α-Adrenoceptor subtypes and Ca2+ mobilization the rabbit ear artery†

The mobilization of cellular and extracellular Ca²⁺ pools by selective α₁-adrenoceptor (phenylephrine) and α₂-adrenoceptor (xylazine) agonists as well as noradrenaline was evaluated in rabbit ear artery. Noradrenaline and phenylephrine possess full instrinsic activity for both types of Ca²⁺ mobilization whereas xylazine up to 1 mM had only a limited contractile effect, being more effective in inducing extracellular Ca²⁺-dependent response. However, extracellular Ca²⁺ was mobilized by xylazine in a concentration 20 times higher than that required to stimulate pre-junctional α₂-adrenoceptors. Noradrenaline (5 μM) and xylazine (1 mM) induced cellular and extracellular Ca²⁺-dependent contractions which were prazosin-sensitive and yohimbine-resistant. Xylazine-induced contractile activity, particularly that dependent upon the extracellular Ca²⁺ pool, was markedly reduced by selective adrenergic denervation with 6-hydroxydopamine, but the actions of noradrenaline were unaffected. These results suggest that: (1) rabbit ear artery contain post-junctional α¹-adrenoceptor but not α₂-adrenoceptors; (2) stimulation of these α₁-adrenoceptors can account for the overall contractile activity of exogenously added noradrenaline and (3) stimulation of α₁-adrenoceptors results in mobilization of cellular as well as extracellular Ca²⁺ pools.