Ca2+-uptake by tissue sections and biochemical characteristics of sarcoplasmic reticulum isolated from fish fast and slow muscles.

Frozen sections (10 micrometer) were cut from fast, slow and cardiac muscles of rainbow trout, frog, and rat. Rates of 45Ca2+-uptake by thin sections were compared at each animal's normal body temperature. Initial rates of Ca2+-uptake were 1.8 and 2.4 times higher, respectively, in trout and rat fast than slow muscles. In spite of a lower body temperature (10 degrees C) rates of Ca2+-uptake by trout fast muscles were 2.8 times higher than for rat Extensor digitorum longus at 37 degrees C. It is suggested that the high functional capacity of fish sarcoplasmic reticulum (SR) is related to adaptations associated with the need for rapid cycles of contraction and relaxation during high speed swimming. The biochemical characteristics of SR isolated from trout fast and slow muscles has also been investigated. The ratio of Ca2+-ATPase activity between fast and slow fibres (2:1) was similar to that obtained for Ca2+-uptake by whole muscle sections. No evidence was obtained for modulation of calcium transport by cAMP dependent protein kinases. The protein composition of highly purified trout SR was investigated by polyacrylamide gel electrophoresis. The concentration of 95 000 to 100 000 dalton (calcium pump protein) was significantly reduced in slow compared to fast muscle SR. Slow muscle SR contains a high proportion of additional protein bands of 48 000 and 31 000 molecular weight.