Evidence for Loss of Capillary Density to be Important in Impaired Muscle Function
Aging is accompanied by a reduction in the density of capillary networks throughout the body, for reasons that are not well understood in depth. The regulation of angiogenesis, the processes of blood vessel formation, is observed to change for the worse, but why does this happen? Which of the underlying forms of accumulating molecular damage cause this? Another important question is the degree to which this loss contributes to specific functional declines, such as that of muscle tissue. Researchers here report on the development of an animal model in which capillary density is first reduced, and then somewhat restored via resistance exercise. There is a clear negative effect on muscle function as a result of capillary loss, suggesting that this is an important factor in the loss of strength that accompanies aging. Finding ways to promote capillary network regrowth in older individuals should be a priority for the regenerative medicine community.
To what extent microvascular rarefaction contributes to impaired skeletal muscle function remains unknown. Our understanding of whether pathological changes in the microcirculation can be reversed remains limited by a lack of basic physiological data in otherwise healthy tissue. The principal objectives here were to: (1) quantify the effect of random microvascular rarefaction on limb perfusion and muscle performance, and (2) determine if these changes could be reversed. We developed a novel protocol in rats whereby microspheres injected into the femoral artery allowed a unilateral reduction in functional capillary density in the extensor digitorum longus (EDL), and assessed acute and chronic effects on muscle function.
Simultaneous bilateral EDL force and hindlimb blood flow measurements were made during electrical stimulation. Following functional capillary rarefaction there was an acute microsphere dose-dependent reduction in muscle fatigue resistance, despite preserved femoral artery perfusion. Histological analysis of EDL samples taken from injected animals confirmed a positive correlation between the proportion of functional capillaries and fatigue resistance. Such impaired performance persisted for at least 2 weeks.
Concomitant mechanical overload improved both perfused capillary density and fatigue resistance, confirming that the capacity for muscle remodelling was retained following chronic distributed ischaemia, and that the impact of capillary rarefaction could be alleviated. These results demonstrate that loss of functional capillaries is detrimental to muscle function, even in otherwise healthy tissue, independent of arterial perfusion. Restoration of muscle performance following a mechanical overload stimulus indicates that angiogenic treatments to alleviate microvascular rarefaction may be key to restoring exercise tolerance.