Calcium transport in the ileum of uremic rats.

Duodenal and ileal calcium transport was studied by 45Ca uptake and estimation of unidirectional fluxes in vitro in kidney-intact and 5/6-nephrectomized rats. After normal-calcium diet, calcium transport was impaired by uremia in the duodenum but not in the ileum. However, 5/6-nephrectomized rats on low-calcium diet showed, in concert with impaired growth, a reduced calcium transport both in duodenum and ileum. Comparing data after normal- and low-calcium diets, the ileal adaptation to low-calcium diet was intact in mild renal failure but abolished in severe uremia (urea less than 100 mg/dl). These results suggest that ileal calcium transport after normal-calcium diet is mainly passive. Taking into account reduced food intake as an additional factor, the active ileal calcium transport after low-calcium diet declines with progressive renal failure according to decreasing levels of 1,25-dihydroxyvitamin D3. A suspected enhancement of the distal small intestinal calcium transport by parathyroid hormone in uremic rats as compensation for proximally impaired absorption could not be demonstrated.