Relationship between sodium intake, proximal tubular function and calcium excretion in normal subjects and in...

Proximal tubular function was studied with maximal water clearance studies in 15 controls (C), 22 Ca stone formers with idiopathic hypercalciuria (IH) and 10 normocalciuric Ca stone formers (NC). Distal delivery of glomerular filtrate (ClDD) and Na excretion were higher in IH and NC than in C; NaCl loading (6 g/day) for seven days in C increased Na excretion and ClDD to similar levels as in IH and NC; the distribution of ClDD for any level of Na excretion was similar in C, NC and IH. NaCl loading in C slightly reduced renal phosphate threshold, which still remained higher than in IH on a free diet. It appears that reduced tubular reabsorption of glomerular filtrate in Ca stone formers is related to habitual high Na intake and is not peculiar to hypercalciuric patients. Habitual high Na intake is unlikely to be responsible for all the metabolic spectrum of idiopathic hypercalciuria.