The role of DA1- and DA2-receptors in the control of blood pressure

1. Endogenous dopamine, acting through specific receptors on blood vessels, renal tubules and adrenal cortical cells, may play a role in the development and maintenance of the hypertensive state. 2. In hypertensive patients, activation of the DA1-receptor with intravenous fenoldopam produces a rapid and sustained reduction in blood pressure, in contrast to the tachyphylaxis seen in some rat models. 3. Activation of the DA2-receptor also represents a viable antihypertensive approach, based on the chronic efficacy of hydergine. However, none of the recently developed selective DA2-receptor agonists has been shown to reduce blood pressure in man. 4. Since animal experiments suggest qualitatively different antihypertensive profiles for quinpirole and SK&F 101468, both presumed to be selective agonists at the DA2-receptor, it may be possible to design new DA2-receptor agonists more effective as antihypertensive drugs. 5. Dopamine receptors at different sites, even if of the same subtype, can be differentially regulated; this may be a consequence of activation of multiple second messenger systems by receptor occupation.

SK&F 89124, a potent and selective agonist at prejunctional dopamine receptors

SK&F 89124 (4-[2-(N,N-di-n-propylamino)ethyl]-7-hydroxy-2(3H) indolone) can be considered as a derivative of N,N-di-n-propyldopamine (DPDA) in which the meta-hydroxyl is replaced by a cyclic amide function. SK&F 89124 is at least one order of magnitude more potent than DPDA as an agonist at peripheral inhibitory prejunctional dopamine receptors (DA2 receptors) in the isolated perfused rabbit ear artery. A potent agonist action of SK&F 89124 at the DA2 receptor can also be demonstrated by inhibition of radioactive overflow from prelabelled canine coronary artery or saphenous vein, and in the anesthetized dog as an inhibition of the tachycardia induced by cardioaccelerator nerve stimulation or the increase in hind-limb perfusion pressure induced by stimulation of the lumbar sympathetic chain. SK&F 89124 is a potent inhibitor of the binding of [3H]spiroperidol to D2 receptors in bovine pituitary homogenates. High concentrations of SK&F 89124 do not activate the adenylate cyclase D1 receptor in rat caudate homogenates, nor produce activation of alpha 2-adrenoceptors or H2-histamine receptors in the guinea pig atrium. Although some alpha 1-adrenoceptor mediated vasoconstriction is produced in the rabbit ear artery and rabbit aorta, the concentrations required are several orders of magnitude higher than those active at the DA2 receptor. From these data it is evident that this structural modification can increase both the potency and selectivity of DPDA as a DA2 receptor agonist. The potency and selectivity of SK&F 89124 make this agent a useful tool for determination of the functional role of the DA2 receptor.

Effects of neuropeptide Y on the response of isolated blood vessels to norepinephrine and sympathetic field stimulation

Although neuropeptide Y (NPY) is not a potent vasoconstrictor in many vascular beds, nanomolar concentrations of this peptide can potentiate the response of isolated blood vessels to sympathetic stimulation or exogenous vasoconstrictors. In the isolated perfused rabbit ear artery, NPY produces a concentration-related potentiation of the response to field stimulation. The potency of NPY in inducing potentiation of nerve-mediated vasoconstriction was not dependent on whether the NPY was administered intra- or extraluminally. In these perfused vessels the endothelium was intact. Similar potentiation of the response to field stimulation is also observed in superfused ring segments of rabbit ear artery, provided that the endothelium was not damaged mechanically. The response to field stimulation in the rabbit ear artery is mediated by the alpha-1 adrenoceptor; in contrast, stimulation induced contraction in the canine saphenous vein results from the activation of vascular alpha-2-adrenoceptors. Despite this difference, the effect of NPY in ring segments of canine saphenous vein was similar to that observed in the ear artery. NPY also produced an endothelium-dependent potentiation of the constrictor response to exogenous norepinephrine in superfused segments of both ear artery and saphenous vein. Although the NPY-induced potentiation of stimulation-induced vasoconstriction was not inhibited by indomethacin, suggesting that an arachidonic acid metabolite is not involved, the release of another endothelial-derived vasoconstrictor substance, as yet unidentified, by NPY may explain its ability to potentiate the responses to both nerve stimulation and a variety of exogenous vasoconstrictors.

