SKINNED FIBRES OF SKELETAL MUSCLE AND THE MECHANISM OF MUSCLE CONTRACTION - Chronological Account of the Author's Investigations into Muscle Physiology - PROPERTIES OF SARCOLEMMA-FREE MYOFIBRIL BUNDLES (SKINNED MUSCLE FIBRES)

1. Method of isolation of skinned muscle fibres：With the aforesaid motive in mind, I tried to isolate myofibrils which still retained their in situ properties (Natori 1949*, Natori 1950, Natori 1950*, Natori 1951 and Natori 1952a). At the beginning of this trial, on the basis of the chemical composition of the sarcoplasm reported previously, various solutions such as Ringer's solution, the Na and K concentrations of which were exchanged reciprocally, using 120 mM KCl solution and so on, were applied for removal of the sarcolemma. However, the mechanical removal of the sarcolemma in the above solutions induced irreversible contractions. For instance, if we tried to remove the sarcolemma in 120 mM KCl solution, the myofibrils contracted markedly as soon as the sarcolemma was injured. If we stretched them for a certain length of time, the fibres became extended until they equalled the slack length of the intact muscle fibre, but their properties were demonstrated in various tests to be considerably different from the properties of living myofibrils. In the course of these trials, I wondered whether contraction would occur if the sarcolemma were to be removed in oil, thus avoiding both diffusion of the solution into and out of the muscle fibre and the generation of injury current. The application of oil immersion was found to be successful and this marked the beginning of a series of experiments of skinned muscle fibres. In the early years, I used the name “isolated myofibrils”, “denuded myofibrils” or “sarcolemma-free bundle of myofibrils” for muscle fibres with the sarcolemma removed. Since the term “skinned fibre” later became popularized, I have recently been using the name “skinned muscle fibre”. In Japan, the term “Natori's fibre” is also used to mean “skinned muscle fibre”.