Naunyn-Schmiedeberg's

Archivesof

Naunyn-Schmiedeberg's Arch. Pharmacol. 309, 217-224 (1979)

Pharmacology 9 by Springer-Verlag 1979

Effects of Ring Substitution on the Pre- and Postjunctional Alpha-Adrenergic Activity of Aryliminoimidazolidines J. Paul Hieble and Robert G. Pendleton Research and Development Division, Smith Kline and French Laboratories, Philadelphia, Pennsylvania 19101, USA

Summary. The pre- and postjunctional e-adrenergic agonist potency of a series of aryliminoimidazolidines was determined in the isolated rabbit ear artery. This series included clonidine, an antihypertensive agent thought to act by stimulating brainstem a-receptors and known to be a preferentially prejunctional eadrenergic agonist. Although all of the compounds acted preferentially on the prejunctional eadrenoceptor, ring substitution had a dramatic effect on both potency and the degree of selectivity. 2-(3,4Dihydroxyphenylimino) imidazolidine was both the most potent and most selective prejunctional e-agonist in this series.

Key words: Prejunctional e-adrenoceptors Postjunctional ~-adrenoceptors - Imidazolidines Clonidine - Rabbit ear artery.

Introduction In addition to the classical postjunctional ~.-receptor, the presence of e-adrenoceptors at or near sympathetic nerve terminals is now well recognized. These prejunctional a-receptors form part of a local feedback system modulating sympathetic neurotransmission, since their activation results in a decrease in the amount of norepinephrine released from the terminals by nerve impulses (for reviews see Langer, 1977; Starke; 1977). Starke et al. (1975) found the antihypertensive drug clonidine [2-(2,6-dichlorophenylimino) imidazolidine] and some other related compounds to be more potent as inhibitors of the norepinephrine release evoked by nerve stimulation in the rabbit pulmonary artery than as direct vasoconstrictors. Other e-adrenergic agonists Send offprint requests to J. P. Hieble at the above address

such as phenylephrine and methoxamine had the opposite selectivity; i.e., they were more potent as vasoconstrictors than as inhibitors of release. Norepinephrine and epinephrine were essentially nonselective. Steinsland and Nelson (1975) showed that clonidine inhibited the constrictor response of the rabbit ear artery to sympathetic nerve stimulation at much lower concentrations than those required to produce vasoconstriction, whereas phenylephrine and methoxamine produced vasoconstriction, but no inhibition of the response to sympathetic nerve stimulation. The conclusion of both of these studies was that the prejunctional e-adrenoceptor had pharmacological characteristics different from those of the postjunctional a-receptor. Numerous reports have confirmed this conclusion and demonstrated the prejunctional selectivity of clonidine and other iminoimidazolidines in various tissues (Drew, 1976; Steinsland and Hieble, 1976; Medgett et al., 1978 ; Pichler and Kobinger, 1978; Walland, 1978; Wikberg, 1978). Although Haeusler (1976) could not demonstrate the prejunctional selectivity of clonidine in vivo using the perfnsed hind limb of the cat, sirhilar experiments using the rabbit have pharmacologically differentiated pre- and postjunctional a-receptors in vivo (Steppler et al., 1978). Unlike most of the classical ~-adrenergic agonists, clonidine can readily penetrate the blood brain barrier (Timmermans et al., 1977a) and it is generally accepted that clonidine produce its antihypertensive action by stimulating postsynaptic c~-adrenoceptors in the brainstem (see review by Kobinger, 1978). Several reports have described attempts to correlate the antihypertensive activity of aryliminoimidazolidines with their molecular structure, lipophilicity and/or peripheral postjunctional e-agonist potency (Hoefke et al., 1975; Struyker Boudier et al., 1975a; Rouot et al., 1976; Timmermans and Van Zwieten, 1977a-d, 1978; Timmermans et al., 1977a, b).

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218

Recently it has been proposed that clonidine acts on central c~-adrenoceptors which, although located postsynaptically, have pharmacologic characteristics similar to those of the peripheral e-receptor (Berthelsen and Pettinger, 1977). Therefore the determination of peripheral prejunctional a-activity may be useful for predicting central e-adrenoceptor activity. In this study the in vitro potency on both the pro- and postjunctional e-adrenoceptors of the rabbit ear artery was determined for a series of ring substituted aryliminoimidazolidines in order to relate molecular structure to c~adrenergic potency and pre/postjunctional selectivity.