Stimulation of postjunctional alpha-1 and alpha-2 adrenoreceptors increases proximal urethral perfusion pressure in the pithed rat

1. In pithed rats, the effects of postjunctional alpha-1 and alpha-2 adrenoreceptor stimulation on prostatomembranous urethral perfusion pressure (UPP) were characterized by using selective adrenoreceptor agonists and antagonists. 2. Dose-dependent increases in UPP were elicited by the intravenous administration of selective alpha-1 and alpha-2 adrenoreceptor agonists, phenylephrine (1.0-30.0 micrograms kg-1) and BHT-933 (10-3000 micrograms kg-1), respectively. 3. The effects of phenylephrine on UPP were antagonized, in a dose-related manner, by pretreatment with prazosin (0.1 and 0.5 mg kg-1, iv), a selective alpha-1 adrenoreceptor antagonist. In contrast, pretreatment with rauwolscine 0.1 and 0.5 mg kg-1, iv), a selective alpha-2 adrenoreceptor antagonist, did not alter phenylephrine-induced increases in UPP. 4. BHT-933 elicited increases in UPP that were antagonized, in a dose-related manner, by pretreatment with rauwolscine (0.1 and 0.5 mg kg-1, iv). Neither dose of prazosin altered the effects of BHT-933 on UPP. 5. These results demonstrate increases in the resistance to flow (constriction) through the proximal urethra following selective stimulation of postjunctional alpha-1 and alpha-2 urethral adrenoreceptors. 6. The role of urethral alpha-adrenoreceptor subtypes in the neurohumoral control of the urethra remains to be determined.

The interaction of the enantiomers of carvedilol with alpha 1- and beta 1-adrenoceptors

The stereoselectivity of carvedilol, a novel beta-adrenoceptor antagonist and vasodilator with one asymmetric carbon atom, was examined at alpha 1- and beta 1-adrenoceptors in vitro and in vivo. (-)-(S)-Carvedilol is a potent, competitive antagonist of the beta 1-adrenoceptor-mediated positive chronotropic response to isoproterenol in guinea pig atrium, with a dissociation constant (KB) of 0.4 nM. (+)-(R)-Carvedilol was more than 100-fold less potent than the (-)-S-enantiomer as an antagonist of beta 1-andrenoceptors, having a KB of approximately 45 nM. Consistent with these findings (-)-(S)-carvedilol (0.1 mg/kg, i.v.) produced a 25-fold rightward shift in the beta 1-adrenoceptor-mediated positive chronotropic response to isoproterenol in pithed rats, whereas the (+)-R-enantiomer had no beta 1-adrenoceptor blocking activity in vivo at this dose. In contrast to the marked degree of stereoselectivity observed at beta 1-adrenoceptors, both (-)-(S)- and (+)-(R)-carvedilol produced equal antagonism of the alpha 1-adrenoceptor-mediated vasoconstrictor response to norepinephrine in rabbit aorta, with KB values of 14 and 16 nM, respectively. Furthermore, in the pithed rat, the alpha 1-adrenoceptor-mediated pressor dose-response curve to cirazoline was shifted approximately 6-fold to the right by both the (+)-R- and (-)-S-enantiomers of carvedilol at a dose of 1 mg/kg, i.v. In anesthetized spontaneously hypertensive rats, (-)-(S)-carvedilol was 6-fold more potent as an antihypertensive than (+)-(R)-carvedilol.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro pharmacologic profile of the novel beta-adrenoceptor antagonist and vasodilator, carvedilol

