The Effect of Age, Stretchtension During Thaw Rigor and Heat of Selected Quality Characteristics of Beef Sternomandibularis Muscle

Some aspects of the phenomenon of thaw rigor have been studied in order to understand how it affects skeletal muscle as a food. Specifically, studies were conducted to evaluate the tenderness and other quality characteristics of muscle strips retained at rest length or stretched by 50 percent during thaw rigor, and to see whether any observed differences were related to the age or sex of the animals. No statistically significant differences due to the stretched state, age or sex were noted in the 24 hr. post thawing pH or water holding capacity as determined by the Carver Press method, although in the latter parameter, individual animals differed significantly (P<0.05) from one another. Differences in the stretched state affected the cooking loss, cooking contraction, shear values, sarcomere length and fiber diameter. Variations in the cooking loss, cooking contraction and sarcomere length were not significant when considered on between group basis while significant variations were observed in the shear values (P <0.05) and fiber diameters (P<0.01). Significant differences in the response of individual animals within t!e different groups were noted in the cooking loss, cooking contraction (P<0.01) and shear values (P<0.05). Within group) differences were not observed in the estimates of sarcomere length and fiber diameter. Significant differences were not noted in the total collagen, alkali-soluble collagen, and elastin contents of the cow, steer and heifer groups. However, the collagen content of the veal group was significantly greater (P<0.005) and was more readily soluble in dilute alkali than the three other groups. No significant differences in the collagen content or solubility was attributable to the stretched state. Heating of the muscle strips significantly affected col!agen solubility (P<0.005) in all the different groups

Roles of Titin in the Structure and Elasticity of the Sarcomere

The giant protein titin is thought to play major roles in the assembly and function of muscle sarcomeres. Structural details, such as widths of Z- and M-lines and periodicities in the thick filaments, correlate with the substructure in the respective regions of the titin molecule. Sarcomere rest length, its operating range of lengths, and passive elastic properties are also directly controlled by the properties of titin. Here we review some recent titin data and discuss its implications for sarcomere architecture and elasticity.

Condensin Regulates the Stiffness of Vertebrate Centromeres

When chromosomes are aligned and bioriented at metaphase, the elastic stretch of centromeric chromatin opposes pulling forces exerted on sister kinetochores by the mitotic spindle. Here we show that condensin ATPase activity is an important regulator of centromere stiffness and function. Condensin depletion decreases the stiffness of centromeric chromatin by 50% when pulling forces are applied to kinetochores. However, condensin is dispensable for the normal level of compaction (rest length) of centromeres, which probably depends on other factors that control higher-order chromatin folding. Kinetochores also do not require condensin for their structure or motility. Loss of stiffness caused by condensin-depletion produces abnormal uncoordinated sister kinetochore movements, leads to an increase in Mad2(+) kinetochores near the metaphase plate and delays anaphase onset.

Modulation of Radula Opener Muscles in Aplysia

We observed fibers immunoreactive (IR) to serotonin (5-HT), the myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine mollusk Aplysia californica. The I7–I10 muscle complex, which produces radula opening, is innervated primarily by one motor neuron, B48. B48 is MM-IR and synthesizes authentic MMA. When B48 is stimulated in a physiological manner, cAMP levels are increased in opener muscles. cAMP increases also are seen when the MMs are applied to opener muscles but are not seen with application of the B48 primary neurotransmitter acetylcholine (ACh). Possible physiological sources of 5-HT and FMRFamide are discussed. When modulators are applied to resting opener muscles, changes in membrane potential are observed. Specifically, 5-HT, MMB, and low concentrations of MMA all depolarize muscle fibers. This depolarization is generally not sufficient to elicit myogenic activity in the absence of neural activity under “rest” conditions. However, if opener muscles are stretched beyond rest length, stretch- and modulator-induced depolarizations can summate and elicit contractions. This only occurs, however, if “depolarizing” modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MMA) hyperpolarize opener muscle fibers and can prevent depolarizing modulators from eliciting myogenic activity. All modulators tested affected parameters of motor neuron-elicited contractions of opener muscles. MMB and 5-HT increased contraction size over the range of concentrations tested, whereas MMA potentiated contractions when it was applied at lower concentrations but decreased contraction size at higher concentrations. FMRFamide decreased contraction size at all concentrations and did not affect relaxation rate. Additionally, the MMs and 5-HT increased muscle relaxation rate, decreased contraction latency, and decreased the rate at which tension was developed during motor neuron-elicited muscle contractions. Thus these modulators dramatically affect the ability of opener muscles to follow activity in the opener motor neuron B48. The possible physiological significance of these findings is discussed.

