Age-related potentiation of the cutaneous capillary blood flow response to heat stress.

This study tested the hypothesis that the rarefaction of cutaneous capillaries with age is compensated by an increase in the response of remaining capillaries to heat stress. Experiments were performed on unanesthetized male Sprague Dawley rats of the following age groups: young (Y--age 4 mo.), middle aged (M--age 16 mo.) and old (O--age 27 mo.). The subepidermal vascular plexus of the tail was viewed by video microscopy from which measurements of blood cell velocity (BCV) in single plexus capillaries and the density of flowing capillaries (DFC) were made. The animal's body and tail were housed in separate chambers for respectively indirect and direct heat stress trials. At a neutral temperature of 25 degrees C there were no significant age differences in BCV, but DFC was decreased from the Y to the M to the O groups. During an indirect heat stress of 35 degrees C, there was a modest increase in DFC and the Y greater than M greater than O rank order persisted. By contrast, all groups experienced a marked increase in BCV during both indirect and direct heat stress, and the degree of the response was significantly greater for the O rats compared to the M or Y groups. These results support the hypothesis being tested.