Orthophosphate transport in the erythrocyte of normal subjects and of patients with X-linked hypophosphatemia.

Abstract

A B S T R A C T We have examined the mechanism of TCA-soluble orthophosphate (P.) transfer across the membrane of mature human erythrocytes in normal subjects and in patients with X-linked hypophosphatemia (X-LH). The studies were carried out largely at pH 7.4 and 370C, in partial simulation of conditions in vivo. (a) At physiological concentrations (1-2 mM) P. enters the intact normal erythrocyte down its chemical gradient and under no conditions could we identify a steady-state trans-membrane gradient for Pi greater than 0.6. Calculations of the phosphate anion distribution ratio using the Nernst equation yield theoretical values that closely approximate observed values. (b) Glycolytic inhibitors have little effect on total entry of 'P, into erythrocytes but they do affect the intracellular distribution of P.. In the presence of iodoacetamide, label accumulates almost exclusively in the orthophosphate pool and less than 1% enters the organic phosphate pool. (c) Specific activity measurements in unblocked cells indicate that P. anion equilibrates first with its intracellular P. pool. These initial findings imply that neither group translocation, nor energy coupling, influence P. permeation into the human erythrocytes. (d) The relationship between 'P entry and extracellular P. concentration is parabolic in the presence of chloride, and linear in the presence of sulfate.

Hypophosphatemic nonrachitic bone disease: An entity distinct from X‐linked hypophosphatemia in the renal defect, bone involvement, and inheritance

“…These findings indicate that the abnormality in phosphate flux in HBD, evident in some epithelial membranes, is not present in the erythrocyte plasma membrane. The same finding has been reported for XLH (21).…”

“…The distribution of phosphate between erythrocytes and serum, and the uptake of 32 P-labeled phosphate anion into inorganic and organic pools in erythrocytes, have been measured in Patient 3 (21); they are both normal. These findings indicate that the abnormality in phosphate flux in HBD, evident in some epithelial membranes, is not present in the erythrocyte plasma membrane.…”

Sodium-phosphate cotransport in human red blood cells. Kinetics and role in membrane metabolism.

“…Several studies have attempted to investigate the role of extracellular phosphate in these membrane-associated glycol) tic reactions by examining the rate of incorporation of extracellular phosphate into the three different c)tosolic phosphate pools: Pi, nucleotide phosphates, and other organic phosphates (Gerlach et al, 1958;Bartlett, 1958;Chedru and Cartier, 1966;Reed and Young, 1967;Tenenhouse and Scriver, 1975). There is a discrepancy in these reports as to whether extracellular phosphate is first equilibrated into the c)tosolic Pi pool and subsequently into cytosolic nucleotides, or whether the cytosolic nucleotide pools reach isotopic equilibrium with extracellular phosphate before the c)tosolic Pi.…”

The defect in transcellular transport of phosphate in the nephron is located in brush-border membranes in X-linked hypophosphatemia (Hyp mouse model)

“…Parathyroid hormone, which normally inhibits tubular reabsorption of phosphate, is not secreted in excess and has little effect on the residua1 transport activity in XLM patients (3,4). Three tissues appear to be directly affected in the XLH phenotype; in addition to kidney, both intestine (5) and bone (6) are implicated; on the other hand, the XLH erythrocyte, a nonepithelial structure, transports phosphate normally (7) The Hyp phenotype is very similar to the XEM phenotype. Renal handling of phosphate is impaired in the mouse (2,8), bone mineralization is affected ( 2 ) , and there is a primary disorder of phosphate transport in the intestine (9).…”