The tension response to stretch of intact skeletal muscle fibres of the frog at varied tonicity of the extracellular medium.

Ramp stretches (2-3% of the fibre length; velocity < 0.3 fibre lengths per s) were imposed on tetanically stimulated skeletal muscle fibres from the frog (1.4-3.0 degrees C; sarcomere length 2.1-2.2 microns). The fibre was immersed in normal Ringer solution (osmotic strength 1.00 R) or in solutions made hypotonic by reduction of the sodium concentration (0.81 R) or hypertonic by addition of different amounts of sucrose (1.22 R and 1.44 R). The shape of the force response to stretch was similar at the different tonicity levels but the force enhancement (E) during stretch was significantly increased by raised tonicity and reduced by lowered tonicity. Since the steady state isometric force (T0), in contrast to E, is markedly reduced by raised tonicity and increased by lowered tonicity the total force during stretch (T0 + E) was little affected by changes in tonicity. After the end of stretch tension decayed towards the isometric level with a time course that could be approximated by a double exponential function. The rate constant of both the fast (t1/2 approximately 10 ms) and the slow (t1/2 approximately 300 ms) exponential process was reduced by increased tonicity suggesting reduced rate of crossbridge detachment. The different effects of varied tonicity on T0 and on E would be consistent with the idea that varied tonicity affects the distribution between high-force and low-force crossbridge states during isometric contraction but not during stretch. The effect may be simulated by assuming that increased tonicity reduces the difference in the strength of binding (energy of binding) between the two attached crossbridge states in the model of Huxley and Simmons.