The ultrastructure of cat papillary muscle was studied with respect to the organization of the contractile material, the structure of the organelles, and the cell junctions. The morphological changes during prolonged work in vitro and some effects of fixation were assessed. The myofilaments are associated in a single coherent bundle extending throughout the fiber cross-section. The absence of discrete "myofibrils" in well preserved cardiac muscle is emphasized. The abundant mitochondria confined in clefts among the myofilaments often have slender prolongations, possibly related to changes in their number or their distribution as energy sources within the contractile mass. The large T tubules that penetrate ventricular cardiac muscle fibers at successive I bands are arranged in rows and are lined with a layer of protein-polysaccharide. Longitudinal connections between T tubules are common. The simple plexiform sarcoplasmic reticulum is continuous across the Z lines, and no circumferential "Z tubules" were identified. Specialized contacts between the reticulum and the sarcolemma are established on the T tubules and the cell periphery via subsarcolemmal saccules or cisterns. At cell junctions, a 20 A gap can be demonstrated between the apposed membranes in those areas commonly interpreted as sites of membrane fusion. In papillary muscles worked in vitro without added substrate, there is a marked depletion of both glycogen and lipid. No morphological evidence for preferential use of glycogen was found.

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