Naunyn-Schmiedeberg's Arch. Pharmacol. 309 (1979) To

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Methods Isolated Perfused Rabbit Ear Artery. A modification of the preparation described by Steinsland et al. (1973) was used for these studies; see the above reference for a drawing of the perfusion apparatus. Male rabbits weighing from 2 - 3 kg were sacrificed by cervical concussion, and a 2 - 4 cm portion of the central ear artery was dissected free at the base of the ear, cannulated with polyethylene tubing (Intramedic PE-50) at both ends, and mounted in a perfusion chamber designed to allow simultaneous intraluminal perfusion and extraluminal superfusion of the artery. The chamber itself was formed from a glass tube 125 mm long x 3 mm inside diameter, q-he major portions of both the intraluminal and extraluminal flows were delivered through small diameter Tygon tubing at a constant rate, usually about 2 ml/min, from 2 channels of a 4-channel peristaltic pump (Buchler PolystaItic). The perfusate in these two main channels came from a reservoir of Krebs-Hcnseleit solution. The rate of flow in each of the two other channels of the pump (minor channels) was kept at one-half of the rate in each of the main channels by using for the minor channels tubing with a smaller internal diameter than that used for the main channels. The minor channels were used for the administration of drugs; the flow from these channels could be delivered either extraluminally or intraluminally. The outflow of the extraluminal superfusion was connected to a 90 cm length of tubing, which was elevated to exert an extraluminal pressure of 50 mm Hg (70 cm H20) on the artery. The application of this extraluminal pressure improved the stability of the response to nerve stimulation. The intraluminal inflow perfusion pressure was measured with a Statham P23AA transducer and recorded on a Physiograph (Narco Biosystems). Since the intraluminal flow rate remained constant, changes in perfusion pressure reflected changes in the resistance to flow, i. e., the degree of vasoconstriction. Once an artery was mounted in the chamber, perfusion and superfusion were continued for at least one hour before an experiment was begun. During this preliminary stabilization period, the sensitivity of the artery to both field stimulation of the sympathetic nerves and administration of vasoconstrictor drugs gradually increased from a low level up to full sensitivity. The sensitivity then remained constant for 10-15 h, provided that the artery was not subjected to excessive intraluminal pressures. The composition of the Krebs-Henseleit solution was as follows: NaC1, 119 mM; NaHCO3, 25 mM; KC1, 4.7 mM; MgSO4, 1.5 mM; KH2PO~, 1.2 mM; glucose, 11 mM; ascorbic acid, 5 gM; disodium EDTA, 30 gM. This solution was gassed with 95 ~ 02, 5 ~ CO2 and maintained at 35~ The pH under these conditions was 7.4. The periarterial sympathetic nerves were excited by field stimulation. Rectangular pulses of'0.7 ms duration and supramaximal voltage (75 volts) from an American Electronics Laboratory Model 104A stimulator, were delivered through platinum electrodes sealed through the glass at the top and bottom of the perfusion chamber.

Fig. 1. Schematic drawing of apparatus for superfusion of rabbit ear artery segment. Superfusion chamber dimensions were 6x12 • 15 ram; capacity was 1.5 ml. The support for the upper wire was a loop of 26 gauge platinum wire; this support held the tissue and upper wire while the superfusion chamber was being assembled and the tissue attached to the transducer

The nerves were stimulated at 4-min intervals by a train of pulses at 10-15 Hz with a duration of 300-500 ms. To accomplish this automatically, the stimulator was triggered every 4 rain by an external timing device. Except for norepinephrine, given intratuminalIy to produce constrictor responses, all drugs were administered via the extraluminal superfusion. Drugs were given in increasing concentration, each concentration remaining in contact with the tissue for 4 min. The drug concentration was increased immediately following the response of the artery to nerve stimulation.

