Ultrastructural and gene-expression changes in the calcium regulation system of rat skeletal muscles under exhausting exercise

The expression of genes responsible for the synthesis of essential proteins regulating the calcium-ion balance and ultrastructural characteristics of fast-twitch (m. extensor digitorum longus, EDL) and slow-twitch (m. soleus, SOL) skeletal muscles under prolonged exercise were studied in an experimental model of forced-swimming rats. A day after the end of the exercise, no significant changes in any of the five investigated genes were revealed in the SOL. A few triad elements (T-tubules and cisternae of sarcoplasmic reticulum) were revealed. A small number of excitation-contraction coupling (ECC) structures in the control and a slight increase in their amount after exercises were noticed. Polymorphism and mitochondrial defects within SOL muscles indicate the importance of these structures in the regulation of calcium balance. In EDL muscles, adaptation mechanisms are aimed mainly at pumping Ca²⁺ ions to the sarcoplasmic reticulum, where the main calcium buffer is calsequestrin. Expression of SERCA1 gene increased by an order of magnitude, and that of CASQ1 increased by three times. Electron microscopy showed a major role of triads in the maintenance of calcium homeostasis in the EDL muscles, as well as a greater destruction of these muscles compared to SOL after exhausting exercise. The high level of triads and a possible activation of the CICR (calcium-induced calcium release) mechanism in fast-twitch muscles can cause damage to them during exhausting exercise. Adaptation of SOL muscles is associated with structural rearrangements of the mitochondrial apparatus, while adaptation of the EDL muscles is caused by calcium removal from the sarcoplasm with Ca-ATPase and its retention in the sarcoplasmic reticulum by calsequestrin.