The anisotropic elastic properties of the sarcolemma of the frog semitendinosus muscle fiber.

Tension and curvature of the sarcolemmal tube of the frog muscle fiber were measured at different extensions and were used to calculate the anisotropic elastic properties of the sarcolemma. A model was derived to obtain the four parameters of the elasticity matrix of the sarcolemma. Sarcolemmal thickness was taken as 0.1 mum. Over the range of reversible sarcolemmal tube extension, the longitudinal elastic modulus E(L) = 6.3 x 10(7) dyn/cm(2), the circumferential modulus E(c) = 0.88 x 10(7) dyn/cm(2), the longitudinal Poisson's ratio sigma(L) = 1.2, and the circumferential Poisson's ratio sigma(c) = 0.18. At tubular rest length E(L) = 1.2 x 10(7) dyn/cm(2). The sarcolemma is less extensible in the longitudinal direction along the fiber axis than in the circumferential direction. It can be extended reversibly to 48% of its rest length, equivalent to extending the intact fiber from a sarcomere length of 3 mum to about 4.5 mum. The sarcolemma does not contribute to intact fiber tension at fiber sarcomere lengths <3 mum, and between 3 and 4 mum its contribution is about 20%. It also exerts a pressure on the myoplasm, which can be calculated by means of the model. The longitudinal elastic modulus of the whole fiber is 1 x 10(5) dyn/cm(2) at a sarcomere length of 2.33 mum.