X-ray diffraction patterns were recorded from EDL muscle of 1–2 month old mice at rest and during isometric contraction (force T0) at temperatures 10- 35°C. Muscles were vertically mounted at the BioCAT beamline (APS, Argonne, USA) in a trough containing physiological solution gassed with carbogen (95% O2 plus 5% CO2, pH=7.4) and at the in situ rest length.

X-ray exposure time was 20 ms for both rest and T₀, repeated twice at each temperature. T₀ was 251±17 kPa (mean±S.E.M., n=3) at 30°C (T_0,30) and not significantly different at 35°C. Below 30°C force decreased with decreasing temperature, reaching 0.25 T_0,30 at 10°C. The M3 meridional reflection from the axial repeat of the myosin heads was sampled by X-ray interference between the two halves of the sarcomere. At rest the M3 reflection consisted of a main peak with two small satellite peaks; during contraction it consisted of two main peaks with an intensity ratio (high/low angle) varying from ~0.8 at 30°C to ~1.1 at 10°C. The M3 intensity at 10°C was ~1/5 that at 30°C, both at rest and during contraction. The M3 spacing increased by 1.3% from rest to isometric contraction at all temperatures; the spacing of its second order (S_M6) had a specific temperature dependence at rest, increasing by ~ 0.4% going from 30° to 10°, consequently, from rest to T₀, S_M6 increased by ~1.3% at 30°C but by only ~0.6% at 10°C.

Supported by Ente Cassa di Risparmio di Firenze, MIUR, CNISM, (Italy), MRC (U.K.), NIH.