On the Pathogenetic Mechanism of Hypercalciuria in Genetically Hypertensive Rats of the Milan Strain

The aim of this study was to investigate the pathogenesis of hypercalciuria in the Milan strain of genetically hypertensive rats. Dietary calcium intake and urinary and fecal calcium output were measured simultaneously with indices of sodium and phosphate homeostasis in male rats of the Milan hypertensive and normotensive strains. In addition, urinary calcium and creatinine excretion rates, calcium, phosphate and creatinine serum concentrations, and bone calcium content were also measured in these rats after an overnight fast. Under fed steady-state conditions dietary calcium, sodium, and phosphate intakes, were similar in the two groups of rats, but hypertensive rats had twofold higher urinary calcium excretion and normal urinary excretion of sodium and phosphate. Fecal calcium output was slightly but significantly higher in the adult hypertensive rats while fecal sodium and phosphate excretion was normal. Because of increased urinary and fecal calcium loss, net calcium balance was significantly less positive in hypertensive than in control rats. Under fasting conditions hypertensive rats were confirmed to have hypercalciuria despite normal serum calcium concentrations and normal creatinine clearance. In accordance with balance data and fasting hypercalciuria, bone calcium content was found to be significantly reduced in hypertensive rats. These findings confirm that hypercalciuria in the Milan hypertensive rats is explained by an altered renal calcium handling; it is also associated with a slightly increased fecal calcium output and, therefore, with a less positive calcium balance and reduced bone calcium content. Am J Hypertens 1989; 2:741 – 746