Intestinal calcium transport: parathyroid hormone and adaptation to dietary calcium.

Thyroparathyroidectomy prevents the elevation of intestinal calcium transport in response to low dietary levels of calcium. Removal of the thyroparathyroid glands reduces elevated intestinal calcium transport of rats on low calcium diets to the levels found in rats fed high calcium diets. This reduction took place 4 days after surgery. The chronic administration of a constant exogenous source of parathyroid hormone to thyroparathyroidectomized rats fed either a high or low calcium diet resulted in high rates of intestinal calcium transport independent of dietary calcium. Since 1,25-dihydroxyvitamin D₃ supplementation eliminates adaptation in a similar manner, these results strongly support the idea that parathyroid glands mediate intestinal adaptation to low dietary calcium presumably by the stimulation of 1,25-dihydroxyvitamin D₃ biosynthesis by secreted parathyroid hormone.