Discrimination between fast- and slow-twitch fibres of guinea pig skeletal muscle using the relative surface density of junctional transverse tubule membrane

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Abstract
The black reaction of Golgi was used to infiltrate transverse (T) tubules in fast-twitch glycolytic (FW), fast-twitch oxidative-glycolytic (FR) and slow-twitch (S) type fibres in muscles of guinea pigs. Non-junctional (fT) and junctional (jT) segments of the T-rubule network are clearly demarcated by this technique. Digitized planimetry and direct measurements were used to determine the proportion of T-tubule network forming junctions with the sarcoplasmic reticulum (%L jT ) and to estimate the surface density (surface area per fibre volume) of total and junctional T membrane. From these data, the volume density (number per fibre volume) of junctional feet was calculated. All three types of fibres have approximately equal surface density of T tubules, but the FW and FR fibres have a much higher proportion of jT. The calculated volume density of feet is twice as high in fast-twitch as in slow-twitch fibres.

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