Urinary calcium excretion at extremes of sodium intake in normal man

To elucidate the relationship between the renal regulation of sodium and calcium excretion at extremes of sodium intake, we studied 6 normal men ingesting a fixed, 400 mg/day calcium intake and four levels of sodium intake from 10 to 1,500 mEq/day. Serum ionized calcium was not influenced by sodium intake. Blood pressure and cardiac index increased modestly. Urinary calcium excretion increased to 262 +/- 53 mg/day (mean +/- SE). Plasma norepinephrine concentration, serum PTH levels, hematocrit, total serum protein concentrations and CO2 content decreased with increasing sodium intake. Urinary cAMP increased as sodium intake was raised from 10 to 300 mEq/day, but subsequently decreased to basal values. Urinary calcium and sodium excretion were related (p less than 0.001) in a nonlinear fashion as were the fractional excretions of these cations (p less than 0.001). The filtered calcium load and the fractional calcium excretion were directly and linearly related (p less than 0.001). We conclude that the effect of sodium intake on urinary calcium excretion principally reflects changes in the filtered calcium load rather than changes in renal sodium handling. Calcium homeostasis at extremes of sodium intake does not appear to be critically dependent upon PTH-mediated mechanisms. The data suggest that the proximal tubule has a remarkable capacity to dissociate calcium resorption from that of sodium.