A comparison of the microcirculation in rat fast glycolytic and slow oxidative muscles at rest and during contractions

We compared the microcirculation of the predominantly glycolytic (cortex of tibialis anterior, TA) and purely oxidative (soleus) muscles of the rat. The TA has wider (3.4 +/- 0.1 microns diameter compared to 2.7 +/- 0.05 microns), longer (405 +/- 29 and 205 +/- 17 microns), and straighter capillaries. Velocity of RBCs at rest is higher in TA (0.30 +/- 0.02 mm/sec) and reaches a higher value during contractions at 1 Hz (0.38 +/- 0.04 mm/sec) more quickly than in soleus (0.21 +/- 0.02-0.28 +/- 0.03 mm/sec). The number of continuously perfused capillaries in TA increased during contractions, but there was little change in soleus. A computer program was devised to estimate the proportion of time spent stationary by RBCs in the capillaries. This was greater in soleus than in TA at rest and was reduced in TA only during contractions. The transit time (TT) through capillaries was much reduced in TA during contractions (from 1.69 +/- 0.17 to 0.78 +/- 0.13 sec) but remained unchanged in soleus (1.17 +/- 0.21 and 0.97 +/- 0.13 sec). The lack of functional hyperemia in soleus may be a direct consequence of this invariability in the TT.