Ultrastructure studies of the contraction-relaxation cycle of glycerinated rabbit psoas muscle: I. The ultrastructure of glycerinated fibers contracted by treatment with ATP

Abstract

Studies of ultrastructural changes of glycerinated rabbit psoas muscle during contraction induced by ATP have shown that the sarcomere patterns of striated muscle seen in electron micrographs can be directly correlated with the physiological state of the tissue at the time of fixation. The relationships of band and sarcomere lengths of rigid control fibers agree well with those upon which the postulated sliding model is based. This is not true for tissue fixed during contraction. Sarcomeres fixed during ATP-induced contraction are characterized by the appearance of contraction bands at the A—I-boundary of sarcomeres with patent I-bands. The contraction band formation usually is accompanied by a slight decrease in thick filament length with no increase in filament diameter. In fibers which were held at extended length during glycerination, the ATP-contracted sarcomeres often have thin filaments which are shorter than those of rigid controls; and the contraction bands usually are extremely dense. Some of these sarcomeres which are rather short still have wide H-bands. Thus, although filament sliding usually accompanies contraction when the fibers contract from rest length, the contraction can occur with no sliding in connection with shortening of stretched muscle fibers with a narrow zone of filament overlap. Contraction band formation, on the other hand, is a feature which clearly distinguishes fibers fixed during contraction from those which have not been contracted.