that caused by methoxamine (alpha1). It is concluded that B-HT 958 is a partial agonist at peripheral alpha 2-adrenoceptors. In doses of about 1 mg/kg and with low frequency sympathetic stimulation (less than 6.4 Hz) it acts presynaptically as agonist; in this dose the drug acts postsynaptically mainly as antagonist.

alpha-Adrenoceptor subtypes in cardiovascular regulation

A great deal of experimental observations at alpha-adrenoceptors may be explained if two points are considered: (1) the alpha 1-/adrenoceptor ratio (selectivity ratio) of a given drug and (2) the ratio of alpha 1-/adrenoceptors in the target organ or tissue. The alpha 1-/alpha 2-selectivity ratio for 10 agonists was determined in pithed rats by their hypertensive effect in the presence or absence of the antagonists rauwolscine or prazosin, respectively. Treatment with a beta-blocking agent made it possible to test drugs with additional beta-adrenergic potencies (phenylethylamines). Methoxamine exerted the highest alpha 1-/alpha 2-selectivity ratio, the azepine derivative B-HT 920 the lowest ratio, i.e., the highest selectivity for alpha 2-adrenoceptors. Using the two selective agonists methoxamine and B-HT 920, respectively, the alpha 1-/alpha 2-adrenoceptor ratio (importance) in several target systems was estimated (determination of equieffective doses). Results indicated the prevalence of alpha 1-adrenoceptors in the nictitating membrane, and the equal importance of both subtypes at peripheral postsynaptic vascular sites in rats, as well as in cats. At presynaptic sites (cardiac nerve, rats), alpha 2-adrenoceptors are prevalent. A comparison of equieffective doses of B-HT 920 and clonidine in cats indicated the alpha 2-nature of the central adrenoceptors which mediate hypotension.

alpha 2-Adrenoceptor agonistic effect of B-HT 920 in isolated perfused hindquarters of rats

In isolated perfused hindquarters of rats pretreated with reserpine the alpha 2-adrenoceptor selective agonist B-HT 920 decreased the flow dose-dependently. This effect was strongly antagonized by yohimbine and less by prazosin, and the reverse potency was demonstrated for methoxamine. With increasing concentrations of B-HT 920, a half maximal effect was achieved with 10(-6.64) M. This value is discussed wih respect to the antagonistic potency of B-HT 920 at alpha 1-adrenoceptors with a pA2 of 4.9.

Modulation of motor activity by alpha 1- and alpha 2-adrenoceptor stimulation in mice

The influence of two alpha-adrenoceptor agonists, clonidine and B-HT 920, on motor activity was tested in mice. Both, clonidine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine) in the dose range 30--300 micrograms/kg s.c. effectively inhibited exploratory activity. On the other hand only clonidine, which stimulates alpha 2- and alpha 1-adrenoceptors increased locomotor activity in mice treated with reserpine (5 mg/kg) and apomorphine (3 mg/kg) in the doses of 0.3 and 1 mg/kg i.p. The highly selective alpha 2-agonist B-HT 920 was ineffective under these conditions up to 30 mg/kg i.p. It is concluded, that in mice "sedative" alpha-adrenoceptors are of the alpha 2- and "excitatory" of the alpha 1-type.

AQ-A 39 (5,6-dimethoxy-2-[3[[alpha-(3,4-dimethoxy)-phenylethyl]methylamino]propyl]phtalimidine), a specific bradycardic agent with direct action on the heart