The pharmacologic profile of the novel beta-adrenoceptor antagonist/vasodilator, carvedilol, has been investigated in vitro. Carvedilol produced competitive antagonism of the beta 1-adrenoceptor mediated positive chronotropic response to isoproterenol in guinea pig atria, and the beta 2-adrenoceptor mediated relaxation to isoproterenol in carbachol (1 mumol/l) precontracted guinea pig trachea, with a dissociation constant (KB) for beta 1-adrenoceptors of 0.8 nmol/l and beta 2-adrenoceptors of 1.3 nmol/l. At slightly higher concentrations, carvedilol produced competitive inhibition of the alpha 1-adrenoceptor mediated contractile response to norepinephrine in rabbit aorta with a KB of 11 nmol/l. Carvedilol had no significant effect on the contractile response to angiotensin II in rabbit aorta at concentrations up to 10 mumol/l, thus demonstrating the lack of nonspecific vasodilator actions in arteries. In canine saphenous vein, carvedilol produced noncompetitive blockade of alpha 2-adrenoceptor mediated vasoconstriction, indicative of some additional activity. In estrogen-primed rat uterus precontracted by depolarization with KCl (70 mmol/l), carvedilol produced concentration-dependent relaxation (IC50 of 7.6 mumol/l), consistent with the notion that carvedilol may be a calcium channel antagonist. Support for this hypothesis was obtained in KCl (70 mmol/l) depolarized rabbit aorta where carvediol (10 mumol/l) produced a 10-fold parallel rightward shift in the concentration-response curve to calcium chloride. These studies demonstrate that carvedilol is a potent beta 1-, beta 2- and alpha 1-adrenoceptor antagonist, and a moderately potent calcium channel antagonist. These multiple activities of carvedilol may contribute to the antihypertensive activity of the compound.

The role of neuropeptide Y in vascular sympathetic neurotransmission may be enhanced in hypertension

The effect of neuropeptide Y on the contractile response to field stimulation was examined in isolated blood vessels. Exogenous neuropeptide Y at a concentration of 100 nmol/l significantly potentiated the response to field stimulation in rabbit ear artery and canine saphenous vein. Administration of neuropeptide Y antiserum to tissues not previously exposed to neuropeptide Y significantly reduced the response to field stimulation; greater effects were observed in the rabbit ear artery than in the canine saphenous vein. Furthermore, this antiserum depressed the response to field stimulation in caudal arteries from spontaneously hypertensive rats (SHR), but not in those from normotensive Wistar-Kyoto rats (WKY). The depression by the anti-neuropeptide Y antibody did not appear to be produced by a non-specific mechanism, since antiserum against either thyrotrophin-releasing factor or platelet-derived growth factor did not affect the response to field stimulation. It is concluded that endogenous neuropeptide Y can contribute to the vascular contractile response to field stimulation, and that this contribution may be enhanced in hypertensive animals.

Involvement of vascular endothelium in the potentiation of vasoconstrictor responses by neuropeptide Y

Although neuropeptide Y is not a potent vasoconstrictor in many vascular beds, nanomolar concentrations of this peptide can potentiate the response of isolated blood vessels to sympathetic stimulation or exogenous vasoconstrictors. Potentiation of the response to field stimulation was observed in superfused ring segments of rabbit ear artery or canine saphenous vein, provided that the endothelium was not mechanically damaged. The ability of neuropeptide Y to potentiate the constrictor response to noradrenaline in the ear artery was also dependent on the presence of an intact endothelium. The release of an endothelial-derived vasoconstrictor substance, as yet unidentified, by neuropeptide Y may explain its ability to potentiate the responses to both nerve stimulation and a variety of exogenous vasoconstrictors.

Pharmacologic characterization of SK&F 104078, a novel alpha-2 adrenoceptor antagonist which discriminates between pre- and postjunctional alpha-2 adrenoceptors

