N-substituted imidazolines and ethylenediamines and their action on alpha- and beta-adrenergic receptors.

A series of N-substituted imidazolines and ethylenediamines were synthesized and examined for their activity in alpha- and beta-adrenergic systems. The length of the intermediate side chain between the catechol and imidazoline ring or the amine of the ethylenediamine segment was shown to affect the adrenergic activity. N-[2-(3,4-Dihydroxyphenyl)ethyl]imidazoline hydrochloride (2) and N-[2-(3,4-dihydroxyphenyl)ethyl]ethylenediamine dihydrochloride (4), both with two methylene groups between the catechol and amine segment, were found to be somewhat selective for alpha 2-adrenergic receptors while 1-(3,4-dihydroxybenzyl)imidazoline hydrochloride (1) and N-2-(3,4-dihydroxybenzyl)ethylenediamine dihydrochloride (3), both with one methylene group between the catechol and amine segment, were more selective for alpha1-adrenergic receptors in a pithed rat model. Of the four compounds examined, only compound 2 showed significant direct activity on beta1- and beta2-adrenergic receptors.

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)

Characterization of the alpha adrenoceptor-mediated effects and antihypertensive activity of ICI 106270: comparison with clonidine.

Clonidine and ICI 106270 are centrally acting antihypertensive agents which act through stimulation of medullary alpha-2 adrenoceptors. Both compounds are equipotent agonists at alpha-2 adrenoceptors as assessed in functional studies in isolated organs and in radioligand binding studies. In addition, clonidine and ICI 106270 possess the same degree of selectivity for alpha-2 adrenoceptors over alpha-1 adrenoceptors. Clonidine and ICI 106270 are equipotent antihypertensive agents after intracisternal administration to spontaneously hypertensive rats, consistent with the observations made in vitro that both compounds are equipotent alpha-2 adrenoceptor agonists. Both compounds were less potent in lowering blood pressure after i.v. administration than after intracisternal administration, thus confirming a central mechanism of action. Interestingly, clonidine was a more potent antihypertensive agent than ICI 106270 after i.v. administration, which, in view of the equal potencies observed after intracisternal administration, suggests that diffusion of ICI 106270 into the central nervous system is selectively retarded, relative to clonidine, by the blood-brain barrier. The differences observed in the rate of penetration of the blood-brain barrier are consistent with the higher pka and corresponding higher extent of ionization (at physiological pH) and lower lipophilicity of ICI 106270 relative to clonidine. A relatively large difference between the i.v. and p.o. antihypertensive potencies was observed for ICI 106270 which indicates poor p.o. absorption of ICI 106270 relative to clonidine, again likely resulting from the greater proportion of ICI 106270 existing in the ionized species.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Blockade and postjunctional vascular alpha 1- and alpha 2-adrenoceptors in pithed rat by the enantiomers of WB-4101.

The enantiomers of WB-4101 were evaluated for their ability to antagonize the alpha 1-adrenoceptor mediated pressor effect of cirazoline and the alpha 2-adrenoceptor mediated pressor effect of UK-14,304 in pithed rats. The (S)-stereoisomer of WB-4101 was more potent than the enantiomeric (R)-isomer in antagonizing both cirazoline and UK-14,304. The difference in potency between the enantiomers in blocking cirazoline was 37-fold in contrast to an enantiomeric difference of less than 3-fold for antagonizing UK-14,304. Based on DR2 values (i.e., dose of WB-4101 isomer in mg/kg required to produce a 2-fold rightward shift in the dose-response curves of cirazoline and UK-14,304) obtained in vivo from Schild regressions, alpha 1/alpha 2 selectivity ratios were calculated. While the (S)-enantiomer displays a 187-fold selectivity for alpha 1-adrenoceptors, the (R)-enantiomer is only approximately 13-fold selective for alpha 1-adrenoceptors. These results indicate that the alpha 1- and alpha 2-adrenoceptor blocking activity of WB-4101 resides predominantly in the (S)-enantiomer and that both enantiomers of WB-4101 are selective alpha 1-adrenoceptor antagonists in vivo. However the degree of alpha 1-adrenoceptor selectivity differs from each enantiomer, and the enantiomeric activity ratios differ for each alpha-adrenoceptor subtype.

Postsynaptic alpha adrenergic receptor subtypes differentiated by yohimbine in tissues from the rat. Existence of alpha-2 adrenergic receptors in rat aorta.

