Keiko Maruyama

α1‐Adrenoceptor Subtypes in Bovine Prostate

Abstract— The object of this study was to examine the existence and characteristics of α1‐adrenoceptor subtypes in the bovine prostate using the radioligand binding assay method. [3H]Prazosin was used as the radioligand and its binding sites in bovine prostate were classified into two subtypes. One subtype showed a high affinity (α1High, Kd: 101·1 Pm and Bmax: 11·8 fmol (mg protein)−1) and the other had a low affinity (α1Low, Kd: 3371·4 Pm and Bmax: 50·5 fmol (mg protein)−1). Although the same pKi values of clorethylclonidine, p‐aminoclonidine, benoxathian and dibenamine to both α1High and α1 Low binding sites in bovine prostate tissue were observed, other α1 antagonists used in this study had different pKi values for the two α1‐adrenoceptor subtypes. The existence and binding characteristics of α1‐adrenoceptor subtypes in bovine prostate were clarified. It is possible that agents selective for one site may contribute to the development of better drugs for the treatment of bladder outlet obstructions of men with benign prostatic hyperplasia.

Discrimination of α1-Adrenoceptor Subtypes in Rat Aorta and Prostate

This study was designed to further discriminate α1-adrenoceptor subtypes in rat aorta and prostate using functional experiments. Responses induced by phenylephrine were equilibrated in both tissues. The pA2 values and slope factors of several α1-antagonists were assessed using concentration-response curves. The antagonists used were prazosin, WB-4101, 5-methylurapidil (5-MU), HV-723, and tamsulosin. In addition, the effects of chloroethylclonidine (CEC) and nifedipine on phenylephrine-induced contractions were investigated. A high pA2 value for prazosin was observed in both tissues (aorta 9.84, prostate 9.19) and the ranking of each drug’s pA2 value is as follows: tamsulosin > prazosin > WB-4101 > HV-723 > 5-MU in the aorta, and tamsulosin > prazosin > 5-MU > WB-4101 = HV-723 in the prostate. A significant difference between the pA2 value of each drug except for tamsulosin in the aorta and in prostate was observed (p < 0.01). Inhibition of contraction by pretreatment with CEC was 83.9 ± 2.42% in the aorta, and 6.17 ± 0.94% in the prostate. On the other hand, inhibition of maximal response by pretreatment with nifedipine (1 µmol/l) was 35.1 ± 2.2% in the aorta and 24.5 ± 3.1% in the prostate. A good correlation between these pA2 values and pKi values for recombinant human α1b-adrenoceptor expressed in CHO cells (aorta) and α1a-subtypes of CEC pretreated rat hippocampus (prostate) were observed. In conclusion, these results suggest that: (1) the contraction of these two tissues is mediated by α1H-adrenoceptor with a high affinity for prazosin; (2) α1H-adrenoceptors correspond to α1b-(aorta) and α1a-subtypes (prostate), and (3) each α1-adrenoceptor subtype in the aorta and prostate may be α1b-(aorta) and α1a-subtypes (prostate), respectively.

α1-adrenoceptor subtypes in the rat ventricular muscle

Abstract— Scatchard analyses of [3H]prazosin binding in rat ventricular muscle membranes showed biphasic curves, which identified α1High‐ and α1Low‐affinity sites. The α1High‐affinity site was completely inhibited by 1 μm phenoxybenzamine. The displacement potencies of α1‐adrenergic antagonists were characterized by [3H]prazosin binding to α1High. and α1Low‐affinity sites in the absence and presence of 1 μm phenoxybenzamine. The affinities of most chemicals for α1Low‐affinity sites were significantly lower than those for α1High‐affinity sites, but WB‐4101 (2‐(2,6‐dimethoxy‐phenoxyethyl)aminomethyl‐1,4‐benzodioxane), arotinolol, cinanserin, nifedipine, and p‐aminoclonidine had the same affinities for both α1Low‐ and α1High‐affinity sites. These results show that two α1‐adrenoceptor subtypes, α1High‐ and α1Low‐affinity, are present in the rat heart, and that there are physical variations in α1‐adrenoceptor binding sites, based on their selectivity to antagonists.

