Depolarization of the Internal Membrane System in the Activation of Frog Skeletal Muscle

Abstract

"Skinned" muscle fibers, single fibers from the frog semitendinosus muscle in which the sarcolemma had been removed, could be reversibly activated by electrical stimulation. Electrical responsiveness was abolished when the skinned fiber was prepared from a muscle exposed to a cardiac glycoside, and the development of responsiveness was delayed when the muscle was bathed in high potassium solution. The findings were taken as evidence that active sodium-potassium exchange across the internal membranes restored electrical excitability, after the sarcolemma had been removed, by establishing a potential gradient across the internal membranes. In general, the contractions were graded with the strength of the applied current. On occasion, however, "all-or-none" type responses were seen, raising the possibility that the internal membranes were capable of an electrically regenerative response. Activation could also be produced by an elevation of the intraceUular chloride ion concentration or a decrease in the intracelhilar potassium ion concentration, suggesting that depolarization of some element of the internal membrane system, that is, a decrease in the potential of the lumen of the internal membrane system relative to the potential of the myofibrillar space, was responsible for activation in these experiments. The distribution of both the electrically induced contractions and those produced by changes in the intracellular ion concentrations indicated that the responsive element of the internal membrane system was electrically continuous over many sarcomeres. Csapo (1959) and Natori and Isojima (1962) have reported that single fibers from a m p h i b i a n muscle could still be activated by electrical stimulation following removal of the surface membrane. It seemed possible that current flow across some component of the internal m e m b r a n e system could account for this p h e n o m e n o n and the experiments described in this paper are an attempt to examine this question. Preliminary accounts of these results have been published (Costantin and Podolsky, 1965, 1966).

Calcium-Induced Calcium Release in Skeletal Muscle

“…When another medium of different ionic composition is substituted for the medium in which skinned fibers or FSR is immersed, Ca 2ϩ release can be evoked, if the new medium causes depolarization of the internal membrane systems, namely, resealed t-tubules (34,251) and/or the SR.…”

Sodium and water contents of sarcoplasm and sacroplasmic reticulum in rat skeletal muscle: effects of anisotonic media, ouabain and external sodium

“…Secondly, in glycerol-treated frog muscle, in which T tubules have been disrupted and the ion movements between SR and extracellular space interrupted in at least some of the fibres presumably, K influx was reduced more than was K efflux, as if active K transport had suffered (Henderson, 1970). Thirdly, skinned muscle fibres, which lack sarcolemma, are responsive to strophanthidin (Costantin & Podolsky, 1967). All of these suggest that ouabain-sensitive Na-K transport can take place across the internal membranes separating sarcoplasm from SR.…”

Contractile activation by voltage clamp depolarization of cut skeletal muscle fibres.

“…However, since they lack the membrane barrier between fibre interior and exterior which is present in intact fibres, the possibility of initiating calcium release by controlled T-tubule depolarization is made difficult or impossible. Instead, calcium release in skinned or split fibres is generally initiated by various types of solution change which are thought to act directly on the sarcoplasmic reticulum (Costantin & Podolsky, 1967;Endo, Tanaka & Ogawa, 1970;Ford & Podolsky, 1972;Nakajima & Endo, 1973). In the present report we consider a new preparation for studying E-C coupling, the cut muscle fibre.…”