Influence of intercellular tissue connections on airway muscle mechanics

Contraction of smooth muscle in visceral organs is modified by structures external to the muscle. Within muscle tissue itself, connective tissue plays an important role in force transference among the contractile cells. Connections arranged radially can affect contractile mechanics by limiting tissue expansion at short lengths. Previous work suggests that increased stiffness at extreme shortening is due to such radial constraints. Two approaches to further study of these effects are reported. To increase radial constraints, very thin Silastic bands were placed loosely about strips of canine trachealis muscle at rest length. The strips were allowed to shorten under light afterloads, expanding until restrained by the bands. Subsequent removal of the bands allowed increased shortening, with less increase in stiffness at short lengths. Related isometric effects were observed. To reduce constraints, muscle strips were partially digested with collagenase. Compared with control conditions, this treatment permitted further shortening, with less increase in stiffness at short lengths. These results emphasize the role of extracellular structures in determining mechanical function of smooth muscle.

Oscillating mitotic newt lung cell kinetochores are, on average, under tension and rarely push

Experimentally introduced tension on kinetochores and their centromeres has been shown to stabilize kinetochore attachment to microtubules, modify kinetochore directional instability, and regulate cell-cycle progression into anaphase. In mitosis, kinetochore tension and the stretch of centromere chromatin are produced by the movement of sister kinetochores toward opposite poles and astral ejection forces on the chromosome arms. However, newt lung cell kinetochores oscillate between poleward and away from the pole motility states throughout mitosis, indicating kinetochores are not under constant tension. To test whether kinetochores are under net tension while they are oscillating, and how often they are under compression and pushing into the chromosome, we measured the distance between sister kinetochores in newt lung cells using both video-enhanced differential interference contrast microscopy (VE-DIC) and immunofluorescence microscopy. We found that for chromosomes in which sister kinetochores are attached to opposite spindle poles, centromeres are, on average, stretched (2.2 microns in living cells and 1.8 microns in fixed cells) with respect to the inter-kinetochore ‘rest’ length (1.1 microns in living and fixed cells). For chromosomes in which only one kinetochore is attached to the spindle, the centromere chromatin associated with the tethered kinetochore is, on average, stretched to approximately half of the average inter-kinetochore distance measured for chromosomes in which both kinetochores are attached. We conclude that while newt lung cell kinetochores oscillate between states of P and AP movement, they are under tension approximately 90% of the time and under compression less than 6% of the time.

Influence of extent of muscle shortening and heart rate on work from frog heart trabeculae

Shortening, lengthening, and net work done by frog (Rana pipiens) heart trabeculae were measured over a range of strain amplitudes (length change) and cycle frequencies. Net work, the product of muscle strain and force over a full lengthening/shortening cycle, increased with strain to strains well over 25% of the muscle's rest length, a value greater than optimum strains reported for most skeletal muscles. A distinct optimum strain for net work was not found. Maximum net work per cycle averaged 7.5 J/kg for ventricular muscle and 2.0 J/kg for atrial muscle. Isometric twitch stress was maximal at 0.4-0.6 Hz twitch frequency in ventricular trabeculae (average 51 kN/m2) and 0.6-1.4 Hz in the atrium (average 14 kN/m2). The twitch duration decreased with increasing twitch frequency. Shortening and net work were maximal at 0.7-Hz cycle frequency in ventricular trabeculae and 0.9 to 1.4 Hz in the atrium. The decline in work per cycle at slower frequencies was due in part to a decline in twitch force. Maximum power for the ventricle was approximately 5 W/kg and occurred at 0.8 Hz and 26% strain, and was 1/3 to 1/4 the power of most skeletal muscles studied at similar temperatures.