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

Abstract

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.