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

Abstract

Previously we reported that intestinal cells contain estrogen receptors, and that 17 β-estradiol enhanced calcium uptake by these cells in vitro. The current study was undertaken to examine the in vivo effects of 17 β-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 17 β-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 17 β-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 17 β-estradiol ( P < 0.05). Serum calcium and phosphorus levels were significantly greater in 17 β-estradiol treated than in control animals. The urinary excretion of calcium and phosphorus was also increased in a dose-dependent manner by 17 β-estradiol, and was significant for both calcium and phosphorus in animals that received high-dose 17 β-estradiol ( P < 0.05). In contrast, 17 β-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.