- Abstract 1,25-Dihydroxyvitamin D-3 (1,25(OH) 2 D 3 ) had direct effects on the HL-60 cell membrane. Treatment of HL-60 cells with 1,25(OH) 2 D 3 for short time periods (2–4 hours) caused an increase in calcium efflux. This phenomenon was found to be unrelated to new protein synthesis since it was not inhibited in the presence of RNA and protein synthesis inhibitors. The treatment of the HL-60 cells with 1,25(OH) 2 D 3 for four hours caused changes in their membrane phospholipid composition. The phosphatidylcholine:phosphatidylethanolamine ratio increased from 1.2 to 1.5. Thus the alteration in the phospholipid composition in the membrane induced by 1,25(OH) 2 D 3 may be responsible for the changes in the permeability of the membrane to calcium ions.