Isolated Superfused Rabbit Ear Artery Segment. Rabbits were sacrificed as described above. The central ear artery was isolated from the surrounding connective tissue at the base of the ear and a polyethylene cannula (Intramedic PE-50) inserted to a depth of about I cm. The cannula and artery were then cut to leave 0.5 cm segment of the artery on the end of the cannula. A schematic drawing of the snperfusion apparatus is shown in Fig. 1. The arterial segment was suspended between two parallel 0.25 mm tungsten wires, the lower wire attached to a removable section of the superfusion chamber and the upper wire connected to a force-displacement transducer (Statham G-10B). When the apparatus was assembled, the lower tungsten wire fit into a small hole drilled into the rear wall of the tissue chamber, thus providing support for this wire on both ends. The arterial segment was mounted on the two support wires using the technique described by Hooker et al. (1977) in which the cannula and artery are stipped over the wires and the cannula is then carefully removed. An initial tension of 2 g was applied to the tissue; as the arterial segment relaxed, tension was increased until it remained constant at 1.5 g. The tissue was superfused at a rate of 5 ml/min with Krebs-Henseleit solution having the composition described above through three inflow tubes (see Fig. l). The flow in each of the minor channels was exactly one-half of the flo~, ~n the main inflow channel. This channel arrangement was used so that two drugs, e.g., an agonist and an antagonist could be administered simultaneously. Two of the superfusion chambers described above could be operated with a 6-channel Brinkmann peristaltic pump.

J. P. Hieble and R. O. Pendieton: Pre- and Postjunctional Alpha-Agonist Activity of Imidazolidines Drugs were administered in increasing concentration, the concentration being increased when an equilibrium response was produced or after 5 min if no response was produced. After an initial increase in sensitivity during the first hour after mounting the tissue, the sensitivity of the ear artery segment to the constrictor action of ~adrenoceptor agonists remained constant for an extended period (greater than 12 h).

Isolated SupeJfused Guinea Pig Left Atrium. Guinea pigs weighing from 300 - 600 g were injected with sodium pentobarbital, 60 mg/kg intraperitoneally. As soon as the animal was anesthetized, the heart was quickly removed and placed in Krebs-Henseleit solution which had been cooled to 10~ and was continuously gassed with 95 % Oz, 5 % CO 2. The left atrium was removed, dissected free from fat and extraneous tissue and mounted in a superfusion chamber in the following manner. A length of thread was tied to the apex of the atrium; the tissue was then impaled on an electrode constructed of 0.5 mm diameter tungsten wire. The electrode assembly with tissue attached was mounted in a 2 ml superfusion chamber similar to that used for the ear artery segment. The apex of the atrium was tied to a forcedisplacernent transducer (Statham G-10 B). The initial tension was set at 1 g and a diastolic tension of 1 g was maintained throughout the experiment. The atrium was superfused at 5 ml/min as described above for the ear artery segment; to increase the longevity of the preparation the solution was maintained at 33~ The atrium was paced by rectangular pulses of I ms duration and supramaximal voltage ( 1 0 - 2 5 volts) delivered from an American Electronics Laboratories Model 751-B stimulator through the impaling electrode and a second electrode at the rear of the chamber, The atrium was paced at 60 pulses/min. At regular intervals ( 4 - 6 min) the sympathetic nerves were excited by field stimulation using a 300 500 ms train of high voltage ( 8 0 - 1 0 0 volt), 0.7 ms pulses at 1 0 15 Hz delivered from an American Electronics Laboratories Model 104A stimulator through a separate set of tungsten electrodes located on either side of the tissue bath. It has been reported (Blinks, 1966) that field stimulation of guinea pig atria produces excitation of both sympathetic and parasympathetic nerves. In the initial experiments with the atrium, atropine (10 7 M) was used to block the effects of parasympathetic nerve activation. It was later found that omission of the atropine had little or no effect on the response of the atrium to field stimulation using the stimulus parameters described above. All of the data reported in this paper were obtained in the absence of atropine. As in the perfused ear artery, drugs were administered immediately following a stimulation, and drug concentration increased following the next stimulation.

Miscellaneous. All drug concentrations are expressed as the molar concentrations attained at the tissue after mixing of the flows from the major and minor channels. Each variability term refers to the standard error of the mean. Clonidine was obtained fiom Boehringer-Ingelheim Ltd. other imidazolidines were prepared at Smith Kline & French Laboratories (Jen et al., 1975). Haloperidol was obtained from McNeil Laboratories, cocaine hydrochloride from Merck Sharp and Dohme and phentolamine methanesnlfonate and reserpine from Ciba Pharmaceutical Company.

