Direct, rapid effects of 25-hydroxyvitamin D3 on isolated intestinal cells

Scattered reports in the literature have suggested that the metabolite 25-hydroxyvitamin D(3) [25(OH)D(3)] has biological activity. In the present work, perfusion of isolated duodenal loops of normal chickens with 100 nM 25(OH)D(3) resulted in enhanced transport of (45)Ca within 2 min relative to the vehicle controls. We then tested the effect of a range of 25(OH)D(3) concentrations on (45)Ca handling by isolated intestinal cells in time course studies. Following a basal uptake period, cell suspensions from 7-week old chicks were treated either with 25, 100, or 300 nM 25(OH)D(3), or the vehicle ethanol (0.01%, final concentration). Both 25 and 100 nM 25(OH)D(3) resulted in a significant (P < 0.05) reduction in (45)Ca levels, relative to controls, between 1-10 min after treatment, while 300 nM 25(OH)D(3) resulted in a significant increase in (45)Ca levels, relative to controls, after 10 min of incubation. The effect of 100 nM 25(OH)D(3) (a physiological level) on cell calcium was abolished by the presence of 6.5 nM 24,25-dihydroxyvitamin D(3). In cell preparations from 14- or 28-week old birds 100nM 25(OH)D(3) had no effect, relative to vehicle controls. Incubation of cells with 2 microM BAY K8644, a calcium channel activator, stimulated (45)Ca uptake within 3 min relative to vehicle controls (P < 0.05), while addition of either 20 microM forskolin or 100 nM phorbol ester (stimulators of the PKA and PKC pathways, respectively) resulted in enhanced radionuclide levels after 10 min of incubation (P < 0.05, relative to corresponding controls). Finally, cells were treated with 100 nM 25(OH)D(3) or vehicle and samples taken at various times for analyses of protein kinase C and A activities. No effect of 25(OH)D(3) on protein kinase C activity was observed, while protein kinase A activity was stimulated to nearly 200% of controls at 1 min after 25(OH)D(3) addition (P < 0.05, relative to corresponding controls) and began declining at 3 min, returning to control levels 5 min after additions. We conclude that 25(OH)D(3) has a direct effect on calcium handling in enterocytes of young animals that may in part be mediated by the protein kinase A signal transduction pathway.