Influence of Parathyroidectomy and Calcium on Rat Renal Function

Abstract

Parathyroid hormone (PTH) has multiple effects on water and electrolyte transport along the nephron. However, the influences of PTH and calcium on the urinary concentration ability are not fully understood. In this study, clearance and microperfusion studies were performed in thyroparathyroidectomized (TPTX) rats either supplemented (TPTX+Ca²⁺) or not with calcium added to the ingested food as CaCl₂ (1.6 g/100 g). Acid-base data and renal functional parameters were measured in TPTX and TPTX+Ca²⁺ rats. Additional studies were performed in the isolated inner medullary collecting tubules of intact and TPTX rats to evaluate the osmotic permeability of this segment in the presence of 10^–6 M PTH added to the bath. In these experiments the possible influence of PTH on antidiuretic hormone induced changes of the osmotic permeability in TPTX and TPTX+Ca²⁺ rats was also investigated. In the TPTX+Ca⁺ group, the glomerular filtration rate increased significantly when compared to the TPTX group (6.04 ± 0.42 vs. 4.88 ± 0.20 ml·min^–1·kg^–1; p < 0.05), but the U/P inulin ratio remained lower than control values (30.8 ± 1.48 vs. 54.0 ± 3.5; p < 0.05), which suggests that normal levels of PTH are necessary to maintain the concentrating ability. In a group of TPTX rats, an acute infusion of PTH (0.5 µg·min^–1·kg^–1) significantly decreased the urinary flow and increased the renal plasma flow, results that agree with the vasomodulator action of this hormone on the renal vasculature. A significant increase in the fractional K⁺ excretion observed in the TPTX+Ca²⁺ group as compared with both control and TPTX, groups suggests that the excreted load of Ca²⁺ may interfere with tubular K⁺ handling in the absence of PTH. PTH (10^–6 M) added to the bath of the isolated inner medullary collecting tubules did not change the osmotic permeability, of intact, TPTX, and TPTX+Ca²⁺ rats. Furthermore, it did not modify the antidiuretic hormone induced changes in the osmotic permeability. These results suggest that this segment of the nephron is PTH insensitive as far as water and ion transport are concerned.