ANGIOTENSIN-II REGULATION OF INTRACELLULAR CALCIUM IN ASTROGLIA CULTURED FROM RAT HYPOTHALAMUS AND BRAIN-STEM

This study examines the angiotensin II (Ang II) regulation of intracel lular free calcium concentration ([Ca2+](i)) in astroglia cultured fro m the hypothalamus and brainstem of the adult rat. Bath perfusion or r apid puffer application of angiotensin II (Ang II) (1-100 nM) increase d [Ca2+](i) in both polygonal and stellate astroglia when measured usi ng fura-2 imaging fluorescence microscopy. Ang II increased [Ca2+](i) in 96.1 and 95.6% of the polygonal and stellate glial cells, respectiv ely. In normal Tyrode's solution (containing 2 mM CaCl2), the Ang II-s timulated increase in [Ca2+](i) characteristically showed a biphasic r esponse, i.e., an initial rapid transient peak followed by a sustained , steady-stale plateau of free Ca2+. In both cell types, the selective Ang II type 1 receptor subtype (AT(1)) antagonist losartan (1 mu M) i nhibited the Ang II-stimulated increase in [Ca2+](i). The selective AT (2) antagonist PD 123319 (1 mu M) did not inhibit the Ang II-stimulate d increase in [Ca2+](i) in either cell type. To define the sources of Ca2+ that participate in the Ang II-stimulated increase in [Ca2+](i) i n astroglia, experiments were performed in a nominally Ca2+-free Tyrod e's solution. In either cell type, this resulted in only an initial tr ansient increase of Ca2+ and no sustained plateau of Ca2+ when challen ged with Ang II. Thapsigargin (5 mu M), cyclopiazonic acid (10 mu M), and ryanodine (10 mu M), but not caffeine (1-10 mM), inhibited the ini tial rise in [Ca2+](i). The plateau increase of [Ca2+](i) caused by An g II (100 mu M) was reversibly inhibited by both cadmium (100 mu M) an d nifedipine (10 mu M); in contrast, gadolinium (100 mu M) had no effe ct on the plateau increase of [Ca2+](i). These results indicate that A ng II, in physiological concentrations, can activate AT(1) receptors t o stimulate both Ca2+ release from intracellular stores and Ca2+ influ x from the extracellular space to increase [Ca2+](i) of polygonal and stellate astroglia.