Altered kinetics of an intravenous calcium load in idiopathic hypercalciuria

Increased gut calcium absorption or reduced renal tubular calcium reabsorption have been alternatively reported in idiopathic hypercalciuria with kidney calculi. The present study aimed to investigate the handling of an exogenous calcium load in hypercalciuric stone formers to detect possible differences with regard to tissue calcium metabolism in vivo. A constant rate intravenous calcium infusion (0.2 mmol kg bodyweight per two hours) was carried out on six absorptives and six renals, defined according to the reported criteria, as well as on normal controls from clinical staff. Serum ionized calcium concentration were determined at regular intervals during the infusion and in the four hours after the end of calcium load. Over the same period, urinary calcium excretion was evaluated in timed urine collections. Absorptive and renal hypercalciurics had lower serum ionized calcium levels compared with normal controls at all experimental times, a finding that suggests a faster disappearance of calcium from the circulation. The total body calcium clearance calculated from the area under the curve of the serum calcium concentrations was enhanced in hypercalciuric patients (P less than .001). Although renal calcium excretion was higher in hypercalciurics, renal calcium clearance accounted for only a minor fraction of the total body clearance, suggesting that the reduced serum calcium levels found in the hypercalciurics could not be explained by the renal calcium leak. The enhanced total body calcium clearance found in hypercalciuric subjects is therefore due to an increased tissue calcium uptake. This finding provides indirect evidence of altered cell calcium handling in idiopathic hypercalciura with no difference between the so-called absorptives and renals in terms of the pathophysiologic mechanism.