Reduced exercise capacity in senescent beagles: an evaluation of the periphery.

This study investigated the effect of age on peripheral factors involved in the systemic response to maximal exercise. Skeletal muscle was analyzed and regional blood flow distribution was determined at rest and during maximal exercise in senescent (old) and in younger mature (young) beagles. Maximal exercise capacity was significantly reduced (P less than 0.05) in old and was associated with a reduction in cardiac output (CO), as well as a tendency for arteriovenous O2 difference to be reduced, with a concomitant reduction in maximal O2 consumption. In each regional circulation evaluated, resting blood flow was similar in young and old. During exercise, blood flow was similar in young and old to the diaphragm, heart, tongue, and six of seven locomotory muscles. Concomitant blood flow reductions in splanchnic regions tended to be more pronounced in old than in young. Skeletal muscle analyses of triceps, semitendinosus, and gastrocnemius muscles disclosed similar percent fiber type distribution in young and old but a reduction in type II fiber area in old. In addition, both muscle capillary density and capillary-to-fiber ratio were reduced in old. These results demonstrate that age-related changes in blood flow distribution during maximal exercise enable skeletal muscle blood flow to be maintained in old, despite reductions in maximal CO and in muscle capillary density. However, this pattern of blood flow distribution only partially compensates for the combined effects of age-related changes in metabolic potential of the periphery, O2 content of arterial blood, and cardiac function during maximal exercise in old.