Reabsorption of lithium and phosphate by the rat kidney: role of the parathyroids

Abstract

Because of the possibility that lithium reabsorption may parallel proximal tubular reabsorption in general, the fractional excretions of lithium (FE(Li)), phosphate (FE(p)) and sodium (FE(Na)) were compared in rats preloaded intraperitoneally with lithium during hydropenia and during stepwise NaCl/HCO3 volume expansion at 150 and 600 μl/min. In intact rats, all three indexes increased significantly during volume expansion. After acute parathyroidectomy, FE(p) values during hydropenia were 93% less than in the intact animals. FE(Li), FE(p) and FE(Na) all increased significantly during progressive volume expansion, but the increase in FE(p) was less than 1% of the increase in the intact rats. When chronically parathyroidectomized rats were studied, FE(Li) and FE(Na) again increased during volume expansion. However, FE(p) values did not increase, but remained at very low levels similar to those in the acutely parathyroidectomized animals. Calcium and magnesium infusion, separately and together, in other acutely parathyroidectomized volume expanded rats did not decrease FE(Li). The results indicate that lithium reabsorption in the rat is not affected by the functional status of the parathyroid glands or the presence of parathyroid hormone. Parathyroidectomy dissociates changes in lithium and phosphate reabsorption during volume expansion.