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.