Calcium ATPase and Intestinal Calcium Transport in Uremic Rats

Calcium ATPase, an enzyme involved in intestinal calcium transport, was measured in homogenates of duodenal mucosal scrapings of normal and uremic rats. The effects of calcium deprivation and treatment with 1 alpha,25-dihydroxycholecalciferol [1,25-(OH)2D3] were investigated as well. Uremia decreased the enzyme activity and impaired the rise after calcium deprivation as observed in intact rats. The 1,25-(OH)2D3 treatment increased the enzyme activity in uremic animals and resulted in an identical response to calcium deprivation as observed in intact rats; parathyroidectomy abolished this effect. A striking correlation between everted duodenal gut sac calcium transport and calcium ATPase activity could be demonstrated for all groups of rats studied. It is concluded that the calcium ATPase activity is linked to the production of 1,25-(OH)2D3 as well as to an additional factor, probably parathyroid hormone. The close relationship between enzyme activity and in vitro calcium transport, even during constant physiological supplementation with 1,25-(OH)2D3, suggests an autonomous role of the calcium ATPase activity for mediation of calcium transport in the duodenum in addition to the well-known mechanisms related to vitamin D and its metabolites.