Alpha1-Adrenoceptors in the Albino Rabbit Ciliary Process

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Abstract
3H-Prazosin and 3H-rauwolscine binding sites were identified in a membrane suspension prepared from albino rabbit iris + ciliary body. Scatchard analysis of saturation binding experiments demonstrated that both 3H-prazosin and 3H-rauwolscine bind to a single population of binding sites with KD values of 0.87 nM and 5.33 nM, respectively. Bmax values of 65.7 and 198 fmol/mg protein were obtained for 3H-prazosin and 3H-rauwolscine, respectively. Displacement studies by several adrenergic agonists and antagonists indicated that 3H-prazosin and 3H-rauwolscine labelled α1- and α2-adrenoceptors, respectively, in the iris + ciliary body. Epinephrine, norepinephrine and phenylephrine were able to stimulate the synthesis of 3H-inositol phosphates in ciliary processes labelled with 3H-inositol, with EC50 values of 2.4, 12 and 10 μM, respectively. The corresponding maximum stimulations of basal activity were 433, 430 and 283%, respectively. Phenylephrine behaved like a partial agonist in this assay. The norepinephrine response could be potently antagonized by prazosin (Ki = 27 nM), with rauwolscine being 285-fold less potent. An epithelial cell suspension was prepared from the ciliary process. Stimulation of phosphatidylinositol turnover by norepinephrine (0.1 mM) was observed, and this could be blocked by prazosin (10 μM), thus, indicating the presence of α1-adrenoceptors, coupled to phosphatidylinositol turnover, in epithelial cells of the rabbit ciliary process.