We investigated the role of extracellular Ca2+in 1, 25-dihydroxyvitamin D3[1, 25-(OH)2D3] rapid stimulation of intestinal Ca2+transport (termed transcaltachia) in the perfused duodena of vitamin D-replete chicks. The carboxylic ionophore ionomycin (2 nM) was found to stimulate 46Ca2+transport from the lumen to the vascular effluent to the same extent as physiological levels of 1, 25-(OH)2D3. The increase in duodenal 45Ca2+transport caused by 1, 25-(OH)2D3was dependent on the presence of medium Ca2+, since it was abolished by prior addition of EGTA and was restored upon the addition of Ca2+. Depolarization of the basal lateral membrane of intestinal epithelial cells with 70 mM K+caused a rapid increase in 45Ca2+transport (30% above control values within 2 min and 250% after 20 min of vascular perfusion). The rise was also abolished by prior addition of EGTA.Intracellular calcium concentrations ([Ca2+]i) were measured in isolated duodenal cells from vitamin D-replete chicks using the fluorescent dye fura 2. A 1-min incubation with physiological concentrations of 1, 25-(OH)2D3(130 pM) caused an increase in [Ca2+]ifrom a basal level of 168 ± 23 nM to 363 ± 44 nM. Pretreatment of intestinal epithelial cells with the protein kinase- C activator tetradeconyl-phorbol acetate (100 nM) or the adenylate cyclase activator forskolin (10 nM), both shown to induce acute stimulation of intestinal 45Ca2+transport in the perfused duodenum, also mimicked the stimulatory effect of 1, 25-(OH) on [Ca2+]i.The increase in [Ca2+]ielicited by the 1, 25-(OH)2D3was due to Ca2+influx from the extracellular medium, since it was blocked by the Ca2+chelator EGTA (5 mM) and the Ca2+channel antagonist nifedipine (1 nM). These results suggest that the acute effects of 1, 25-(OH)2D3on duodenal 45Ca2+transport are triggered by the influx of Ca2+through voltage-operated Ca2+channels and that both protein kinase-C and protein kinase-A play an important role in mediating or modulating 1, 25-(OH)2D3effects on transcaltachia. © 1990 by The Endocrine Society.
