Ca2+-activated force-generating properties of mammalian skeletal muscle fibres: histochemically identified single peeled rabbit fibres

Abstract

Single peeled (sarcolemma removed) rabbit skeletal muscle fibres, identified histochemically from their myofibrillar ATPase and oxidative staining pattterns, were characterized according to their Ca²⁺-activated steady-state force-generating properties at normal intracellular pH (7.0) and under acidotic (pH 6.5) conditions. Maximum force-generating capacity of each fibre was assessed by measuring steady-state isometric force generation at saturating Ca²⁺ concentration at both pH values. The Ca²⁺ sensitivity of each fibre was ascertained by determining the percentage of maximum force generated at each of several subsaturating Ca²⁺ concentrations at both pH values. Fibres were selected from soleus, tibialis anterior and adductor magnus muscles. At subsaturating Ca²⁺ concentrations only two functional groups of fibres were distinguishable, corresponding to the histochemical classifications type I and type II. Type I fibres were more sensitive to Ca²⁺ and less depressed by acidosis than type II fibres in the subsaturating range of Ca²⁺ concentrations. At saturating Ca²⁺ concentrations, the acidotic depression of maximum force was significantly less for type I fibres than type II nonoxidative fibres regardless of their muscle of origin. Type II oxidative fibre maximum force properties depended upon the muscle of origin and demonstrated subgroups of these fibres that were different from type II nonoxidative fibres and similar to type I fibres.