Pharmacological and immunocytochemical characterization of subtypes of alpha-1 adrenoceptors in dog aorta.
In this study, the effects of nine alpha-1 adrenoceptor antagonists [prazosin, WB 4101 (WB), chloroethylclonidine (CEC), 5-methylurapidil (5-MU), BMY 7378 (BMY), MDL 73005EF (MDL73), MDL 72832 (MDL72), RS 17053 (RS) and SK&F 105854 (SKF)] were studied on contractile responses to phenylephrine (PE) of the endothelium-denuded dog aorta in vitro. All antagonists, except CEC, 5-MU and RS, produced concentration-dependent competitive inhibition of contractile responses of the aorta to PE. The rightward shift of the concentration-response curves of PE yielded constant pKB values with increasing antagonist concentrations in most cases allowing a single pooled value to be determined: for prazosin, a pKB of 8.99 +/- 0.11 (n = 20, KB of 1.03 nM); for WB, a pKB of 8.75 +/- 0.08 (n = 23, KB of 1.76 nM); for BMY, a pKB of 7.21 +/- 0.13 (n = 13, KB of 62 nM); for MDL72, a pKB of 7.95 +/- 0.15 (n = 12, KB of 11.2 nM); and for SK&F 105854, a pKB of 5.82 +/- 0.08 (n = 15, KB of 1.52 microM). For MDL73, pKB values decreased with antagonist concentration: 7.88 +/- 0.06 at 10 nM, 7.56 +/- 0.28 at 100 nM and 6.92 +/- 0.18 at 1000 nM, which suggests the presence of more than one receptor subtype. CEC (10 and 100 microM) almost completely inhibited responses to PE; lower concentrations had no significant effect. 5-MU (10-300 nM) and RS (3-300 nM) were ineffective antagonists in this tissue. Because WB, a highly selective alpha-1D and alpha-1A adrenoceptor subtypes inhibitor, blocked PE responses (with less affinity than for alpha-1A adrenoceptors), and 5-MU and RS, which are selective blockers for alpha-1A adrenoceptor, were ineffective, we conclude that alpha-1A adrenoceptors are absent in the dog aorta. The effects of the less selective MDL72 were inconsistent with actions at alpha-1B or alpha-1D adrenoceptors. Although WB shifted the PE concentration-response curve to the right, the abilities of BMY, MDL73 and SKF to inhibit competitively PE contraction were of lower affinity compared with expectations for interaction with alpha-1D adrenoceptors; they are not the predominant subtype. The complete inhibition of PE responses by CEC suggests that the dog aorta contains the alpha-1B adrenoceptor subtype. In immunocytochemical studies of the expression of alpha-1B adrenoceptor, all cells apparently expressed this protein. Moreover, Western blot studies of the microsomal fractions confirmed the presence of alpha-1B adrenoceptors. In the dog aorta, the alpha-1 adrenoceptors predominantly resemble alpha-1B rather than alpha-1D adrenoceptors as reported in the rat aorta.
Low
A M
AM
Department of Biomedical Sciences, McMaster University, Hamilton, ON, Canada.
Lu-Chao
H
H
Wang
Y F
YF
Brown
R D
RD
Kwan
C Y
CY
Daniel
E E
EE
D000818:Animals; D001011:Aorta / Q000737:chemistry* / Q000187:drug effects / Q000502:physiology; D003000:Clonidine / Q000031:analogs & derivatives / Q000494:pharmacology; D004146:Dioxanes / Q000494:pharmacology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D007150:Immunohistochemistry; D007211:Indoles / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D018340:Receptors, Adrenergic, alpha-1 / Q000032:analysis / Q000145:classification*; D013141:Spiro Compounds / Q000494:pharmacology
D004146:Dioxanes; D007211:Indoles; D010879:Piperazines; C099495:RS 17053; D018340:Receptors, Adrenergic, alpha-1; D013141:Spiro Compounds; C056635:MDL 72832; D010656:Phenylephrine; C051731:chlorethylclonidine; C010654:(2-(2',6'-dimethoxy)phenoxyethylamino)methylbenzo-1,4-dioxane; C053428:BMY 7378; D003000:Clonidine; D011224:Prazosin