Received February 11, 2007
Revised March 21, 2007
Accepted after revision May 23, 2007
Effect of hypoxia on fatigue development in rat muscle composed of different fiber types
Richard A Howlett 1
Michael C Hogan 1*
1 University of California at San Diego
* To whom correspondence should be addressed. E-mail: mchogan{at}ucsd.edu.
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Abstract |
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This study investigated the relationship between hypoxia and the rate of fatigue development in contracting rat hindlimb muscles primarily composed of different fiber types. Hindlimb muscles of 11 rats were exposed and the soleus (SOL) and gastrocnemius/plantaris (GP) were each isolated with circulation intact and attached to individual force transducers. Rats were then equilibrated with either normoxic (N; arterial PO2 = 87.7 ± 1.5 Torr) or hypoxic (H; arterial PO2 = 30.0 ± 2.4 Torr) conditions using an inspired O2 fraction of 0.21 and 0.10, respectively. The stimulation protocol consisted of 2 min each at 0.125, 0.25, 0.33 and 0.5 tetanic contractions/sec sequentially for both conditions. Following the 8 minute stimulation period, relative developed muscle tension (% of maximal) was nearly identical for both H and N in SOL (54.2 ± 3.5 vs. 54.3 ± 4.2 %), but was significantly (p < 0.05) lower in H than N (10.8 ± 0.9 vs. 43.0 ± 8.9 %) in GP, indicating a greater amount of fatigue during hypoxia only in the GP. SOL PCr content fell to similar levels (24.1 ± 1.6 vs. 21.1 ± 4.9 mmol/kg dry weight (dw)) during both H and N, but in the white portion of the gastrocnemius (WG), PCr was significantly lower following H than N (14.3 ± 1.5 vs. 34.0 ± 6.0 mmol/kg dw). Similarly, muscle lactate increased in both fiber types at fatigue, but only in WG was the increase significantly greater with H (SOL 7.1 ± 2.0 vs. 5.3 ± 1.1; WG 13.7 ± 4.5 vs. 5.3 ± 2.2 mmol/kg dw). Increases in calculated muscle [H+], free ADP, and free AMP were similar between N and H in SOL but were significantly greater during H in WG, compared to N. These data demonstrate that hypoxia induces greater fatigue and disruption of cellular homeostasis in rat hindlimb muscle composed primarily of fibers with low oxidative capacity compared to those of a more oxidative type.
Key Words:
Hypoxia, Metabolism, Skeletal muscle
