Exercise-activated Ca²⁺ entry and enhanced risk of heat stroke: Calcium Signaling and Excitation–Contraction in Cardiac, Skeletal and Smooth Muscle

Exertional/environmental heat strokes (EHSs) are hyperthermic crises triggered by strenuous physical exercise and/or exposure to environmental heat, and are caused by an altered intracellular Ca²⁺ homeostasis in muscle. We recently demonstrated that a single bout of exercise on treadmill leads to formation of calcium entry units (CEUs), intracellular junctions that promote interaction between STIM1 and Orai1, the two proteins that mediate store-operated Ca²⁺ entry (SOCE). SOCE is a mechanism that is activated during muscle fatigue and that allows for recovery of extracellular Ca²⁺ during prolonged activity. The hypothesis underlying this work is that assembly of CEUs during prolonged exercise may predispose to EHSs when exercise is performed in challenging environmental conditions. To test this hypothesis, 4-mo-old mice were (1) divided into three experimental groups: control, trained-1m (1 mo of voluntary running in wheel cages), and exercised-1h (1 h of incremental treadmill run); and (2) subjected to an exertional stress (ES) protocol consisting of an incremental 45-min treadmill run at 34°C and 40% humidity. We then (a) measured the internal temperature of mice, which was higher in the two pre-exercised groups (trained-1m: 38.9°C ± 0.33; exercised-1h: 38.7°C ± 0.40) compared with control animals (37.9°C ± 0.17); (b) applied an ex vivo ES protocol to isolated EDL muscles (tetanic stimulation performed at 30°C) and verified that samples from trained-1m and exercised-1h mice generated a tension significantly greater than control samples; and (c) analyzed CEUs by electron microscopy (EM) and verified that EDL muscles of trained-1m and exercised-1h mice contained a greater number of membranes elements forming CEUs. The data collected indicates that the presence of CEUs correlates with a greater increase in body temperature and could, in principle, predispose to EHS when exercise is performed in challenging environmental conditions.