Bidirectional allosteric effects of agonists and GTP at alpha(2A/D)-adrenoceptors

Agonists and GTP exert reciprocal effects on the stability of the G protein-coupled receptor/G protein complex, implying bidirectional control over the receptor/G protein interface. To investigate this relationship, we compared the ability of a series of hydroxyl-substituted phenethylamine and imidazoline agonists to stimulate [(35)S]guanosine 5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) binding in membranes from alpha(2A/D)-adrenergic receptor-transfected PC12 cells with the magnitude of the GTP-induced reduction in agonist affinity in [(3)H]rauwolscine-binding studies. Agents previously described as full and partial agonists in functional studies showed similar relative efficacies in promoting GTP binding (r = 0.97) as well as similar relative potencies (r = 0.94). Efficacy among agonists for promotion of [(35)S]GTPgammaS binding was closely correlated with the relative influence of GTPgammaS on agonist binding (r = 0.97), consistent with a bidirectional allosteric influence by agonists and GTP on receptor/G protein complexation. In an additional series of tolazoline derivatives, a range in efficacy from full agonism to strong inverse agonism was observed, depending on the presence or absence of hydroxyl substituents. Together these results suggest that agonist-induced repositioning of transmembrane helices via their hydroxyl interactions is a critical determinant of the stability of the receptor/G protein complex and therefore of agonist efficacy.

Depolarization of membrane potential and the smooth muscle contraction by isothiocyanatobenzyl imidazoline in guinea-pig stomach circular muscle

Isothiocyanatobenzyl imidazoline (IBI) produces characteristic slowly developing contraction of many smooth muscle preparations including the circular smooth muscle of the guinea-pig stomach. Changes in the membrane potential were recorded intracellularly, and the muscle contraction induced by IBI was investigated. IBI at 100 micromol/l slowly produced a sustained depolarization of the membrane with a maximum change of approximately 15 mV. This depolarization could not be blocked by 1-hyoscyamine, 100 nmol/l. An imidazoline analogue, oxymetazoline at 1 micromol/l, did not change the resting membrane potential as observed after IBI. Significant membrane depolarization after IBI still occurred in Ca2+-free medium. During IBI-induced depolarization, sudden reduction of Na+ to 30 mmol/l in the medium reduced the depolarization slightly. IBI-induced depolarization was additive with that produced by 20 mmol/l K+ in the medium. In the presence of tetraethylammonium chloride or levcromakalim or nifedipine, IBI continued to depolarize the membrane although functional pharmacological experiments showed that the contractile effects of IBI were significantly inhibited by 30 micromol/l levcromakalim and abolished by 100 nmol/l nifedipine. At 100 micromol/l phentolamine (reported by others as an inhibitor of ATP-sensitive potassium channels) completely blocked IBI-induced contraction. Phentolamine (30 micromol/l) blocked the contractile effects of IBI by 50%. On the other hand, S(-)-Bay K 8644, a voltage-dependent calcium channel activator, was additive with the contractile response of IBI. These results indicated that IBI produced membrane depolarization and contraction of the guinea-pig stomach circular muscle, by a mechanism not involving muscarinic receptors or alpha-adrenoceptors. Even though levcromakalim, an ATP-sensitive potassium channel opener, could not inhibit IBI-induced depolarization, the ATP-sensitive potassium channel and the voltage-dependent calcium channel may be intrinsically linked with the action of IBI.

Activation of L-type calcium channel by tolazoline derivatives: role of isothiocyanate moiety