Alpha-1 and Beta-Adrenergic receptor blocking potencies of bopindolol and its two metabolites (18–502 and 20–785) as assessed by radioligand binding assay methods

1. The pKi value of bopindolol for alpha 1High-subtypes in canine aorta, rat hearts and rat brain was 5.71, 5.52 and 6.56, respectively. In addition, the pKi values of these drugs in canine aorta, rat hearts and rat brain for the alpha 1Low-subtype was very low. 2. The phenylephrine induced-contractions of aortae of guinea pigs and rats were not inhibited by these agents. 3. Both bovine hearts and tracheal smooth muscles indicated that 18-502 had the highest pKi value to beta 1- and beta 2-adrenoceptor subtypes and the rank order of these beta-blocking potencies were 18-502 > bopindolol > 20-785.

Tamsulosin: Assessment of Affinity of 3H-Prazosin Bindings to Two α1-Adrenoceptor Subtypes (α1H and α1L) in Bovine Prostate and Rat Heart and Brain

Abstract 1. The present study was designed to assess the displacement potencies of tamsulosin to 3H-prazosin bindings in two α1-adrenoceptor (AR) subtypes (α1H and α1L) in bovine prostate, rat heart and brain compared with those of amosulalol, labetalol, ketanserin, clonidine and propranolol. 2. The pKi values of tamsulosin to α1H and α1L subtypes in bovine prostate were 9.13 and 8.99 and these values were almost the same as those of prazosin. On the other hand, low pKi binding values of amosulalol, labetalol, ketanserin, clonidine and propranolol to these subtypes were observed. 3. Low pKi values of tamsulosin to α2- and β-ARs and muscarinic and 5HT2 receptors in the rat brain were observed. 4. These results suggest that tamsulosin has high affinities to α1L-AR subtypes in bovine prostate and rat hearts as well as α1H-AR subtypes, implying an inhibitory effect of this drug on the contraction of the prostate.

Two distinct α1-adrenoceptor subtypes in the human prostate: Assessment by radioligand binding assay using 3H-prazosin

1. We showed that there were two distinct alpha(1)-adrenoceptor subtypes (alpha(1H) and alpha(IL)) in the human prostate which show different affinities for 3H-prazosin. 2. WB4101, tamsulosin, 5-methylurapizil, phentolamin, and terazosin, but not nifedipine, had significantly higher pKi values for the alpha(1H)-subtype than for the alpha(IL)-subtypes. 3. There was good correlation (r = 0.92, P < 0.05) between the pKi values obtained for the alpha(1H)-receptors in membrane fractions and the cloned human alpha(1c)-adrenoceptor subtype.

Tamsulosin: assessment of affinityof (3)H-P razosin binding to two alpha-1- adrenoceptor subtypes in the canine aorta.

This study was performed to assess the affinity of tamsulosin to the α1L- in addition to α1B-adrenoceptor (α1-AR) subtypes coexisting in the canine aorta using the radioligand binding assay. The antagonistic effects of this drug on contraction of the rat aorta were also assessed, and the results were compared with those obtained with prazosin, amosulalol, labetalol, ketanserin, clonidine and propranolol. The pKi value of tamsulosin to the α1L-subtype was lower than those of prazosin and HV-723, but higher than those of amosulalol, ketanserin and labetalol. The pKi value of tamsulosin for the α1B-subtype in the canine aorta was similar to that of prazosin. However, this drug showed a higher pKi value than amosulalol, HV-723, labetalol and ketanserin. On the other hand, the order of inhibition potencies for contraction of the rat aorta by phenylephrine was as follows: prazosin > tamsulosin > amosulalol > HV-723 > labetalol > ketanserin > clonidine > propranolol. Thus, although the affinity of tamsulosin to the α1B-AR subtype in the canine aorta was as high as that in the bovine prostate reported in our previous study, the affinity (pKi 7.87) of this drug to α1L-AR in the canine aorta was lower than that (pKi 8.99) in the bovine prostate. These observations suggested that the pharmacological potencies of tamsulosin in the aorta and prostate may be different.