Abstract

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 pK_Bvalues with increasing antagonist concentrations in most cases allowing a single pooled value to be determined: for prazosin, a pK_Bof 8.99 ± 0.11 (n = 20,K_B of 1.03 nM); for WB, a pK_B of 8.75 ± 0.08 (n = 23,K_B of 1.76 nM); for BMY, a pK_Bof 7.21 ± 0.13 (n = 13,K_B of 62 nM); for MDL72, a pK_Bof 7.95 ± 0.15 (n = 12,K_B of 11.2 nM); and for SK&F 105854, a pK_B of 5.82 ± 0.08 (n = 15,K_B of 1.52 μM). For MDL73, pK_Bvalues 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 μM) 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 andalpha-1A adrenoceptor subtypes inhibitor, blocked PE responses (with less affinity than foralpha-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 oralpha-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 ofalpha-1B adrenoceptors. In the dog aorta, the alpha-1 adrenoceptors predominantly resemblealpha-1B rather than alpha-1D adrenoceptors as reported in the rat aorta.