Effects of High and Low Aerobic Capacity Phenotype on Muscular and Vascular Adaptive Responses Following Hind Limb Ischemia
Authors: Granier Elizabeth , Katwa Laxmansa C. , Zakari Madaniah O. , Alsahly Musaad B. , Koch Lauren G. , Britton Stephen L. , Lust Robert M. 
Published: November 2023

Rationale

Active exercise programs generally are beneficial in Peripheral Arterial Occlusive Disease (PAOD), but often generate heterogeneous responses, suggesting an additional contribution from the intrinsic aerobic exercise capacity. A novel rat strain, developed using artificial selection for intrinsic endurance running capacity (High Capacity Running, HCR; Low Capacity Running, LCR), provides a unique tool that enables assessment of intrinsic aerobic capacity and the effects of these phenotypes on adaptive responses to ischemic stress.

Purpose

To determine the influence of intrinsic exercise capacity on the vascular and metabolic adaptive responses to hind limb ischemia.

Methods

Muscle samples from both the ischemic and the non‐ischemic limb in both strains were compared, histologically for the anatomical vascular capillarity supporting perfusion, and functionally using both perfusion tracers (microspheres) to track blood flow, and direct muscle stimulation to test fatigue characteristics. PCR was used to screen for potential gene associations that might suggest potential pathways accounting for any observed differences between the phenotypes following occlusive ischemia.

Results

Prior to ligation, there were not significant differences between the phenotypes in the time to exhaustion with high frequency pacing (LCR time (sec) = 2291 ± 186 vs. HCR time (sec) = 2571 ± 218; p = n.s.). Following ligation, both strains decreased the time to muscle exhaustion with pacing, but LCRs decreased significantly more than HCRs (437 ± 47 vs. 824 ± 56, p <0.001). There were no differences in resting blood flow between LCR and HCR. The immediate decrease in flow following ligation was significantly more severe in LCRs than HCRs (52.5% vs 37.8%, p < 0.001), and did not recover after 7 days in LCRs, but did in HCRs (48.8% vs 24.2% reduced from baseline, p < 0.001). VEGF, eNOS, and ANG2 (but not ANG1) gene expression were decreased in LCRs vs HCRs before occlusion, and increased significantly in LCRs 14 days after occlusion, but not in HCRs. LCRs had reduced capillary density compared to the HCR in non‐ischemic gastrocnemius muscle (417.81 ± 23.6 vs 614.6 ± 57.5, p<0.05). The HCR phenotype increased capillary density at both 7 (918.54 ± 197.6) and 14 days (1015.41 ± 174.9) after occlusion. Capillary density in LCRs was significantly lower at all time points after occlusion (LCR 7 Day = 564.76 ± 40.5, LCR 14 Day = 507.48 ± 54.2, both p < 0.05 vs HCR for respective time point). In contrast to HCRs, capillary density did not change in LCRs after occlusion. There were no differences in capillary contacts per fiber (NCAF) between phenotypes under control conditions (LCR = 4.547 ± 0.220 vs. HCR = 4.280 ± 0.217), but both phenotypes increased NCAF significantly in response to ischemia. However, the increase in the HCRs was earlier in onset, present at both 7 (5.170 ± 0.276) and 14 days after occlusion (5.210 ± 0.255), while the LCR was significantly elevated only at the 14 day endpoint (6.220 ± 0.481), but at that time was actually greater than HCR.

These results suggest that low endurance running capacity confers increased risk for ischemic injury and is subject to delayed and less effective adaptive response to ischemic stress.

Support or Funding Information

NIH P40 OD 021331