| Phosphate is a necessary component for every cell. Because it forms the high energy bonds of ATP and phosphocreatine (PCr), it serves a vital role in energy utilization. This is particularly important in skeletal muscle, as this tissue experiences rapid and dramatic increases in its rate of ATP hydrolysis. However, few studies have examined how skeletal muscle manages its cellular phosphate content. Since phosphate is not synthesized in the cell and cannot easily pass through the plasma membrane, it is acquired by a sodium/phosphate (Na/Pi) cotransporter. To evaluate this uptake process, inorganic phosphate (Pi) uptake rates in different skeletal muscle sections were measured in isolated rat hindlimbs perfused with [ 32P]Pi.; Cellular phosphate content depends on the balance of the rates of P i uptake and Pi efflux; at steady state, these rates are necessarily matched. Since the rate of efflux is expected to be a function of the intracellular Pi concentration, Pi uptake rate should be proportional to cellular Pi content. This proved true, as Pi uptake rates (n = 11) were greatest in soleus (2.42 ± 0.17 μmol/g/h), lower in red gastrocnemius (1.31 ± 0.11), and lowest in white gastrocnemius (0.49 ± 0.06). Pi uptake rates at plasma Pi concentrations of 0.3–1.7 mM suggest that the P i uptake process in skeletal muscle is nearly saturated at normal plasma Pi levels. This implies that the main determinant of Pi uptake rates may be expression of Na/Pi transporter protein. However, while protein expression of PiT-1, one of the two Na/Pi transporter isoforms in skeletal muscle, was 7- to 8-fold greater in soleus and red gastrocnemius than in white gastrocnemius, the expression pattern across fiber types did not match the pattern of Pi uptake rates, suggesting other factors are involved in regulating Pi uptake in skeletal muscle. At steady state, fractional turnover of the cellular Pi pool (0.67, 0.57, and 0.33 min−1 for soleus, red gastrocnemius, and white gastrocnemius, respectively) varied among fiber types, indicating differential management of intracellular Pi.; In experiments designed to analyze potential acute modulation of the Pi uptake process, Pi uptake was measured in muscles contracting at 0.5, 3, or 5 Hz for 30 min. Because there is a net efflux of phosphate from skeletal muscle during contractions, likely due to an elevated intracellular Pi concentration, it was expected that Pi uptake rates would increase and, thus, minimize loss of phosphate. However, contractions caused reductions (20–75%) in Pi uptake in a manner dependent upon stimulation frequency. These data imply that P i uptake in skeletal muscle is acutely modulated during contractions and that decreases in Pi uptake rates, in combination with expected increases in Pi efflux, exacerbate the net loss of phosphate from the cell. |
