Relationship between ultrastructural profiles of membrane systems involved in excitation-contraction coupling and signal transduction in skeletal muscle cells following exercise training

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The t-tubules and SR are structurally and functionally joined at Ca^<2+> release units(CRUs), known as triads. We have studied the effects of short term denervation followed by reinnervation on the ultrastructural architectures in CRUs of fast-twitch(FT) and slow-twitch(ST) muscle fibers. Ischiadic nerve freezing resulted in total lack of neuromuscular transmission for 3 days followed by a slow recovery, but no decline in twitch force elicited by direct stimulation. The latter measurements indicate no significant atrophy within this time frame. The membrane systems of skeletal muscle fibres were visualized using a high voltage electron microscope. [^3H]nitrendipine binding was used to detect levels of dihydropyridine receptor(DHPR) expression. The Ca^<2+> pumping free sarcoplasmic reticulum domains were not affected by the denervation, but the Ca^<2+> release domains were dramatically increased, particularly in the FT fibres. The increase is evidenced by a doubling up of the areas of contacts between SR and t-tubules, so that in place of the normal triadic arrangement, pentadic and heptadic junctions, formed by multiple interacting layers of ST and t-tubules are seen. Frequency of pentads and heptads increases and declines in parallel to the denervation and reinnervation but with a delay. Immunofluorecence and electron microscopy observations show presence of DHPR and ryanodine receptor clusters at pentads and heptads junctions. A significant positive correlation between the level of [^3H]nitrendipine binding component and the frequency pentads and heptads was observed in both the FT and ST fibres indicating that overexpression of DHPRs accompanies the build up extra junctional contacts. It is possible that the multiple sites of pentad and heptad formation appear in response to muscle disuse to compensate for the normal CRUs, thus acting to increase the released Ca^<2+> for muscular contraction from the SR.