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

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Abstract
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.