Further evidence that the classical alpha 1A- and cloned alpha 1c-adrenoceptors are the same subtype

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Alpha1L-adrenoceptors in canine pulmonary artery

The aim of this study was to characterize the alpha-adrenoceptors of the canine pulmonary artery. Arterial rings from lower lung lobes were suspended for isometric-tension recording in the presence of cocaine (5 x 10(-6) M), hydrocortisone (3 x 10(-5) M), propranolol (5 x 10(-6) M), and rauwolscine (10(-7) M) to inhibit neuronal uptake, extraneuronal uptake, and beta- and alpha2-adrenoceptors, respectively. Prazosin was more potent against contractions evoked by phenylephrine (pA2 of 9.7) compared with methoxamine (pA2 of 8.4). SZL49 (10(-8) and 3 x 10(-8) M), an irreversible alpha1-adrenergic antagonist, inhibited responses to phenylephrine but not methoxamine. With norepinephrine, low concentrations of prazosin (3 x 10(-10) M and 10(-9) M) caused inhibition of the concentration-response curve; a higher concentration (3 x 10(-9) M) failed to produced further inhibition, whereas increasing the concentration of the antagonist (to 10(-8) and 3 x 10(-8) M) caused further rightward shifts in the concentration-response curve. The Arunlakshana and Schild plot revealed two components corresponding to pA2 values of 9.8 and 8.4. After SZL49 (3 x 10(-8) M), the Arunlakshana and Schild plot for the interaction between norepinephrine and prazosin was linear and generated a pA2 of 8.3. Contractions evoked by phenylephrine were inhibited by the alpha1B/alpha1D-adrenoceptor antagonist, chloroethylclonidine (10(-5) M), or by the alpha1B-antagonist, risperidone (pA2 value of 8.5), but were relatively resistant to inhibition by the selective alpha1D-antagonist, BMY7378 (-log K(B) of 6.1). The results suggest that two alpha1-adrenoceptor subtypes mediate contraction of the canine pulmonary artery. One subtype has high affinity for prazosin (alpha1H, likely to be alpha1B), is activated by phenylephrine, and is inhibited by SZL49. The other subtype has lower affinity for prazosin (alpha1L), is stimulated by methoxamine, and is relatively resistant to SZL49. The physiologic agonist, norepinephrine, causes contraction by activating both subtypes.

Synthesis and alpha-adrenoceptor blocking activity of the enantiomers of benzyl-(2-chloroethyl)-[2-(2-methoxyphenoxy)-1-methylethyl]amine hydrochloride

The enantiomers of benzyl-(2-chloroethyl)-[2-(2-methoxyphenoxy) -1-methylethyl]amine hydrochloride (1, CM18) were synthesized and studied pharmacologically for their irreversible antagonism at rat vas deferens alpha-adrenoceptors. In addition, assignment of the absolute configuration of the two enantiomers of 1 was made by X-ray crystallographic analysis performed on the intermediate amine (+)-2 hydrochloride. The enantiomer (R)-(+)-1 [(R)-(+)-CM18] (a) had a 10-fold preferential blocking activity for alpha 1-versus alpha 2-adrenoceptors, (b) discriminated, like racemic 1, between two possible alpha 1-adrenoceptor subsites/subtypes, with a selectivity ratio of 6.5 and (c) was 10-23 times as potent as the (S)-(-)-enantiomer at alpha 2- and alpha 1-adrenoceptors. Thus, it may be a valuable tool for the characterization of rat vas deferens alpha 1-adrenoceptor subtypes.

A possible structural determinant of selectivity of boldine and derivatives for the alpha 1A-adrenoceptor subtype

1. The selectivity of action of boldine and the related aporphine alkaloids, predicentrine (9-O-methylboldine) and glaucine (2,9-O-dimethylboldine) and alpha 1-adrenoceptor subtypes was studied by examining [3H]-prazosin competition binding in rat cerebral cortex. WB 4101 and benoxathian were used as selective alpha 1A-adrenoceptor antagonists. 2. In the competition experiments [3H]-prazosin (0.2 nM) binding was inhibited by WB 4101 and benoxathian. The inhibition curves displayed shallow slopes which could be subdivided into high and low affinity components (pKi = 9.92 and 8.29 for WB 4101, 9.35 and 7.94 for benoxathian). The two antagonists recognized approximately 37% of the sites with high affinity from among the total [3H]-prazosin specific binding sites. 3. Boldine, predicentrine and glaucine also competed for [3H]-prazosin (0.2 nM) binding with shallow and biphasic curves recognizing 30-40% of the sites with high affinity. Drug affinities (pKi) at the high and low affinity sites were, 8.31 and 6.50, respectively, for boldine, 8.13 and 6.39 for predicentrine, and 7.12 and 5.92 for glaucine. The relative order of selectivity for alpha 1A-adrenoceptors was boldine (70 fold alpha 1A-selective) = predicentrine (60 fold, alpha 1A-selective) > glaucine (15 fold, alpha 1A-selective). 4. Pretreatment of rat cerebral cortex membranes with chloroethylclonidine (CEC, 10 microM) for 30 min at 37 degrees C followed by thorough washing out reduced specific [3H]-prazosin binding by approximately 70%. The CEC-insensitive [3H]-prazosin binding was inhibited by boldine monophasically (Hill slope = 0.93) with a single pKi value (7.76). 5. These results suggest that whereas the aporphine structure shared by these alkaloids is responsible for their selectively of action for the alpha 1A-adrenoceptor subtype in rat cerebral cortex, defined functional groups, namely the 2-hydroxy function, induces a significant increase in alpha 1A-subtype selectivity and affinity.

