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 (P(i)) and high-energy organic pools of ATP and phosphocreatine (PCr). At steady state, uptake of P(i) into the cell must equal the rate of efflux, which is expected to be a function of intracellular P(i) concentration. We measured (32)P-labeled P(i) uptake rates in different muscle fiber types to determine whether they are proportional to cellular P(i) content. P(i) uptake rates in isolated, perfused rat hindlimb muscles were linear over time and highest in soleus (2.42 +/- 0.17 micromol x g(-1) x h(-1)), lower in red gastrocnemius (1.31 +/- 0.11 micromol x g(-1) x h(-1)), and lowest in white gastrocnemius (0.49 +/- 0.06 micromol x g(-1) x h(-1)). Reasonably similar rates were obtained in vivo. P(i) uptake rates at plasma P(i) concentrations of 0.3-1.7 mM confirm that the P(i) uptake process is nearly saturated at normal plasma P(i) levels. P(i) uptake rate correlated with cellular P(i) 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 P(i) uptake across fiber types implies that other factors are involved in regulating P(i) uptake in skeletal muscle. Furthermore, fractional turnover of the cellular P(i) 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 P(i), likely due to differences in resistance to P(i) efflux from the fiber.