Studies have investigated the pharmacologic mechanism of 2-(4'-isothiocyanatobenzyl) imidazoline (IBI) and analogs for interaction with imidazoline receptors (IRs), alpha-adrenergic receptors (alpha-ARs), and calcium channels in cardiovascular muscle systems. IBI differs from tolazoline by substitution of an electrophilic isothiocyanato (NCS) group. Unlike tolazoline, which is a partial alpha-AR agonist, IBI produced an irreversible, slow-onset, and sustained contraction of rat aorta with an median effective concentration (EC50) value of 5 microM, and a maximal contraction (116%) greater than that of phenylephrine (100%) and tolazoline (59%). The IBI-induced contractions were dependent on calcium channels and independent of alpha-ARs or IRs. Similarly, structure-activity relation studies in rat aortic smooth muscles on a series of synthesized IBI analogs indicated that NCS analogs, but not those without the NCS group, exhibited effects by a non-alpha-AR, non-IR, but a calcium channel-dependent mechanism. Thus the presence of an intact IBI ring in these analogs is not a requirement for these activities. Further, IBI inhibited dihydropyridine (DHP, [3H]PN 200-110 and [3H]Bay K 8644) binding to L-type calcium channels of T-tubule membranes in rabbit skeletal muscle. In contrast to nifedipine, IBI and NCS derivatives (nifedipine-NCS, naphazoline-NCS) only partially (50-88%) displaced specific binding of these radioligands. A single site of noncooperative interaction was observed for nifedipine (nH = 0.97), whereas tolazoline-NCS (IBI, nH = 1.46) and nifedipine-NCS (nH = 1.37) exhibited a positive cooperativity in binding to DHP sites. These receptor-binding data indicate that NCS derivatives bind to L-type calcium channels and interact allosterically with DHP-binding sites. Direct binding of the NCS group to specific nucleophilic protein sites of the calcium channel may be responsible for its activation and the subsequent contractile effects of IBI.

Isothiocyanatobenzyl imidazoline is an alkylating agent for I2-imidazoline binding sites in rat and rabbit tissues

Isothiocyanatobenzyl imidazoline (IBI), the 4'-NCS analogue of tolazoline, has been used to alkylate several receptor sites in rabbit iris muscles. Because of the high affinity of tolazoline for the I2-imidazoline binding sites (Ki = 16-130 nM), this study was designed to assess whether IBI is also an alkylating agent for these sites. In competition studies, IBI displayed moderate affinity (Ki approximately 2-3 microM) against I2A-imidazoline sites in the rabbit cerebral cortex and I2B-imidazoline sites in the rat cerebral cortex labelled by [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI). However, preincubation (30 min at 25 degrees C) of rat cortical and liver membranes with IBI (10(-7) M to 10(-3) M), followed by extensive washing, markedly decreased (17% to 96%) the specific binding of [3H]2-BFI to I2B-imidazoline sites. IBI (10(-5) M to 10(-3) M) also bound irreversibly to I2A-imidazoline sites in rabbit cerebral cortex but with a lesser efficacy (27% to 83% reduction of [3H]2-BFI binding). Saturation curves of [3H]2-BFI binding in the rat cerebral cortex indicated that preincubation with 10(-6) M IBI reduced the total density (Bmax) without affecting the affinity (Kd) of I2B-imidazoline sites for IBI. Acute treatments (6 h) with IBI (10 and 30 mg/kg, i.p.) also dose-dependently reduced (26% and 41%; respectively) the total density of I2B-imidazoline sites. These results demonstrate the ability of IBI to alkylate I2-imidazoline binding sites in vitro and in vivo and provide evidence for the use of IBI as a new tool for the study of the functional implications of imidazoline binding sites.

Irreversible agonist and antagonist properties of isothiocyanatobenzyl imidazoline in albino rabbit iris muscles

The alkylating agent isothiocyanatobenzyl imidazoline (IBI) was synthesized to investigate the unique receptor interacting properties of imidazolines. On the isolated rabbit iris sphincter, IBI produced concentration-dependent responses with an EC50 of 18 mumol/1, and at the highest concentration tested the maximum contraction of the tissue was 50% of the carbachol maximum. At equiactive concentrations with the similar washing procedure, the total duration of responses to IBI and carbachol was 24 and 3 min, respectively. After repeated washing, the sphineter relaxes to the control baseline of tone but, after reexposure to IBI for 6 h, failed to contract, indicating that desensitization or irreversible block has developed. Unlike with carbachol, the sphincter contraction to IBI was not affected by atropine 1 mumol/1, indomethacin 1 mumol/1, verapamil 10 mumol/1, or nifedipine 10 mumol/1. At a higher concentration of nifedipine and papaverine 100 mumol/1, the response to IBI was blocked. Furthermore, the contractile response to IBI was abolished by Ca++ removal from the medium. Under similar conditions, 26 +/- 8% of the maximum response to carbachol was preserved. Thus influx of extracellular as well as rise in intracellular Ca++ appears vital for the contractile response to IBI. IBI did not contract the iris dilator, but shifted the concentration-response curve to the alpha-adrenoceptor activator, phenylephrine, to the right with a reduction in the maximum response. The tissue failed to regain the sensitivity to phenylephrine after 6 h of repeated washing. Phentolamine and nifedipine provided a small but significant protection of the response to phenylephrine against the irreversible block by IBI. Based on chemical and pharmacological properties of IBI, it is concluded that the molecule acts in the rabbit as an irreversible agonist on unidentified receptors of the iris sphineter and an irreversible antagonist of multiple receptors on the iris dilator. These molecular properties of IBI are clearly different from that of the parent imidazoline molecule tolazoline.

