Ultrastructural features of the sarcoplasmic reticulum during rapid cooling contracture and tetanus in frog skeletal muscle.

Electron microscopic studies on the ultrastructural changes of the sarcoplasmic reticulum (SR) of the skeletal muscle were performed during caffeine rapid cooling contracture (RCC). The swelling of the SR was clearly induced by RCC. The diameter of the SR fixed for the electron microscopic study at the peak tension of RCC increased with graded caffeine concentration. Mean diameters of the SR (terminal cisternae) were (29.42 +/- 9.27) X 10(-2) micron at 1 mM, (31.37 +/- 10.68) X 10(-2) micron at 2 mM, (35.33 +/- 8.60) X 10(-2) micron at 4 mM caffeine, respectively, and (20.71 +/- 6.94) X 10(-2) micron at peak tetanus. On returning to room temperature (after RCC), the enlarged SR by RCC returned to its initial state (ca. 19 X 10(-2) micron) except in the case of RCC at 4 mM caffeine concentration level. Rupture of the SR and other irreversible changes during RCC occurred at 4 mM caffeine concentration level. On the other hand, there were no remarkable alterations in ultrastructure of the SR in tetanus. No morphological changes were observed at the SR-T tubule junction (junctional gap) or in the intra-SR granules (globular particles) during RCC and tetanus. It might be suggested that the morphological changes of the SR on caffeine rapid cooling associated with Ca released from the SR.