SK&F 104078 has been reported to be a moderately potent antagonist at postjunctional alpha-2 adrenoceptors in canine saphenous vein, rabbit saphenous vein and canine saphenous artery (KB = 76-150 nM). In contrast, SK&F 104078 has been found to have essentially no affinity for prejunctional alpha-2 adrenoceptors in the guinea pig atrium. To characterize further the pharmacology of SK&F 104078 we have examined its effects in several additional alpha-2 adrenoceptor models and on several non alpha adrenoceptor-mediated vascular responses. SK&F 104078 does not block the neuroinhibitory effect of alpha methylnorepinephrine in the guinea pig ileum. In contrast, in the rat vas deferens, high concentrations of SK&F 104078 (3-30 microM) antagonized the neuroinhibitory effect of UK 14,304; however, the antagonism was not competitive. At concentrations up to 1 microM, SK&F 104078 did not potentiate [3H]overflow from guinea pig vas deferens or guinea pig atrium prelabeled with [3H]norepinephrine, indicating no prejunctional alpha-2 adrenoceptor blocking activity in these tissues. SK&F 104078 is a competitive antagonist at alpha-1 adrenoceptors, and at 5-hydroxytryptamine2 receptors, as demonstrated by blockade of norepinephrine- and serotonin-induced contraction in the rabbit aorta, with KB values of 155 and 20 nM, respectively. At concentrations up to 10 microM, SK&F 104078 does not depress angiotensin II-induced contraction of rabbit aorta. At 1 microM, no depression of the response to Ca++ in depolarized rabbit aorta is observed, although a significant inhibition of this response is seen at 10 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for heterogeneity between pre- and postjunctional alpha-2 adrenoceptors using 9-substituted 3-benzazepines

A series of alpha adrenoceptor antagonists, including both reference compounds and the novel benzazepine antagonists, SK&F 86466 (6-chloro-2,3,4,5-tetrahydro-3-methyl-1H-3-benzazepine) and two of its 9-substituted derivatives, SK&F 101253 and SK&F 104078, were tested in vitro for affinity at central and peripheral alpha adrenoceptor subtypes. Peripheral alpha-1 adrenoceptor antagonist potency of these agents, as assessed by the receptor dissociation constant (KB) against norepinephrine-induced contraction in the rabbit aorta, correlated with the Ki value for inhibition of [3H]prazosin binding to central alpha-1 adrenoceptors in rat brain homogenates. Central alpha-2 adrenoceptor affinity, measured as the Ki for inhibition of [3H]rauwolscine binding to rat brain homogenates, correlated well with antagonist activity at peripheral postjunctional alpha-2 adrenoceptors as reflected by the KB against B-HT 920-induced contraction in canine saphenous vein. The 9-substituted benzazepines, SK&F 101253 and SK&F 104078, produce preferential blockade of postjunctional vs. prejunctional alpha-2 adrenoceptors in peripheral models. The high affinity of SK&F 104078 for postjunctional alpha-2 adrenoceptors in the canine saphenous vein was confirmed by its ability to inhibit [3H]rauwolscine binding to postjunctional alpha-2 adrenoceptors in this tissue. The observation that the Ki values for these antagonists against [3H] rauwolscine binding correlate with their KB values at the postjunctional alpha-2 adrenoceptors, rather than those at the prejunctional neuroinhibitory alpha-2 adrenoceptor, suggests a pharmacologic similarity between the postjunctional vascular alpha-2 adrenoceptors and the central [3H]rauwolscine binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the thromboxane receptor antagonist SK&F 88046 in the canine, monkey and human coronary vasculature

U-46619, a stable "functional" thromboxane/endoperoxide receptor agonist, produced potent contractile responses in isolated canine, rhesus monkey and human left circumflex coronary arteries (EC50 = 9.11 x 10(-9)M, 1.98 x 10(-8)M and 3.50 x 10(-9)M, respectively). Canine intrapulmonary veins were also contracted potently by U-46619 (EC50 = 1.22 x 10(-9)M). SK&F 88046, a thromboxane A2 (TxA2) end-organ receptor antagonist, blocked the vasoconstrictor effects of U-46619 in the canine circumflex artery (KB = 1.33 x 10(-8)M), canine intrapulmonary vein (KB = 1.46 x 10(-9)M), monkey circumflex artery (KB = 8.47 x 10(-8)M), and human circumflex artery (KB = 8.49 x 10(-7)M). SK&F 88046 was 10-60 times more potent in the canine and rhesus monkey coronary vasculature than in the human coronary preparations. Intracoronary administration of U-46619 to anesthetized, open chest dogs produced a dose-related decrease in left circumflex coronary artery blood flow which resulted in decreases in left ventricular developed pressure, left ventricular positive and negative dP/dt, ascending aortic blood flow, and an increase in left ventricular end-diastolic pressure. The decrease in coronary blood flow and the hemodynamic changes were either attenuated or completely inhibited by i.v. administration of SK&F 88046 (2.5 mg/kg + 0.05 mg/kg/min or 5.0 mg/kg + 0.1 mg/kg/min). SK&F 88046 was compared to two other TxA2 receptor antagonists in canine isolated intrapulmonary veins. SQ 29,548 was approximately 2-times more potent than SK&F 88046 as an antagonist of U-44619 mediated contractions (KB = 7.0 x 10(-10)M). In contrast, BM 13.177 was 150-fold less potent (KB = 2.19 x 10(-7)M) than SK&F 88046. Thus, the present study demonstrates species variability in response to TxA2 agonists and antagonists and reconfirms the relative importance of species selection in studying these agents.

