Short-term heat therapy enhances heart thermotolerance: Cyclic thermal stress reveals adaptive resilience in cardiac function.

Short-term heat therapy is emerging as a promising non-pharmacological strategy to enhance cardiovascular resilience, yet its effects on intrinsic cardiac thermotolerance remain unclear. This study investigated whether 5 or 10 days of hot-water immersion (40 °C) is sufficient to induce functional thermotolerance in isolated rat hearts. Wistar rats were assigned to the control (CTR), 5-day heat therapy (HT5), or 10-day heat therapy (HT10) groups and subsequently evaluated in a Langendorff preparation under a progressive thermal challenge (37 - 40 °C). Mechanical performance was assessed through left ventricular systolic pressure (LVSP), end-diastolic pressure (LVEDP), developed pressure (LVDP) and heart rate (HR), all expressed as a percentage of baseline. HT10 exhibited a markedly superior hemodynamic profile during heating, characterized by a smaller decline in LVSP and LVDP and reduced elevation of LVEDP compared with CTR and HT5. HT5 showed only partial protection, with greater variability and late-phase diastolic impairment. HR decreased transiently at 38 °C in both treated groups, but HT10 displayed faster chronotropic recovery and greater stability at 39-40 °C. These patterns indicate that repeated heat exposure for 10 days promotes a robust thermotolerant cardiac phenotype, improving systolic stability, diastolic compliance and chronotropic control during acute heat stress. Collectively, the results demonstrate that short-term cyclic thermal stress, particularly 10 days, is sufficient to enhance myocardial thermal resilience, aligning with early acclimation mechanisms described for mitochondrial protection, HSP-mediated cytoprotection and anti-apoptotic modulation. These findings support the translational potential of short-term heat therapy as a viable intervention to strengthen cardiac tolerance to thermal challenges.