The effects of aging on capillary hemodynamics in contracting rat spinotrapezius muscle.

Advancing age alters the structural and functional determinants of convective and diffusive muscle oxygen (O(2)) flux. However, capillary red blood cell (RBC) hemodynamics have not been investigated during contractions in muscles of old animals. Therefore, we tested the hypothesis that aging induces significant capillary hemodynamic alterations during electrically-induced contractions in the spinotrapezius muscle of old Fischer 344 x Brown Norway rats when compared to younger counterparts. The spinotrapezius muscle was observed via intravital microscopy under both resting and contracting conditions in 8 old (O: 26-30 months) and 5 young (Y: 6-8 months) animals. Capillary RBC flux (F(RBC)), velocity (V(RBC)), hematocrit (H(CAP)), and lineal F(RBC) (F(RBC) x capillary lineal density) were determined in all visible capillaries and fields at rest and during the steady-state of muscle contractions. At rest F(RBC) and V(RBC) were elevated in O compared to Y rats, while there was no difference in H(CAP) or lineal F(RBC) between groups. During the contracting steady-state, Delta F(RBC) (Y: 28.8+/-7.7, O: -2.9+/-1.4 cells/s), Delta V(RBC) (Y: 253+/-68, O: -4+/-15 microm/s), Delta H(CAP) (Y: 0.02+/-0.02, O: -0.03+/-0.01), and the Delta lineal F(RBC) (Y: 892+/-255, O: -24+/-30 cells/s/mm) were all lower (P<0.05) in O compared to Y rats. These results indicate that despite maintained total perfusive and diffusive O(2) transport at rest, advancing age results in significant impairments in capillary hemodynamics during electrically-induced contractions. These alterations likely contribute to the mechanisms responsible for the reduced exercise capacity commonly found in elderly populations.