Recovery of microvascular Po₂during the exercise off-transient in muscles of different fiber type

The speed with which muscle energetic status recovers after exercise is dependent on oxidative capacity and vascular O(2) pressures. Because vascular control differs between muscles composed of fast- vs. slow-twitch fibers, we explored the possibility that microvascular O(2) pressure (Pmv(O(2)); proportional to the O(2) delivery-to-O(2) uptake ratio) would differ during recovery in fast-twitch peroneal (Per: 86% type II) compared with slow-twitch soleus (Sol: 84% type I). Specifically, we hypothesized that, in Per, Pmv(O(2)) would be reduced immediately after contractions and would recover more slowly during the off-transient from contractions compared with Sol. The Per and Sol muscles of six female Sprague-Dawley rats (weight = approximately 220 g) were studied after the cessation of electrical stimulation (120 s; 1 Hz) to compare the recovery profiles of Pmv(O(2)). As hypothesized, Pmv(O(2)) was lower throughout recovery in Per compared with Sol (end contraction: 13.4 +/- 2.2 vs. 20.2 +/- 0.9 Torr; end recovery: 24.0 +/- 2.4 vs. 27.4 +/- 1.2 Torr, Per vs. Sol; P <or= 0.05). In addition, the mean response time for recovery was significantly faster for Sol compared with Per (45.1 +/- 5.3 vs. 66.3 +/- 8.1 s, Sol vs. Per; P < 0.05). Despite these findings, Pmv(O(2)) 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 O(2) driving pressure (i.e., Pmv(O(2))) is reduced in fast-compared with slow-twitch muscle. In conclusion, the results of the present investigation may partially explain the slowed recovery kinetics (phosphocreatine and O(2) uptake) found previously in 1) fast- vs. slow-twitch muscle and 2) various patient populations, such as those with congestive heart failure and diabetes mellitus.