Influx of Extracellular Calcium Mediates 1,25-Dihydroxyvitamin D3-Dependent Transcaltachia (the Rapid Stimulation of Duodenal Ca2+ Transport)

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)₂D₃ was 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⁺]i from 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)₂D₃ on [Ca²⁺]i.

The increase in [Ca²⁺]i elicited by the 1,25-(OH)₂D₃ was 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)₂D₃ on 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)₂D₃ effects on transcaltachia. (Endocrinology127: 2475–2480,1990)