In vivo effect of 17β-estradiol on intestinal calcium absorption in rats

Previously we reported that intestinal cells contain estrogen receptors, and that 17β-estradiol enhanced calcium uptake by these cells in vitro. The current study was undertaken to examine the in vivo effects of 17β-estradiol on intestinal absorption of calcium and phosphorus. Three groups of rats were studied. Group 1 received solvent vehicle. Groups 2 and 3 received 5 μg and 40 μg 17β-estradiol/kg body weight/day, respectively, for 21 days. Hormone and solvent vehicle injections were given subcutaneously. Rats were fed a Teklad diet containing 0.4% Ca, 0.3% P and 3.0 U vitamin D/g during the study. Intestinal absorption of calcium and phosphorus was assessed over a 5-day period from day 15–19. Carmine red (25 mg/100 g diet) was added to the rat feed to mark the beginning and end of fecal collections. Administration of 17β-estradiol caused an increase in intestinal absorption of calcium and phosphorus. The increase was significant only for calcium, and in the animals that received high-dose 17β-estradiol (P < 0.05). Serum calcium and phosphorus levels were significantly greater in 17β-estradiol treated than in control animals. The urinary excretion of calcium and phosphorus was also increased in a dose-dependent manner by 17β-estradiol, and was significant for both calcium and phosphorus in animals that received high-dose 17β-estradiol (P < 0.05). In contrast, 17β-estradiol treatment did not significantly alter the serum levels of parathyroid hormone and l,25(OH)₂vitamin D. These findings indicate that estrogen administration promotes intestinal absorption of calcium in vivo. The enhanced calcium absorption, in spite of unaltered serum l,25(OH)₂vitamin D levels, suggests that estrogen does not promote calcium absorption mainly by increasing the circulating levels of l,25(OH)₂vitamin D.