Distribution of red blood cell velocity in capillary network, and endothelial ultrastructure, in aged rat skeletal muscle.

Although age-related structural and functional changes in skeletal muscle have been described extensively, little is known about the accompanying hemodynamic and structural changes in the microvasculature. The objective of this study was to use the extensor digitorum longus muscle in mid-aged (12 months) and old (28 months) Fisher 344 male rats to evaluate (1) the distribution of microvascular flow in the resting state, (2) the distribution response to a complete 30-min tourniquet ischemia, and (3) the extent of damage of capillary endothelium. Using intravital video microscopy, the mean resting velocity of red cells in capillaries was found to be 3x larger in old rats while the distribution of velocity within the microvascular bed was as heterogeneous as that in mid-aged rats. The postischemic response was characterized by the same mean peak velocity, but a slower return to velocity to the preischemic level. Within the microvascular bed, there was a less uniform postischemic response among capillaries. No long-term effect of ischemia was seen as velocity was already stable at the preischemic level 20 min after the tourniquet release. There were no differences in the pre- and postischemic densities of perfused capillaries, wet/dry weight ratios, or the occurrence of damaged capillaries. Thus, in this muscle model, aging was associated with an increased resting flow but a remarkably unaffected long-term flow response to a vasodilatory stimulus and endothelial ultrastructure.