Synthesis and biological activities of a new set of irreversibly acting 2-(4'-isothiocyanatobenzyl)imidazoline analogs in rat thoracic aorta

IBI [2-(4'-isothiocyanatobenzyl)imidazoline, 3] has been shown to cause slow-onset, long-lasting contractions of rat thoracic aorta through a non-alpha-adrenergic receptor (non-alpha-AR) mediated mechanism. A series of IBI-related anlogs 7-14 and 16 was prepared to determine the structural requirements for the interaction with non-alpha-AR in rat aortic strips. All IBI analogs produced concentration-dependent contractile responses on rat thoracic aorta. Whereas the actions of analogs 7, 14, and 16 were partly mediated by alpha-ARs, the stimulatory activities of the remaining IBI analogs were unaffected by phenoxybenzamine pretreatment, suggesting that a non-alpha-adrenergic mechanism is involved. We have shown that the contractile actions of IBI and analogs 10-13 were not blocked with the imidazoline/guanidinium receptive site (IGRS) ligands idazoxan, cirazoline, or clonidine. However, the calcium channel blockers nifedipine or verapamil shifted the concentration-response curve of IBI and its analogs 10-13 to the right and reduced the maximal contractile responses. The action of IBI on rat thoracic aorta was reduced by the omission of extracellular calcium in the medium. These results suggest that the stimulatory activities of IBI and analogs 10-13 are not related to the activation of alpha-AR or IGRS receptors and are likely coupled to the voltage-dependent Ca2+ channels.

Diversity of the pharmacological actions of some tolazoline analogues in human platelets and rat aorta

Tolazoline and two 4'-substituted benzyl analogues, 2-(4'-aminobenzyl) imidazoline (ABI) and 2-(4'-isothiocyanatobenzyl)imidazoline (IBI) were synthesized and evaluated for adrenoceptor activity in human platelets (alpha 2) and rat aorta (alpha 1), respectively. IBI was prepared as an affinity label for alpha-adrenoceptors and compared with chloroethylclonidine. Tolazoline, IBI, ABI and chloroethylclonidine inhibited the primary and secondary waves of epinephrine-induced human platelet aggregation. In aspirin treated platelets, primary wave aggregatory responses to epinephrine were blocked in a competitive manner by tolazoline, ABI. IBI and chloroethylclonidine giving pA2 values of 6.33, 6.12, 4.71 and 5.70, respectively. Only IBI blocked the aggregation responses to ADP (secondary wave only) arachidonic acid and U46619 (a thromboxane A2 agonist). Arachidonic acid-induced serotonin release and malondialdehyde formation and thrombin-induced release of [3H]arachidonic acid from membrane phospholipids were also blocked by IBI. These data indicate that IBI blocks arachidonic acid release, prostaglandin biosynthesis and the action of thromboxane A2. One hour exposure of aspirin treated platelets with IBI abolished inhibitory effects against epinephrine induced aggregation. In contrast to human platelets, both ABI and IBI produced contractions of rat aorta; however, only the responses to ABI were blocked in a competitive manner by the alpha-antagonists, phentolamine, prazosin, and SKF 104078. Moreover, idazoxan blocked the stimulatory actions of IBI, cirazoline and phenylephrine on rat aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Paradoxical effects of isothiocyanate analog of tolazoline on rat aorta and human platelets

