Early effects of 1 alpha,25-dihydroxy-vitamin D3 on phosphate absorption. A role for alkaline phosphatase?

Using ex vivo perfused intestinal loop from vitamin D-deficient rats, we studied the lumen-to-vein transport of 32Pi in intestines perfused intraluminally with [gamma-32P] ATP, [alpha-32P] ATP or 32Pi, and infused through their vascular connections with either 1,25(OH)2D3 (6nM) or ethanol vehicle. For intestines perfused with [gamma-32P] ATP in which the gamma-32P is easily liberated by alkaline phosphatase, 1,25(OH)2D3 infusion resulted in a significant and early (15 min) increase in net lumen-to-vein 32Pi transport. Pretreatment of rats with levamisole, an inhibitor of alkaline phosphatase, partially inhibited such 32Pi transport. For intestines infused with [alpha-32P] ATP, the lumen-to-vein transport of 32Pi did not differ in 1,25(OH)2D3-treated and in control intestines. For intestines perfused with 32Pi, 1,25(OH)2D3 infusion resulted in an early (15 min) increase in lumen-to-vein transport of 32Pi. It is concluded that 1,25(OH)2D3 rapidly stimulates intestinal Pi transport both directly and also by increasing inorganic phosphate content inside the intestinal lumen. The latter effect probably involves 1,25(OH)2D3-stimulated alkaline phosphatase activation.