Velocity of shortening

Figure 4. When a muscle contracts isotonically or a constant resisting force is imposed on it during a contraction, the velocity at which it shortens quickly comes to a constant. The force-velocity curve shows the relationship between the force applied to a muscle and the steady-state velocity of shortening. As in all other muscles, the force-velocity curve of smooth muscle is a rectangular hyperbola for all positive shortening velocities. In order to compare the behavior of muscles of different lengths and diameters, it is common to normalize force and velocity by dividing each by its maximum value and expressing the result as a percentage, nd...

Figure 8. For any set of conditions, the greatest velocity that a muscle can shorten is attained when the total force opposing shortening is zero. Empirically, the maximum velocity of shortening increases with the degree of phosphorylation of myosin. This is seen as the straight line in the velocity-phosphorylation plane. The maximum force that a smooth muscle can develop is not increased by phosphorylation beyond about 25% phosphorylation. It seems therefore that past a point, phosphorylation regulates the rate at which work is being done rather than the force that can be developed. The force a muscle can develop if 25% myosin is phosphorylated is maximal and saturated however, the rate of doing work is not saturated and continues to increase with further phosphorylation.

In terms of muscle function, muscle is very adaptable. Depending on the type of stimulation, muscle can either twitch or contract tetanically for a variable length of time. If the ends are held fixed, then it contracts isometrically and the force produced is maximal. If one or both ends of the muscle are not held fixed then the muscle is able to shorten. The muscle can shorten at a fixed load (isotonic contraction) where the velocity of shortening is also constant. Power output (force X velocity) is maximum where the velocity of shortening is about one third of the maximal rate. Finally, the muscle can shorten at maximum velocity (unloaded shortening). However, the molecular basis of the interaction of myosin with actin to produce force, or shortening, is the same in each case. 

Cardiostimulation. By stimulating Pi-receptors, hence activation of ade-nylatcyclase (Ad-cyclase) and cAMP production, catecholamines augment all heart functions, including systolic force (positive inotropism), velocity of shortening (p. clinotropism), sinoatrial rate (p. chronotropism), conduction velocity (p. dromotropism), and excitability (p. bathmotropism). In pacemaker fibers, diastolic depolarization is hastened, so that the firing threshold for the action potential is reached sooner (positive chronotropic effect, B). The cardiostim-ulant effect of p-sympathomimetics such as epinephrine is exploited in the treatment of cardiac arrest Use of p-sympathomimetics in heart failure carries the risk of cardiac arrhythmias. 

Larsson L, Moss RL (1993) Maximum velocity of shortening in relation to myosin isoform composition in single fibres from humtm skeletal muscles. J Physiol 472 595-614... 

When the crystalloids and most of the globular proteins and enzymes are removed from the skeletal muscle fiber by extraction with glycerol-water, the tension developed in ATP-contraction remains as high as before, but the maximal shortening is now comparable to that of a muscle fiber in the delta state, and the velocity of shortening is even less. And finally, the thread model of purified, oriented actomyosin develops only a low tension of a few hundred grams per square centimeter, which is of the same order as that for some smooth muscles. 

Effect of Hypothyroidism on Velocity of Shortening In isolated fibers from hypothyroid muscles, is dramatically slowed across fiber types. Hypothyroidism results in a slower for the rat diaphragm muscle (Figure 111.6a) (Gosselin et al., 1996 Herb et al., 1996) and hind-limb muscles (Caiozzo... 

Figurel 11.6 Maximum velocity of shortening (Umax) and power. Umax (a) and maximum power (b) for rat diaphragm muscle bundles. Hypothyroidism was induced for 3 months in rats prior to tissue extraction and in vitro evaluation. Hypothyroidism resulted in declines in Umax and maximum power. "Indicates statistically significant difference from control muscle strips. Error bars are SEM.

Slowed maximum velocity of shortening, rate of apparent cross-bridge attachment to actin filaments, rate constant for force redevelopment and rate of ATP consumption. 

The above estimates are mainly intended to give the idea of the orders of magnitudes of the quantities. It is seen from Eq. (8) that ot/C h + o) must be less than unity for the expression to converge rapidly. With the above given estimates it is true only for small velocities of shortening (t 2cms ). Presumably both Fh and gn have been underestimated above by about 10% or more and the value of s would be less than that given above. 

Hill, AV. The variation of total heat production in a twitch with velocity of shortening. Proc. R. Soc. Lond. B. Biol. Sci. 1964, 159, 596—605. 

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