Altered osteoblast gluconeogenesis in X-linked hypophosphatemic mice is associated with a depressed intracellular pH

We have studied gluconeogenesis and intracellular pH levels in normal (+/Y) and X-linked hypophosphatemic (Hyp/Y) mice. Compared with +/Y littermates, Hyp/Y mouse osteoblasts showed a higher rate of glucose production from fructose (10-fold), glutamine, and malate, but no significant difference when α-ketoglutarate was used as substrate. The activities of the pentose cycle enzymes, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase, were not different in the two osteoblast preparations. Examination of intracellular pH (pH_i) using the double excitation of the pH-sensitive dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) revealed a significantly lower pHi in Hyp/Y mouse osteoblasts compared with +/Y mouse osteoblasts (7.01±0.03 n=10 versus 7.15±0.04 n=8, respectively; P<0.05). These results show for the first time that osteoblasts are capable of glucose production and that glucose production is altered in the Hyp/Y mouse osteoblast. As altered gluconeogenesis has been associated with reduced intracellular pH in other systems, a similar mechanism may be operative in the Hyp/Y mouse osteoblast. The observed defects may be intrinsic to the Hyp phenotype as the alterations in intracellular pH and gluconeogenesis persisted in vitro, or they may represent impressed memory from the in vivo state and the presumed circulating factor that influences phosphate transport.