The active metabolite of D vitamin, 1,25(OH)(2)D-3, has been suggested to promote acute uptake Of Calcium through the intestinal lining in cell lines and murine models. In this Study, the effects of D vitamin on the cytoplasmic Ca2+ of single human jejunal enterocytes, obtained with LOC-1-GUT technique, was analyzed in vivo in 1 fluorometric system using fura-2 as the Ca2+-sensing probe. Vitamin-promoted acute Ca2+ influx exhibited dual kinetics, indicating initial release from intracellular Ca2+ pools and fast entry from the extracellular space. Furthermore, providing a chemical clamp of membrane potential close to 0 mV did not activate voltage-sensitive calcium channels in the cellular membrane, neither was the hormone-induced Ca2+ influx affected by verapamil. This advocates that voltage-operated channels like L-type Ca2+ channels do not participate in the process of Ca2+ uptake. In fact, the existence of calcium-release-activated-calcium channels (I-CRAC) was implied by the findings that irreversible depletion of intracelhilar Ca2+ stores by thapsigargin promoted Ca2+ entry. In the thapsigargin-treated enterocytes, D vitamin lost its ability to promote calcium entry indicating all important role for intracellular store-operated Ca2+ stores in the acute effects of 1,25(OH)(2)D. (c) 2005 Elsevier Inc. All rights reserved.