The isothiocyanate analog (IBI) of tolazoline produced contraction of the rat aortic strip, with an ED50 value of 1.63 x 10(-5) M. The maximum contraction of the analog was nearly equal to that of tolazoline or phenylephrine. At 27 degrees C the tissue reactivity of phenylephrine and IBI was similar. When compared at equiactive concentrations, the total duration of contraction of IBI was three times longer than that of tolazoline. Thus, the longer duration of action of IBI may be attributed to the S=C=N group substitution of the molecule. IBI at 10(-6) M shifted the dose-response curve of phenylephrine to the right with reduction in maxima. Phentolamine and other alpha 1 or alpha 2 adrenoceptor blockers failed to block the responses of IBI in aorta, whereas verapamil or nifedipine blocked the response significantly. It appears that IBI is acting through calcium-channel-sensitive or calcium-receptor-related mechanism(s). In aspirin-treated platelets from human plasma, a distinct phase of aggregation induced by epinephrine can be blocked by IBI with KB of 2 x 10(-5) M. This indicates a small but selective alpha 2 related action of IBI. The aggregation induced by ADP or second component of aggregation induced by epinephrine were also blocked by IBI at concentrations comparable to that of the alpha 2-adrenoceptor-mediated response. This indicates a lack of specificity of IBI in differentiating various phases of aggregation. Therefore, as compared to tolazoline, IBI presents an interesting paradox in its interaction with various receptors or mechanisms in the vascular tissue and platelets.

Effects of benzylic hydroxyl substitution on the alpha-adrenoceptor blocking activity of tolazoline

The R(-)- and S(+)-enantiomers of alpha-hydroxytolazoline, the benzylic hydroxy-substituted derivative of the alpha-adrenoceptor antagonist, tolazoline, were evaluated at alpha 1- and alpha 2-adrenoceptors in canine saphenous vein. Benzylic hydroxyl substitution of tolazoline in either the R(-) or S(+) configuration significantly decreased affinity at both alpha 1- and alpha 2-adrenoceptors. Differences in affinity between the R(-)- and S(+)-enantiomers were small, which is characteristic of imidazolines, but in marked contrast to phenethylamines where enantiomeric differences are large. The rank order of affinities at alpha 1- and alpha 2-adrenoceptors is tolazoline greater than S(+)-alpha-hydroxytolazoline = R(-)-alpha-hydroxytolazoline, which is different from that order predicted by the Easson-Stedman hypothesis (i.e., R(-) greater than S(+) = desoxy). The findings support our contention that phenethylamines and imidazolines interact differently with alpha-adrenoceptors.

Alpha-1 adrenergic coupling events induced by full and partial agonists in rabbit aorta

Differences in the ability of full vs. partial agonists to initiate alpha-1 adrenergic receptor-mediated coupling events were studied in isolated segments of rabbit aorta. Mono- and dimethoxysubstituted tolazolines produced contractile responses which, at their maximum, were 27 to 100% of the response produced by the full agonist phenylephrine. In addition to differences in maximum response, contraction kinetics varied between full and partial agonists. Responses to partial agonists displayed a slower approach to peak tension and loss of the rapid phase of tension development which is associated with release of intracellular Ca++. Among the tolazoline series 3,5 dimethoxy-, 3 methoxy-, and 2 methoxy derivatives were compared further with phenylephrine for their ability to cause phosphatidylinositol cycle turnover, intracellular Ca++ release and Ca++ influx. For each of these coupling events, a rank of phenylephrine greater than or equal to 3, 5 greater than 3 greater than 2 was observed. However, a higher percentage of Ca++ influx vs. Ca++ release was observed for the partial agonists, suggesting that their contractile responses may be more dependent upon extracellular Ca++ than intracellular Ca++. Our results indicate that partial agonists initiate the same coupling events as full agonists; however, the relative proportion of Ca++ release and influx may be different for partial agonists because of the reduced rate of second messenger production.

Interaction of enantiomers of hydroxy tolazoline with adrenoceptors

Adrenoceptor-mediated effects of the enantiomers of hydroxytolazoline and tolazoline (i.e., desoxy derivative) have been investigated in vitro. The enantiomers and tolazoline were partial agonists of postjunctional alpha 1-adrenoceptors in rat aorta. The rank order of potencies of the compounds in this system was as follows: tolazoline greater than R(-)-hydroxytolazoline greater than S(+)-hydroxytolazoline. The efficacy of R(-)-hydroxytolazoline was higher than that of tolazoline, though its affinity for the receptor was less. The KB values for prazosin against these agonists were nearly equal, which indicated that these imidazolines activate the same type of receptor in rat aorta. The S(+)-isomer, however, produced both a prazosin sensitive and resistant component of the response. The interactions of the derivatives with presynaptic alpha 2-adrenoceptors were studied in field-stimulated myenteric plexus-longitudinal muscle of guinea-pig ileum. These substances were blockers at presynaptic alpha 2-adrenoceptors. Based on KB values, the order of affinity in this system was as follows: tolazoline greater than S(+)-isomer greater than or equal to R(-)-isomer. beta-Adrenoceptor mediated activity was quantitated in guinea-pig and rat atria. R(-)-hydroxytolazoline lacked chronotropic effects either in guinea pig or rat atria. At 3 X 10(-4) M the isomer did not antagonize the effect of isoproterenol in the atria. On the other hand, S(+)-hydroxytolazoline produced a variable chronotropic effect in guinea-pig atria, but failed to show any significant activity in rat atria. Thus, the beta-adrenoceptor mediated action appears to be insignificant. Steric aspects of alpha-adrenoceptor mediated events are discussed.