In isolated guinea-pig atria AQ-A 39 (5,6-dimethoxy-2[3[[alpha-(3,4-dimethoxy)-phenylethyl]methylamino]propyl]phtalimidine) decreased the rate of spontaneously beating preparations, the contraction amplitude and maximal driving frequency of electrically driven preparations. However, the concentrations which reduced the parameters by 30% were different: 1.4 microgram/ml, 110 microgram/ml and 19 microgram/ml respectively. The bradycardic action was not affected by atropine (0.05 microgram/ml). In the ECG of anaesthetized cats (0.1-10 mg/kg i.v.) the prominent effect of AQ-A 39 was the increase in heart period (PP') and QT in contrast to the chemically related verapamil which mainly increased PQ. Blood pressure and ejection time were slightly affected whereas the diastolic period was markedly prolonged (5 mg/kg i.v.). The 'triple product' of heart rate X ejection time X blood pressure was decreased by AQ-A 39. In cats with acute occlusion of a coronary artery branch, AQ-A 39 diminished the elevation of the ST-segment of the epicardial electrogram. AQ-A 39 decreased the heart rate in conscious dogs (5 mg/kg i.v.), provided the initial heart rate was higher than approximately 130 beats/min, but increased the heart rate when the initial rates were lower. The drug revealed an anticholinergic effect by antagonising the bradycardic action of carbachol on isolated atria from guinea pigs. The prevalent effect on heart rate differentiated AQ-A 39 from other drugs with direct action on the heart such as antiarrhythmics, the so-called 'calcium antagonists' and cholinergic drugs. The profile resembled that of alinidine (St 567) and indicated a decrease in myocardial oxygen demand.

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.

Chemistry, pharmacology, and structure-activity relationships with a new type of imidazolines exerting a specific bradycardic action at a cardiac site

The reaction of alkyl halides with 2-(arylimino)imidazolidines (I) leads to imidazoline derivatives II, in which the side chain is situated at the bridge nitrogen atom between the phenyl group and the imidazoline ring. The new imidazolines (II) exert a specific bradycardic action at a cardiac site. Syntheses and pharmacology are shown and structure-activity relationships discussed. The results reveal that the imidazoline derivatives (II) represent a class of compounds with a novel type of cardiac action.

Central cardiovascular alpha-adrenoceptors. Relation to peripheral receptors

Clonidine and related drugs cause a specific pattern of cardiovascular depression by an agonistic effect on alpha-adrenoceptors within the central nervous system (CNS). For further characterization of the central adrenoceptors, the actions of three substances of the "clonidine-type" derived from two different chemical groups were studied at peripheral vascular sites. Clonidine and two azepin derivatives had very weak vasoconstrictor activity in isolated perfused rat hindquarters. Signs of desensitization were observed when these drugs were administered repeatedly. However, these compounds exerted antagonism against the vasoconstricting effect of noradrenaline. The ranking order of these drugs in their antagonistic potency was the same as with their central cardiovascular depressor potency. In spinal rats, the three compounds raised blood pressure due to their alpha-adrenoceptor stimulation. The ranking order was the same for the pressor potency as for their central cardiovascular depressor potency. It is concluded that in isolated preparations the agonistic activity of clonidine-like substances on postsynaptic alpha-adrenoceptors might not be representative for their CNS effect.

Binding of an imidazolidine (clonidine), an oxazoloazepin (B-HT 933) and a thiazoloazepin (B-HT 920) to rat brain alpha-adrenoceptors and relation to cardiovascular effects

The specific binding to alpha-adrenoceptors in crude plasma membrane preparations of the rat brain was studied by means of 3H-clonidine (specific radioactivity 26.7 Ci/mmole). Equilibrium binding of 3H-clonidine could be described adequately according to a two-site model with a minor population of high affinity sites (KD1 = 0.4 nM) and a major population of low affinity sites (KD2 = 6.1 nM). The heterogeneity of 3H-clonidine binding was also indicated by a biphasic association rate in kinetic binding studies. In competition experiments with 3H-clonidine concentrations of 0.5 or 4.0 nM respectively, concentration-dependent displacement was observed with the non-radioactive compounds: clonidine, B-HT 920 (2-amino-6-allyl-5, 6, 7, 8-tetrahydro-4H-thiazolo-[5, 4-d]-azepin-dehydrochloride) and B-HT 933 (2-amino-6-ethyl-4, 5, 7, 8-tetrahydro-6H-oxazolo-[5, 4-d]-azepin-dihydrochloride). IC50 values of 3, 21 and 160 nM or 10, 63 and 380 nM respectively were thereby evaluated. Cardiovascular effects were estimated in rats. The blood pressure increase in spinal animals was taken as parameter for alpha-adrenoceptor stimulation at peripheral vascular sites. The bradycardic effect in vagotomized animals was taken as parameter for central nervous sympathoinhibition. The ranking order of the potency of the three drugs was the same in both in vivo tests and parallels the in vitro binding affinities at both binding sites: clonidine greater than B-HT 920 greater than B-HT 933. These results indicate the similarity of the alpha-adrenoceptor structures in brain membrane preparations, at peripheral vascular sites and at central sympathoinhibitory sites.