All

Results

When increasing concentrations of clonidine were administered to the isolated perfused rabbit ear artery two actions were seen. These were inhibition of the constrictor response to nerve stimulation, followed by vasoconstriction at higher concentrations (Fig. 2).

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Fig. 3. Concentration-effect curve for clonidine as an inhibitor of neurotransmission in the rabbit ear artery. Experimental conditions as described in Fig. 2

Clonidine produced a concentration-related inhibition of the constrictor response of the ear artery to brief, intermittent stimulation of the sympathetic nerves (Fig. 3). The Ecs0 for this inhibitory effect was 1.8 • 10-8 M. The inhibitory potency of clonidine was unaffected by cocaine at a concentration of 3 • 10- ~ M; nevertheless, this concentration of cocaine was routinely present in all experiments with the ear artery to block neuronal uptake, since cocaine increased both the magnitude and the uniformity of the responses of the artery to field stimulation of the sympathetic nerves. Clonidine's inhibitory effect on neurotransmission was not the result of a postjunctional action such as c~adrenergic blockade, since clonidine did not reduce the constrictor response of the artery to intraluminal administration of norepinephrine (Fig. 4).

220

Naunyn-Schmiedeberg's Arch. Pharmacol. 309 (1979)

The effect of ~-adrenergic antagonists on clonidineinduced inhibition of neurotransmission could not be determined in the rabbit ear artery, since the effector response to nerve stimulation was abolished by eblockade. Clonidine was also found to be an effective inhibitor of neurotransmission in the isolated superfused guinea pig atrium, producing a concentrationrelated inhibition of the response to sympathetic nerve stimulation (Fig. 5). The Ecso for clonidine in this tissue (2.3 • 10- s M) was essentially equal to the Ecs0 determined in the ear artery. As in the ear artery, the atrial experiments were carried out in the presence of 3 x 1 0 - 6 M cocaine to block neuronal uptake. This concentration of cocaine produced a significant potentiation (> 50 % in most preparations) of the response of the atrium to field stimulation of the sympathetic nerves. Since the effector response in the atrium results from stimulation of/~-adrenoceptors, the effect of ~blockade on clonidine-induced inhibition could be determined. Figure 6 shows that phentolamine competitively antagonized the inhibitory effect of clonidine on neurotransmission in the atrium. Clonidine-induced vasoconstriction in the ear artery was not attenuated by either cocaine (3 • i0-6 M) or by treatment of the rabbits with a dose of reserpine which causes almost complete depletion of neuronal

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Fig. 4. Effect of clonidine on the response of the ear artery to intraluminat norepinephrine infusion. Infusion of 3 x 10-7 M norepinephrine for 5 s at 6 min intervals. Experiment carried out in the presence of cocaine (3 • 10 -6 M). Dotted line represents control response (53 + 5 m m Hg)

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norepinephrine storage (5 mg/kg, i.v.; Shore et al., 1957) administered 18 h prior to sacrifice and could be blocked by a low concentration (3 x 10 -8 M) ofphentolamine (data not shown). This postjunctional effect could not be measured quantitatively in a perfused vessel, since any near-maximal constriction would irreversibly damage the vessel due to sustained high intraluminal pressures. Therefore, a preparation was used in which circular smooth muscle tension could be measured directly in a superfused segment of the rabbit ear artery. Using this preparation, a concentrationresponse curve for clonidine as an agonist on postjunctional ~-adrenoceptors was obtained (Fig. 7). As in all experiments previously described, cocaine (3 x 10 -6 M) was employed to block neuronal uptake. Clonidine was a partial agonist on postjunctional c~receptors in the ear artery, producing a maximum response of about 60% of the maximum response produced by norepinephrine (Fig. 7). Using the perfused ear artery and superfused ear artery segment, the pre- and postjunctional c~adrenergic potencies of a series of aryliminoimidazolidines were determined. As a result of the longevity of both of the above in vitro preparations, concentrationresponse curves for several imidazolidines could be determined using one preparation. In the superfused ear artery segment, a concentration-response curve for norepinephrine was determined both before and after determination of concentration-response curves for the imidazolidines. Comparing the initial and final norepinephrine curves showed no decrease in sensitivity or maximum response of the arterial segment. Table 1 shows the E%0 for each compound on the pre- and postjunctional c~-adrenoceptors as well as the efficacy on the postjunctional receptor. Additional experiments showed that all the compounds produced vasoconstriction which was not attenuated by reserpine pretreatment and could be blocked by low concentrations ofphentolamine (data not shown). Table I also shows the ratio of post- to prejunctional E%o values, a measure of prejunctional selectivity. A selectivity ratio is not reported for the unsubstituted phenyliminoimidazolidine since an accurate prejunctional E%o could not be determined for this compound because of

