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

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.