AJP - Renal Physiology, Vol 258, Issue 5 1336-F1344, Copyright © 1990 by American Physiological Society

ARTICLES

Parathyroid hormone action on phosphate transport is inhibited by high osmolality

S. A. Kempson, C. Helmle, M. I. Abraham and H. Murer
Physiology Institute, University of Zurich, Switzerland.

Parathyroid hormone (PTH) produces rapid inhibition of Na(+)-phosphate cotransport, characterized by a decreased maximal rate of transport, and the inhibition is independent of de novo protein synthesis. The present study determined whether the action of PTH on Na(+)-phosphate cotransport is mediated, at least in part, by rapid endocytic internalization of Na(+)-phosphate cotransporters present in the plasma membrane. Horseradish peroxidase, a fluid-phase marker, was used to demonstrate the presence of endocytosis in opossum kidney (OK) epithelial cells in monolayer culture. An increase in medium osmolality to 500 mosmol/kgH2O, by addition of sucrose, produced 80% inhibition of endocytosis within 1 h. The inhibition was reversed on returning the cells to normal medium. Incubation of OK cell monolayers with PTH (10(-8) M) for 3 h at normal osmolality (281 mosmol/kgH2O) inhibited Na(+)-phosphate cotransport (4 min uptakes) by 56-67%. In hyperosmolar medium (513 mosmol/kgH2O), when endocytosis was inhibited, PTH inhibited Na(+)-phosphate cotransport by only 25-39%, a change that was significantly different from the inhibition in normal medium. Hyperosomolality had no effect on PTH inhibition of Na(+)-H+ exchange or on PTH stimulation of intracellular adenosine 3',5'-cyclic monophosphate. We conclude that the full inhibitory action of PTH on Na(+)-phosphate cotransport may require an intact endocytic mechanism.

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