LONG-TERM ACTIVATION OF CAPACITATIVE CA2+ ENTRY IN MOUSE MICROGLIAL CELLS

The cytoplasmic free calcium concentration ([Ca2+](i)) was measured in cultured microglial cells with the Ca2+-sensitive fluorescent dye Fur a-2 using a digital imaging system. Stimulation of P-2 purinergic rece ptors by ATP or UTP always evoked a [Ca2+](i) elevation. The ATP-induc ed Ca2+ response involved both Ca2+ influx through ionotropic receptor s and Ca2+ release from intracellular pools, whereas UTP selectively s timulated intracellular Ca2+ release. When intracellular Ca2+ release was stimulated in the absence of extracellular Ca2+, the readmission o f extracellular Ca2+ caused a large rebound [Ca2+](i) increase. Follow ing this rebound, [Ca2+](i) did not return to the initial resting leve l, bur remained for long periods of time (up to 20 min), at a new; hig her steady-state level. Both the amplitude of the rebound Ca2+ transie nt and the new plateau level strongly correlated with the degree of in tracellular Ca2+ depletion, indicating the activation of a store-opera ted Ca2+ entry pathway. The elevated steady-state [Ca2+](i) level was associated with a significant increase in the plasma membrane permeabi lity to Ca2+, as changes in extracellular Ca2+ were reflected in almos t immediate changes of [Ca2+](i).Similarly, blocking plasma-lemmal Ca2 + channels with the non-specific agonist La3+ (50 mu M) caused a decre ase in [Ca2+](i), despite the continuous presence of Ca2+ ions in the extracellular medium. After the establishment of the new, elevated ste ady-state [Ca2+](i) level, stimulation of P-2U metabotropic purinorece ptors did not induce a [Ca2+](i) response. In addition, application of either thapsigargin (1 mu M) or carbonyl cyanide chlorophenyl hydrazo ne (10 mu M) failed to affect [Ca2+](i). We conclude that the maximal depletion of intracellular Ca2+ stores in mouse brain microglia determ ines the long-term activation of a plasma membrane Ca2+ entry pathway. This activation appears to be associated with a significant decrease in the capability of the intracellular Ca2+ stores to take up cytosoli c Ca2+ once they have been maximally depleted. (C) 1998 IBRO. Publishe d by Elsevier Science Ltd.