Store-operated Ca²⁺ entry (SOCE) is a mechanism that allows muscle fibers to recover external Ca²⁺, which first enters the cytoplasm and then, via SERCA pump, also refills the depleted intracellular stores (i.e., the sarcoplasmic reticulum, SR). We recently discovered that SOCE is mediated by Calcium Entry Units (CEUs), intracellular junctions formed by: (i) SR stacks containing STIM1; and (ii) I-band extensions of the transverse tubule (TT) containing Orai1. The number and size of CEUs increase during prolonged muscle activity, though the mechanisms underlying exercise-dependent formation of new CEUs remain to be elucidated. Here, we first subjected isolated extensor digitorum longus (EDL) muscles from wild type mice to an ex vivo exercise protocol and verified that functional CEUs can assemble also in the absence of blood supply and innervation. Then, we evaluated whether parameters that are influenced by exercise, such as temperature and pH, may influence the assembly of CEUs. Results collected indicate that higher temperature (36 °C vs. 25 °C) and lower pH (7.2 vs. 7.4) increase the percentage of fibers containing SR stacks, the n. of SR stacks/area, and the elongation of TTs at the I band. Functionally, assembly of CEUs at higher temperature (36 °C) or at lower pH (7.2) correlates with increased fatigue resistance of EDL muscles in the presence of extracellular Ca²⁺. Taken together, these results indicate that CEUs can assemble in isolated EDL muscles and that temperature and pH are two of the possible regulators of CEU formation.

Exertional heat stroke (HS) is a hyperthermic crisis triggered by an excessive accumulation of Ca²⁺ in skeletal muscle fibers. We demonstrated that exercise leads to the formation of calcium entry units (CEUs), which are intracellular junctions that reduce muscle fatigue by promoting the recovery of extracellular Ca²⁺ via store-operated Ca²⁺ entry (SOCE). Here, we tested the hypothesis that exercise-induced assembly of CEUs may increase the risk of HS when physical activity is performed in adverse environmental conditions (high temperature and humidity). Adult mice were: (a) first, divided into three experimental groups: control, trained-1 month (voluntary running in wheel cages), and acutely exercised-1 h (incremental treadmill run); and (b) then subjected to an exertional stress (ES) protocol, a treadmill run in an environmental chamber at 34 °C and 40% humidity. The internal temperature of the mice at the end of the ES was higher in both pre-exercised groups. During an ES ex-vivo protocol, extensor digitorum longus(EDL) muscles from the trained-1 month and exercised-1 h mice generated greater basal tension than in the control and were those that contained a greater number of CEUs, assessed by electron microscopy. The data collected suggest that the entry of Ca²⁺ from extracellular space via CEUs could contribute to exertional HS when exercise is performed in adverse environmental conditions.

Tubular aggregates (TAs), ordered arrays of sarcoplasmic reticulum (SR) tubes, are the main morphological alteration found in muscle biopsies from patients affected by TA myopathy (TAM). TAM has been linked to mutations in the genes encoding for STIM1 and ORAI1, which are two proteins that mediate Store-Operated Ca²⁺ entry (SOCE). SOCE is a mechanism that allows recovery of extracellular Ca²⁺ during fatigue, when the SR becomes depleted. As TAs also form in fast-twitch muscle fibers of aging male mice (not in females), we studied the effect of sex hormones on the aggregation of TAs during aging. We administered estrogen (ad libitum in drinking water) to male mice from 10 to 18 months of age and then evaluated the following: (a) the presence of TAs using histology and electron microscopy (EM); (b) oxidative stress, a mechanism that could underlie damage to proteins and membranes (and possibly their accumulation in TAs); and (c) SOCE function during ex vivo stimulation in the presence or absence of external Ca²⁺ or SOCE blocker (BTP-2). The results collected indicate that treatment with estrogen (a) significantly reduced the formation of TAs; (b) reduced oxidative stress, which was elevated in aging male mice; and (c) restored SOCE, i.e., the capability of aged EDL muscles to use external Ca²⁺ by promoting maintenance of Ca²⁺ Entry Units (CEUs, the intracellular junctions that mediate SOCE). Finally, we also show that formation of TAs is reduced by treatment of mice with N-acetilcysteine (NAC), a potent antioxidant also administered ad libitum in drinking water.