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Properties of the Sarcolemma*

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Properties of the Sarcolemma* 313 PROPERTIES OF THE SARCOLEMMA* C. Y. MATSUSHWA California State Polytechnic University, Pomona Like other surface membranes, the sarcolem of skeletal muscle cells serves to maintain the integrity of the sarcoplasm and to regulate the constant exchange of ions and metabolites between the inter- and intracellular environments. Its vital role in excitation-contraction coupling is well documented, but its role in the conversion of muscle to meat remains unclear. Advances in membranology indicate that the chemical constituents are intimately associated with structure and function of membranes. Thus, knowledge of the chemical and structural characteristics of the sarcolemma would greatly increase our compre- hension of such phenomena as bioelectrogenesis, metabolite transport and permeability changes in this surface membrane. Since Dr. Stromer has provided us with an insight into the structure of membranes, I will limit my discussion to some chemical properties of skeletal muscle sarcolemma. As a prelude to describing these properties, some time should be devoted to discussing available isolation procedures. "his is necessary because the chemical characteristics of sarcolemmal preparations appear to be influenced by the isolation procedure utilized (Abood -et ,',a1 1966; Heffron et al., 1967; Hultin and Westort, 1969; Sulakhe Let a1-*, 1973a). Westort and Hultin (1966) also inferred that the procedure of choice may be governed by the objectives of the study. Isolation of Sarcolemmal Membranes Since 1961, numerous isolation procedures for skeletal muscle sarcolemma have been published (Abood et al., 1966; Boegman et al., 1970; Koketsu et al., 1964, Kono and Colowick, 1961; McCollester, 1962; McNamara et a1 ., 1971; Peter, 1970; Rosenthal et al ., 1965; Westort and iIultin, 136r These procedures share the common objective of solubilizing and extracting the myofibrillar proteins without membrane disruption. Furthermore, these procedures usually represented modifications to those originally reported by Kono and Colowick (1961) and McCollester (1962) who utilized high and low ionic strength extracting solutions, respectively, and hence, sarcolemmal isolation procedures can be classified as being either high ionic strength or low ionic strength extraction methods. * Presented at the 27th Annual Reciprocal Meat Conference of the American Meat Science Association, 1974. 314 The basic scheme to Kono and Colowick's procedure (Kono and Colowick, 1961) is shown in Figure 1. Essentially, this procedure entails repeated homogenization in Tris buffer and the myofibrillar proteins extracted with 0.4 M LiBr followed by overnight extraction in 1.0 M KC1. Final separation of the cell membrane fragments from extraneous material is accomplished by differential centrifugation through a series of KBr solution differing in density. The major modifications to this method have been to homogenize in buffered 50 mM CaC12, to centrifuge at lower gravitation forces, i.e., 900 to 2000 x g, and eliminate the differential centrifugation through KBr (Abood et al., 1966; Koketsu et e.,1964; Sulakhe et al., 1973a). According to Koketsu --et al. (1964r the use of CaC12 during homogenization was necessary to prevent tangling and destruction as well as adhesion of the membrane fragments to the container during isolation. The elimination of differential centrifugation through KBr was facilitated by utilizing lower centrifugal forces which separates the sarcolemmal fragments from particulate matter (Sulakhe et al., 1973a). Because strong salt solutions may have adverse effects on membranes, McCollester (1962) developed a milder procedure for preparing sarcolemmal membranes . This procedure maintained near physiological conditions by employing a maximum ionic strength of 0.15 M and pH values between 6.4 and 7.8. The rationale to this procedure, however, is difficult to reconcile. As shown in figure 2, muscle tissue was repeatedly homogenized in 50 mM CaC12 to prevent irreversible contraction and/or activate Ca2+- dependent enzymes during the subsequent incubation step. Hultin and coworkers (Hultin and Westort, l969a; Stanley and Hultin, 196%), however, indicated that the primary effect of CaC was to stimulated myosin ATPase activity and, thereby destroying a'it 1 endogenous ATP and permitting actomyosin to form. At any rate, the sediment following centrifugation was washed 4 times in a buffered NaCl solution before the cell segments were incubated at 37OC for 30 minutes. According to McCollester (McCollester, 1962; McCollester and Semente, lw),incubation was necessary to break down the 'lcytoskeletonn of the muscle cell and thereby transforming the contractile proteins from a water-insoluble to a