Fig. 5 Effect of increasing concentrations of clonidine on the response of the superfused guinea pig atrium to sympathetic nerve stimulation. Stimulation at 15 Hz for 0.5 s at 4 rain intervals. Experiment carried out in the presence of cocaine (3 x 10 -6 M). The washout of clonidine from the atrium was considerably slower than from the ear artery (compare with Fig. 2); however, in most experiments the response finally returned to the control value

J. P. Hieble and R. G. Pendleton: Pre- and Postjunctional Alpha-Agonist Activity of Imidazolodines postjunctional ~-antagonist activity. 2-(Phenylimino) imidazolidine inhibited the constrictor response of the ear artery to norepinephrine administration at conc e n t r a t i o n s o n l y s l i g h t l y h i g h e r (Echo = 1 0 - s M ) t h a n those required to inhibit the response to nerve stimul a t i o n (Ech0 --- 1.5 • 10 - 6 M ) ; t h e r e f o r e e - a n t a g o n i s t activity undoubtedly was contributing to the inhibition of the response to nerve stimulation. None of the other iminoimidazolidines inhibited the response of the ear artery to norepinephrine administration.

221

2-(3,4-Dihydroxyphenylimino) imidazolidine (VII) was an extremely potent inhibitor ofneurotransmission (Table 1). This compound has been reported to have dopaminergic agonist properties (Struyker Boudier et al., 1975b). Since dopaminergic agonists are effective inhibitors of neurotransmission in the rabbit ear artery (Steinsland and Hieble, 1978), the effect ofhaloperidol, a dopamine receptor antagonist, on inhibition induced by VII was determined. Haloperidol (10 -8 M) did not block the inhibitory action of VII (data not shown); I00-

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Fig. 6. Concentration-effect curve for clonidine as an inhibitor of neurotransmission in the guinea pig atrium and the antagonism of this effect by phentolamine. Experimental conditions as described in Fig. 5. In the 8 experiments where the receptor dissociation constant (Ke) of phentolamine was determined, concentration-effect curves for clonidine in the presence and absence of phentolamine were determined in the same piece of tissue

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Fig. 7. Concentration-effect curves for cionidine and norepinephrine (NE) acting on postjunctional e-adrenoceptors in the superfused rabbit ear artery segment. The response was expressed as ~ of maximum response of the artery to norepinephrine (3.4 _+ 0.4 g). Experiments carried out in the presence of cocaine (3 x 10 6 M). A concentration-effect curve for norepinephrine was determined in each tissue segment used for determination of a clonidine concentration-effect curve

Table 1, Pre- and Postjunctional c~-adrenergic activity of aryliminoimidazolidines

R

Compound

[ [[ III IV V VI VII

R

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Efficacy

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>1.5• -6 7.8 • 10 -8 1.8 x 10 -8 1.5x 10 8 5.1 x 10 8 5.0x10-* 3.6• 10 -9

1.0• 4 3.1 x 10 .6 2.9 x 10 -7 2.5x10 7 1.0x 10 s 6.5x10 -7 1.9x10 -6

u

0.6 0.6 0.65 0.8 0.6 0.8 0.9

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40 16 17 196 13 528

N E = 1.0 u Ratio could not be determined; see results Ecso values calculated from averaged concentration-effect curves (see Fig. 3 and 7). The number of experiments included in each averaged curve 9 is shown in parenthesis after the Ecso. Concentration-effect curves for several compounds were usually determined in each piece of tissue

222 this concentration of haloperidol produced a significant blockade of the inhibitory response to dopamine or apomorphine (Steinsland and Hieble, 1978). Therefore it appears that VII, like the other compounds in the serie s, inhibited neurotransmission in the ear artery by an c~-adrenergic mechanism.

