Myosin head orientation: a structural determinant for the Frank-Starling relationship

The cellular mechanism underlying the Frank-Starling law of the heart is myofilament length-dependent activation. The mechanism(s) whereby sarcomeres detect changes in length and translate this into increased sensitivity to activating calcium has been elusive. Small-angle X-ray diffraction studies have revealed that the intact myofilament lattice undergoes numerous structural changes upon an increase in sarcomere length (SL): lattice spacing and the I1,1/I1,0 intensity ratio decreases, whereas the M3 meridional reflection intensity (IM3) increases, concomitant with increases in diastolic and systolic force. Using a short (∼10 ms) X-ray exposure just before electrical stimulation, we were able to obtain detailed structural information regarding the effects of external osmotic compression (with mannitol) and obtain SL on thin intact electrically stimulated isolated rat right ventricular trabeculae. We show that over the same incremental increases in SL, the relative changes in systolic force track more closely to the relative changes in myosin head orientation (as reported by IM3) than to the relative changes in lattice spacing. We conclude that myosin head orientation before activation determines myocardial sarcomere activation levels and that this may be the dominant mechanism for length-dependent activation.

Frog skeletal muscle thick filaments are three-stranded.

A procedure has been developed for isolating and negatively staining vertebrate skeletal muscle thick filaments that preserves the arrangement of the myosin crossbridges. Electron micrographs of these filaments showed a clear periodicity associated with crossbridges with an axial repeat of 42.9 nm. Optical diffraction patterns of these images showed clear layer lines and were qualitatively similar to published x-ray diffraction patterns, except that the 1/14.3-nm meridional reflection was somewhat weaker. Computer image analysis of negatively stained images of these filaments has enabled the number of strands to be established unequivocally. Both reconstructed images from layer line data and analysis of the phases of the inner maxima of the first layer line are consistent only with a three-stranded structure and cannot be reconciled with either two- or four-stranded models.

Some Low-Angle X-Ray Evidence on the Structural Changes in Thermally- and Plasma-Treated Wool

A series of events occurred when wool was heated in vacuum. At some stages structural changes in the thermally-treated wool were observed, as reflected in the low-angle x-ray diffraction patterns. An increase in the intensity of the 39 Å meridional reflection and the appearance of a 4-point diagram with azimuthal angle of 45° at about 46 Å spacing were observed when wool was heated near 170°C for 90 min. Similar results were found in samples heated at higher temperatures but for shorter time. The most heat-resistant meridional reflection is the 66 Å. The low-angle x-ray diffraction patterns of plasma-treated wool showed only disappearance of the sharp lipid ring at 48 Å. No intensification of any reflection was observed. Prolonged treatment with plasma destroys the low-angle x-ray diffraction pattern of wool.

Crystallite Orientation in Natural Cellulose Fibers

Experimental methods are described for measuring the azimuthal intensity distribution of the 040 meridional reflection from cellulose fibers. The recording of the profile has been carried out with a tilted fiber diffractometer. Methods are given for correcting the recorded profile for the effect of other contaminating reflections which happen to be at nearly the same Bragg angle. The results so obtained for ramie and a number of cottons are compared quantitatively with the azimuthal profiles of the 002 equatorial diffraction from these fibers by making use of the Kratky formula. It is shown that the details of the spiral structure of cotton are more clearly brought out in the 040 profile than in that of the 002 reflection. The Kratky formula does not appear to hold good for cottons. It is shown that the usual practice of estimating the degree of orientation of the b-axis of crystallites from equatorial profiles can give rise to misleading results.

DEVELOPMENT OF FEATHER KERATIN DURING EMBRYOGENESIS OF THE CHICK

The development of keratin in 9 to 17 day embryonic chick feathers has been studied by x-ray diffraction and cytochemical methods. The x-ray diffraction pattern given by the 9-day feathers contains none of the features seen in the adult pattern. In the 10 to 11 day patterns, besides two diffuse rings centered at 4.7 A and 10 A, two sharp, rather weak rings are seen at 35 A and 4.2 A with slight preferred orientations about the equator and the meridian, respectively. At 12 days, in addition to the foregoing, a sharp intense equatorial reflection at ∼56 A is observed. On treatment with lipid solvents, the 35 A ring is removed; prolonged extraction removes the 4.2 A ring, while blurring the 56 A reflection and enhancing the central low angle scatter. The 14-day pattern shows, besides all the features seen in the earlier patterns, a 23 A meridional reflection and other meridional and near meridional reflections. All the basic features of the adult pattern are seen at this stage and remain essentially intact on lipid extraction. Beyond 14 days, the pattern remains essentially the same, only improving in clarity and detail. The 4.2 A ring seen in the 10 to 15 day pattern is scarcely detectable in the 16-day pattern. Cytochemical evidence indicates that extensive —S—S bond formation occurs between the 13th and 14th days. It is suggested that lipids serve as a framework for the developing keratin structure which acquires permanent stability through hydrogen bonds and disulfide cross-links. The relation between keratin synthesis and tissue architecture as well as cytodifferentiation is discussed.