The plasmalemmal area of striated muscle fibres is greater than the apparent surface area (A = circumference x length) because of variable folds and the invaginations of the caveolae and T-tubules. Freeze-fracture replicas of the surface membrane of sartorius and semitendinosus muscles from Rana pipiens have been used to determine the numbers and distribution of folds and caveolae at different sarcomere lengths. (1) The plasmalemma folds are variable in size and shape, but are always oriented perpendicular to the long axis of the fibre. The folds vary with stretch, being more prominent at short sarcomere lengths. The caveolae are elliptical invaginations of the plasmalemma which open to the outside by a narrow "neck" of approximately 20 nm. The caveolar lumen has an average long dimension of 81.6 +/- 11.7 nm and an average short dimension of 66.9 +/- 7.9 nm. The caveolar "necks" only can be seen in freeze-fracture replicas and these are distributed in two circumferential bands on either side of the Z-line, and in longitudinal bands separated by distances of 1-5 mum. In the sartorius muscle, at a sarcomere length of 2.8 mum, there is an average number of thirty-seven caveolae per square micrometer of fibre surface. (2) During passive stretch the opening of folds provides membrane for the necessary increase in surface area up to a sarcomere length of about 3.0 mum. This length is defined as the critical sarcomere length (Sc). The number of caveolae remains constant at all sarcomere lengths less than Sc and thus their "necks" have been used as membrane markers to determine the amount of folding at different sarcomere lengths. The membrane area contained in folds and caveolae is expressed as a fraction of the apparent surface area (A). For example, in the sartorius muscle, at a sarcomere length of 2.4 mum, the membrane area, excluding the T-tubules, is: A + 0.1A (folding) + 0.7A (caveolae) = 1.8A. (3) For stretch beyond Sc membrane is provided by the opening of caveolae. At a sarcomere length of about 8 mum all the caveolae are open and the fibres rupture with further stretch. (4) The relative contributions of folds and caveolae vary with sarcomere length in a way that is consistent with assumptions of constant volume and plasmalemma area. The maintenance of constant plasmalemma area, even after excessive stretch, suggests that the plasmalemma is relatively inelastic in this situation
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