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24,25‐Dihydroxyvitamin D3 Suppresses the Rapid Actions of 1,25‐Dihydroxyvitamin D3 and Parathyroid Hormone on Calcium Transport in Chick Intestine

Studies were undertaken to determine whether 24,25‐dihydroxyvitamin D3 (24,25(OH)2D3) modulates the rapid effects of 1,25‐dihydroxyvitamin D3 (1,25(OH)2D3) and parathyroid hormone (PTH) on calcium transport in the perfused chick intestine. Perfusion with control media resulted in a transport ratio (treated/average basal) of 1.07 ± 0.06 at t = 40 minutes, while perfusion with 65, 130, 300, or 650 pM 1,25(OH)2D3 yielded ratios of 1.92 ± 0.23, 2.6 ± 0.4, 2.8 ± 0.08, and 3.34 ± 0.37, respectively. Simultaneous perfusion with each of these doses and 6.5 nM 24,25(OH)2D3 reduced treated/average basal ratios to ∼1.4 after 40 minutes of perfusion. Vascular perfusion with 65 pM bovine PTH [bPTH(1–34)] stimulated intestinal calcium transport ratios to 3.0 ± 0.5 after 40 minutes, while the inclusion of 6.5 nM 24,25(OH)2D3 reduced ratios at this time point to 0.56 ± 0.19. To investigate the effect of these agents on signal transduction, isolated intestinal cells were monitored for intracellular calcium changes using the indicator dye fura‐2. After establishing a stable baseline, addition of 130 pM 1,25(OH)2D3 induced rapid calcium oscillations. Intestinal cells exposed to 6.5 nM 24,25(OH)2D3 also exhibited rapid oscillations in fluorescence, which were not further altered by subsequent addition of 1,25(OH)2D3. Incubation of isolated cells with 130 pM 1,25(OH)2D3 was found to increase protein kinase C (PKC) activity within 5 minutes, and protein kinase A (PKA) activity within 7 minutes. Exposure of cells to 65 pM bPTH(1–34) had minimal effect on PKC activity, but resulted in pronounced increases in PKA activity. Stimulation of protein kinases by either secosteroid or peptide hormone was inhibited in the presence of 6.5 nM 24,25(OH)2D3. It is concluded that 24,25(OH)2D3 may exert endocrine actions on intestine.