Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P-i) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P-i uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [P-32]P-i uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P-i uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [P-32]P-i uptake rates declined by similar to25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P-i uptake are not likely confounded by changes in precursor [P-32]P-i specific activity in the interstitium. In soleus and red gastrocnemius, declines in P-i uptake rates were related to energy expenditure over the contraction duration. These data imply that P-i uptake in skeletal muscle is acutely modulated during contractions and that decreases in P-i uptake rates, in combination with expected increases in P-i efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P-i must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.