Parathyroid hormone effect on chicken renal brush-border membrane phosphate transport

Renal clearance studies showed that parathyroidectomy (PTX) stimulated renal PO4 reabsorption within 90 min in 3-wk-old chicks. Subsequent parathyroid hormone (PTH) infusion caused net PO4 secretion. Kidneys were taken for brush-border membrane (BBM) preparations 3 h after PTX or sham operations and 1 h after PTH or saline injections, thus yielding four categories of PTH status: PTX/saline, sham/saline, PTX/PTH, and sham/PTH. Efficacy of PTX and PTH was confirmed by examination of serum Ca concentrations. A 100 mM NaSCN gradient, out greater than in, caused concentrative PO4 uptake by BBM from sham/saline animals. Concentrative uptake was not produced by 100 mM KSCN, out greater than in; pH 7.4in vs. pH 5.4out; or 100 mM NaC1, out = in. Removal of endogenous PTH (PTX/saline) significantly stimulated Na-dependent PO4 uptake compared with sham/saline. PTH infusion significantly depressed Na-dependent PO4 uptake in PTX/PTH and sham/PTH groups compared with sham/saline and PTX/saline. Na-dependent, concentrative glucose uptake was present and unchanged by PTH. Kinetic analysis, based on 5-s uptakes, showed that both apparent affinity and maximal velocity (Vmax) for Na-dependent PO4 transport were decreased by PTH treatment. The data suggest that PTH influences avian renal PO4 excretion at least in part through a Na-dependent PO4 transport system in proximal tubule BBM.