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 degrees C), we characterized cellular contractile mechanisms under normothermic (37 degrees C) and hyperthermic (39-42 degrees 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).

Heat acclimation-induced elevated glycogen, glycolysis, and low thyroxine improve heart ischemic tolerance.

Based on our observations of energy sparing in heat-acclimated (AC) rat hearts, we investigated whether changes in preischemic glycogen level, glycolytic rate, and plasma thyroxine level mediate cardioprotection induced in these hearts during ischemia-reperfusion insults. Control (C) (24 degrees C), AC (34 degrees C, 30 days), acclimated-euthyroid (34 degrees C + 3 ng/ml l-thyroxine), and control hypothyroid (24 degrees C + 0.02% 6-n-propyl-2-thiouracil) groups were studied.