IDENTIFICATION OF BETA-LIKE ADRENOCEPTORS ASSOCIATED WITH BIOLUMINESCENCE IN THE SEA PANSY RENILLA-KOELLIKERI

Previous studies have reported pharmacological and biochemical evidenc e for the involvement of adrenergic substances in the regulation of ne uroeffector activities in the bioluminescent cnidarian Renilla koellik eri (Cnidaria, Anthozoa). Therefore, direct radiobinding assays were d eveloped to identify and characterize beta-adrenergic binding in membr ane preparations from this species, using the two beta-antagonists [H- 3]dihydroalprenolol and [H-3]CGP12177 as tracers. In addition, the eff ect of various beta-adrenergic agents on luminescence was examined. Bi nding of the radioligands at 25-degrees-C was rapid, reversible, satur able and specific. Saturation studies revealed the presence of two dif ferent and independent classes of binding site, site1 and site2, in th e body of the colony (rachis). In contrast, homogeneous populations of binding sites corresponding to site1 were detected in autozooid polyp s and to site2 in the peduncle. The pharmacological profile of beta-ad renergic binding in R. koellikeri membrane preparations displayed prop erties consistent with the presence of two sites and followed a patter n similar to beta2- and beta1-adrenergic receptor subtypes for site1 a nd site2, respectively. Bioluminescence in polyps was induced by beta- agonists as well as by one beta1-selective antagonist, atenolol, and w as blocked by several beta-blockers including (+/-)CGP12177. The speci ficity pattern of the physiological effect of beta-adrenergic drugs on luminescence mirrors that of the radioligand interaction with site1. This suggests that radioligand binding to site1 represents binding to the receptor that mediates luminescence excitation in R. koellikeri. B lockade of the luminescent responses to site1 agonists by isotonic MgC l2 indicates that this beta-adrenergic mechanism must rely on interneu ronal transmission. Collectively, these results suggest the evolutiona ry conservation of beta-adrenoceptors and of their dual character from coelenterates to higher vertebrates.