Effects of intracellular acidification and varied temperature on force, stiffness, and speed of shortening in frog muscle fibers

This study aimed to establish whether the temperature-dependent effect of acidification on maximum force observed in mammalian muscles also applies to frog muscle. Measurements of force, stiffness, and unloaded velocity of shortening in intact single muscle fibers from the anterior tibialis muscle of Rana temporaria were performed between 0 and 22°C during fused tetani in H2CO3-CO2-buffered Ringer solution with pH adjusted to 7.0 and 6.3, respectively. The force-to-stiffness ratio increased as a rectilinear function of temperature between 0 and 20°C at pH 7.0. Lowering the pH to 6.3 reduced the tetanic force by 13.5 ± 1.2 and 11.5 ± 1.4% at 2.8 and 20.5°C, respectively, with only a minor reduction in fiber stiffness. The maximum speed of shortening was decreased by lowered pH by 12.9 ± 1.5 and 7.8 ± 1.1% at low and high temperature, respectively. Acidification increased the time to reach 70% of maximum force by 18.0% at ∼2°C; the same pH change performed at ∼20°C in the same fibers reduced the rise time by 24.1%. The same increase in the rate of rise of force at high temperature was also found at normal pH after the fibers were fatigued by frequent stimulation. It is concluded that, in frog muscle, the force-depressant effect of acidification does not vary significantly with temperature. By contrast, acidification affects the onset of activation in a manner that is critically dependent on temperature.

The Filament Lattice of Striated Muscle

Millman, Barry M. The Filament Lattice of Striated Muscle. Physiol. Rev. 78: 359–391, 1998. — The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Maximum speed of shortening and ATPase activity in atrial and ventricular myocardia of hyperthyroid rats

The kinetic properties of the myofibrillar system of atrial and ventricular myocardia of hyperthyroid rats were analyzed by determining ATPase activity and maximum shortening velocity. Hyperthyroidism was induced by daily subcutaneous injections of triiodothyronine (0.2 mg/kg body wt) for 2 wk. The treatment induced a marked atrial and ventricular hypertrophy and, in ventricular myocardium, an isomyosin shift toward a homogeneous V1 composition. Skinned trabeculae and purified myofibrils were prepared from atrial and ventricular myocardia. Enzymatic assays on the myofibrils showed that both Ca-stimulated ATPase activity and Ca-Mg-dependent ATPase activity had equal values in atrial and ventricular myocardia. In skinned trabeculae during maximal Ca activations, force-velocity curves were determined by load-clamp maneuvers, and unloaded shortening velocity (Vo) was obtained with the slack-test method. Both maximum shortening velocities extrapolated from the force-velocity curves (Vmax) and Vo were significantly higher (+68 and +52%, respectively) in atrial than in ventricular preparations. Developed tension was significantly greater in ventricular preparations. Maximum power output was not significantly different. Previous findings (V. Cappelli, R. Bottinelli, C. Poggesi, R. Moggio, and C. Reggiani. Circ. Res. 65: 446-457, 1989) had led to the conclusion that variations in ATPase activity and shortening velocity of ventricular myocardium can be accounted for by changes in isomyosin composition. In this light, the present results suggest that 1) ATPase activity is equal in atrial and ventricular myocardia as the two tissues contain the same myosin heavy chain isoform, 2) the difference in maximum speed of shortening between atrium and ventricle might be due to the presence of tissue-specific isoforms of myosin light chains.

Relative magnitude and asynchrony of directional components of contraction in sedated dog left ventricle

The purpose of this study was to quantitate the temporal relationships and the extent and speed of shortening in segments of myocardium responsive to contraction in circumferential, longitudinal, and oblique fiber groups. Measurements were made in five sedated dogs (morphine, diazepam) with and without alterations in preload and afterload (nitroprusside, phenylephrine). The measurement interval was the phase of rapid contraction, determined by differentiation of the segment length vs. time. In the control state, percentage segment shortening was greater in circumferential than in longitudinal [15.2 +/- 0.24 (SE) vs. 10.5 +/- 0.80%; P = 0.0020] and in the subepicardial oblique than in the subendocardial oblique fiber directions (16.6 +/- 0.65 vs. 9.7 +/- 0.36%; P = 0.0010). Shortening was proportional to both maximum speed and duration of shortening (r = 0.735 +/- 0.015 and 0.757 +/- 0.017, respectively). Duration of shortening was significantly longer in circumferential than in longitudinal (mean difference 39.3 +/- 6.6 ms; P = 0.0039) and in subepicardial oblique than in subendocardial oblique directions (mean difference 27.7 +/- 5.5 ms; P = 0.0072). Velocities of up to 3.0 segment lengths/s were attained in response to nitroprusside. These data reveal the local anisotropy and asynchrony of contraction in the myocardium; however, they also support the concept of the myocardium as a functional continuum. The dominance of circumferential over longitudinal and subepicardial over subendocardial oblique contractile components indicates their relative contributions to the constriction of the midmyocardial shell.

Maximum shortening speed of motor units of various types in cat lumbrical muscles

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.

Contractile properties of cat skeletal muscle after repetitive stimulation

The isometric and force-velocity properties of the fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus muscles were investigated immediately after and during recovery from a fatiguing stimulus regime (40 Hz for 330 ms every second for 180 s) in the anesthetized cat. The amplitude of the isometric twitch of FDL was unaffected but in soleus it remained depressed for much of the recovery period. Immediately after stimulation the twitch time to peak of FDL increased to 140% of the control (prefatigue) value and then reverted to control values. The maximum isometric tetanic tension (Po) developed by FDL was reduced to 67% of control values immediately after the stimulus regime, whereas soleus declined to 93% of control. Recovery of maximum force development was achieved after 45 min in FDL and after 15 min in soleus. The maximum speed of shortening of FDL was reduced to 63% of control values immediately after fatigue; despite some recovery within the first 30 min, it remained depressed during the remainder of the recovery period (up to 300 min). Maximum speed of shortening was unaltered in soleus. The a/Po value transiently increased to 176% of control values in FDL immediately after the fatigue regime but promptly returned to control values. Force-velocity properties of soleus were not affected by the stimulus regime. It is concluded that in FDL changes in the maximum speed of shortening and maximum isometric tension as a result of the stimulus regime are attributable to changes in the intrinsic behavior of cross-bridges and the metabolic status of the fibers, particularly in the fast-twitch fatigue-resistant fibers.

Force-velocity properties of fatigue-resistant units in cat fast-twitch muscle after fatigue

The isometric and force-velocity properties of an identified and uniform population of fast-twitch, fatigue-resistant (FR) fibers within the flexor digitorum longus (FDL) muscle were investigated before, immediately after, and during recovery from a fatiguing repetitive isometric stimulus regime (40 Hz for 330 ms every s for 180 s) in the anesthetized cat. It was necessary to determine the smallest fraction of muscle that had the same force-velocity properties as the whole muscle. This was approximately 15% for FDL; if the fraction was less, the maximum speed of shortening was depressed and the a/Po value increased. Motor units were enlarged by partial denervation of the muscle, causing the intact motoneurons to sprout and incorporate more muscle fibers; FR units showed the greatest increase. Immediately after the fatigue regime, maximum isometric tetanic tension declined to 67% but subsequently recovered to 90% of the control value by the end of the 60-min recovery period. Maximum speed of shortening dropped to 71% of the control but after 30 min had recovered and did not differ significantly from control values. It is concluded that the capacity for recovery from fatigue is greater for FR units than for a whole muscle, which also contains fast-fatiguable units, and that the mechanisms involved in the recovery of the maximum isometric tension and maximum speed of shortening are independently regulated.