Pharmacologic differentiation between pre- and postjunctional alpha 2-adrenoceptors by SK&F 104078

Most alpha 2-adrenoceptor antagonists do not discriminate between pre- and postjunctional alpha 2-adrenoceptors, and this has led to the commonly held belief that pre- and postjunctional alpha 2-adrenoceptors may represent one homogeneous population of receptors. SK&F 104078 has been shown to be a potent antagonist at postjunctional alpha 2-adrenoceptors at concentrations that do not block prejunctional alpha 2-adrenoceptors. Thus, SK&F 104078 is a competitive postjunctional alpha 2-adrenoceptor antagonist in canine and rabbit saphenous veins, canine saphenous artery and human platelet with a dissociation constant of approximately 100 nmol/l. Conversely, SK&F 104078 is inactive as a prejunctional alpha 2-adrenoceptor antagonist in atria from dog, guinea pig, rabbit and rat, and in guinea-pig ileum at concentrations up to 10,000 nmol/l. Likewise, SK&F 104078 has the ability to block postjunctional arterial alpha 2-adrenoceptors in vivo in the pithed rat at doses that do not inhibit prejunctional alpha 2-adrenoceptors in the same model. The results suggest that pre- and postjunctional alpha 2-adrenoceptors may not represent one homogeneous class, but rather are discrete subtypes of the alpha 2-adrenoceptor that may be differentiated by SK&F 104078.

Synthesis and evaluation of non-catechol D-1 and D-2 dopamine receptor agonists: benzimidazol-2-one, benzoxazol-2-one, and the highly potent benzothiazol-2-one 7-ethylamines

Our interest in identifying D-1 and D-2 dopamine receptor agonists that are not catechols led us to extend previous studies with oxindoles by investigating analogues of dopamine, N,N-dipropyldopamine, m-tyramine, N,N-dipropyl-m-tyramine, and epinine in which the m-hydroxyl is replaced by the NH portion of a thiazol-2-one, oxazol-2-one, or imidazol-2-one group fused to the 2,3-position. These compounds were evaluated for their affinity and agonist activity at D-1 and D-2 receptors by using in vitro assays. Replacement of the m-hydroxy in N,N-dipropyldopamine with the thiazol-2-one group resulted in a dramatic increase in D-2 receptor affinity and activity compared to that of N,N-dipropyldopamine itself or that of the corresponding oxindole, 1. The resulting compound, 7-hydroxy-4-[2-(di-n-propylamino)ethyl]benzothiazol-2(3H)-one (4), is the most potent D-2 receptor agonist reported to date in the field-stimulated rabbit ear artery (ED50 = 0.028 nM). The benzoxazol-2-one (6), benzimidazol-2-one (5), and isatin (51) analogues showed D-2 receptor agonist potency similar to that of 1. The des-7-hydroxyl analogue of 4 (21) also has enhanced D-2 receptor activity compared to that of the corresponding oxindole, 8. 7-Hydroxy-4-(2-aminoethyl)benzothiazol-2(3H)-one, 27, a non-catechol, has enhanced D-1 and D-2 receptor activity in vitro compared to that of the corresponding oxindole, 7. In vivo, 27 increased renal blood flow and decreased blood pressure in the dog. However, these effects were mediated primarily by D-2 receptor agonist activity. This may be a result of the D-1 partial agonist activity of 27 coupled with its potent D-2 receptor activity.

