Stimulation of avian renal phosphate secretion by parathyroid hormone

Phosphate buffers (ammonium, sodium, potassium, and calcium phosphate, pH 5.5, 7.2, 8.5) and 32P were infused unilaterally into the renal portal systems of intact, parathyroidectomized (PTX), and parathyroid hormone-infused (PTH) domestic fowl to study the secretory flux for inorganic phosphate (Pi). Urine samples were collected simultaneously from both kidneys, with the uninfused kidney serving as a control for the portal-perfused kidney (modified Sperber technique). No consistent unilateral excess of Pi or 32P excretion occurred for any of the experimental groups. For intact birds, fractional 32P excretion by both kidneys (FE32p) was identical to fractional Pi excretion (FEpi) (determined by chemical analysis) and reflected net reabsorption (0.64). However, during PTH infusion, FE32p was 0.82 (net reabsorption) while FEPi was 1.21 (net secretion). These results indicate that a) the peritubular-to-lumen flux for Pi and 32P is a minor component of net tubular transport, regardless of the parathyroid status, counterion availability, or peritubular Pi concentration; b) plasma Pi and 32P enter the tubule lumen predominantly by filtration; c) PTH stimulates tubular Pi secretion; and d) the secreted Pi is derived from an organic or inorganic pool that does not readily equilibrate with infused 32P (or presumably peritubular Pi).