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1 Department of Anatomy & Physiology, Kansas State University, Manhattan, KS, USA
2 Department of Anatomy & Physiology, Kansas State University, Manhattan, KS, USA; Department of Kinesiology, Kansas State University, Manhattan, KS, USA
* To whom correspondence should be addressed. E-mail: pjmcdono{at}vet.ksu.edu.
The speed with which muscle energetic status recovers following exercise is dependent on oxidative capacity and vascular O2 pressure. Because vascular control differs between muscles comprised of fast versus slow-twitch fibers, we explored the possibility that microvascular O2 pressure (Po2m: proportional to the oxygen delivery (Qo2) to oxygen uptake (Vo2) ratio; Qo2/Vo2) would differ during recovery in fast-twitch peroneal (Per: 86% type II) compared to slow-twitch soleus (Sol: 84% type I). Specifically, we hypothesized that in Per Po2m would be reduced immediately following contractions and would recover more slowly during the off-transient from contractions compared to Sol. The Per and Sol muscles of six female Sprague-Dawley rats (wt = ~220g) were studied following the cessation of electrical stimulation (120s; 1 Hz) to compare the recovery profiles of Po2m. As hypothesized, Po2m was lower throughout recovery in Per when compared to Sol (end-contraction: Per, 13.4±2.2 vs. Sol, 20.2±0.9; end-recovery: Per, 24.0±2.4 vs. Sol, 27.4±1.2 Torr, p
0.05). In addition, the mean response time (MRT) for recovery was significantly faster for Sol when compared to Per (Sol, 45.1±5.3 vs. Per, 66.3±8.1 s; p0.05). Despite these findings, Po2m rose progressively in both muscles and at no time fell below end-exercise values. These data indicate that during the recovery from contractions (which is prolonged in Per), capillary O2 driving pressure (i.e. Po2m) is reduced in fast compared to slow-twitch muscle. In conclusion, the results of the current study may explain partially the slowed recovery kinetics (phosphocreatine and Vo2) found previously in 1) fast vs. slow-twitch muscle and 2) various patient populations, such as congestive heart failure and diabetes mellitus.
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