Demonstration of alpha 2-adrenoceptor-mediated contraction in the isolated canine saphenous artery treated with Bay K 8644

Contractions to B-HT 920 in the plantar branch of the canine saphenous artery in vitro occurred consistently only in the presence of the calcium channel agonist, Bay K 8644 (30 nM). These results indicate that prior activation of the calcium channel may be required to elicit a subsequent response to B-HT 920 in this artery. Furthermore, the contractile response was inhibited by rauwolscine (10 nM) and was unaffected by prazosin (10 nM), indicating that the response is mediated by postjunctional alpha 2-adrenoceptors.

Dopamine agonists related to 3-allyl-6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzaz epi ne-7, 8-diol. 6-Position modifications

The N-allyl derivative (SK&F 85174) of 6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-diol (SK&F 82526) retains the DA-1 agonist potency of the latter compound but unlike the parent also shows substantial DA-2 agonist activity. In a previous study of N-substituted benzazepines these combined agonist effects were shown to be uniquely associated with the N-allyl group. A continuation of this research has examined dependency of combined DA-2/DA-1 agonist activities on 6-position modification with the specific objective of developing an agonist with maximum effectiveness and potency at the DA-2 receptor subtype. DA-2 agonist activity was measured in a rabbit ear artery assay, and DA-1 agonist activity was determined in an adenylate cyclase assay. Replacing chloro with bromo retains the activity pattern and the potency of the chloro compound; replacement with a hydrogen causes a decrease of both DA-1 and DA-2 receptor activating potency. Introduction of a 6-methyl group causes loss of DA-2 agonist activity and reduction in DA-1 agonist potency. Substitution with a 6-fluoro provides the best balance of DA-2 and DA-1 agonist activities; this compound was moderately potent in both assays.

Hemodynamic effects of selective dopamine receptor agonists in the rat and dog

The availability of highly selective dopamine receptor agonists has allowed the characterization of the role of DA1 and DA2 receptors in the cardiovascular system. Fenoldopam (SK&F 82526) is a potent agonist at DA1 receptors, with much less agonist activity at the DA2 subtype or at alpha and beta adrenoceptors. In contrast, SK&F 89124 activates only the DA2 subtype. SK&F 85174, the N-allyl derivative of fenoldopam, retains potent DA1 agonist activity but also has moderately potent agonist activity at the DA2 receptor. All three compounds will reduce blood pressure in hypertensive rats. In the anesthetized dog, each agonist will reduce blood pressure and total peripheral resistance. The overall hemodynamic profile is remarkably similar, despite the marked difference in dopamine receptor subtype selectivity. The principal pharmacologic difference is enhanced bradycardia with the compounds having DA2 agonist activity, resulting from activation of neuroinhibitory DA2 receptors on cardiac sympathetic nerve terminals. In the dog, each compound will increase renal blood flow. Studies in the anesthetized rat with fenoldopam and SK&F 89124, using radiolabelled microspheres to measure blood flow to various vascular beds, also show a significant increase in renal flow, with a tendency toward increased blood flow in the splenic and intestinal beds. Hence, dopamine receptor agonists offer a useful approach to cardiovascular therapy via DA1 mediated vascular dilation, DA2 mediated modulation of sympathetic tone or a combination of both activities.

Prejunctional alpha 2-adrenoceptors modulate stimulation-evoked norepinephrine release in rabbit lateral saphenous vein

Segments of rabbit lateral saphenous vein prelabelled with [3H]noradrenaline were perfused and superfused with physiological salt solution. Tritium overflow evoked by transmural nerve stimulation (3 Hz for 2 min) was abolished by tetrodotoxin (1 microM). The selective alpha 2-adrenoceptor agonist UK 14,304 inhibited stimulation-evoked 3H-overflow in a concentration-dependent manner, with an IC50 of 71 nM. In contrast, the alpha 2-adrenoceptor agonist B-HT 933 had no effect on 3H-overflow in concentrations up to 10 microM. The selective alpha 2-adrenoceptor antagonists idazoxan and SKF 86466, as well as the non-selective alpha-antagonist phentolamine, facilitated the nerve stimulation evoked 3H-overflow, with an order of potency of idazoxan greater than or equal to phentolamine greater than SK&F 86466. Prazosin (100 nM) had little effect on 3H-overflow. These findings suggest that stimulation-evoked neurotransmitter release is modulated via prejunctional alpha 2-adrenoceptors.

