Phosphate uptake and PiT-1 protein expression in rat skeletal muscle

Abstract

Skeletal muscle fiber types differ in their contents of total phosphate, which includes inorganic phosphate (Pi) and high-energy organic pools of ATP and phosphocreatine (PCr). At steady state, uptake of Piinto the cell must equal the rate of efflux, which is expected to be a function of intracellular Piconcentration. We measured32P-labeled Piuptake rates in different muscle fiber types to determine whether they are proportional to cellular Picontent. Piuptake rates in isolated, perfused rat hindlimb muscles were linear over time and highest in soleus (2.42 ± 0.17 μmol·g−1·h−1), lower in red gastrocnemius (1.31 ± 0.11 μmol·g−1·h−1), and lowest in white gastrocnemius (0.49 ± 0.06 μmol·g−1·h−1). Reasonably similar rates were obtained in vivo. Piuptake rates at plasma Piconcentrations of 0.3–1.7 mM confirm that the Piuptake process is nearly saturated at normal plasma Pilevels. Piuptake rate correlated with cellular Picontent ( r = 0.99) but varied inversely with total phosphate content. Sodium-phosphate cotransporter (PiT-1) protein expression in soleus and red gastrocnemius were similar to each other and seven- to eightfold greater than PiT-1 expression in white gastrocnemius. That the PiT-1 expression pattern did not match the pattern of Piuptake across fiber types implies that other factors are involved in regulating Piuptake in skeletal muscle. Furthermore, fractional turnover of the cellular Pipool (0.67, 0.57, and 0.33 h−1in soleus, red gastrocnemius, and white gastrocnemius, respectively) varies among fiber types, indicating differential management of intracellular Pi, likely due to differences in resistance to Piefflux from the fiber.