Calcium influx in a rat mast cell (RBL-2H3) line. Use of multivalent metal ions to define its characteristics and role in exocytosis

An increase in concentration of cytosolic Ca2+ ([Ca2+]i) is associated with an accelerated influx of 45Ca2+ when cultured RBL-2H3 cells are stimulated with either antigen or analogs of adenosine although these agents act via different receptors and coupling proteins (Ali, H., Cunha-Melo, J.R., Saul, W.F., and Beaven, M.A. (1990) J. Biol. Chem. 265, 745-753). The same mechanism probably operates for basal Ca2+ influx in unstimulated cells and for the accelerated influx in stimulated cells. This influx had the following characteristics. 1) It was decreased when cells were depolarized with high external K+; 2) it was blocked by other cations (La3+ greater than Zn2+ greater than Cd2+ greater than Mn2 = Co2+ greater than Ba2+ greater than Ni2+ greater than Sr2+) either by competing with Ca2+ at external sites (e.g. La3+ or Zn2+) or by co-passage into the cell (e.g. Mn2+ or Sr2+); and 3) the inhibition of influx by K+ and the metal ions had exactly the same characteristics whether cells were stimulated or unstimulated even though influx rates were different. The dependence of various cellular responses on influx of Ca2+ was demonstrated as follows. The stimulated influx of Ca2+, rise in [Ca2+]i, and secretion, could be blocked in a concentration-dependent manner by increasing the concentration of La3+, but concentrations of La3+ (greater than 20 microM) that suppressed influx to below basal rates of influx markedly suppressed the hydrolysis of inositol phospholipids (levels of inositol 1,4,5-trisphosphate were unaffected). Some metal ions, e.g. Mn2+ and Sr2+, however, supported the stimulated hydrolysis of inositol phospholipid and some secretion in the absence of Ca2+. Thus a basal rate of influx of Ca2+ was required for the full activation of inositol phospholipid hydrolysis, but in addition an accelerated influx was necessary for exocytosis.