Regulation of Na+-K+-Cl−cotransporter in primary astrocytes by dibutyryl cAMP and high [K+]o

Abstract

In this study, we examined the Na+-K+-Cl− cotransporter activity and expression in rat cortical astrocyte differentiation. Astrocyte differentiation was induced by dibutyryl cAMP (DBcAMP, 0.25 mM) for 7 days, and cells changed from a polygonal to process-bearing morphology. Basal activity of the cotransporter was significantly increased in DBcAMP-treated astrocytes ( P < 0.05). Expression of an ∼161-kDa cotransporter protein was increased by 91% in the DBcAMP-treated astrocytes. Moreover, the specific [3H]bumetanide binding was increased by 67% in the DBcAMP-treated astrocytes. Inhibition of protein synthesis by cyclohexamide (2–3 μg/ml) significantly attenuated the DBcAMP-mediated upregulation of the cotransporter activity and expression. The Na+-K+-Cl−cotransporter in astrocytes has been suggested to play a role in K+ uptake. In 75 mM extracellular K+concentration, the cotransporter-mediated K+ influx was stimulated by 147% in nontreated cells and 79% in DBcAMP-treated cells ( P < 0.05). To study whether this high K+-induced stimulation of the cotransporter is attributed to membrane depolarization and Ca2+ influx, the role of the L-type voltage-dependent Ca2+ channel was investigated. The high-K+-mediated stimulation of the cotransporter activity was abolished in the presence of either 0.5 or 1.0 μM of the L-type channel blocker nifedipine or Ca2+-free HEPES buffer. A rise in intracellular free Ca2+ in astrocytes was observed in high K+. These results provide the first evidence that the Na+-K+-Cl− cotransporter protein expression can be regulated selectively when intracellular cAMP is elevated. The study also demonstrates that the cotransporter in astrocytes is stimulated by high K+ in a Ca2+-dependent manner.