Studies on intestinal calcium absorption in patients with idiopathic hypercalciuria

Using an in vitro method, the uptake of radio-labelled Ca2+ by jejunal biopsy specimens from control subjects, patients with idiopathic hypercalciuria and patients with renal stones without hypercalciuria, were compared. Radio-labelled Ca2+ uptake was investigated over the concentration range 0.1-5.0 mmol/l. For all subjects there was a linear relationship between Ca2+ uptake and medium concentration suggesting that Ca2+ uptake was a passive process. There was no significant difference in Ca2+ uptake between control subjects and patients with renal stones without hypercalciuria. Patients with idiopathic hypercalciuria, both absorptive and renal subtypes, showed increased Ca2+ uptake at all incubation medium concentrations. Assays of various biochemical parameters including alkaline phosphatase, Ca2+-activated ATPase, cyclic AMP and Ca2+-binding protein, in jejunal biopsy specimens showed no significant differences between control subjects and patients with idiopathic hypercalciuria. The results suggest that the intestinal abnormality in idiopathic hypercalciuria is due to enhanced permeability of the brush border membrane to Ca2+, possibly mediated by alterations in membrane lipids.