In skeletal muscle cells, a synchronised myosin S1 power stroke causes a change in intensity (I_M3) of the meridional X-ray reflection at 14.5 nm, which is thought to indicate both elastic and active changes in the myosin lever arm tilt. During sinusoidal length oscillations (0.1-1 kHz), I_M3 signals are approximately sinusoidal and in phase opposition to fibre length, but are distorted during shortening by the passage of I_M3 through an intensity maximum (I_{M3 max}), forming a secondary I_M3 minimum at the point of maximum shortening. The extent of shortening (Δy) required to reach I_{M3 max} has been shown to depend upon the force per cross-bridge (P_S1), when isometric tetanic tension is varied by temperature change (Griffiths et al. 2002). However, since temperature effects on relaxed I_M3 (i.e. unrelated to the power stroke) have been reported (Lowy et al. 1991), we have now examined the effect of Ringer’s tonicity on Δy, which is an alternative method of varying P_S1.

Intact fibre bundles from tibialis anterior muscles of Rana temporaria (sacrificed by decapitation) were mounted horizontally between a moving coil motor and a capacitance force transducer. Synchrotron radiation (source Elettra, Trieste, Italy; wavelength 0.15 nm, dimensions 0.3 X 3.0 mm) was admitted to the experimental chamber by a fast shutter mechanism at the plateau of an isometric tetanic contraction, while sinusoidal length oscillations were imposed simultaneously at 1 kHz. I_M3 was monitored by a 1D delay line detector at 2.6 m from the preparation, with a sampling time resolution of 50 µs (Fig. 1). Bundles were exposed to 0.8 or 1.4 fold standard Ringer’s tonicity by variation of [NaCl]. Hypertonicity reduced P_S1 by 17 % and increased Δy by 0.15 nm (12 °C); hypotonicity increased P_S1 by 12 % and reduced Δy by 0.24 nm (2 °C). These findings are consistent with a P_S1 dependence of S1 lever arm tilt, but indicate a less steep dependency under hypertonic conditions than that observed by temperature variation.

This work was supported by the E.U. Human Potential Programme.