IN-VIVO EFFECT OF 17-BETA-ESTRADIOL ON INTESTINAL CALCIUM-ABSORPTION IN RATS

Previously we reported that intestinal cells contain estrogen receptor s, and that 17 beta-estradiol enhanced calcium uptake by these cells i n vitro. The current study was undertaken to examine the in vivo effec ts of 17 beta-estradiol on intestinal absorption of calcium and phosph orus. Three groups of rats were studied. Group 1 received solvent vehi cle. Groups 2 and 3 received 5 mu g and 40 mu g 17 beta-estradiol/kg b ody weight/day, respectively, for 21 days. Hormone and solvent vehicle injections were given subcutaneously. Rats were fed a Teklad diet con taining 0.4% Ca, 0.3% P and 3.0 U vitamin D/g during the study. Intest inal absorption of calcium and phosphorus was assessed over a 5-day pe riod from day 15-19. Carmine red (25 mg/100 g diet) was added to the r at feed to mark the beginning and end of fecal collections. Administra tion of 17 beta-estradiol caused an increase in intestinal absorption of calcium and phosphorus. The increase was significant only for calci um, and in the animals that received high-dose 17 beta-estradiol (P < 0.05). Serum calcium and phosphorus levels were significantly greater in 17 beta-estradiol treated than in control animals. The urinary excr etion of calcium and phosphorus was also increased in a dose-dependent manner by 17 beta-estradiol, and was significant for both calcium and phosphorus in animals that received high-dose 17 beta-estradiol (P < 0.05). In contrast, 17 beta-estradiol treatment did not significantly alter the serum levels of parathyroid hormone and 1,25(OH)(2)vitamin D . These findings indicate that estrogen administration promotes intest inal absorption of calcium in vivo. The enhanced calcium absorption, i n spite of unaltered serum 1,25(OH)(2)vitamin D levels, suggests that estrogen does not promote calcium absorption mainly by increasing the circulating levels of 1,25(OH)(2)vitamin D.