S T U D I E S OF T H E T R I A D IV. Structure of the Junction in Frog Slow Fibers

The structure of the triadic junction in frog slow fibers has been studied and compared with that of twitch fibers. The junct ional gap is wider (by approximately 13 %) in slow fibers. The junct ional feet have the same size and disposition as in twitch fibers, al though the size and shape of the junct ional areas are different. I t is concluded that the role of triads in slow fibers is the same as in twitch fibers I N T R O D U C T I O N In frog slow fibers the development and maintenance of tension after depolarization in high potassium strongly depends on the concentration of calcium in the outside solution (Liittgau, 1963 a). In this respect, slow fibers apparently differ from twitch fibers, where the calcium necessary for activation of the contractile material is all liberated from an internal store, the sarcoplasmic reticulum, or SR (see Lfittgau, 1963 b, and Sandow, 1955, for reviews). On the other hand, it has been shown that slow fibers in the frog (Page, 1965), as well as in other vertebrates (Peachey, 1965; Hoyle et a l , 1966, Hess, 1967; Page, 1969), possess structures equivalent to those thought to form the intermediary links between excitation and contraction in twitch fibers. Thus, slow fibers are pervaded by tubules, whose walls are continuous with the plasma membrane and which probably carry the surface depolarization to the interior of the fiber, similarly to the transverse (T) system tubules of twitch fibers (Page, 1965). The SR is present in frog slow fibers (Peachey and Huxley, 1962) and is fairly well developed (Page, 1965). Similarly to that of twitch fibers (Costantin et al., 1965), the SR of slow fibers contains a fairly high concentration of calcium, which can be visualized in the electron microscope after precipitation with oxalate (Costantin et a l , 1967, Page, 1969). I t is reasonably speculated that at least some of the calcium for excitation of slow fibers is released from this internal store (Page, 1969). In twitch fibers it is thought that the signal to release calcium is transmitted from the T system to the SR at the triads, where specialized areas of junction exist between the two systems of membranes. In slow fibers, close proximity between SR and T system elements occurs at triads (and dyads) which differ in number and location from those of twitch fibers (Page, 1965) This paper shows that the fine structural details of the triadic junctions of slow fibers are not basically different from those of twitch fibers, thus providing further evidence that morphologically, at least, slow fibers have an excitation-contraction (e-c) coupling pathway analogous to that of twitch fibers. M A T E R I A L S A N D M E T H O D S Lumbricahs muscles from the big toe of Rana flip~ens hind legs were dissected and fixed either in 3% glutaraldchyde in 0.1 2vi phosphate buffer, pH 7.2 (Figs. 1, 3-6, 11, 12), or in the same fixative with the addi120 ~rHE JOT~AL OF CELL BIOLOGY • VOtiVE 56, 1973 • pages 120-1~8 on N ovem er 7, 2017 jcb.rress.org D ow nladed fom tlon of peroxide (Peracchia and l~Iittler, 1972) for 2 h at room temperature After a brief wash in buffer, small bundles of fibers were postfixed in 2% OsOa in the same buffer for 2 h at room temperature. Dehydration was in ethanol and propylene oxide and embedding was in Epon. Sections were stained m alcoholm uranyl acetate, followed by lead (Sato, t968). Sections were cut on an A. F. Huxley pattern Cambridge microtome and examined in Stamens Elmiskop 101 and AEI 801 microscope. Where accurate measurements were required, the magnification of the AEI microscope was standardized by eliminating hysteresis of the lenses, as recommended by the manufacturer. This supposedly eliminates variations an magnifieauon to within -4-2%.