Pharmacological differentiation of pre- and post-junctional alpha 2-adrenoceptors

It is now recognized that two post-junctional alpha-adrenoceptors mediate vascular constriction. The vascular alpha 2-adrenoceptors seem to be particularly sensitive to circulating catecholamine levels, in contrast to the alpha 1-adrenoceptors, which are activated primarily by neuronally released norepinephrine. Most alpha 2-adrenoceptor antagonists do not discriminate between the pre-junctional neuroinhibitory alpha 2-adrenoceptor and the post-junctional vascular alpha 2-adrenoceptor. However, we have synthesized and characterized a compound (SK&F 104078: 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-2,3,4,5-tetrahydro-1H-3- benzazepine) which is a potent antagonist at post-junctional vascular alpha 2-adrenoceptors in vitro but has no effect at pre-junctional neuroinhibitory alpha 2-adrenoceptors. The post-junctional selectivity of SK&F 104078 has been confirmed by in vivo studies determining pre- and post-junctional alpha 2-adrenoceptor antagonist activity in the pithed rat. The ability to selectively block post-junctional alpha 2-adrenoceptors offers a novel approach to antihypertensive therapy, since the vasoconstrictor effects of circulating catecholamines can be attenuated without influencing the feedback control of transmitter release operating via pre-junctional alpha 2-adrenoceptors, and excess sympathoadrenal tone can be reduced without affecting normal neurovascular transmission.

6,7-Dichloro-1-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline . A structurally novel beta-adrenergic receptor blocking agent

Replacement of the catecholic hydroxyl groups of the beta-adrenergic receptor agonist 6,7-dihydroxy-1-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline (trimetoquinol) with chloro substituents results in a compound with marked beta-adrenoceptor antagonist properties. This, therefore, parallels the similar transformation of the beta-adrenoreceptor agonist isoproterenol into the antagonist dichloroisoproterenol. In a test for inhibition of isoproterenol-induced enhancement of the rate of contraction of spontaneously beating guinea pig atrial pairs the resultant 6,7-dichloro-1-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline (6b) had a KB value of (6.7 +/- 2.3) X 10(-8) M. Although this is nearly 2 orders of magnitude less potent than propranolol (KB = 6.2 X 10(-10) M in this test), this compound represents the prototype of a new class of beta-adrenergic receptor blockers, and unlike dichloroisoproterenol it is not a partial agonist. It has physicochemical properties, e.g., pKa and distribution and partition coefficients, that differ from the prototypic beta-blockers. These altered properties might impart advantageous tissue distribution and altered pharmacological properties to the new molecule. This new beta-adrenoreceptor antagonist is suggested to merit further study.

Comparison of the alpha-adrenoceptor characteristics in human and canine prostate

Alpha adrenoceptors, mediating contraction, have been shown to be present in strips of hypertrophic prostate surgically removed from patients with benign prostatic hypertrophy (BPH), providing a rational explanation for the demonstrated effectiveness of alpha antagonists in the symptomatic treatment of this disease. Inasmuch as the dog develops spontaneous and hormonally induced prostatic enlargement, studies were performed to compare the adrenoceptor characteristics of canine and human prostate to determine whether the dog represents a useful model to search for more effective alpha-adrenolytic therapy for human BPH. Norepinephrine produces contraction in isolated strips of canine prostate although it is only one-tenth as potent as previously reported in human tissue. In contrast, several selective alpha 1-adrenoceptor agonists are potent contractile agents in canine prostate, but are nearly inactive in the human tissue. This difference may be a consequence of their partial agonist character. The potency of selective alpha-adrenoceptor antagonists in blocking the norepinephrine-induced contractile response in both canine and human tissue is consistent with an action of norepinephrine on the alpha 1 adrenoceptor. The receptor dissociation constants for these antagonists are similar in prostatic tissue from dogs and humans, and the values in canine tissue correlate well with those obtained in the rabbit ear artery, a standard model for vascular alpha 1 adrenoceptors. Hence the dog may represent a useful model for studies of the potential responsiveness of human prostate to adrenergic agents.

