Decoupling of the excitation-contraction link by glycerol procedure in volume-clamped muscle fibers.

The role of "osmotic shock" and vesiculation in the mechanism of disconnection of the T-system tubules by the glycerol procedure was studied in volume-clamped muscle fibres of the crayfish. The transient dilatation of fibres during the wash-out of glycerol was prevented by application of hypertonic sucrose solutions of decreasing osmolarity to hold the fibre volume constant. Under these conditions the fibres were decoupled only partially; the twitch tension was decreased to a steady level of about 40% of the control tension. The decay in tension developed without the transient potentiation observed in fibres allowed to dilate. The decoupling could not be reversed by reapplication of glycerol saline. The structure of volume-clamped fibres differs from the dilated ones in two respects; the vesicles appear smaller and are absent in continuous layers (about 4 micrometer in width) just under the surface and the cleft membranes. Electron microscopy of these regions has revealed the presence of normal T-system tubules communicating with the fibre exterior. The results provide evidence for the secondary role of the osmotic factor and the essential role of the vesiculation in the induction of the detubulation by glycerol procedure.