Alpha-adrenoceptor-mediated actions of optical isomers and desoxy analogs of catecholimidazoline and norepinephrine in human platelets: in vitro

Adrenoceptor-mediated effects of the enantiomers of optically active imidazoline, 2-(3,4,alpha-trihydroxybenzyl imidazoline (catecholimidazoline; CI), and norepinephrine (NE), and the corresponding desoxy derivatives, 2-(3,4-dihydroxybenzyl)imidazoline (desoxy-CI) and dopamine, have been investigated in human platelets. Differences between responsiveness of platelets from donor to donor were observed in the presence of the isomers and the desoxy analogs of NE and CI. In certain platelet preparations, all compounds gave concentration-dependent stimulatory responses, whereas in other preparations, only R(-)-NE and R(-)-CI were inducers of platelet aggregation and serotonin release. The rank order of stimulatory potencies (EC50; microM) for CI and NE was R(-)-NE (1.3) greater than R(-)-CI (7.5) greater than S(+)-NE (19) = S(+)-CI (20) = dopamine (22) greater than desoxy-CI (greater than 35). Unlike R(-)-CI, both S(+)-CI and desoxy-CI were either agonists or antagonists of human platelet function. In preparations unresponsive to the S(+)-isomers or desoxy analogs, the potencies (EC50) for R(-)-NE and R(-)-CI were 1.7 and 7.7 microM respectively. The corresponding inactive CIs [S(+)-CI and desoxy-CI] were inhibitors of both primary and secondary phases of aggregation and serotonin release responses to R(-)-CI and R(-)-NE, respectively. In contrast, the aggregation responses to ADP, arachidonic acid or U46619 were not blocked by S(+)-CI or desoxy CI. The rank order of inhibitory potencies for selected alpha-adrenoceptor agents against R(-)-NE was phentolamine greater than clonidine greater than desoxy-CI greater than S(+)-CI. Moreover, the relative inhibitory potencies of phentolamine and desoxy-CI against aggregation responses to R(-)-NE and R(-)-CI, respectively, were the same. These results suggest that the enantiomers and desoxy derivatives of CI and NE mediate their effects in human platelets by an interaction with alpha-adrenoceptors; catecholamines and imidazolines interact with the same alpha-adrenoceptors in human platelets; the stereochemical requirements of both chemical classes for stimulatory activity in human platelets adhere to the Easson-Stedman hypothesis in this alpha 2-adrenoceptor system; and desoxy-CI possessed the highest potency as an antagonist of alpha-adrenoceptors which suggests that the hydroxy group at the benzylic carbon atom of these imidazolines may not be required for maximal binding to adrenoceptors in platelets.

Effects of para-substitution on tissue levels and alpha-adrenoceptor antagonist properties of tolazoline

Tolazoline and a series of para-substituted analogues were examined in mice to determine the effects of para substitution on alpha-adrenoceptor antagonist potency and tissue disposition. alpha-Adrenoceptor antagonism was measured as abilities to attenuate the hypothermic or sedative actions of clonidine. In general, para substitution by electron-withdrawing metabolically stable groups (Cl, F) resulted in increased or unchanged brain levels of drug relative to tolazoline. The para substitution by electron-donating metabolically labile groups (CH3, OCH3) leads to reduced brain levels. Effects on alpha-adrenoceptor antagonist properties did not follow a similar pattern. Thus, increased or decreased antagonism of clonidine effects by para-chloro- or para-methyl-tolazoline, respectively, could be attributed solely to increased or decreased brain levels of drug. para-Fluorotolazoline did not antagonize clonidine but was present in brain at levels equivalent to those of tolazoline. para-Methoxytolazoline on the other hand could not be detected in any tissue but antagonised hypothermia more readily than sedation. These data indicate that the factors governing the disposition or alpha-adrenoceptor antagonist properties of tolazoline analogues are different and independent of each other.

