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 P-32-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 mumol.g(-1).h(-1)), lower in red gastrocnemius (1.31+/-0.11 mumol.g(-1).h(-1)), and lowest in white gastrocnemius (0.49+/-0.06 mumol.g(-1).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.