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Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

Department of Physiology, College of Medicine, Department of Biomedical Sciences, College of Veterinary Medicine, and Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211

Submitted 3 December 2003 ; accepted in final form 23 February 2004

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 [³²P]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, [³²P]P_i uptake 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 P_i uptake are not likely confounded by changes in precursor [³²P]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.

P_iT-1; P_iT-2; phosphate efflux; Na-P_i; cotransport

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