Calcium modulation of phorbol ester-induced alterations in murine macrophage morphology.

The phorbol ester tumor promoter phorbol-12-myristate-13-acetate (PMA) induced mouse resident peritoneal macrophage spreading in an in vitro system in a time- and dose-dependent manner; this process was modified by agents which alter intracellular calcium metabolism. After a 35-min incubation with PMA, 50% of the macrophages were spread (as classified by at least a 2-fold increase in cell surface area). Also at 35 min, the median effective concentration for PMA induction of spreading was 1.6 ng/ml. The intracellular calcium antagonist 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate inhibited PMA-induced cell spreading with a one-half maximal inhibitory concentration of 8.0 microM. The calcium ionophore A23187 enhanced PMA-induced spreading by 30% at 0.1 to 10 nM. The histological dye ruthenium red, which purportedly increases intracellular calcium by displacing membrane-bound calcium stores, enhanced PMA-induced spreading up to 75% at 1.0 pM. The cationic chelator ethyleneglycolbis(beta-aminoethylether)-N,N-tetraacetic acid (1.8 and 3.6 mM) had no effect on PMA-induced spreading. Thus, PMA-induced spreading was independent of extracellular calcium but was modulated by agents altering intracellular calcium metabolism. Microfilament formation, a proposed mechanism of cell spreading, also depends on intracellular calcium availability. The microfilament inhibitor cytochalasin B inhibited PMA-induced spreading with a one-half maximal inhibitory concentration of 1 microM. Future experiments should investigate the hypothesis that calcium availability to the cytoskeletal elements regulates the morphological effects of PMA on macrophages.