Proximal and distal intestinal calcium transport in vitro as influenced by low calcium diet, uremia, parathyroidectomy and 1,25-dihydroxycholecalciferol treatment in rats.

Duodenal and ileal in vitro calcium transport was studied in 5/6 nephrectomized rats. Low calcium diet resulted in an increase of the duodenal calcium transport in kidney intact control animals but not in uremic rats; the ileal transport however increased in controls as well as in uremic rats. Parathyroidectomy decreased this adaptive transport at both gut sites in intact and uremic rats. Supplementation with low doses of 1,25-dihydroxycholecalciferol (1,25-DHCC) restored the duodenal transport in nephrectomized rats to the level of controls; nephrectomized plus parathyroidectomized animals increased the duodenal but not the ileal calcium transport. A close correlation between calcium transport and serum phosphorus concentrations of all groups of rats studied points towards a possible role of phosphate for intestinal calcium transport. It is concluded that duodenal calcium transport is regulated by the parathyroid-1,25-DHCC axis with 1,25-DHCC as effector; ileal calcium transport in contrast is regulated by combined direct effects of 1,25-DHCC and parathyroid hormone, the latter being essential for active transport.