Synthesis and biological profile of the enantiomers of [4-(4-amino-6,7-dimethoxyquinazolin-2-yl)-cis-octahydroquinoxalin- 1-yl]furan-2-ylmethanone (cyclazosin), a potent competitive alpha 1B- adrenoceptor antagonist

The enantiomers of [4-(4-amino-6, 7-dimethoxyquinazolin-2-yl)-cis-octahydroquinoxalin-1-yl]-fu ran- 2-ylmethanone (cyclazosin, 1) were synthesized from the chiral furan-2-yl(cis-octahydroquinoxalin-1-yl)methanone [(+)-2 and (-)-2], which were obtained by resolution of the racemic amine with (S)-(+)- and (R)-(-)-mandelic acid. The binding profile of the enantiomers of 1 was assessed at alpha 1-, alpha 2-, D2, and 5-HT1A receptors as well as at native alpha 1A- and alpha 1B- and cloned alpha 1a-, alpha 1b-, and alpha 1d-adrenoceptor subtypes in comparison with prazosin, spiperone, and AH11110A. (+)-1 displayed a 40-90-fold selectivity for the alpha 1B(alpha 1b)-adrenoceptor relative to alpha 1A(alpha 1a) and alpha 1d subtypes. A significant enantioselectivity was observed at the alpha 1A(alpha 1a)-adrenoceptor and particularly at alpha 1d-adrenoceptors since (-)-1 was 11-14- and 47-fold, respectively, more potent than (+)-1. Furthermore the enantiomer (+)-1 displayed selectivities of 1100-, 19000-, and 12000-fold in binding to alpha 1b-adrenoceptors relative to alpha 2-adrenoceptors and 5-HT1A and D2 receptors. These results indicate that (+)-1, [(+)-cyclazosin] is the most potent and selective ligand for the alpha 1B-adrenoceptor subtype so far described and may be a valuable tool in the characterization of alpha 1-adrenoceptor subtypes.

Effect of KMD-3213, an alpha 1a-adrenoceptor-selective antagonist, on the contractions of rabbit prostate and rabbit and rat aorta

KMD-3213, (-)-(R)-1-(3-hydroxypropyl)-5-[2-[[2-[2-(2,2,2-trifluoroethoxy) phenoxy]ethyl]amino]propyl]indoline-7-carboxamide, a newly synthesized alpha 1-adrenoceptor antagonist, has been shown to have potent action toward, and to be selective for human cloned and native alpha 1-adrenoceptors. In the present study, we characterized the inhibitory effect of KMD-3213 on the phenylephrine (alpha 1-adrenoceptor-selective agonist)-induced contraction of rabbit prostate, rabbit thoracic aorta and rat thoracic aorta to functionally confirm the tissue selectivity of KMD-3213. The mean pA2 value for KMD-3213 for the inhibition of the rabbit prostatic contraction was 10.05, whereas the values for the rabbit and rat aortic contractions were 9.36 and 8.13, respectively. The order of mean pA2 values for the inhibition of the rabbit prostatic contraction was KMD-3213 > or = tamsulosin >> prazosin, whereas that for the rabbit and rat aortic contractions was tamsulosin > KMD-3213 > prazosin and tamsulosin > or = prazosin >> KMD-3213, respectively. KMD-3213 produced a sigmoidal inhibition curve for single-dose phenylephrine-induced contractions of rabbit prostate, whereas it produced a non-sigmoidal curve for that of rabbit aorta. KMD-3213 had no effect on isoproterenol-induced chronotropic action in guinea-pig atria, and 5-hydroxytryptamine-, histamine- and acetylcholine-mediated contractions of rabbit aorta. These results indicate that the potency of the inhibitory activity of KMD-3213 depends on the tissue subtype expression and that KMD-3213 preferentially antagonizes prostatic contraction.

