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 less than 0.01) and the Paf (10 nM) stimulated increase in [Ca2+]i from 143 +/- 15 to 406 +/- 44 nM (P less than 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 microM) 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 microM) inhibited [3H]Paf binding only in vitamin D-3-treated cells. The specific binding reached a plateau of 28 +/- 3 fmol per 2.5.10(6) 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.