Evidence for an intestinal mechanism in hypercalciuria of spontaneously hypertensive rats.

To define the mechanism for the hypercalciuria in spontaneously hypertensive rats (SHR), Ca clearance was evaluated in fasted 23-wk-old SHR and normotensive Wistar Kyoto (WKy) controls. There was no exaggerated calciuria before or after parathyroidectomy. Ca balance was therefore measured in the nonfasted animals, which revealed hyperabsorption in SHR of both sexes with increments 10-fold that of Ca excretion, supporting the primacy of intestinal hyperabsorption. In situ duodenal Ca uptake was also increased in the SHR. Parathyroidectomy did not affect the hyperabsorption. Hypercalcemia (total and ionized) and hypercalciuria in SHR associated with reduced adenosine 3',5'-cyclic monophosphate excretion, were abolished by fasting. Correction of hypertension for 6 mo failed to abolish the hypercalciuria. Bone Ca deposits were increased in 1-yr-old SHR. Ten-week-old SHR, in contrast, displayed mild malabsorption. Our data do not support the "renal leak" hypothesis. Instead, the adult SHR is characterized by increased Ca retention due to primary hyperabsorption, absorptive hypercalciuria, and increased bone Ca deposition. These phenomena are independent of sex, parathyroid hormone, and treatment of the established hypertension.