Ž . European Journal of Pharmacology 430 2001 9–17 www.elsevier.comrlocaterejphar Dopamine induces intracellular Ca 2q signals mediated by a -adrenoceptors in rat pineal cells 1B Elkin Rey a , Francisco J. Hernandez-Dıaz b , Pedro Abreu b , Rafael Alonso b , Lucıa Tabares a, ) ´ ´ ´ a Department of Physiology and Biophysics, School of Medicine, UniÕersity of SeÕille, AÕda. Sanchez Pizjuan, 4, 41009 SeÕille, Spain ´ ´ b Department of Physiology, School of Medicine, UniÕersity of La Laguna, Tenerife, Spain Received 3 May 2001; received in revised form 16 July 2001; accepted 27 July 2001 Abstract We have studied the functional interaction of dopamine with a -adrenoceptor subtypes by measuring intracellular Ca 2q levels in 1 pineal cells, a cell type where adrenoceptors are well characterized. We show that dopamine induces transient intracellular Ca 2q signals in only 70% of cells responding to phenylephrine. Dopamine-induced Ca 2q signals desensitise faster than Ca 2q transients elicited with phenylephrine and are selectively blocked by desipramine, imipramine, and a -adrenoceptor antagonists. These results suggest that 1B dopamine induced Ca 2q signals are mainly due to the activation of one subtype of a-adrenoceptor, the a . q 2001 Elsevier Science 1B B.V. All rights reserved. Keywords: Dopamine; a-Adrenoceptor; Pineal cell; Catecholamine 1. Introduction Noradrenaline binds at least to 10 different adrenocep- tors, which either activate or inhibit adenylate cyclase, or are coupled to phospholipase C. All adrenoceptors share three conserved residues within their transmembrane seg- ments, which are thought to be the minimal structure for ligand binding. This residues are aspartate in the third transmembrane segment which may act as a counterion for the amine moiety in biogenic amines, and two serines in the fifth transmembrane segment thought to be involved in the recognition of the two hydroxyl groups of adrenaline Ž . and noradrenaline Strader et al., 1987, 1989 .On the other hand, dopamine binds to a different family of receptors formed by at least five subtypes. Dopamine receptors, which are mainly coupled to the activation or the inhibi- tion of adenylate cyclase, share with adrenoceptors the same three conserved residues within their transmembrane segments for the ligand binding. This is not surprising given the structural similarity between adrenaline, nor- adrenaline and dopamine. The possibility that all three catecholamines may indis- tinctly activate dopamine receptors and adrenoceptors has ) Corresponding author. Tel.: q 34-954-55-1768; fax: q 34-954-55- 1769. Ž . E-mail address: ltabares@cica.es L. Tabares . been long timed suspected. So far, b-adrenergic-like ef- fects of dopamine have been demonstrated both on the Ž vascular system and the heart Rajfer et al., 1988; Oue- . draogo et al., 1998 . Similarly, in human platelets, it has been shown that dopamine increases intracellular cAMP Ž levels through the activation of b-adrenoceptors Anfossi . et al., 1993 and, in rat adipocytes, dopamine lowers glucose uptake through the activation of b -adrenoceptors 3 Ž . Lee et al., 1998 . We have studied the effect of dopamine on intracellular Ca 2q levels in rat pinealocytes, a cell type having a- and b-adrenoceptors and where the intracellular pathways acti- vated by noradrenaline have been deeply studied. Our results show that dopamine, at low micromolar concentra- tions, is able to elicit intracellular Ca 2q increments due to the activation of a-adrenoceptors. The pharmacological characterisation of receptors mediating the Ca 2q increment triggered by dopamine points to the preferential activation of the a -adrenoceptor over other a subtypes. 1B 1 2. Materials and methods 2.1. Cell dissociation Pineal glands were obtained from male Wistar rats Ž . 100–150 g housed in a temperature-controlled room 0014-2999r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0014-2999 01 01250-X