Robert B. Sharman

Metabolic fluctuation during a muscle contraction cycle

Gated31P-nuclear magnetic resonance followed the metabolic fluctuation in rat gastrocnemius muscle during a contraction cycle. Within 16 ms after stimulation, the phosphocreatine (PCr) level drops 11.3% from its reference state. The PCr minimum corresponds closely to the time of maximum force contraction. Pi increases stoichiometrically, while ATP remains constant. During a twitch, PCr hydrolysis produces 3.1 μmol ATP/g tissue, which is substantially higher than the reported 0.3 μmol ATP ⋅ twitch-1 ⋅ g tissue-1 derived from steady-state experiments. The results reveal that a substantial energy fluctuation accompanies a muscle twitch.

Effect of hind-limb suspension on young and adult skeletal muscle: I. Normal mice

The purpose of this study was to determine the effect of hind-limb suspension (HS) on morphometric, histologic, and contractile characteristics of fast extensor digitorum longus (EDL) and slow soleus (SOL) twitch muscles in adult and immature mice. Hind-limb suspension for 2 weeks was used to produce atrophy in two groups of mice, ages 4 and 12 weeks, with nonsuspended animals serving as controls. Young HS mice exhibited marked decreases in SOL weight, length, cross-sectional area (CSA), twitch and tetanic tensions, and rates of tension development and relaxation, with increases in fatigue resistance. HS reduced the diameter of both type I and IIA fibers, increased the percentage of type I fibers, and decreased the percentage of type IIA fibers in both young and adult SOL. Muscle weight, length, CSA, IIA and IIB fiber areas, and maximum rate of tetanic tension development were decreased in EDL of young HS mice; fatigue resistance and EDL half-relaxation times were increased. For most parameters evaluated, slow twitch muscle was more affected than fast twitch. HS affected contractile characteristics less than morphometric or histologic parameters. Rates of tension development and relaxation were the contractile parameters most affected by HS, and the time parameters of contraction were least affected. For all measurements young mice were more affected than adult mice.

The effect of ischemic preconditioning on the recovery of skeletal muscle following tourniquet ischemia.

&NA; It has been well documented that ischemic preconditioning limits ischemic‐reperfusion injury in cardiac muscle, but the ability of ischemic preconditioning to limit skeletal muscle injury is less clear. Previous reports have emphasized the beneficial effects of ischemic preconditioning on skeletal muscle structure and capillary perfusion but have not evaluated muscle function. We investigated the morphologic and functional consequences of ischemic preconditioning, followed by a 2‐hour period of tourniquet ischemia on muscles in the rat hindlimb. The 2‐hour ischemia was imposed without preconditioning, or was preceded by three brief (10 minutes on/10 minutes off) preischemic conditioning intervals. We compared muscle morphology, isometric contractile function, and muscle fatigue properties in predominantly fast‐twitch, tibialis anterior muscles 3 (n = 8) and 7 (n = 8) days after ischemia‐reperfusion. Two hours of ischemia, followed by reperfusion, results in a 20 percent reduction of muscle mass (p < 0.05) and a 33 percent reduction in tetanic tension (p < 0.05) when compared with controls (n = 8) at 3 days. The same protocol, when preceded by ischemic preconditioning, results in similar decreases in muscle mass and contractile function. Neuromuscular transmission was also impaired in both ischemic groups 7 days after ischemia. Nerve‐evoked maximum tetanic tension was 69 percent of the tension produced by direct muscle stimulation in the ischemia group and 65 percent of direct tension in the ischemic preconditioning/ischemia group. In summary, ischemic preconditioning, using the same protocol reported to be effective in limiting infarct size in porcine muscle, had no significant benefit in limiting injury or improving recovery in the ischemic rat tibialis anterior. The value of ischemic preconditioning in reducing imposed ischemic‐reperfusion‐induced functional deficits in skeletal muscle remains to be demonstrated. (Plast. Reconstr. Surg. 100: 1767, 1997.)

Fast and slow skeletal muscles: Effect of ethanol on contractility of muscles from mice

We examined in vitro the effect of ethanol at four concentrations (0g%, 0.1g%, 0.2g%, and 0.4g%) on contractile parameters of 40 fast extensor digitorum longus (EDL) and 40 slow soleus muscles from healthy mice at 35C. Preparations were curarized to avoid the possible effect of ethanol on the terminal axons or skeletal neuromuscular junction. Contractile parameters measured included: (1) twitch and tetanic tension; (2) rate of tension development; (3) time to peak tension and half relaxation for twitch; (4) time to first evidence of relaxation in the tetanus; and (5) maximum rate of relaxation. The three lower concentrations of ethanol had no significant effect on muscle contractility; however, the 0.4g% dose reduced EDL twitch tension by 9%. High doses of ethanol (2.5g%) reduced the tetanic tension produced by the EDL and soleus muscles 31% and 26%, respectively. Ethanol at 2.5g% also reduced the twitch tension of the EDL and soleus by 50% and 38%, respectively. The data suggested that the 0.4g% is the highest dose of ethanol that should be used to dilute drugs in a solution that will bathe directly stimulated curarized muscle without confounding effects. In addition, it is highly unlikely that a direct effect of ethanol on muscle contractility in humans is related to an impairment in driving.

A reperfusion interval reduces the contractile deficit in skeletal muscle following tourniquet ischemia

Bloodless surgical procedures on the extremities are achieved by application of a pneumatic tourniquet. The ischemia produced has deleterious effects on nerve and muscle function. It has been suggested that temporary interruption of ischemia by a reperfusion interval can prevent muscle and nerve injury. We investigated the muscle and nerve response to 3 hours of tourniquet ischemia, with and without a reperfusion interval after the first 2 hours of application, in a rodent model. Morphometric, contractile, and histologic parameters were measured. Tourniquet ischemia, with and without a reperfusion interval, results in muscle injury and a transient depression of muscle function. Introduction of a reperfusion interval reduces the severity of injury and increases the early rate of recovery. However, the later stages of recovery appear to be unaffected by reperfusion.

Effect of hind-limb suspension on young and adult skeletal muscle. II. Dystrophic mice.

Disuse atrophy induced by limb immobilization reportedly protects dystrophic mouse muscle from histopathological changes. This study was conducted to determine whether disuse atrophy induced by hind-limb suspension (HS) limits the histopathology and contractile abnormalities typically observed in the dystrophic mouse. Two weeks of hind-limb suspension were applied to dystrophic mice (line 129B6F1) at two ages, 4 weeks (6 mice) and 12 weeks (8 mice). Thirty-one untreated dystrophics served as controls. In general, HS exaggerated the dystrophic signs, especially in the younger mice; it reduced animal weight, muscle weight, maximum tetanic and twitch tensions, and rates of tetanic and twitch tension development. HS further slowed the contractile properties of soleus (SOL) and extensor digitorum longus (EDL) muscles, and increased their fatigue resistance. HS reduced the size of type I and IIA fibers in the 6-week SOL and EDL, but not in the 14-week muscles. HS produced a preferential atrophy of SOL type I fibers, with a parallel increase in type IIA fibers. However, it did not alleviate the fiber size variability, degree of necrosis, central nucleation, inflammation, or muscle fibrosis in dystrophic muscles. These data demonstrate that disuse by hind-limb suspension does not prevent the histopathological deterioration or loss of muscle function in 6- and 14-week dystrophic mice.