The selective alpha-2 adrenergic receptor antagonist, yohimbine, was used to differentiate postsynaptic alpha-adrenergic receptors in five peripheral tissues of the rat. Three distinct postsynaptic receptor subtypes were observed based on the affinity of the receptors for yohimbine. Receptors with high affinity for yohimbine were detected in the aorta, whereas low affinity receptors were observed in the vas deferens. The affinity for yohimbine in these two tissues differed by over 50-fold. Receptors with intermediate affinity for yohimbine were found in the portal vein, spleen and bladder. The Schild plot for yohimbine in the bladder suggests that blockade of alpha receptors in this tissue is not competitive or that there exists more than one type of alpha-adrenergic receptor present. Comparison of the dissociation constant of yohimbine in the aorta with dissociation constants obtained from the literature for this compound in a variety of tissues containing alpha-1 or alpha-2 adrenergic receptors indicates that the postsynaptic alpha adrenergic receptor in the aorta is of the alpha-2 type. Conversely, the postsynaptic alpha adrenergic receptor in the portal vein appears to be alpha-1. The differences in postsynaptic alpha appears to be alpha-1. The differences in postsynaptic alpha adrenergic receptors in these two vascular tissues may reflect the marked differences in adrenergic innervation and the possible relative lack of neuroeffector junctional alpha adrenergic receptors in the rat aorta.

Receptor interactrions of imidazolines. VI. Significance of carbon bridge separating phenyl and imidazoline rings of tolazoline-like alpha adrenergic imidazolines.

The pharmacological significance of the carbon bridge separating the imidazoline and phenyl rings of tolazoline-like alpha adrenergic imidazolines has been investigated. Extending the carbon bridge to two carbon atoms, or deleting the carbon bridge, lowers affinity of the imidazolines for the alpha receptor and markedly decreases or abolishes efficacy (i.e., agonist activity), suggesting that a single carbon atome optimallyu separates the phenyl and imidazoline rings. Although one carbon is optimal for alpha adrenergic activity, this particular atom does not appear to be essential since nitrogen may substitute for carbon with no marked or consistent changes observed in affinity or efficacy. Hydroxylation of the carbon bridge decreases affinity for the receptor approximately 10-fold but does not alter efficacy, whereas a similar substitution made in the norepinephrine-series of phenethylamines markedly increases affinity (Patil et al., 1974). With both the imidazolines and phenethylamines, this carbon atom may stereoselectively influence binding to the receptor. These results suggest that the carbon atom bridging the phenyl and imidazoline rings of tolazoline-like imidazolines serves only to provide optimal separation between these rings and does not contribute directly to the binding process. It is proposed that alpha adrenergic imidazolines interact differently with the alpha adrenergic receptor than the norepinephrine-like phenethylamines.

Receptor interactions of imidazolines. V. clonidine differentiates postsynaptic alpha adrenergic receptor subtypes in tissues from the rat.

The alpha-adrenergic effects of clonidine were studied in five tissues of the rat. Postsynaptic alpha adrenergic receptors in these tissues may be divided into three distinct classes based on affinity of the receptors for clonidine. High affinity receptors are located in the aorta and low affinity receptors in the vas deferens. Receptors with intermediate affinity are found in the portal vein, spleen and bladder. The bladder may also contain, to a small extent, a population of receptors similar to those found in the vas deferens which may mediate a different response. Available data suggest that the alpha adrenergic receptor of the rat aorta with high affinity for clonidine may be of the alpha-2 type but located postsynaptically in this tissue.

Receptor interactions of imidazolines. IV. Structural requirements for alpha adrenergic receptor occupation and receptor activation by clonidine and a series of structural analogs in rat aorta.

Clonidine and a series of nine halogen and alkyl-substituted structural analogs were evaluated for alpha adrenergic receptor agonist activity in rat aorta. Phenylephrine was included for comparison. All the imidazolines were partial agonists with respect to phenylephrine due to the fact that they only weakly activated the receptor. Whereas phenylephrine requires approximately 0.2% receptor occupancy to produce a response 20% of maximum, the imidazolines required from 25 to 100% receptor occupancy to produce the same response. Although clonidine and its structural analogs are from 125 to 500 times weaker activators of the alpha adrenergic receptor than phenylephrine, they have from 2 to 60 times higher affinity for the receptor than phenylephrine. The results support our previous conclusions that a dichotomy exists between the factors that direct receptor occupation and receptor activation since the structural requirements for each of these parameters differ.