Adenosine Triphosphatase of Sarcoplasmic Reticulum Triads and Sarcolemma identified histochemically

Abstract

THE adenosine triphosphatase (ATPase) activity of human skeletal muscle is confined to the A-bands (except the H-band portion)¹ when fresh-frozen tissue is cut in a cryostat, the sections dried on coverslips, and the Padykula–Herman² standard incubation medium used (20 min 37° C, pH 9.4, 405 mM ATP, 18 mM Ca⁺⁺, no Mg⁺⁺). However, if the sections are put in the incubating medium before being dried, additional ‘wet ATPase’ activity is seen in the intermyofibrillar regions of all the muscle fibres (Fig. 1)³. Further investigations have now disclosed that the wet ATPase activity in the intermyofibrillar region is located at the level of the A-band and I-band junction, as seen in longitudinal sections (Fig. 2). This is the precise location of the sarcoplasmic reticulum triads and of no other constituent in human skeletal muscle cells, as disclosed by electron microscopy⁴. Wet ATPase activity also occurs in the sarcolemmal membrane (Fig. 3) and is qualitatively like the sarcoplasmic reticulum triad ATPase. There is increased sarcolemmal activity periodically at the level of triad activity seen in longitudinal sections (Fig, 3), The wet ATPase occurs in the muscle nucleoli but not in the muscle mitochondria. Under the conditions used, the sarcoplasmic reticulum triad and sarcolemmal ATPase produce a darker staining than the myofibrillar A -band ATPase.