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EXERCISE AND RESPIRATORY PHYSIOLOGY

Elevation in heat shock protein 72 mRNA following contractions in isolated single skeletal muscle fibers

Division of Physiology, Department of Medicine, University of California San Diego, La Jolla, California

Submitted 28 November 2007 ; accepted in final form 2 June 2008

The purpose of the present study was 1) to develop a stable model for measuring contraction-induced elevations in mRNA in single skeletal muscle fibers and 2) to utilize this model to investigate the response of heat shock protein 72 (HSP72) mRNA following an acute bout of fatiguing contractions. Living, intact skeletal muscle fibers were microdissected from lumbrical muscle of Xenopus laevis and either electrically stimulated for 15 min of tetanic contractions (EX; n = 26) or not stimulated to contract (REST; n = 14). The relative mean developed tension of EX fibers decreased to 29 ± 7% of initial peak tension at the stimulation end point. Following treatment, individual fibers were allowed to recover for 1 (n = 9), 2 (n = 8), or 4 h (n = 9) prior to isolation of total cellular mRNA. HSP72, HSP60, and cardiac -actin mRNA content were then assessed in individual fibers using quantitative PCR detection. Relative HSP72 mRNA content was significantly (P < 0.05) elevated at the 2-h postcontraction time point relative to REST fibers when normalized to either HSP60 (18.5 ± 7.5-fold) or cardiac -actin (14.7 ± 4.3-fold), although not at the 1- or 4-h time points. These data indicate that 1) extraction of RNA followed by relative quantification of mRNA of select genes in isolated single skeletal muscle fibers can be reliably performed, 2) HSP60 and cardiac -actin are suitable endogenous normalizing genes in skeletal muscle following contractions, and 3) a significantly elevated content of HSP72 mRNA is detectable in skeletal muscle 2 h after a single bout of fatiguing contractions, despite minimal temperature changes and without influence from extracellular sources.

exercise; heat shock; stress