AJP - Gastrointestinal and Liver Physiology, Vol 248, Issue 6 718-G725, Copyright © 1985 by American Physiological Society
Intestinal absorption of 1,25-dihydroxyvitamin D3 in the rat
M. D. Sitrin, K. L. Pollack and M. J. Bolt
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.
