Influence of parathyroidectomy and calcium on rat renal function

Parathyroid hormone (PTH) has multiple effects on water and electrolyte tra nsport along the nephron. However, the influences of PTH and calcium on the urinary concentration ability are not fully understood. In this study, cle arance and microperfusion studies were performed in thyroparathyroidectomiz ed (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 functio nal parameters were measured in TPTX and TPTX+Ca2+ rats. Additional studies were performed in the isolated inner medullary collecting tubules of intac t and TPTX rats to evaluate the osmotic permeability of this segment in the presence of 10(-6) MPTH added to the bath. In these experiments the possib le influence of PTH on antidiuretic hormone induced changes of the osmotic permeability in TPTX and TPTX+Ca2+ rats was also investigated. In the TPTXCa+ group, the glomerular filtration rate increased significantly when comp ared 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 o f PTH are necessary to maintain the concentrating ability. In a group of TP TX rats, an acute infusion of PTH (0.5 mu g.min(-1).kg(-1)) significantly d ecreased the urinary flow and increased the renal plasma flow, results that agree with the vasomodulator action of this hormone on the renal vasculatu re. A significant increase in the fractional K+ excretion observed in the T PTX+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 ab sence of PTH. PTH (10(-6) M) added to the bath of the isolated inner medull ary collecting tubules did not change the osmotic permeability, of intact, TPTX, and TPTX+Ca2+ rats. Furthermore, it did not modify the antidiuretic h ormone induced changes in the osmotic permeability. These results suggest t hat th is seg ment of the nephron is PTH insensitive as far as water and io n transport are concerned.