Discussion

The pre- and postjunctional ~-adrenoceptor activity of clonidine observed in this study is consistent with previously reported data in the rabbit ear artery and in other tissues. The E%o for clonidine acting on postjunctional e-receptors (2.9x 10 -TM; Table1) was similar to those reported for clonidine acting on postjunctional a-receptors in the rabbit aorta (5 x 10-7 M; Constantine and McShane, 1968) and rabbit pulmonary artery (3.8• .7 M; Starke et al., 1974). Clonidine was found to be significantly more potent on the prejunctional e-receptors in the ear artery, having a selectivity ratio of 16 (see Table 1); the prejunctional selectivity of clonidine in the ear artery has been reported by Steinsland and Nelson (1975), The ability of clonidine to inhibit adrenergic neurotransmission in the guinea pig atrium is consistent with the report of Medgett et al. (1978). The fact that cocaine neither attenuated nor potentiated the e-adrenergic agonist activity of clonidine indicates that clonidine acts directly on the a-receptor and is not a substrate for neuronal uptake. The concentration of cocaine used (3x 10 -6 M) is well above that found by Iversen (1973) to produce 50% inhibition of neuronal uptake in rat heart tissue (3.8 x 10- 7 M) and close to that found by Hughes (1972) to produce maximal potentiation of norepinephrine outflow from the rabbit portal vein (6 x 10 .6 M). The fact that reserpine pretreatment did not reduce the postjunctional a-activity of clonidine provides further evidence that clonidine acts by a direct mechanism, The receptor dissociation constant (KB) for phentolamine as an antagonist of clonidine-induced inhibition of neurotransmission in the guinea pig atrium was calculated as described in the review by Furchgott (1972) (see Fig. 6). Phentolamine has been found to be either equipotent on pre- and postjunctional ereceptors (Cubeddu et al., 1974) or slightly more potent on the prejunctional receptor (Doxey et al,, 1977), Our KB for phentolamine antagonism of clonidine's inhibitory effect in the guinea pig atrium (7.8 nM) is similar to the KB reported by Doxey et al. (1977) for phentolamine as an antagonist of clonidine's inhibitory effect in the rat vas deferens (4.2 nM) and to the KB values reported for phentolamine as an antagonist of a variety of e-adrenergic responses (see review by

Naunyn-Schmiedeberg'sArch. Pharmacol.309 (1979) Furchgott, 1972). These results show that, in the atrium, the inhibitory action of clonidine results from stimulation of prejunctional e-adrenoceptors, and it is reasonable to assume that its inhibitory action in the ear artery is also due to prejunctional e-receptor stimulation. Table 1 shows that most of the imidazolidines are partial agonists on the postjunctional e-adrenoceptor. Clonidine has been reported to be a partial agonist on postjunctional e-receptors in several tissues (Constantine and McShane, 1968; Hodge and Robinson, 1972; Starke et al., 1974). According to Medgett et al. (1978), clonidine acts as a partial agonist on prejunctional as well as postjunctional a-receptors. Although all of the imidazolidines produced essentially complete inhibition of the constrictor response of the ear artery to nerve stimulation at 10 Hz for a short duration of 300- 500 ms (3 - 5 pulses) these results do not allow a conclusion on the intrinsic activity of the compounds on the prejunctional receptor, since the intrinsic activity of a prejunctional e-agonist as an inhibitor of neurotransmission is dependent on the stimulus parameters employed. Low frequency and/or short duration stimuli are inhibited more effectively than stimuli of higher frequency or longer duration (Starke et al., 1974; Steinsland and Nelson, 1975; Medgett et al., 1978). Ortho-substitution results in a dramatic potentiation of both the pre- and postjunctional e-adrenergic potency of 2-(phenylimino) imidazolidine (I). This suggests that the angle of rotation between the planes of the phenyl and imidazolidine rings is an important determinant of e-adrenergic activity. By using molecular orbital calculations, Timmermans et al. (1977b) showed that with no ring substituents the two rings are nearly coplanar (interplanar angle = 15~ and that diortho substitution with either chloro or methyl groups results in an increase in the interplanar angle to 45 - 50~ (see Fig. 8). It therefore seems that imidazolidines with larger interplanar angles must interact more favorably with c~-adrenoceptors. The 2-chloro derivative (II) has e-agonist potency greater than I but less than III or IV. In the lowest energy conformation II would be expected to have an interplanar angle of 15~ with the chlorine directed away from the imidazolidine ring (Timmermans et al., 1977b); however, the percentage of molecules of II existing in this conformer would be lower than in the case of I, since I can exist in two energetically equal conformations due to the lack of any ortho-substituent (see Fig. 8). The fact that the 2,6-dichloro (III) and 2,6-dimethyl (IV) derivatives have essentially equal activity on both e-adrenoceptors suggests that the ortho-substituent potentiates activity via a steric rather than an electronic inductive effect since chloro and methyl groups have

