X-RAY EVIDENCE THAT IN CONTRACTING LIVE FROG MUSCLES THERE EXIST 2 DISTINCT POPULATIONS OF MYOSIN HEADS

Using synchrotron radiation and whole muscles, 2 ms time-resolved x-ra y diffraction patterns were recorded at 8 degrees C. The results show that in both isotonic and isometric contractions, as well as in length changes imposed at maximum tension [P-0], the meridional third myosin layer line consists of two distinct reflections with different intens ities and spacings that measure similar to 14.623 and 14.412 nm at P-0 . Although the intensity behavior of the two reflections is strikingly different during quick releases, it is very similar during stretches. Study of the time courses indicates that myosin heads diffracting at P-0 with the similar to 14.623 nm periodicity are actively involved in tension production. Those diffracting at P-0 with the periodicity of similar to 14.412 nm appear not be associated with tension production during isometric contraction and releases, but the results suggest tha t they are recruited during stretches and here contribute to tension p roduction. Our most important conclusion is that under all conditions of contraction we have investigated there exist two populations of myo sin heads, each with a well defined axial disposition and configuratio n.