CHARACTERIZATION OF THE 1,25-DIHYDROXYCHOLECALCIFEROL-STIMULATED CALCIUM INFLUX PATHWAY IN CACO-2 CELLS

The present studies were conducted to investigate the mechanisms under lying the 1,25-dihydroxycholecalciferol (1,25(OH)2D3)-induced increase in intracellular Ca2+ ([Ca2+]i) in individual CaCo-2 cells. In the pr esence of 2 mm Ca2+ 1,25(OH)2D3-induced a rapid transient rise in [Ca2 + in Fura-2-loaded cells in a concentration-dependent manner, which de creased, but did not return to baseline levels. In Ca2+-free buffer, t his hormone still induced a transient rise in [Ca2+]i, although of low er magnitude, but [Ca2+]i then subsequently fell to baseline. In addit ion, 1,25(OH)2D3 also rapidly induced Ca-45 uptake by these cells, ind icating that the sustained rise in [Ca2+]i was due to Ca2+ entry. In M n2+-containing solutions, 1,25(OH)2D3 increased the rate of Mn2+ influ x which was temporally preceded by an increase in [Ca2+]i. The sustain ed rise in [Ca2+]i was inhibited in the presence of external La3+ (0.5 mm). 1,25(OH)2D3 did not increase Ba2+ entry into the cells. Moreover , neither high external K+ (75 mm), nor the addition of Bay K 8644 (I mum), an L-type, voltage-dependent Ca2+ channel agonist, alone or in c ombination, were found to increase [Ca2+]i. 1,25(OH)2D3 did, however, increase intracellular Na+ in the absence, but not in the presence of 2 mm Ca2+, as assessed by the sodium-sensitive dye, sodium-binding ben zofuran isophthalate. These data, therefore, indicate that CaCo-2 cell s do not express L-type, voltage-dependent Ca2+ channels. 1,25(OH)2D3 does appear to activate a La3+inhibitable, cation influx pathway in Ca Co-2 cells.