A TRICk to Improve the Effectiveness of RIC: Role of Limb Temperature in Enhancing the Effectiveness of Remote Ischemic Conditioning

SIMPLE SUMMARY: Remote ischemic conditioning is a simple cardioprotective practice consisting in brief intermittent ischemia applied to a limb. Remote ischemic conditioning has been repeatedly validated in animal models. However, translation from animal experiments to clinics for remote ischemic conditioning has been disappointing. We have demonstrated that keeping the animal’s limb warm while performing intermittent ischemia reduces infarct size more effectively than cold intermittent ischemia; thus, we propose that a more accurate temperature control of the limb undergoing remote ischemic conditioning can increase the efficacy of this cardioprotective maneuver. A simple thermal blanket around the ischemic limb while performing remote ischemic conditioning could be an easy approach to test in humans, as it is simple and safe. ABSTRACT: Background: Treatment of myocardial ischemia/reperfusion (IR) injury is still an unmet clinical need. A large variability of remote ischemic conditioning (RIC) protection has been reported; however, no studies have considered the temperature of the ischemic limb. We analyzed the effects of temperature on RIC protection. Methods: Left hind-limbs of anesthetized male mice were immersed in warm (40 °C, warm-RIC) or cold (20 °C, cold-RIC) water and subjected to a RIC protocol (4 × 5 min limb ischemia/reperfusion). In the control groups (warm-CTR or cold-CTR), the limbs underwent thermic conditions only. Isolated hearts underwent 30 min ischemia and 60 min reperfusion. A PI3K-inhibitor, LY294002 (5 µM), was infused in warm-RIC hearts before the IR protocol (warm-RIC LY). Infarct size was evaluated by nitro blue tetrazolium staining and expressed as the percent of risk area. Results: While cold-RIC did not reduce the infarct size compared to cold-CTR (51 ± 1.62% vs. 54 ± 1.07% of risk area, p = NS), warm-RIC (44 ± 1.13%) significantly reduced the infarct size with respect to either cold-RIC (p < 0.001) or warm-CTR (58 ± 1.41%, p < 0.0001). LY294002 infusion revealed the PI3K/Akt involvement in the warm-RIC protection. Infarct size reduction was abrogated by LY294002 pretreatment (warm-RIC: 44 ± 1.13% vs. warm-CTR 58 ± 1.41% p < 0.0001; vs. warm-RIC LY 54 ± 1.69% p = 0.0002). Conclusion: our study shows a remarkable difference between warm-RIC and cold-RIC in terms of infarct size reduction, supporting a pivotal role for limb temperature in RIC-induced cardioprotection.