Normal milk composition in lactating X-linked hypophosphatemic mice despite continued hypophosphatemia

Patients with X-linked hypophosphatemia and mice bearing the Hyp gene have reduced renal tubular reabsorption of phosphate and an osteomalacic bone disease. To test if altered phosphate transport also exists in the mammary glands, milk was analyzed from normal (n = 9) and heterozygous Hyp (n = 8) mice 14 days after giving birth. Inorganic phosphate, total phosphate, calcium, magnesium, sodium, and potassium were measured; percent cream, fat, water, and nonfat organic solids were measured; and protein was measured. No significant differences (NSD) were found except for greater sodium in Hyp milk. There was also NSD in litter weight. The lactating Hyp had a lower body weight and remained hypophosphatemic relative to lactating normals, but both groups had higher plasma phosphate than nonlactating controls of the same genotype. The data suggest that Hyp mice can accumulate a normal amount of phosphate in their milk despite the plasma phosphate being two-thirds of normal. These data, with other recent reports of different organ systems, suggest that the altered phosphate transport activity may be restricted to the kidney.