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Effects of aging on capillary geometry and hemodynamics in rat spinotrapezius muscle

Departments of Kinesiology, Anatomy, and Physiology, Kansas State University, Manhattan, Kansas 66506-5802

Submitted 16 December 2002 ; accepted in final form 21 February 2003

The effects of aging on muscle microvascular structure and function may play a key role in performance deficits and impairment of O₂ exchange within skeletal muscle of senescent individuals. To determine the effects of aging on capillary geometry, red blood cell (RBC) hemodynamics, and hematocrit in a muscle of mixed fiber type, spinotrapezius muscles from Fischer 344 x Brown Norway hybrid rats aged 6–8 mo [young (Y); body mass 421 ± 10 g, n = 6] and 26–28 mo [old (O); 561 ± 12 g, n = 6] were observed by high-resolution transmission light microscopy under resting conditions. The percentage of RBC-perfused capillaries (Y: 78 ± 3%; O: 75 ± 2%) and degree of tortuosity and branching (Y: 13 ± 2%; O: 13 ± 2%, additional capillary length) were not different in O vs. Y muscles. Lineal density of RBC-perfused capillaries in O was significantly reduced (Y: 30.7 ± 1.8, O: 22.8 ± 3.1 capillaries/mm; P < 0.05). However, RBC-perfused capillaries from O rats (n = 78) exhibited increased RBC velocity (V_RBC) (Y: 219 ± 12, O: 310 ± 14 µm/s; P < 0.05) and RBC flux (F_RBC) (Y: 27 ± 2, O: 41 ± 2 RBC/s; P < 0.05) vs. Y rats (n = 66). Thus O₂ delivery per unit of muscle was not different between groups (Y: 894 ± 111, O: 887 ± 118 RBC · s^-1 · mm muscle^-1). Capillary hematocrit was not different in Y vs. O rats (Y: 26 ± 1%, O: 28 ± 1%: P > 0.05). These data indicate that in resting spinotrapezius muscle, aging decreases the lineal density of RBC-perfused capillaries while increasing mean V_RBC and F_RBC within those capillaries. Whereas muscle conductive O₂ delivery and capillary hematocrit were unchanged, elevated V_RBC reduces capillary RBC transit time and may impair the diffusive transport of O₂ from blood to myocyte particularly under exercise conditions.

capillary hematocrit; microvascular oxygen exchange; Brown Norway rat; red blood cell velocity and flux

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