Effects of phosphate loading and parathyroid hormone on starling renal phosphate excretion

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Abstract
The effects of phosphate loading were examined in intact, parathyroidectomized (PTX), and PTX starlings infused with parathyroid extract (PTX +/- PTE). In intact starlings, there was no correlation between the filtered phosphate load and the quantity of phosphate excreted. Net secretion predominated at lower filtered loads (2.5-20 mu mol . kg-1 . min-1) but net reabsorption occurred at filtered loads between 20 and 35 mu mol . kg-1 . min-1. Phosphate-loaded PTX starlings reabsorb phosphate over a wide range of filtered loads (5-45 mu mol . kg-1 . min-1). Excretion was significantly correlated with the filtered load in PTX birds (r = 0.943). Phosphate loaded PTX + PTE starlings secreted phosphate when filtered loads ranged from 2.5 to 30 mu mol . kg-1 . min-1. Excretion also was significantly correlated with the filtered load in this group (r = 0.972). The results of these experiments indicate that a) variations in endogenous PTH release in intact starlings result in wide variations in the renal pattern of phosphate excretion during phosphate loading; b) net phosphate secretion rarely occurs during phosphate loading in PTX starlings, but always occurs in PTX + PTE starlings; and c) both phosphate secretion and phosphate reabsorption are limited by maximal tubular transport capacities. Additional evidence indicates that PTH inhibits tubular phosphate reabsorption in starlings.

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