Relation between central sympathoinhibitory and peripheral pre- and postsynaptic alpha-adrenoceptors as evaluated by different clonidine-like substances in rats

A series of 8 clonidine-like substances were tested in rats on central sympathoinhibition (heart rate decrease in vagotomized rats, D50 = dose which decreased heart rate by 50 beats/min), peripheral postsynaptic activity (blood pressure increase in spinal rats, PD30 = dose which increased blood pressure 30 mm Hg) and peripheral presynaptic activity (inhibition of tachycardia as elicited by electrical stimulation in the spinal canal of pithed rats, ID50 = dose which inhibited for 50%). The D50, PD30 and ID50 were converted in moles/kg and the negative logarithms thereof were correlated. There was a good correlation between the central effect (D50) and both peripheral effects (PD30 and ID50, respectively; r = 0.89 and 0.91, respectively) provided one drug with low lipoid affinity was omitted from calculation (St 600). There was also good correlation between both peripheral effects (PD30 and ID50; r = 0.94) and this could be extended to other imidazolines which are not acting centrally as antihypertensives (oxymetazoline, naphazoline, St 91, tramazoline; r = 0.90). The results were interpreted by (1) the agonistic activity of the clonidine-like drugs mainly at alpha 2-adrenoceptors and (2) the presence of alpha 2-adrenoceptors at peripheral presynaptic, peripheral postsynaptic as well as central cardiovascular sites.

Differentiation of drugs acting centrally upon the cardiovascular system by means of sympathetic and vagal responses

The response pattern of the autonomic nervous system was investigated after central administration (intra-cisternal, vertebral artery) of amphetamine, morphine, fentanyl, dextromoramide and the substance R 28935, chemically related to the neuroleptic agent pimozide. Effects on the sympathetic system were measured by recording electrical discharges of fibres of the (preganglionic) major splanchnic nerve in anaesthetized cats; those on the vagal system by recording the heart rate in anaesthetized dogs under beta-adrenoceptor blockade; the baroreceptor reflex was elicited by the blood pressure increase of i.v. injected angiotensin. All substances decreased the spontaneous discharge rate of the splanchnic nerve. Amphetamine facilitated the vagally mediated reflex bradycardia and this was antagonized by the alpha-adrenoceptor blocking agent piperoxan. Amphetamine did not affect the resting heart rate, as has already been shown for clonidine and related substances. The narcotic analgesics lowered the resting heart rate but did not facilitate the baroreceptor reflex response. R 28935 neither influenced resting heart rate nor the baroreceptor reflex response in beta-blocked dogs. On the basis of the vagal response pattern it was therefore possible to distinguish between 3 groups of central hypotensive drugs.

[New concepts on central regulation of the blood pressure. Mode of action of clonidine and alpha-methyl DOPA (author's transl)]

A review is presented of several reports which strongly indicate that the reaction pattern of "cardiovascular depression" (decrease in blood pressure, heart rate and cardiac output) is triggered off by the stimulation of alpha-adrenoceptors in the brain. The efferent information is mediated by an inhibition of the sympathetic and by an activation of the cardiac vagal system. The central alpha-adrenoceptors, which are localized within the medulla, are stimulated by the antihypertensive agents clonidine and alpha-methyl-DOPA, and also by central administration of other sympathomimetic drugs such as naphazoline, oxymetazoline, amphetamine and others. It can be assumed that these drugs mimic the action of endogenous nor-adrenaline, which is present in considerable amounts in the brain stem, and which might, under physiological conditions, mediate the pattern of cardiovascular depression.

