Effect of long-term heat strain on calcium signaling in the murine myocardium

The physiological effects of prolonged heat strain (HS) insufficient to cause heat stroke on the heart remain poorly understood. We previously established this causes cellular injury and stress in the murine heart. Given that calcium dysregulation can lead to these previously detected changes, the purpose of this investigation was to determine the extent to which HS altered calcium homeostasis in the murine myocardium. We hypothesized that HS would undermine calcium homeostatic control. To test this hypothesis, 7 wk old female mice were assigned to either TN (Thermoneutral; n=8, 31.2 ± 1.0 °C, 35 ± 1% humidity) or HS (n=9; 37.6 ± 0.0 °C, 42 ± 1 % humidity) conditions for 24 h. In comparison to TN, HS increased protein abundance of plasma membrane calcium ATPase by 19% ( P=0.05), phosphorylated (p)-phospholamban by 426% ( P<0.01), calmodulin-dependent protein kinase II by 142% ( P<0.01), CaM by 37% ( P<0.01) and calsequestrin by 58% ( P<0.05); however, HS decreased phopholamban by 25% ( P=0.05) and p-protein kinase A by 58% ( P<0.01) compared to TN. These data demonstrate that 24 h of sub-heat stroke HS is sufficient to induce calcium signaling protein changes in the myocardium that favors calcium clearance from the cytosol into the SR with concomitant desensitization to β-adrenergic stimulation. However, how calcium dysregulation may lead to functional impairments is unknown.