Normal insulin sensitivity and substrate oxidation in hearts from rats with low intrinsic aerobic exercise capacity and systemic insulin resistance

Background: In patients, inactivity, obesity and insulin resistance (IR) are associated with increased incidence of heart failure. Rats selectively bred for low (LCR) intrinsic aerobic exercise capacity show signs of the metabolic syndrome including IR, compared to their counterparts bred for high intrinsic aerobic capacity (HCR).

Hypothesis: We reasoned that systemic IR in LCR should translate to impaired substrate oxidation and reduced insulin sensitivity in the heart.

Methods: Isolated hearts were perfused in the working mode to analyze cardiac function, substrate oxidation patterns, insulin response, and oxygen consumption.

Results: After 22 generations of selective breeding, LCR displayed reduction of exercise capacity (LCR vs. HCR: distance 280±12 vs. 1968±63m, time 19.5±0.6 vs. 71.7±1.4min, speed 19.2±0.3 vs. 45.3±0.7m/min; all p<0.05). At 21 weeks, body weight (+34%), tibia length (+6%), heart weight (+31%), and heart weight to tibia length ratio (+24%; all p<0.05) were increased. LCR display higher random glucose, higher fasting glucose, and higher insulin levels in serum than HCR indicating the presence of insulin resistance in LCR. Here, in contrast, isolated hearts showed no differences in glucose (0.22±0.02µmol/min/gdry) or fatty acid oxidation (0.79±0.10µmol/min/gdry), oxygen consumption (28.3±4.1nmolO₂/min/gdry) or cardiac power (18.6±1.6mW/gdry). Furthermore, sensitivity to insulin (Δglucose oxidation: +0.57±0.095µmol/min/gdry) was not different between the two populations.

Conclusion: We conclude that systemic insulin resistance is not always linked to cardiac insulin resistance and function, suggesting an independent mechanism. It may be speculated that diabetes or pre-diabetes may not immediately have detrimental effects on the heart.