DEPLETION OF INTRACELLULAR CALCIUM STORES FACILITATES THE INFLUX OF EXTRACELLULAR CALCIUM IN PLATELET-DERIVED GROWTH-FACTOR STIMULATED A172GLIOBLASTOMA CELLS

Calcium signaling in non-excitable cells is the consequence of calcium release from intracellular stores, at times followed by entry of extr acellular calcium through the plasma membrane. To study whether entry of calcium depends upon the level of saturation of intracellular store s, we measured calcium channel opening in the plasma membrane of singl e confluent A172 glioblastoma cells stimulated with platelet derived g rowth factor (PDGF) and/or bradykinin (BK), We monitored the entry of extracellular calcium by measuring manganese quenching of Indo-1 fluor escence, PDGF raised intracellular calcium concentration ([Ca2+](i)) a fter a dose-dependent delay (t(del)) and then opened calcium channels after a dose-independent delay (t(ch)). At higher doses (> 3 nM), BK i ncreased [Ca2+](i) after a t(del) similar to 0 s, and t(ch) decreased inversely with both dose and peak [Ca2+](i), Experiments with thapsiga rgin CTG), BK, and PDGF indicated that BK and PDGF share intracellular Ca2+ pools that are sensitive to TG, When these stores were depleted by treatment with BK and intracellular BAPTA, t(del) did not change, b ut t(ch) fell to almost 0 s in PDGF stimulated cells, indicating that depletion of calcium stores affects calcium channel opening in the pla sma membrane. Our data support the capacitative model for calcium chan nel opening and the steady-state model describing quantal Ca2+ release from intracellular stores. (C) 1996 Wiley-Liss, Inc.