Functional evidence for an alpha 1B-adrenoceptor mediating contraction of the mouse spleen

alpha 1-Adrenoceptor agonists ((-)-adrenaline = (-)-noradrenaline > > L-phenylephrine > methoxamine > (-)-(4a R, 10a R)-3,4,4a,5,10,10a-hexahydro-6-methoxy-4-methyl-9-methylthio-2 H-naphth[2,3-b]-1,4-oxazine (SDZ NVI 085) > cirazoline) evoked contraction of isolated mouse spleen strips, whereas oxymetazoline and indanidine were nearly inactive. Splenic contractions elicited by (-)-noradrenaline were inhibited by chloroethylclonidine (3 x 10(-6) - 6 x 10(-5) M) and partially attenuated by SZL-49 (10(-7) -10(-6) M), but remained resistant to (+/-)-isradipine (10(-9) -10(-7) M). The contractions were competitively antagonized by low concentrations of the alpha 1B-adrenoceptor-selective antagonist, spiperone (pA2 = 8.29), but by relatively high concentrations of the alpha 1A-adrenoceptor-selective receptor antagonists, tamsulosin (pA2 = 8.62), 5-methyl-urapidil (pA2 = 7.03), (+)-niguldipine (pA2 = 6.26) and the alpha 1D-adrenoceptor-selective antagonist, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro-[4.5]dec ane-7, 9-dione (BMY 7378) (pA2 = 6.76). Functional antagonist affinities at mouse spleen alpha 1-adrenoceptors were consistent with those at guinea-pig splenic alpha 1B-adrenoceptors, but not with those of either rat vas deferens alpha 1A- or rat aortic alpha 1D-adrenoceptors. Antagonist affinities at mouse spleen alpha 1-adrenoceptors correlated also best with published antagonist data on cloned and expressed alpha 1b-adrenoceptors but less well with those for either alpha 1a- or alpha 1d-adrenoceptors. The results provide pharmacological evidence that the alpha 1-adrenoceptor mediating smooth muscle contraction of mouse spleen is the B subtype.

Alpha 1B-adrenoceptor-mediated excitation of piriform cortical interneurons

Pharmacological techniques have defined the existence of two different alpha 1-adrenoceptors, the alpha 1A- and alpha 1B-adrenoceptor subtypes and both of these receptors have been cloned in addition to a cloned alpha 1d-adrenoceptor. A subpopulation of interneurons in layer III of the rat piriform cortex that are excited by 5-hydroxytryptamine (5-HT) via 5-HT2A receptors are also excited by norepinephrine via alpha 1-adrenoceptors. In the present study we determined the pA2 values against the norepinephrine-mediated excitation of piriform cortical interneurons for a number of antagonists that are (1) not selective for alpha 1A- or alpha 1B-adrenoceptors (prazosin), (2) selective for alpha 1A-adrenoceptors (5-methyl urapidil, 2-(2,6-dimethoxy-phenoxyethyl)- aminomethyl-1,4-benzodioxane hydrochloride (WB 4101), benoxathian, phentolamine) and (3) selective for alpha 1B-adrenoceptors (spiperone and risperidone). The pA2 values for the antagonist blockade of norepinephrine-mediated interneuron excitation were significantly correlated to literature values for the pKi values of antagonist binding to the alpha 1B-adrenoceptor (r = 0.919) and the cloned alpha 1b-adrenoceptor (r = 0.849) but were not correlated to the pKi values of antagonist binding to the alpha 1A-adrenoceptor or the cloned alpha 1a- and alpha 1d-adrenoceptor. Thus, we conclude that this population of piriform cortical interneurons is excited by norepinephrine via alpha 1B-adrenoceptors.

Analysis of the activity of alpha 1-adrenoceptor antagonists in rat aorta

1. In this study, the effect of seven alpha 1-adrenoceptor antagonists (tamsulosin, phentolamine, prazosin, WB-4101, 5-methylurapidil, spiperone and HV723) have been examined on the contractile response to noradrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2. NA and PE, when administered using a cumulative dosing schedule, both produced concentration-dependent contraction of aortic rings. It was possible to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[A]50 = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (nH = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, the Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (p[A50] = 6.09 +/- 0.08, nH = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judged by calculation of the mean sum of squares of the differences between the observed and predicted values. 3. All of the antagonists investigated produced concentration-dependent inhibition of the contractile responses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tested were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (nH) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves obtained with NA and/or PE. 4. Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A]50 values estimated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameters for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity. 5. In the absence of evidence to suggest that the deviations from simple competitive antagonism were due to failure to satisfy basic experimental conditions for quantitative analysis, an attempt was made to see whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-receptor model was significantly better than that obtained with the one-receptor model. Furthermore, with the exception of the data obtained with phentolamine, the pKB estimates for the two receptors were independent of whether NA or PE was used as agonist. 6. To determine which alpha 1-adrenoceptor subtypes may be associated with those defined by the two receptor model, the mean pKB estimates obtained from the two-receptor model fit were compared with affinities measured by Laz et al. (1994) for rat cloned alpha 1-adrenoceptor subtypes expressed in COS-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both pKB1 and pKB2 in the case of the alpha 1a/d-adrenoceptor (now referred to as alpha 1d-adrenoceptor; Hieble et al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha 1d-adrenoceptor. However, relatively good matches were also found between pKB1 and alpha 1c and between pKB2 and alpha 1b. Therefore, on the basis of these data, it is not possible to rule out the involvement of all three alpha 1-adrenoceptors. The conflicting reports concerning the characteristics of the alpha 1-adrenoceptor population mediating contraction of the rat aorta may, at least in part, be due to the lack of highly selective ligands and to between-assay variation in the expression of multiple alpha 1-adrenoceptors.