ANTIGEN AND THAPSIGARGIN PROMOTE INFLUX OF CA2-2H3 CELLS BY OSTENSIBLY SIMILAR MECHANISMS THAT ALLOW FILLING OF INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE AND MITOCHONDRIAL CA2+ STORES( IN RAT BASOPHILIC RBL)
H. Ali et al., ANTIGEN AND THAPSIGARGIN PROMOTE INFLUX OF CA2-2H3 CELLS BY OSTENSIBLY SIMILAR MECHANISMS THAT ALLOW FILLING OF INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE AND MITOCHONDRIAL CA2+ STORES( IN RAT BASOPHILIC RBL), Biochemical journal, 304, 1994, pp. 431-440
In single, Fura 2-loaded RBL-2H3 cells, antigen and thapsigargin deple
ted the same intracellular pool of Ca2+ in the absence of external Ca2
+, provision of external Ca2+ induced immediate increases in levels of
free Ca2+ ([Ca2+](i)). These increases were dependent on the presence
of external Ca2+ and, presumably, on influx of Ca2+ across the cell m
embrane. Both stimulants enhanced intracellular accumulation of Ca-45(
2+) through ostensibly similar mechanisms because accumulation was blo
cked to similar extents by various multivalent cations or by depolariz
ation with K+. Because thapsigargin blocked reuptake of Ca2+ into inos
itol 1,4,5-trisphosphate sensitive stores, uptake occurred independent
ly of the refilling of these stores. Uptake was dependent instead on s
equestration of Ca-45(2+) in a pool of high capacity that was insensit
ive to thapsigargin, caffeine, GTP and inositol 1,4,5-trisphosphate bu
t sensitive to ionomycin and mitochondrial inhibitors. The existence o
f an inositol 1,4,5-trisphosphate-insensitive pool was also apparent i
n permeabilized cells; at 0.1 mu M [Ca2+](i), uptake of Ca-45(2+) was
largely confined (> 80 %) to the inositol 1,4,5-trisphosphate-sensitiv
e pool, but at 2 mu M [Ca2+](i) uptake was largely (> 60 %) into the i
nositol 1,4,5-trisphosphate-insensitive pool. Provision of mitochondri
al inhibitors along with thapsigargin to block uptake into both pools,
did not impair the thapsigargin-induced increase in [Ca2+](i) or infl
ux of Ca2+, as indicated by changes in Fura 2 fluorescence, but did bl
ock the intracellular accumulation of Ca-45(2+). The studies illustrat
e the utility of simultaneous measurements of [Ca2+](i) and Ca-45(2+)
uptake for a full accounting of Ca2+ homoeostasis as exemplified by th
e ability to distinguish between influx and mitochondrial uptake of Ca
2+.