Combined heat acclimation (AC) and exercise training (EX) enhance exercise performance in the heat while meeting thermoregulatory demands. We tested the hypothesis that different stress-specific adaptations evoked by each stressor individually trigger similar cardiac alterations, but when combined, overriding/trade-off interactions take place. We used echocardiography, isolated cardiomyocyte imaging and cDNA microarray techniques to assay in situ cardiac performance, excitationcontraction (EC) coupling features, and transcriptional programs associated with cardiac contractility. Rat groups studied were controls (sedentary 24°C); AC (sedentary, 34°C, 1 mo); normothermic EX (treadmill at 24°C, 1 mo); and heat-acclimated, exercise-trained (EXAC; treadmill at 34°C, 1 mo). Prolonged heat exposure decreased heart rate and contractile velocity and increased end ventricular diastolic diameter. Compared with controls, AC/EXAC cardiomyocytes demonstrated lower L-type Ca 2ϩ current (ICaL) amplitude, higher Ca 2ϩ transient (Ca 2ϩ T), and a greater Ca 2ϩ T-to-ICaL ratio; EX alone enhanced I CaL and Ca 2ϩ T, whereas aerobic training in general induced cardiac hypertrophy and action potential elongation in EX/ EXAC animals. At the genomic level, the transcriptome profile indicated that the interaction between AC and EX yields an EXACspecific molecular program. Genes affected by chronic heat were linked with the EC coupling cascade, whereas aerobic training upregulated genes involved with Ca 2ϩ turnover via an adrenergic/ metabolic-driven positive inotropic response. In the EXAC cardiac phenotype, the impact of chronic heat overrides that of EX on EC coupling components and heart rate, whereas EX regulates cardiac morphometry. We suggest that concerted adjustments induced by AC and EX lead to enhanced metabolic and mechanical performance of the EXAC heart. echocardiography; isolated cardiomyocytes; Ca 2ϩ signaling; genomic responses HEAT ACCLIMATION (AC) and exercise training (EX) are powerful stressors, causing structural cardiac remodeling and improving mechanical performance. Each stressor has its own adaptive requirements, e.g., increased peripheral blood flow to enhance heat dissipation, required for AC, or increased muscle blood flow to augment the oxygen supply and muscle power output needed during bouts of exercise. Evidence derived from isolated rat heart preparations prompted the hypothesis that different stress-specific adaptive signaling mechanisms trigger the similar cardiac alterations evoked by each stressor alone. Upon AC, an increase in left cardiac ventricular compliance and systolic pressure with a concomitant decrease in oxygen consumption suggests enhanced cardiac efficiency (25-26). At the cellular level, greater Ca 2ϩ transient amplitude compensates for decreased myofilament Ca 2ϩ responsiveness and plays a major role in greater force development (4). Myosin isoform redistribution from the predominantly fast myosin isoform (V1) to slow myosin isoform (V3) predominance and the altered expression of Ca 2ϩ r...