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Effects of Prolactin on Ionic Membrane Conductances in the Human Malignant Astrocytoma Cell Line U87-MG

Institut National de la Santé Et de la Recherche Médicale EMI 0347 "Signalisation et Mécanismes Moléculaires de l'Apoptose," and Laboratoire de Physiologie et Physiopathologie de la Signalisation Cellulaire, Centre National de le Recherche Scientifique UMR 5543, Université de Bordeaux 2, 33076 Bordeaux Cedex, France

Submitted 25 July 2003; accepted in final form 27 October 2003

Prolactin (PRL) is involved in numerous biological processes in peripheral tissues and the brain. Although numerous studies have been conducted to elucidate the signal transduction pathways associated with the PRL receptor, very few have examined the role of ion conductances in PRL actions. We used the patch-clamp technique in "whole cell" configuration and microspectrofluorimetry to investigate the effects of PRL on membrane ion conductances in the U87-MG human malignant astrocytoma cell line, which naturally expresses the PRL receptor. We found that a physiological concentration (4 nM) of PRL exerted a biphasic action on membrane conductances. First, PRL activated a Ca²⁺-dependent K⁺ current that was sensitive to CTX and TEA. This current depended on PRL-induced Ca²⁺ mobilization, through a JAK2-dependent pathway from a thapsigargin- and 2-APB-sensitive Ca²⁺ pool. Second, PRL also activated an inwardly directed current, mainly due to the stimulation of calcium influx via nickel- and 2-APB-sensitive calcium channels. Both phases resulted in membrane hyperpolarizations, mainly through the activation of Ca²⁺-dependent K⁺ channels. As shown by combined experiments (electrophysiology and microspectrofluorimetry), the PRL-induced Ca²⁺ influx increased with cell membrane hyperpolarization and conversely decreased with cell membrane depolarization. Thus PRL-induced membrane hyperpolarizations facilitated Ca²⁺ influx through voltage-independent Ca²⁺ channels. Finally, PRL also activated a DIDS-sensitive Cl^- current, which may participate in the PRL-induced hyperpolarization. These PRL-induced conductance activations are probably related to the PRL proliferative effect we have already described in U87-MG cells.

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