Force-frequency relationship and potentiation in mammalian skeletal muscle.

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.     

Plasticity in mammalian skeletal muscle.

While it has been recognized for many years that different limb muscles belonging to the same mammal may have markedly differing contractile characteristics, it is only comparatively recently that it has been demonstrated that these differences depend upon the motor innervation. By appropriately changing the peripheral nerve innervating a mammalian skeletal muscle, it is possible to change dramatically the contractile behaviour of the reinnervated muscle. The manner by which the motor innervation determines the nature of a muscle fibre's contractile machinery is not completely understood, but it appears that the number and pattern of motor nerve impulses reaching the muscle play an important role. The biochemical changes occurring within muscle fibres whose contractile properties have been modified by altered motor innervation include the synthesis of different contractile proteins.     

Glycerol treatment in mammalian skeletal muscle

Summary Potassium (K-) contractures were recorded from slow-twitch (mouse soleus) and fast-twitch (mouse extensor digitorum longus (EDL) and rat sternomastoid) muscles. The mouse limb muscles responded to a maintained increase in external potassium concentration with a rapid increase in tension (fast contracture) which inactivated and was followed by a slow contracture. Rat sternomatoid muscles responded with fast contractures only. The threshold potassium concentration for contraction was higher in fast-twitch muscles than in soleus muscles, at 22 and at 37°C. After corrections had been made for the more rapid depolarization of soleus fibers, the threshold potential for soleus fiber contraction was 15 mV closer to the resting membrane potential than the threshold for fast-twitch fiber contraction. The K-contracture results were confirmed by two microelectrode voltage-clamp experiments. Activation of fast twitch fibers required depolarizing pulses that were 15 to 20 mV greater than the pulses required to activate soleus fibers. When the time courses of K-contractures were compared it was evident that inactivation with prolonged depolarization was much faster in the fast-twitch muscles than in the soleus muscles. The results suggest that the voltage dependence and kinetics of the process coupling T-tubule depolarization with calcium release from the sarcoplasmic reticulum may depend on fiber type in mammalian skeletal muscle.     

Slow charge movement in mammalian skeletal muscle

Voltage-dependent charge movements were measured in the rat omohyoid muscle with the three-microelectrode voltage-clamp technique. Contraction was abolished with hypertonic sucrose. The standard (ON-OFF) protocol for eliciting charge movements was to depolarize the fiber from -90 mV to a variable test potential (V) and then repolarize the fiber to -90 mV. The quantity of charge moved saturated at test potentials of approximately 0 mV. The steady state dependence of the amount of charge that moves as a function of test potential could be well fitted by the Boltzmann relation: Q = Qmax/(1 + exp[-(V - V)/k]), where Qmax is the maximum charge that can be moved, V is the potential at which half the charge moves, and k is a constant. At 15 degrees C, these values were Qmax = 28.5 nC/microF, V = -34.2 mV, and k = 8.7 mV. Qmax, k, and V exhibited little temperature dependence over the range 7-25 degrees C. "Stepped OFF" charge movements were elicited by depolarizing the fiber from -90 mV to a fixed conditioning level that moved nearly all the mobile charge (0 mV), and then repolarizing the fiber to varying test potentials. The sum of the charge that moved when the fiber was depolarized directly from -90 mV to a given test potential and the stepped OFF charge that moved when the fiber was repolarized to the same test potential had at all test potentials a value close to Qmax for that fiber. In nearly all cases, the decay phase of ON, OFF, and stepped OFF charge movements could be well fitted with a single exponential. The time constant, tau decay, for an ON charge movement at a given test potential was comparable to tau decay for a stepped OFF charge movement at the same test potential. Tau decay had a bell-shaped dependence on membrane potential: it was slowest at a potential near V (the midpoint of the steady state charge distribution) and became symmetrically faster on either side of this potential. Raising the temperature from 7 to 15 degrees C caused tau decay to become faster by about the same proportion at all potentials, with a Q10 averaging 2.16. Raising the temperature from 15 to 25 degrees C caused tau decay to become faster at potentials near V, but not at potentials farther away.(ABSTRACT TRUNCATED AT 400 WORDS)     

Association of tailed acetylcholinesterase to lipidic membranes in mammalian skeletal muscle

Tailed acetylcholinesterase (AChE) was studied in three subcellular membrane fractions of mouse skeletal muscle: a fraction enriched in isolated motor endplates (C), an extrasynaptic membrane fraction (A) and a microsomal fraction (S). In the (C) fraction, tailed asymmetric 16S AChE required high salt conditions to be extracted, while in (A) and (S) microsomal membranes, a collagenase sensitive 16S form, was extracted by detergent alone. This apparent “hydrophobic” property suggests that there is a pool of 16S AChE which is probably bound to lipidic membranes. The detergent extractable (DE) 16S AChE was not concentrated in motor endplate-rich regions and differential inhibition of external and internal AChE demonstrated that it could have both intra- and extracellular locations in the adult differentiated muscle fibres.     

The dynamic properties of mammalian skeletal muscle

The dynamic characteristics of the rat gracilis anticus muscle at 17.5°C have been determined by isotonic and isometric loading. For a fixed initial length these characteristics were represented either as a family of length-velocity phase trajectories at various isotonic afterloads or as a series of force-velocity curves at different lengths. An alternate method of viewing these data, the length-external load-velocity phase space, was also generated. When the muscle was allowed to shorten from different initial lengths, the velocity of shortening achieved at a given length was lower for longer initial lengths. The amount of departure was also dependent upon the isotonic load, the greater the load the greater the departure. The departures were not caused by changes in the elastic elements of the muscle or fatigue in the ordinary sense. When the behavior of the muscle was investigated at different frequencies of stimulation, the shortening velocity was a function of the number of stimulating pulses received by the muscle at a given frequency. The shortening velocity of the rat gracilis anticus muscle is, therefore, not only a function of load and length, but also of an additional variable related to the time elapsed from onset of stimulation.     

Sarcolemma blebs and cell damage in mammalian skeletal muscle

Plasma membrane blebs are an early sign of cellular damage in isolated cells. Phenazine methosulphate (PMS) triggers the production of conspicuous and characteristic sarcolemma blebs in mouse diaphragm skeletal muscle incubated in vitro and also causes severe myofilament damage. It is suggested that PMS activates transmembrane NAD(P)H dehydrogenases and, in turn, a modification of sulphydryl groups of the cytoskeleton, thereby permitting bleb formation in contracting cells.     

An ultrastructural study of the stretch-induced hypertrophy of skeletal muscle

The teres minor muscle of the adult chicken was studied ultrastructurally following tonic stretch-induced hypertrophy. The contralateral control muscle fibres showed compact myofibrils and proliferation of normal Z-bands. Myofibrils of the hypertrophied muscle however, showed Z-band alterations as Z-band expansions and Z-band streaming. Thus Z-band is a highly responsive structure to tonic stretch. Since a number of neuromuscular conditions display Z-band anomalies, the latter occurring in response to a variety of metabolic and physiologic stimuli, including tonic stretch as shown here, represents a non-specific phenomenon.