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

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 Pi into the cell must equal the rate of efflux, which is expected to be a function of intracellular Pi concentration. We measured 32P-labeled Pi uptake rates in different muscle fiber types to determine whether they are proportional to cellular Pi content. Pi uptake 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. Pi uptake rates at plasma Pi concentrations of 0.3–1.7 mM confirm that the Pi uptake process is nearly saturated at normal plasma Pi levels. Pi uptake rate correlated with cellular Pi content ( 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 Pi uptake across fiber types implies that other factors are involved in regulating Pi uptake in skeletal muscle. Furthermore, fractional turnover of the cellular Pi pool (0.67, 0.57, and 0.33 h−1 in soleus, red gastrocnemius, and white gastrocnemius, respectively) varies among fiber types, indicating differential management of intracellular Pi, likely due to differences in resistance to Pi efflux from the fiber.