Centrally induced reduction in sympathetic tone-a postsynaptic alpha-adrenoceptor-stimulating action of imidazolines

Naphazoline or oxymetazoline (both 30 mug/kg) were injected into the cisterna magna of anaesthetized cats and reduced blood pressure, heart rate and the electrical discharge rate of small fibre bundles of the preganglionic sympathetic splanchnic nerve. Cats were depleted of endogenous noradrenaline by pretreatment with reserpine (5 mg/kg, 18 h) and alpha-methyl-p-tyrosine (twice 300 mg/kg, 18 and 2 h). In these animals, intracisternal injection of 30 mug/kg oxymetazoline exerted a decrease of sympathetic discharges similar to that described for non-pretreated animals. In noradrenaline-depleted cats intracisternal injection of 1 mug/kg clonidine also decreased the sympathetic discharges. It is concluded that these imidazolines exert their sympathoinhibitory and cardiovascular effects by stimulation of postsynaptic alpha-adrenoceptors in the CNS.

RA 642, a pyrimido-pyrimidine-derivative with vasodilating and hypertensive potency

2,2'-[(4,8-bis(diethylamino)-pyrimido[5,4-d]-pyrimidine-2,6-diyl)di-(2-methoxyethyl)imino]diethanol), RA 642, combines hypertensive and vasodilating effects. In anaesthetized animals arterial blood pressure was increased by i.v. doses of 0.25-4 mg/kg in cats and 0.025-0.25 mg/kg in dogs. In conscious dogs, 25 mm increase of mean blood pressure was achieved with 0.2 mg/kg i.v. and 18.8 mg/kg p.o. Cerebral blood flow was enhanced and calculated cerebral vascular resistance was reduced by RA 642. Total peripheral resistance was diminished by 0.25-1.0 mg/kg i.v. A vasodilatation of femoral and coronary vessels was shown after intraarterial injection. This effect as well as a BaCl2-antagonism in the isolation ileum is explained by a papaverine-like relaxant effect on smooth muscle. Activity on peripheral adrenergic receptors was excluded. Hypertension was abolished in spinalized cats, indicating a central mechanism of this effect.

Stimulation of sympathetic cardiovascular centres by RA 642, a new pyrimido-pyrimidine derivative

RA 642, 2,2'-[14,8-bis(diethylamino)-pyrimido[5,4-d] pyrimidine-2,6-diyl)di-(2-methoxyethyl)-imino]diethanol, has formerly been described to hypertensive when administered systemically. In the present experiments injection of 50 mug/kg RA 642 into the vertebral artery (i.a. vert.) of the cat exerted hypertension, while the same dose given intravenously (i.v.) elicited no effect. Since the drug was still effective in cats of which the brain was removed rostrad of the myelencephalon but not in spinal animals, its site of action was concluded to be in cardiovascular centres of the medulla oblongata. An i.a. vert. injection increased total peripheral resistance of the vasculature. The electrical discharge rate in preganglionic cervical sympathetic nerve fibres increased in parallel to the arterial blood pressure following the i.v. injection of 1 mg/kg of the substance. 1 mg/kg RA 642 i.v. decreased the reflex bradycardia elicited by the i.v. injection of angiotensin in dogs pretreated with a beta-adrenoceptor blocking drug to study the vagal effector reactions of the baroreceptor reflex, and treated with clonidine to facilitate the reflex. Thus, RA 642 increased sympathetic and, following clonidine treatment, decreased vagal reflex activity. RA 642 stimulated the respiration by a central action. The drug had no convulsant action.

