Swelling of sarcoplasmic reticulum in the periphery of muscle fibres afterisometric contractions in rat semimembranosus lateralis muscle

The decline in isometric force, swelling of sarcoplasmic reticulum and loss of desmin was measured in semimembranosus lateralis muscle of male Wistar rats immediately after a short series of brief (500 ms) maximal isometric c ontractions. For the active muscle, the series ended below (protocol A) and just over muscle optimum length (protocol AA). in one protocol, the muscle remained passive and was extended to lengths just over muscle optimum leng th (protocol P). After all experimental protocols, no loss of desmin was ob served and sarcomere appearance was normal. Protocol A produced swelling (8 7%) of the sarcoplasmic reticulum but no decline in isometric force. Protoc ol AA produced larger swelling (147%) of the sarcoplasmic reticulum and an isometric force decline (<49%) at short muscle lengths. Swelling of sarcopi asmic reticulum was observed mainly in the periphery of muscle fibres. Prot ocol P did not result in swelling of the sarcoplasmic reticulum and isometr ic force decline. It is concluded that swelling of the sarcoplasmic reticul um in the periphery of muscle fibres after brief maximal isometric contract ions is associated with muscle force and not muscle length.