Ca2+Mg2+-ATPase activity in erythrocyte membranes in hypercalciuric nephrolithiasic patients

Ca(2+)Mg(2+)ATPase is an enzyme involved in calcium transport across biological membranes. In order to determine its possible role in the pathogenesis of calcium nephrolithiasis, we assessed Ca2+Mg2+-ATPase activity in erythrocyte membranes from 18 hypercalciuric (HC) nephrolithiasic patients (nine with absorptive hypercalciuria (AHC), nine with renal hypercalciuria (RHC)) and eight normocalciuric healthy controls (C). Participants took no medication for at least 3 months before the study and remained on a diet containing approximately 25 mmol of calcium per day for 14 days. Pump activity was measured in calmodulin-free red blood cell membranes and expressed in mumol of phosphate released per mg of membrane protein per h. There were statistically significant differences in pump activity between groups (1.43+/-0.07; 1.93+/-0.1; 1.65+/-0.06; C, AHC, RHC, respectively, P<0.005 (three groups) and P<0.02 AHC versus RHC. Enzyme activity was positively correlated with 24-h urinary calcium excretion (r=0.64, P<0.01) in HC patients; no such relationship was found in C. In conclusion, erythrocyte membrane Ca(2+)Mg(2+)ATPase activity is increased in HC. Differences in enzyme activity between AHC and RHC may reflect different degrees of a single generalized epithelial calcium transport disturbance.