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1 Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas, USA
* To whom correspondence should be addressed. E-mail: musch{at}vet.ksu.edu.
The effects of aging on muscle microvascular structure and function may play a key role in the 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 Fisher 344 Brown Norway hybrid rats aged 6-8 months (Young (Y); body mass 421±10 g, n=6) and 26-28 months (Old (O); 561±12 g, n=6) were observed by high resolution transmission light microscopy under resting conditions. The % of RBC-perfused capillaries in Y and O (Y, 78±3; O, 75±2), and the degree of tortuosity and branching (Y, 13±2; O, 13±2) were not different in the O compared to Y muscles. The 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 (n=78) compared with Y (n=66) rats exhibited an increased RBC velocity (Y, 219±12; O, 310±14 µm /s, P<0.05) and RBC flux (Y, 27±2; O, 41±2 RBC/ s, P<0.05). Thus, O2 delivery per unit of muscle was not different between groups (Y, 894±111; O, 887±118 RBC/s/mm of muscle). Capillary hematocrit was not different in Y vs O (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 whilst increasing mean RBC velocity and flux within those capillaries. Whereas muscle conductive O2 delivery and capillary hematocrit were unchanged, the elevated RBC velocity will reduce capillary RBC transit time and may impair the diffusive transport of O2 from blood-myocyte particularly under exercise conditions.
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