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1 Department of Anatomy and Physiology, Kansas State University, Manhattan, KS, USA
2 Department of Health and Kinesiology, Texas A&M University, College Station, TX, USA
* To whom correspondence should be addressed. E-mail: poole{at}vet.ksu.edu.
A single bout of eccentric exercise incurs muscle damage, but it is not known whether this is correlated with microcirculatory dysfunction. We tested the following hypotheses in the spinotrapezius muscle of rats either 1 (DH-1; n = 6) or 3 (DH-3; n = 6) days following a downhill run to exhaustion (90-120 min; -14° grade): 1. in resting muscle, capillary hemodynamics would be impaired, and 2. at the onset of subsequent acute concentric contractions, the decrease of microvascular oxygen pressure (PO2mv), which reflects the dynamic balance between O2 delivery and O2 utilization, would be accelerated compared with control (CON, n = 6) rats. In contrast to CON muscles, intravital microscopy observations revealed the presence of sarcomere disruptions in DH-1 and DH-3 and increased capillary diameter in DH-3 (CON: 5.2 ± 0.1; DH-1: 5.1 ± 0.1; DH-3: 5.6 ± 0.1 µm; both P < 0.05 vs. DH-3). At rest, there was a significant reduction in the percentage of capillaries that sustained continuous RBC flux in both DH running groups (CON: 90.0 ± 2.1; DH-1: 66.4 ± 5.2; DH-3: 72.9 ± 4.1 %, both P < 0.05 vs. CON). Capillary tube hematocrit was elevated in DH-1, but reduced in DH-3 (CON: 22 ± 2; DH-1: 28 ± 1; DH-3: 16 ± 1 %; all P < 0.05). Although capillary RBC flux did not differ between groups (P > 0.05), RBC velocity was lower in DH-1 compared to CON (CON: 324 ± 43; DH-1: 212 ± 30; DH-3: 266 ± 45 µm/s; P < 0.05 DH-1 vs. CON). Baseline PO2mv prior to contractions was not different between groups (P > 0.05), but the time constant of the exponential fall to contracting PO2mv values was accelerated in the DH running groups (CON: 14.7 ± 1.4 ; DH-1: 8.9 ± 1.4; DH-3: 8.7 ± 1.4 s, both P < 0.05 vs. CON). These findings are consistent with the presence of substantial microvascular dysfunction following downhill eccentric running which slows the exercise hyperemic response at the onset of contractions and reduces the PO2mv available to drive blood-muscle O2 delivery.
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