Interactions of dimethoxy-substituted tolazoline derivatives with alpha 1- and alpha 2-adrenoreceptors in vitro

The activities of a series of dimethoxy-substituted tolazoline derivatives were investigated at alpha 1-adrenoreceptors in guinea-pig aorta and alpha 2-adrenoreceptors in field-stimulated guinea-pig ileum. Radioligand binding studies to alpha 1- and alpha 2-adrenoreceptors in rat cerebral cortex were also performed to support the pharmacological findings. The 2,5- and 3,5-dimethoxy-substituted tolazoline derivatives were potent full agonists at alpha 1-adrenoreceptors in guinea-pig aorta, whereas the 2,3- and 3,4-dimethoxy-substituted tolazoline derivatives were inactive as alpha 1-adrenoreceptor agonists. 2,3-Dimethoxytolazoline was a partial agonist at alpha 2-adrenoreceptors in field-stimulated guinea-pig ileum. The intrinsic activity of 2,3-dimethoxytolazoline was similar to that of clonidine, but less than that of UK-14,304. The -log ED50 of 2,3-dimethoxytolazoline (7.66) was only 3- to 5-fold lower than that of clonidine or UK-14,304, indicating that this compound has relatively high potency as an alpha 2-adrenoreceptor agonist. The 2,5-, 3,5- and 3,4-dimethoxy-substituted tolazoline derivatives were inactive as alpha 2-adrenoreceptor agonists. 3,4-Dimethoxytolazoline was a moderately potent and selective alpha 2-adrenoreceptor antagonist in field-stimulated guinea-pig ileum. The results indicate that the dimethoxy-substituted tolazoline derivatives possess different pharmacological activities and selectivities at alpha 1- and alpha 2-adrenoreceptors, depending upon the positions of the dimethoxy substitutions. The 2,5- and 3,5-dimethoxytolazoline derivatives are potent and selective alpha 1-adrenoreceptor agonists, whereas 2,3-dimethoxytolazoline is a potent and selective alpha 2-adrenoreceptor agonist. 3,4-dimethoxytolazoline is a moderately potent and selective alpha 2-adrenoreceptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Optically active catecholimidazolines: a study of steric interactions at alpha-adrenoreceptors

The optical isomers and deoxy form of 2-(3,4, alpha-trihydroxybenzyl)imidazoline hydrochloride were examined for their alpha-adrenergic activity on rat aorta. The rank order of stimulant activity was deoxy (2) congruent to (R)-(-)-1 greater than (S)-(+)-1. This is in contrast to catecholamines in which the order of activity is (R)-(-)-epinephrine greater than (S)-(+)-epinephrine = epinine (deoxyepinephrine). The relative order of potency for the isomers of 2-(3,4, alpha-trihydroxybenzyl)imidazoline is different than that predicted by the Easson--Stedman theory for stereoisomers of catecholamines. Also, substitution of the deoxy compound 2 with substituents, methyl or benzyl, in the 4-position lowers the alpha-adrenergic agonist activity, and differences observed between optical isomers were small.

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.

Structure-activity relationships of clonidine- and tolazoline-like compounds at histamine and alpha-adrenoceptor sites

1 Thirty clonidine- and tolazoline-like compounds with differing phenyl ring substituents were tested for agonistic actions at histamine H1-receptors (guinea-pig ileum), histamine H2-receptors (guinea-pig driven right ventricular strips), post-junctional alpha-adrenoceptors (rat desheathed was deferens) and pre-junctional alpha-adrenoceptors (inhibition of sympathetic stimulation in guinea-pig driven left atria). 2 All compounds were inactive at histamine H1-receptors, while 21 of the 30 compounds displayed varying stimulant activity at H2-receptors. 3 At post-junctional alpha-receptors all 30 compounds produced stimulant actions, whereas at prejunctional alpha-receptors the compounds displayed either agonistic or antagonistic actions. 4 Thus structure-activity-relationships (SAR) could only be validated for histamine H2- and post-junctional alpha-receptor effects. These studies show that the most potent compounds are those with 2,6-phenyl substituents in which rotation is restricted so that the two rings are aplanar. Electronic effects of the substituents have a greater influence on activity at H2- than at alpha-receptors. 5 The major difference in SAR involves the influence of substituents in the 3, 4 or 5 positions on the phenyl ring. The presence of these substituents abolish significant activity at H2-receptors, while alpha-receptor stimulant activity is retained.