Pharmacological classification of alpha(1)-adrenoceptors mediating contractions of rabbit isolated ear artery: Comparison with rat isolated thoracic aorta

1 The present study attempted to classify pharmacologically the alpha(1)-adrenoceptor subtype(s) present in two isolated, vascular ring preparations, the rabbit ear artery and rat thoracic aorta. 2 In the ear artery, the agonist effects of phenylephrine were antagonized by 5-methyl urapidil (pA(2)=7.90; Schild slope=0.85) and BMY 7378 (pA(2)=6.11; Schild slope=0.80) but not in a simple competitive manner. The shallow Schild slopes are consistent with the activation of a heterogeneous receptor population. Indeed the 5-methyl urapidil data set could be fitted to a two-receptor model yielding a high antagonist affinity (PKBH) estimate of 7.85 and a low affinity (pK(BL)) estimate of 6.03. 3 The effects of clonidine in the ear artery were competitively antagonised by 5-methyl urapidil (pK(B)=7.91) and BMY 7378 (pK(B)=5.53). These data are consistent with contractions to clonidine being mediated by a single receptor subtype. 4 In the aorta, the effects of phenylephrine were antagonized by 5-methyl urapidil (pA(2)=7.95; Schild slope=1.11) and BMY 7378 (pA(2)=9.08; Schild slope=0.73). Neither data set was consistent with a simple competitive interaction. The BMY 7378 data suggested again, that phenylephrine was acting at a heterogeneous receptor population. Subsequent analysis by the two-receptor model yielded a high affinity (pK(BH)) estimate of 8.95 and a low affinity (pK(BL)) estimate of 7.00. 5 The alkylating agent, chloroethylclonidine (CEC) elicited concentration-dependent contractions in the ear artery with a potency (p[A](50)) of 5.57. Pretreatment of this tissue with CEC (5 mu M, 30 min incubation) had no effect on subsequent responses to phenylephrine. In contrast, in the aorta, CEC demonstrated no agonism but pretreatment with this agent (5 mu M, 15 min incubation) caused a rightward shift and depression of subsequent phenylephrine concentration-effect curves. 6 The affinity of clonidine in the rabbit ear artery (pK(A)=6.17) was found to be significantly different to its affinity in the rat thoracic aorta (pK(A)=7.12) suggesting that this agonist activates different alpha(1)-adrenoceptor subtypes in the two tissues. 7 These results suggest that heterogeneous populations of alpha(1)-adrenoceptors are present in both tissues. In the ear artery, the profile of antagonist and agonist activity is most consistent with alpha(1A)-adrenoceptors being the predominant receptor subtype. The second receptor population does not appear to correspond to any of the recognized alpha(1)-adrenoceptor subtypes. In the aorta alpha(1D)-adrenoceptors appear to predominate, with alpha(1A)-adrenoceptors being the most likely candidate for the second receptor population.