Microvascular architecture in rat soleus and extensor digitorum longus muscles.

Microvascular architecture was investigated in the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles. Rats (n = 5) were anesthetized and papaverine was infused into a carotid artery cannula to induce vasodilation. Microfil casting compound was then infused at an inflation pressure (caudal artery) of 100 mm Hg. Bilateral SOL and EDL muscles were excised 24-72 hr postcasting, dehydrated in ethanol, and cleared in methyl salicylate. Branch frequencies (BR) and segment lengths (SL) of intramuscular arterioles and venules were quantified along primary (1 degree), secondary (2 degrees), and tertiary (3 degrees) order microvessels using microscopy. In both muscles, BR decreased with increasing vessel order. Regional differences in network organization were observed within the EDL muscle. SL of 1 degrees arterioles was 47% shorter in the SOL muscle indicating more compact microvascular networks compared to the EDL muscle. These findings provide a structural basis for reported differences in blood flow between the SOL and EDL muscles at rest and during exercise.