Relationship between activity and structure in derivatives of clonidine

2-(2,6-Dichlorphenylamino)-(clonidine, Catapresan, Catapres), 2-(2,6-diethylphenylamino- (St 91), 2-(2-chloro-6-methylphenylamino)-, 2-(2-chloro-4-methylphenylamino)-, 2-(2-methyl-5-fluorophenylamino) -and 2-(2-chloro-3-methyl-phenylamino)-2-imidazoline were investigated in various pharmacological tests. 1. All substances increased blood pressure in spinal rats and initially in intact cats and dogs and increased the total peripheral resistance in the latter. As these compounds also showed mydriasis in conscious rats it has been concluded that these effects are due to stimulation of peripheral alpha-adrenoceptors. 2. With the exception of St 91 the substances lowered blood pressure following i.v. injection, they decreased heart rate in cats and dogs and the cardiac output in the latter. These four compounds also decreased heart rate in vagotomised and atropinised rats. It was concluded that these effects were due to a decrease in sympathetic activity of the CNS. 3. In anaesthetised rats with beta-adrenoceptor blockade by toliprolol, a blood pressure increase was elicited by i.v. injection of angiotensin and the resulting bradycardia was recorded as a measure of vagal reflex activity. Clonidine and three derivatives which have shown hypotensive activity facilitated the vagal cardiodepressor reflex; St 91 was inactive in this respect. It has been concluded that decrease in central sympathetic tone and increase in central vagal activity are linked together in these compounds. 4. St 91 did not lower blood pressure and did not facilitate vagal reflex bradycardia after i.v. injection in dogs, but was active after intracisternal injection. It has been concluded, therefore, that this compound is able to act on structures in the CNS like clonidine but these effects usually do not occur after systemic administration because of its poor ability to penetrate the blood-brain barrier. 5. All derivatives decreased gastric acid secretion. 6. All substances increased blood glucose levels and were sedative; St 91 was the least effective compound in both respects pointing to a central mediation of these effects. 7. The results show that clonidine and the derivatives tested have the same reaction pattern. 8. The relationship between the CNS mediated cardiovascular depression and the peripheral alpha-adrenergic stimulating potency in conjunction with the lipoid solubility have been discussed.

Localization in the CNA of adrenoceptors which facilitate a cardioinhibitory reflex

In dogs (pentobarbitone, 25 mg/kg) the brain was removed rostrally to the pons, leaving the cerebellum intact (decerebrate animals). In other animals the cerebellum was additionally removed (bulbar animals). In all animals beta-adrenoceptors were blocked by toliprolol (5 mg/kg s.c.). Angiotensin (0.025-0.3mug/kg) was repeatedly injected i.v. and the resulting maximal reflex bradycardia was recorded. Intracisternal (i.ci.) injection of clonidine, 0.5 to 1 mug/kg, in decerebrate or i.v. injection of 10 or 30 mug/kg in bulbar animals significantly facilitated the reflex bradycardia. This effect was antagonised by a subsequent injection of piperoxan 50 mug/kg i.ci. in decerebrate or 1 mg/kg i.v. in bulbar animals. It is concluded that the facilitatory action of clonidine is mediated by alpha-adrenoceptors within the medulla oblongata.

Investigation into some imidazoline compounds, with respect to peripheral alpha-adrenoceptor stimulation and depression of cardiovascular centers

Peripheral alpha-adrenoceptor stimulation was tested by means of hypertensive effects of the drugs following i.v. injection in spinal rats. Naphazoline (NP), oxymetazoline (OM), St 91-2-(2,6-diethylphenylimino)-2-imidazolidine--and St 1697--2-(2-ethyl, 6-methylphenylimino)-2-imidazolidine--were 3 to 5 times more potent in tthis respect thatn clonidine (CLON) whereas St 363--2-(2,4-dichlorophenylimino)-2-imidazolidine--and xylazine (XY) exerted only approx. 1/20 the effect of that of clonidine. Sympathoinhibitory activity after i.v. injection was tested by the bradycardiac effect in vagotomized rats; St 1697, St 363 and XY were active, approx. 1/10-1/30 of CLON, whereas NP, OM and St 91 were inactive. However, following intracisternal (i.ci.) injection of cardiovascular depression, typical for clonidine: (1) in dogs with blocked beta-adrenoceptors, the drugs facilitated the vagally meditated cardiodepressor reflex in response to baroreceptor stimulation by i.v. injection of angiotensin; (2) in dogs treated with atropine and in (3) vagotomized cats (only NP, OM and St 363) a long lasting decrease in heart rate was observed. Some of the experiments were complicated by increases in blood pressure, due to the "leakage" of small amounts of the highly vasopressor active drugs, from the cisternal spaces into the peripheral circulation. The majority of results indicated, that the central cardiovascular depressor effects of the tested drugs depend on their alpha-adrenoreceptor stimulating potency and on their ability to penetrate from cerebrospinal fluid or from the blood to cardiovascular centers. Relationships between the ability for penetration and the lipoid affinity are discussed.