Effects of 28-day mechanical and chewing stress on content of bound and diffusible ions in muscles of mastication

Type I and type IT muscle fibres have different ion concentrations. Muscles adapt to chronic stress by changing of fibre types and remodelling of the myosin heavy chains in the muscle fibres. The present investigation on ionic change during muscular contraction was c arried out on 10-week-old pigs (6 treated animals, 6 controls) over a 28-da y period. Six pigs received acrylic build-ups to induce mechanical advancem ent of the lower jaw and chronic chewing stress. Muscle tissue was taken fr om the masseter (M1, M2, M3), temporal (TP1, TP2), medial pterygoid (PM) an d geniohyoid (GH) muscles by a standardized method. Eighty-four muscle samp les were used for histological fibre differentiation with mATPase. Energy-d ispersive X-ray microanalysis of muscles was carried out in an environmenta l scanning electron microscope. Endurance stress in the stressed muscles wa s seen as an increase of type I fibres (P < 0.001). This histological chang e and ionic alterations were measured in the anterior region of the massete r (M1 and M2) and in the posterior region of the temporal muscle (TP2), Sma ller changes were found in the medial pterygoid muscle. We measured in this muscles increases in potassium, sulphur, chloride (P < 0.05) and even larg er increases in phosphate (up to 1.5 mmol/g to 2.3 mmol/g. P < 0.001) and s odium (3-fold, P < 0.001). The results reveal the effects of chronic stress on muscle fibres and ion concentration in the muscle. Chronic stress resul ted in an increase of type I fibres and increased ion concentration in the same muscle region. These are considered to be indicators of more efficient contraction. The changes in ion concentration are an important factor in m uscle contraction.