Altered Ca²⁺ handling and myofilament desensitization underlie cardiomyocyte performance in normothermic and hyperthermic heat-acclimated rat hearts

Heat acclimation (AC) improves cardiac mechanical and metabolic performance. Using cardiomyocytes and isolated hearts from 30-day and 2-day acclimated rats (AC and AC-2d, 34°C), we characterized cellular contractile mechanisms under normothermic (37°C) and hyperthermic (39-42°C) conditions. To determine contractile responses, Ca²⁺ transients (Ca²⁺ T), sarcoplasmic reticulum (SR) Ca²⁺ pool size (fura-2/indo-1 fluorescence), force generation [amplitude systolic motion (ASM)], L-type Ca²⁺ channels [dihydropyridine receptor (DHPR)], ryanodine receptors (RyRs), and total (PLBt) and phosphorylated phospholamban [serine phosphorylated (PLBs) and theonine phosphorylated (PLBtr)] proteins and transcripts were measured (Western blot, RT-PCR). Cardiac mechanical performance was measured using a Langendorff system. We demonstrated that AC and AC-2d increased Ca²⁺ T amplitude (148% and 147%, respectively) and twitch force (180% and 130%, respectively) and desensitized myofilaments, as indicated by a rightward shift in the ASM-Ca²⁺ relationships, despite no change in SR Ca²⁺ pool size. Hence, generation of higher Ca²⁺ T underlies greater force development in AC and AC-2d myocytes. In isolated hearts, ryanodine administration eliminated differences between AC and control (C) hearts, implying an important role for RyRs in that acclimation phase. Increased expression of DHPR and RyRs, and decreased PLBs/PLBt in AC hearts only, suggest that different pathways increase force generation in the AC-2d vs. AC myocytes. At basal beating rates, hyperthermia (39-41°C) enhanced pressure generation in AC hearts. C hearts failed to restitute pressure beyond 39°C. Increased beating frequency produced negative inotropic response. In C cardiomyocytes, hyperthermia elevated basal cytosolic Ca²⁺ and tension, Ca²⁺ T, and ASM. AC myocytes enhanced Ca²⁺ T but showed myofilament desensitization, suggesting its involvement in cardiac protection against hyperthermia. Collectively, both Ca²⁺ turnover and myofilament responsiveness are important adaptive acclimatory targets during normothermic and hyperthermic conditions.