1,25-(OH)2 VITAMIN-D-3 ENHANCES PAF-STIMULATED CALCIUM MOBILIZATION IN U937 CELLS BY EXPRESSION OF SPECIFIC PAF BINDING-SITES

The effect of 1,25-(OH)2 vitamin D-3 (10 nM, 72 h) on cytosolic free Ca2+ concentration ([Ca2+]i) in U937 cells before and after stimulation with Paf, LTD4 and ADP was investigated. 1,25-(OH)2 vitamin D-3 increased basal [Ca2+]i from 98 ± 1 nM to 121 ± 5 nM (P < 0.01) and the Paf (10 nM) stimulated increase in [Ca2+]i from 143 ± 15 to 406 ± 44 nM (P < 0.01). These vitamin D-3 effects were time-related and occurred after 24 h (basal [Ca2+]i) and 12 h (Paf stimulated Ca2+-mobilization) but not after 3 h. In comparison, vitamin D-3 failed to modulate Ca2+-mobilization in response to ADP (1-40 μM) and increased it only in response to low leukotriene D4 concentrations (0.1-1 nM). The total binding of [3H]Paf (2.8 nM) was not significantly different in untreated vs. vitamin D-3-treated cells. However, the Paf receptor antagonist Web 2086 (1 μM) inhibited [3H]Paf binding only in vitamin D-3-treated cells. The specific binding reached a plateau of 28 ± 3 fmol per 2.5 · 106 cells between 1.4 and 2.8 nM [3H]Paf. The Paf receptor antagonist Web 2086 (1-1000 nM) also inhibited the Paf-mediated Ca2+-mobilization in vitamin D-3-treated cells (IC50 = 191 ± 55 M). These data suggest that the enhanced Ca2+-mobilization in vitamin D-3-treated U937 cells in response to Paf is mediated by an expression of putative Paf receptors. © 1990.