J. P. Hieble and R. G. Pendleton: Pre- and Postjunctional Atpha-Agonist Activity of lmidazolodines

A Fig. 8 Schematic drawings of two confbrmers of the aryliminoimidazolidine molecule in which the aromatic and imidazolidine rings are either coplanar (interplanar angle = 0~ A) or perpendicaiar (interplanar angle = 90~ B). A conformer approximating A is preferred for molecules in which X = H. Since free rotation is possible about the C1N bond axis, increasing the steric bulk of X will result in an increase in the interplanar angle of the preferred conformer

similar steric p r o p e r t i e s ( R o u o t et al., 1976) b u t opp o s i t e inductive effects (Taft et al., 1958). R i n g s u b s t i t u e n t s in the recta- a n d p a r a - p o s i t i o n s also affect ~-adrenergic p o t e n c y a n d selectivity. T h e p a r a - c h l o r o derivative (V) is m u c h m o r e selective t h a n clonidine (III) as a p r e j u n c t i o n a l a - a g o n i s t ; a d d i n g a s e c o n d chlorine in the m e t a - p o s i t i o n yields the least selective c o m p o u n d in the series. N o e x p l a n a t i o n for these effects is a p p a r e n t . In the p h e n y l e t h a n o l a m i n e s , h y d r o x y l substituents in b o t h the 3 a n d 4 p o s i t i o n s seem to be r e q u i r e d for any significant p r e j u n c t i o n a l a - a g o n i s t activity. P h e n y l e t h a n o l a m i n e s l a c k i n g one or b o t h o f these h y d r o x y l s , e.g. : p h e n y l e p h r i n e , m e t h o x a m i n e a n d syne r p h r i n e are quite selective for the p o s t j u n c t i o n a l ar e c e p t o r (Steinsland a n d Nelson, 1975; Steinsland a n d Hieble, 1976; Hieble a n d P e n d l e t o n , u n p u b l i s h e d observations). Since clonidine selectively activates prej u n c t i o n a l a-receptors, 3 , 4 - d i h y d r o x y l s u b s t i t u t i o n is o b v i o u s l y n o t r e q u i r e d for p r e j u n c t i o n a l a-activity in the i m i n o i m i d a z o l i d i n e s . H o w e v e r , V I I which has this s u b s t i t u t i o n p a t t e r n was b o t h the m o s t p o t e n t a n d m o s t selective p r e j u n c t i o n a l a-agonist, s h o w i n g t h a t 3,4d i h y d r o x y l s u b s t i t u t i o n confers p r e j u n c t i o n a l a - a g o n i s t activity in the i m i n o i m i d a z o l i d i n e s as well as in the phenytethanolamines. This s t u d y d e m o n s t r a t e s t h a t r i n g - s u b s t i t u t i o n can alter the m a g n i t u d e o f the p r e j u n c t i o n a l selectivity t h a t seems to be i n h e r e n t to the a r y l i m i n o i m i d a z o l i d i n e s . Since the r e c e p t o r on which these c o m p o u n d s act to p r o d u c e their a n t i h y p e r t e n s i v e effect m a y be similar to the p e r i p h e r a l p r e j u n c t i o n a l a-receptor, d e t e r m i n a t i o n o f p r e j u n c t i o n a l e - a g o n i s t p o t e n c y m a y be useful in p r e d i c t i n g p o t e n t i a l h y p o t e n s i v e activity, p r o v i d e d the c o m p o u n d has the p h y s i c a l p r o p e r t i e s r e q u i r e d for p e n e t r a t i o n into the central n e r v o u s system.

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Received May 2/Accepted August 10, 1979

Effects of ring substitution on the pre- and postjunctional alpha-adrenergic activity of aryliminoimidazolidines.

Naunyn-Schmiedeberg's Archivesof Naunyn-Schmiedeberg's Arch. Pharmacol. 309, 217-224 (1979) Pharmacology 9 by Springer-Verlag 1979 Effects of Ring...
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