The Sarcoplasmic Reticulum and Transverse Tubules of the Frog's Sartorius

Abstract

The sarcoplasmic reticulum of the frog's sartorius muscle was examined by electron microscopy following sequential fixation in glutaraldehyde and osmium tetroxide and embedding in Epon. The earlier results of Porter and Palade on Ambystoma muscle were confirmed in the sartorius. In addition, the transverse tubules were observed to be continuous across the width of the fiber, a set of flat intermediate cisternae was seen to connect the terminal cisternae to the longitudinal tubules in the A band, and the continuous reticulum collar at the center of the A band was found to be perforated by circular and elongated pores (the fenestrated collar). The transverse tubules have a volume about 0.3 per cent of the fiber volume, and a surface area about 7 times the outer cylindrical surface area for a fiber 100 µ in diameter. The terminal cisternae, the intermediate cisternae, and the longitudinal tubules together with the fenestrated collar each have a volume of 4 to 5 per cent of the fiber volume and a surface area 40 to 50 times the outer surface area of a fiber 100 µ in diameter. Some evidence for continuity of the transverse tubules with the fiber surface is presented, but this is thought to be not so convincing as evidence presented by others. The results are discussed in terms of a possible mechanism for a role of the transverse tubules and sarcoplasmic reticulum in excitation-contraction coupling, as suggested by their morphology and a variety of physiological studies. In this scheme, the transverse tubules are thought to be electrically coupled to the terminal cisternae, so that depolarization of the fiber surface spreads inward along the transverse tubules and to the terminal cisternae, initiating the release of a contraction-activating substance.

Elemental distribution in striated muscle and the effects of hypertonicity: Electron probe analysis of cryo sections

“…Such a compartment has been suggested by previous authors on the basis of ion flux studies (27,46,60,69,79), the distribution of fluorescent dyes and other markers (34), and the finding of fiber CI contents exceeding the amounts predicted by passive electrochemical distribution (46). The apparent correspondence between the size of this kinetically defined compartment and the size of the SR (70,74) led to suggestions (46,79) that the SR is in ionic communication with the extracellular space. This hypothesis is not in agreement with electrophysiological measurements (49) including measurements of membrane capacitance (28,33,36,50), but received support from studies showing the swelling of the SR in muscles fixed after incubation in hypertonic solutions (15,16,53).…”

“…THE JOURNAL OF CELL BIOLOGY' VOLUME 74, 1977 frogs. The X~ ~ values for P, S, C1, and K concentrations measured in all the muscles give probability values of <0.001, indicating that it is highly unlikely that these differences in ion concentrations measured on a dry weight basis in different frogs were due to the statistical error in measurement.…”

“…THE JOURNAL OF CELL BIOLOGY" VOLUME 74,1977 900 to 2,000 /~ as judged by the gray-to-blue interference colors. The thinner, more electron-lucent sections were used for imaging and analyzing terminal cisternae.…”

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

“…The transverse tubules (Ttubules) are invaginations of the plasma membrane and contain the voltage sensing dihydropyridine receptor (DHPR). The portion of the SR terminal cisternae membrane facing the T-tubules is called junctional face membrane and contains the ryanodine receptor Ca 2+ release channel (RyR1), regulatory and structural proteins such as JP-45, triadin and junctin as well as a number of other proteins that are part of the RyR1 Ca 2+ release channel macromolecular structure [1][2][3][4]. In skeletal muscle, the triad, i.e.…”

THE STRUCTURE OF INTERSEGMENTAL MUSCLE FIBERS IN AN INSECT, PERIPLANETA AMERICANA L

“…Much evidence favors the implication of the latter in the control of myofibrillar activation and deactivation, through cyclic exchange of calcium ions between the cavities of the sarcoplasmic reticulum and the fibrils, occurring in phase with the train of excitatory signals distributed, via the celt membrane, along the openmouthed T system tubules, and transferred to the reficulum across the dyad or triad gap. This subject has been discussed, in the context of vertebrate skeletal muscle fibers, by Porter (1961), Ebashi (1961), Huxley (1964), Hasselbach (1964) and Peachey (1965). The effectiveness of a "relaxing factor" fraction with calcium-binding properties, isolated from locust synchronous muscle (Tsukamoto et al, 1965), on both mammalian and insect myofibrillar preparations, together with the comparable membrane distribution in these muscles, points to a common mode of activation control in synchronously responding skeletal muscle fibers.…”