Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

Abstract

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (Pi) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless Piuptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [32P]Piuptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. Piuptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [32P]Piuptake rates declined by ∼25% 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 Piuptake are not likely confounded by changes in precursor [32P]Pispecific activity in the interstitium. In soleus and red gastrocnemius, declines in Piuptake rates were related to energy expenditure over the contraction duration. These data imply that Piuptake in skeletal muscle is acutely modulated during contractions and that decreases in Piuptake rates, in combination with expected increases in Piefflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of Pimust subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.