Influence of osmotic swelling on cross section and resting tension in isolated skeletal muscle fibers.

The causes of change in resting tension with hypotonic swelling of isolated living frog skeletal muscle fibers were studied by observing their cross-sectional shape. The cross-sectional area steadily increased, in an almost osmometer-like manner, to one-third the standard tonicity. When the cross-sectional shape became almost circular and the circumferential length began to extend, the resting tension began to decrease/increase prominently at sarcomere lengths shorter/longer than 2.8 microns. The coincidence of the occurrence of prominent changes in tension and in the cross-sectional shape indicates that tension changes are closely related to the circumferential extension of the sarcolemma. For reference purposes, we studied the effects of reduction in ionic strength and in osmotic compressive force on the resting tension of mechanically and chemically skinned fibers. We concluded that (1) the decrease in resting tension with hypotonic swelling was mediated by an elevation in intracellular hydrostatic pressure due to the circumferential extension of sarcolemma, and that (2) the increase in tension was due to longitudinal contraction of the two-dimensionally tense sarcolemma, arising from its circumferential extension.