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 duodenal of vitamin D-replete chicks. The carboxylic ionophore ionomycin (2 microM) was found to stimulate 45Ca2+ 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 microM), 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 microM). 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.
de Boland
A R
AR
Division of Biomedical Sciences, University of California, Riverside 92521.
Norman
A W
AW
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002117:Calcitriol / Q000494:pharmacology / Q000502:physiology*; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics*; D002645:Chickens; D005576:Colforsin / Q000494:pharmacology; D004386:Duodenum / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism*; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D015759:Ionomycin / Q000494:pharmacology; D008297:Male; D009994:Osmolar Concentration; D011189:Potassium Chloride / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013997:Time Factors
D005456:Fluorescent Dyes; C049925:fura-2-am; D005576:Colforsin; D004533:Egtazic Acid; D015759:Ionomycin; D011189:Potassium Chloride; D002117:Calcitriol; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium; D016257:Fura-2