Role of swelling in muscle contraction

Sarcomere shortening of prerigor muscles and its influence on drip loss

Detailed studies of muscle shortening post mortem at incubation temperatures between -2°C and +38°C revealed that the sarcomeres in unrestrained, excised red bovine muscle (M. sternomandibularis) shortened less than 10 % in the prerigor state between 6°C and 18°C. Below 6°C, sarcomeres contracted up to 70%. Between 20°C and 38°C sarcomere shortening of 40% was observed. In the red porcine M. cleidooccipitalis the minimum of shortening was measured at about 10°C, a higher degree of shortening-up to 50%-being obtained above and below this temperature. The drip loss of both muscle types increased linearly with increasing prerigor shortening. This latter relationship is discussed with regard to changes within the muscle post mortem. The influence of three events on water movement from the interfilamental space into the interfibrillar fluid and from there into the extracellular space is critically evaluated. These events are: (1) the prerigor contraction of sarcomeres depending on the temperature of storage, (2) the changes due to the falling pH post mortem and (3) the onset of rigor mortis, with its irreversible association of actin and myosin.

The myoplasmic fixed charges of the barnacle muscle fiber and the free Ca2+ concentration

An experimental model was recently developed to measure the myoplasm density of fixed charges (Caillé J.P. (1979) Biochim. Biophys. Acta 585, 300-313). The effect of ATP, Mg2+ and Ca2+ suggests that the myoplasmic thermodynamically effective charge density (phi X) decreases during muscular contraction. In order to determine if this reduction was due to either ATP, Mg2+ or Ca2+, the phi X dependence on those substances was studied. The results indicated that the addition of MgCl2 (2 mM) reduced phi X by 10 to 30% depending on the water content of the myoplasm. A reduction of phi X was also observed with the addition of (10(-4) M) Ca2+, but the presence of ATP was necessary for this reduction to occur. Finally when EGTA was added to the external solution in order to control the level of free Ca2+, an important reduction of phi X (40%) was observed for a pCa equal to 5.7 (water content 76.1 +/- 0.3 ml/100 g). These results allowed a correlation between the reduction of the myoplasmic fixed charges and the turning on of the contractile machinery.

Donnan potential of rabbit skeletal muscle myofibrils I: electrofluorochromometric detection of potential

The fluorescence of the dye CC-6 [(3-hexyl-2-(3-hexyl-2-benzoxazolinylidene)-1-propenyl)-benzoxazolium iodide] has been shown to indicate Donnan potentials in rabbit skeletal muscle myofibrils. These results are in agreement with previously published work in which the potentials were measured with microelectrodes on glycerol-extraced muscle fibers. The magnitude of the Donnan potential of the myofibrils has been shown to be dependent on the state (rigor or relaxed) of the system.

The myofilament lattice: studies on isolated fibers. 3. The effect of myofilament spacing upon tension

The effect of ionic strength on the generation of tension and upon the interfilament spacing in living intact and skinned single striated muscle fibers from the walking leg of crayfish (Orconectes) were determined by isometric contraction studies correlated with low-angle X-ray diffraction. Sarcomere lengths were determined by light diffraction. Tensions were induced in intact fibers by caffeine in the bathing medium and by ionophoretic microinjection of calcium. Tensions were induced in skinned fibers by a buffered calcium-EGTA solution. The interfilament spacing of intact and skinned fibers over the range of ionic strengths investigated were determined by X-ray diffraction and correlated with the physiological data. It is demonstrated that the ionic strength affects the tension-generating capacity of the muscle as it affects the chemo-mechanical transform of excitation-contraction coupling. It is further demonstrated that interfilament spacing changes encountered during shortening and with variation in the osmotic strength have no effect upon the tension-generating capacity of muscle.