Intestinal absorption of 1,25-dihydroxyvitamin D3 in the rat

Intestinal absorption of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] from in vivo jejunal sacs was studied in rats with thoracic duct and bile duct cannulas. In 6 h 86.5% of the administered 1,25(OH)2D3 was absorbed, but only 7.3% was recovered in thoracic duct lymph. Appearance in plasma of [3H]-1,25(OH)2D3 after intrajejunal administration was identical in rats with and without diverting lymph cannulas, indicating that the 1,25(OH)2D3 is absorbed almost entirely via portal blood. When 1,25(OH)2D3 was instilled into the jejunum in a fat suspension without bile salts rather than a mixed micellar solution, less was recovered in lymph, but total intestinal absorption was unchanged. High-pressure liquid chromatography of a lymph extract demonstrated that 1,25(OH)2D3 was present only as the unchanged secosteroid. In lymph, only 4.1% of the 1,25(OH)2D3 was in the chylomicrons, with the remainder bound to the plasma protein fraction of lymph. Treatment of rats with cycloheximide to block chylomicron synthesis did not decrease absorption of 1,25(OH)2D3. These results indicate that intestinal absorption of 1,25(OH)2D3 is effective and not very dependent on luminal bile salts. Almost all 1,25(OH)2D3 is released from the intestine directly into portal blood and does not require packaging in chylomicrons for transport into intestine lymph.