Variation in Intrinsic Aerobic Capacity Reveals Distinct Differences in the Maintenance of Rat Knee Joint Integrity

The objective was to test how intrinsic aerobic capacity affects the articular cartilage, infrapatellar fat pad (IFP), subchondral bone, and synovial membrane of the rat knee joint. Thirty-five 24-week-old male N:NIH rats selectively bred for high aerobic capacity (high-capacity-runner [HCR]; n = 17) and low aerobic capacity (low-capacity-runner [LCR]; n = 18) were fed either a high-fat/high-sucrose (HFHS) diet or a chow diet for 12 weeks. Alterations in articular cartilage, subchondral bone microarchitecture, IFP hypertrophy, and synovial membrane cytokine levels were quantified. Body composition, serum cytokines, and glucose tolerance were assessed at Week 12. Compared to HCR rats, LCR rats had higher modified Mankin scores, bone volume, total volume, bone mineral density, bone volumetric fraction, and trabecular thickness, and lower specific bone surface, bone surface density, and connectivity density. LCR rats also had higher RANTES (Regulated on Activation, Normal T Cell Expressed and Secreted) and lower EGF (epidermal growth factor) levels in synovium analytes. Furthermore, in response to the HFHS diet, there were no differences in adipocyte hypertrophy within the IFP for either the HCR or LCR rats. LCR rats demonstrated hyperglycemia, elevated pro-inflammatory marker levels, and decreased anti-inflammatory marker levels compared to HCR rats. It was concluded that intrinsic high aerobic capacity maintains superior knee joint integrity both with and without an obesogenic dietary challenge, likely due to superior metabolic resistance. Whether intrinsic high aerobic capacity might protect joint degeneration in obesity and metabolic syndrome in other species including humans, warrants investigation.