Synthesis and dopaminergic activity of some halogenated mono- and dihydroxylated 2-aminotetralins

In a series of 7,8-dihydroxy-1-phenyltetrahydro-3-benzazepine dopamine receptor agonists introduction of a chloro or fluoro substituent into the 6-position increases dopaminergic potency. Also, in this series replacement of the 7-hydroxyl group with a halogen results in inversion of activity from dopamine receptor agonist to antagonist. The present study was aimed at exploring the possibility that the structure-activity observations in the 3-benzazepine series of dopaminergic agents might be extrapolated to another class of dopamine receptor agonists, the 2-aminotetralins. Thus, a series of chloro- and fluoro-substituted mono- and dihydroxylated 2-aminotetralins was prepared and evaluated for dopaminergic properties in D-1 and D-2 receptor-related tests. Introduction of a chloro substituent into the 8-position of the prototype of this series, i.e. 2-amino-6,7-dihydroxytetralin (ADTN), resulted in a compound with a high degree of selectivity for the D-1 subpopulation of dopamine receptors; it was equally or more potent than ADTN in the D-1 receptor-related tests with greatly decreased effectiveness in the tests involving D-2 receptors. A similar effect was observed with 8-fluoro-ADTN; however, the N-(4-hydroxyphenethyl)-N-propyl derivative 4g of the 8-chloro-substituted ADTN showed marked D-2 binding affinity. Conversely, introduction of a chloro substituent into the 5-position of ADTN markedly decreased D-1 receptor affinity and efficacy. This effect was not seen with the related 5-fluoro derivative, suggesting D-1 receptors are more sensitive to bulk in the 5-position of ADTN than are the D-2 receptors. Replacement of either the 6- or 7-hydroxyl groups of ADTN with a chloro or fluoro substituent, in contrast, did not parallel the response seen in the benzazepine series (i.e., the compounds uniformly demonstrated less receptor affinity and did not have dopamine receptor antagonist activity); however, the decrease in agonist potency was less marked in the case of 2-amino-6-fluoro-7-hydroxytetralins than in the chlorinated monohydroxyaminotetralins. Thus, a parallelism in structure-activity relationships in the benzazepine and aminotetralin series of dopamine receptor agonists was not observed. The differences may reflect altered modes of receptor binding in the two series.

Dopamine receptor agonists: 3-allyl-6-chloro-2,3,4,5-tetrahydro- 1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-diol and a series of related 3-benzazepines

The N-allyl derivative (SK&F 85174) of 6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-dio l (SK&F 82526) not only retains the exceptional D-1 agonist potency of its parent but also displays reasonably potent D-2 agonist activity, as measured by a dopamine-sensitive adenylate cyclase test and a rabbit ear artery assay, respectively. Several additional N-substituted compounds were prepared to explore the D-2/D-1 agonist relationship. The N-methyl analogue retained good D-2 agonist potency, but this substitution converted D-1 agonist activity into antagonist activity. Most other N-substituents sharply decreased D-2 agonist potency including the N-n-propyl group. This observation was surprising since the introduction of mono- or di-N-n-propyl substituent(s) is commonly linked with retention or enhancement of D-2 agonist potency in other series of dopamine agonists. The N-(2-hydroxyethyl) analogue retains about one-fourth the D-2 potency of SK&F 85174. Several synthetic methods were used to prepare these compounds. N-Allylation of a trimethoxybenzazepine followed by cleavage of the methyl ethers with boron tribromide was the preferred method. Other methods used were direct alkylation of the trihydroxy secondary amine, i.e., SK&F 82526, and an acylation-amide reduction-cleavage method.