Parathyroid hormone contributes to volume expansion-induced inhibition of proximal reabsorption

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Abstract
Volume expansion inhibits the proximal reabsorption of H2O, HCO3, and Cl. The present work tested a hypothetical role of parathyroid hormone (PTH) in the expansion effect. Sprague-Dawley rats were studied during a plasma-replete euvolemic state and following 10% body wt colloid-free expansion. In group I, 6 intact rats, volume expansion decreased plasma ionized Ca concentration ([Ca2+]P) from 2.28 .+-. 0.06 to 2.11 .+-. 0.04 meq/l and increased nephrogenous cAMP (NcAMP) from 29 .+-. 5 to 66 .+-. 10 pmol .cntdot. min-1 .cntdot. g kidney wt-1. In group II, 6 acutely thyroparathyroidectomized (TPTX) rats, [Ca2+]P also fell from 2.18 .+-. 0.08 to 1.80 .+-. 0.08 meq/l but NcAMP did not rise significantly (9 .+-. 3 vs. 17 .+-. 5 pmol .cntdot. min-1 .cntdot. g kidney wt-1). Stimulation of PTH activity occurred during expansion in intact animals. In group III, 7 TPTX rats, volume expansion inhibited proximal reabsorption of total CO2 by 11%, of Cl by 24% and of H2O by 19%. Volume expansion-induced reduction in HCO3, Cl and H2O reabsorption was smaller in TPTX than in intact rats previously studied. Volume expansion inhibits proximal reabsorption in part by decreasing the active transcellular NaHCO3 and NaCl transport secondary to stimulation of PTH activity.

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