water-soluble state. He suggested the "cytoskeleton" consisted primarily of the sarcoplasmic reticulum and Z lines. With respect to the latter, a recent phase micro- scopic study indicated that the Z lines were degraded during the incubation step (Matsushima, 1971) . Following incubation and 5 post-incubation washes with histidine-buffered 25 mM NaC1, the muscle cell segments were subjected to several Tris-buffered water extractions. During these water extractions, the contractile proteins were dissolved, and the cell segments were emptied leaving the sarcolemmal membranes. The emptying process was characterized initially by marked swelling and a concomitant increase in viscosity of the myofibrillar proteins; this was followed by a subsequent decrease in both viscosity and volume of the sedimented material containing the sarcolemmal membranes as dissolution of the contractile proteins occurred. Several alternatives have been reported for the dissolution of the acto- myosin gel and liberation of empty cell segments. These include: (1) first extracting with 2 x 10-7 N NaCH md then treating the residue with ATP (Rosenthal et al., 1965) or sodium pyrophosphate (Ferdman et al., 1970) and waterT(3 first extracting with 0.01 mM EGTA followed by FIGUFG 1 ISOLATION OF SKELETAL MUSCLE SARCOLEMMA USING HIGH IONIC STRXNGTH SOLUTIONS (Kono and Colowick, 1961) (a) Suspend in 0.4 M LiBr, 0.01 M Tris (pH 8.2-8.4) (b) Stir 4 hr. (c) Centrifuge at 20,000 x g for 25 min . (a) Repeat previous steps except stir for 3 hr. FIGURF: 2 ISOLATION OF SKELETAL MUSCLE SARCOLEMMA USING LQI*J IONIC STREDGTH SOWIONS (McCollester, 1962) Muscle Tissue (a) Homogenized in 50 mM CaCb for 10 sec. (b) Homogenate filtered through cheesecloth c) Centrifuge at 600 x g, 5-7 sec. Sediment (a) 4 washes in 25 mM NaC1, 2.5 mM DL-histidine chloride (pH 7.4) (b) Centrifuge at 600 x g, 5-7 see. between washes Supernatant Sediment Suspend in the NaC1-histidine solution Incubate at 370C for 30 min. Centrifuge at 600 x g, 5-7 sec. I Supernatant Sediment (a) 5 washes in the NaC1-histidine solution (b) Centrifuge at 600 x g, 5-7 sec . after each wash. Super atant (a) 4-5 "extractions" with Tris-buffered water (PH 7.4-7.8) Centrifuge at 2000 x g, 7 min., after each (b) "extractnt ion .I1 Supernatantrt Sediment = empty cell sewents using 0.1 mM ATP to remve any residual actomyosin (Peter, 1970), or (3) direct application of the Tris-water extract on a double layer of sucrose (21% over 6036 sucrose) and centrifugation at 900 x g (Westort and Hultin, 1966). Regardless of the procedure utilized, the final preparation contains empty cell segments which appear through phase optics as tubular and transparent structures (Figure 3 ) . Occasionally, cable-like elements have been observed coiled around the external surface of these empty tubular segments (Kono and Colowick, 1961; Matsushima, 1971). At the resolution attainable by the electron microscope, the sarcolemma (Figure 4) a pears as electron-dense ribbons, approximately 80 8 wide (Matsushima, 1971P , with a mat of fine fibrils adhering to what probably represents the external surface of the membrane. Several authors (Kono and Colowick, 1961; Kono et al., 1964; Hultin and Westort, 1969b; Rosenthal et al., 1965) have suggested that these fibrils represent the outer layer of fibrous connective tissue often associated with the term, sarcolem (Mauro and Adams, 1961; Robertson, 1956). On the other hand, McCollester (1962) is of the opinion that these fibrils are contaminants which may account for up to one-half of the dry weight of sarcolemmal preparations. Purity of Sarcolemmal Preparations The assessmerit of purity of sarcolemnal preparations has been very difficult to ascertain. One major obstacle is the lack of any effective marker for this membrane. As a result, most investigators have had to rely on microscopic, enzymic markers for and known properties of sub- cellular organelles, and/or exhaustive comparative studies to determine purity of their preparations. An example utilizing enzyme markers is shown in table 1. In accord with other studies utilizing electron microscopy and/or enzyme markers such as succinic dehydrogenase (Matsushima, 1971) or succinic oxidase (Heffron and Duman, 1967; Hultin and Westort, 1969b), contamination from mitochondrial membranes appears minimal at best. The presence of microsomal membranes in sarcolemmal preparations, on the other hand, has been difficult to assess. If glucose-6-phosphatase activity, which is commonly used as a marker for microsomes, is utilized, it would appear that membranes of the sarcotubular system are present in significant quantities. However, as I will show and point out later, Hultin and Westort (l969b), who observed similar activities in their sarcolemmal preparations, have presented suggestive evidence to the contrary.
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