ABSTRACT
Muscle force is generated by myosin crossbridges interacting with actin. As estimated from stiffness and equatorial X-ray diffraction of muscle and muscle fibres, most myosin crossbridges are attached to actin during isometric contraction, but a much smaller fraction is bound stereospecifically. To determine the fraction of crossbridges contributing to tension and the structural changes that attached crossbridges undergo when generating force, we monitored the X-ray diffraction pattern during temperature-induced tension rise in fully activated permeabilized frog muscle fibres. Temperature jumps from 5-6 degrees C to 16-19 degrees C initiated a 1.7-fold increase in tension without significantly changing fibre stiffness or the intensities of the (1,1) equatorial and (14.5 nm)(-1) meridional X-ray reflections. However, tension rise was accompanied by a 20% decrease in the intensity of the (1,0) equatorial reflection and an increase in the intensity of the first actin layer line by approximately 13% of that in rigor. Our results show that muscle force is associated with a transition of the crossbridges from a state in which they are nonspecifically attached to actin to one in which stereospecifically bound myosin crossbridges label the actin helix.
Subject(s)
Actins/physiology , Muscles/physiology , Myosins/physiology , Animals , Biomechanical Phenomena , In Vitro Techniques , Muscle Contraction , Muscle Fibers, Skeletal/physiology , Rana temporaria , Temperature , X-Ray DiffractionABSTRACT
The relationship "heart rate - left ventricular pressure" was investigated in the isolated canine heart perfused with constant pressure at different preloads. Rhythmical stimulation was performed with constant stimulus interval duration and with stimulus intervals randomly changed near the average value (150-200 stimuli in series). Correlation and dispersion function analysis show that rhythm dispersion had a negative inotropic effect which was independent of the preload of the ventricle in the range of 120-180 beat/min, but this dependence occurred with low rats of stimulation. This method is proposed for the assessment of contractility under conditions of heart rate variations (physiological and pathological arrhythmias).
