Ultrastructural studies of frog myocardium.

The ultrastructural organisation of frog heart muscle was investigated using electron microscopy and related techniques. As far as possible, stincture was related to function, with particular emphasis being placed on calcium movements in contraction and relaxation. A basic description of the tissue is presented. Certain features of the cells, such as small size, flattened profiles, the presence of large intercellular clefts and sarcolemmal specialisations, indicate a high degree of reliance on the external medium. Irregularly distributed, short, tubular invaginations were found, which were condidered to increase the efficiency of the movement of solutes within the cell, and possibly to be primitive analogues of T-systems. Pinocytotic vesicles were found to be common and to pinch off from the sarcolemma and move into the cell taking materials from the bathing medium. Apparently anomalous vacuoles were found to be autophagic, and to arise from vacuolating mitochondria and from lysosomal degradation of organelles. Tlie application of cryo-ultramicrotomy to frog myocardium is discussed. Intracellular calcium was investigated using electron probe microanalysis. Close correlation between observed and expected ratios of calcium distribution in myofibrils and reasonable correlation between the absolute calcium content of the tissue foimd by two different methods, indicated the reasonable validity of calcixim localisation in conventionally prepared electron microscope specimens. Relative calcium levels in different organelles following induction of different physiological states, implicated the sarcoplasmic reticulum as a major intracellular site of calcium of activation, although mitochondria where also found to be involved in calcium fluxes, Myofibrillar H lines were found to be sites of calcium concentration, and nuclei, specific granules and mast cells were foimd to be high in calcium. Frog heart muscle organisation is discussed in relation to other myocardia and to the mode of action of striated muscle.