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

Omer Cohen, Hifa Kanana, Ronen Zoizner, Chaya Gross, Uri Meiri, Michael D. Stern, Gary Gerstenblith, Michal Horowitz*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)266-275
Number of pages10
JournalJournal of Applied Physiology
Issue number1
StatePublished - Jul 2007
Externally publishedYes


  • Calcium transients
  • Cardiomyocyte contractility
  • L-type calcium channel
  • Phospholamban
  • Ryanodine receptors


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