Extensor muscle responses to triamcinolone

Interaction of glucocorticoids and activity patterns affect muscle function

The purpose of this study was to determine the effects of glucocorticoids on muscle mass and contractile properties of muscles of similar fiber composition but differing in activity patterns. Rats were divided into two groups and administered prednisolone (5 mg/kg per day) (P; N = 9) or served as controls (C; N = 10) for 10 days. Contractile properties were then determined in the left gastrocnemius-plantaris-soleus (GPS) muscle complex and a strip of costal diaphragm (D). An index of fatigue was also determined in both muscle preparations. Whole-body, GPS, and D weights decreased (P < 0.05) in the P animals (22%, 17%, and 15%, respectively) when compared to C. Specific tension (PO) increased (P < 0.05) in the GPS complex (21%) and decreased in the D (26%). Maximal shortening velocity (Vmax) was not different (P > 0.05) between groups in either the GPS or the D. While the index of fatigue was not different (P > 0.05) between groups in the D, there was a 30% increase (P < 0.05) in the rate of fatigue in the GPS. These data indicate that, although glucocorticoids cause decreased muscle mass in both D and GPS, a change in muscle architecture may prevent a decrease in force-generating ability in some limb muscles. However, glucocorticoids do not increase D fatigability as seen in the GPS.

Effect of dexamethasone on diaphragmatic and soleus muscle morphology and fatigability

In spite of the extensive use of corticosteroids, its myopathic effect on respiratory muscle is unknown with relation to the dose and duration of therapy. The present study examined the effects of dexamethasone on the morphology and function of respiratory (diaphragm) as compared to skeletal (soleus) muscle. Control rats were compared to those that received low, high and prolonged doses of dexamethasone. Body mass was reduced proportionally to the dose and duration of therapy. Atrophy of the diaphragm was greater than that of the soleus when normalized to body mass. Absolute tension and tension as a function of cross sectional area was significantly (P less than 0.05) less in the diaphragm and soleus at the high and prolonged dosages. Dexamethasone had no effect on fatigability except in the soleus after prolonged treatment. Furthermore, after prolonged therapy, it had converse effects on the Pt and 1/2RT in the diaphragm as compared to the soleus muscle. Dexamethasone improved the recovery times in the diaphragm alone. Our data suggest that dexamethasone weakens the diaphragm and soleus by reducing its mass and apparently also through some effect on the intrinsic contractile apparatus.

The combined action of exercise and methylprednisolone sodium succinate on the rat gastrocnemius muscle

1. Methylprednisolone sodium succinate (SM) administration reduces the amount of contractile proteins and increases the collagen in the rat gastrocnemius muscle; it also increases the muscle's isotonic contraction capacity. 2. Exercise in combination with SM treatment reduces this capacity and causes also further reductions of the contractile proteins and increases of collagen. 3. There are no changes of either the relative muscle weight or the number of acetylcholine receptors in any experimental group as compared to controls.

Effects of glucocorticoids on motor units in cat hindlimb muscles

The purpose of this study was to examine the effects of glucocorticoid treatment on the contractile, electrical and fatigue properties of isolated motor units of identified type. Although it is known that glucocorticoid administration induces atrophy and weakness most strongly in fast, pale muscles and to a lesser extent in red muscle, the relationship between steroid effects and motor unit type is not known. Properties of medial gastrocnemius (MG) and soleus (SOL) motor units were studied in normal cats and in cats treated with triamcinolone acetonide (3-4 mg/kg body weight for 10-16 days). Glucocorticoid treatment produced weakness preferentially in fast-twitch motor units. This suggests that catabolic steroids cause a reduction in the amount of contractile protein and hence contractile strength of motor units in inverse proportion to their relative activity or degree of use.

Facilitatory effects of dexamethasone on neuromuscular transmission

The beneficial effects of glucocorticoids in myasthenia gravis are attributed to their immunosuppressive actions. There are also studies reporting direct facilitatory as well as depressant effects of glucocorticoids on neuromuscular transmission. The effects of dexamethasone on neuromuscular transmission were studied by intracellular and extracellular microelectrode recording techniques in the mouse phrenic nerve-diaphragm preparation. Creatinine had to be added to the bathing media to prevent precipitation of the glucocorticoid with Ca2+ and Mg2+; creatinine had no effect. One hour of perfusion with dexamethasone (10(-4) to 10(-3) M) increased the frequency of miniature end-plate potentials (MEPPs), as well as the amplitude and quantum content of end-plate potentials (EPPs), but did not change MEPP amplitude, suggesting an increase in acetylcholine release. Dexamethasone also enhanced presynaptic facilitation and potentiation during repetitive stimulation. It had no effect on muscle resting membrane potential but increased the amplitude, overshoot, and rate of rise of muscle action potentials. The amplitudes of nerve terminal action potentials were also enhanced by dexamethasone. These findings suggest that glucocorticoids have a direct facilitatory action on neuromuscular transmission by a presynaptic action.

Acetylcholine stores in rat diaphragm are increased by higher concentrations of dexamethasone

In indirectly stimulated hemidiaphragms, dexamethasone (Dex, 2 microM) causes a significant increase in tissue acetylcholine (ACh) content. No increase in tissue ACh is found with 0.2, 0.6 or 6 microM Dex. Physostigmine (Physo, 15 microM) also causes an increase in tissue ACh, which is even greater in the presence of Dex (6-25 microM). There is no increase in ACh due to Dex, with 50 microM Dex plus Physo. The order of addition is important, as the Dex-induced increase in ACh is only found when Dex is added before Physo. No increase in twitch tension is found with any of these treatments. No Dex-induced increase in ACh is found with unstimulated hemidiaphragms. Similar increases in ACh are also found with other glucocorticoids (plus Physo), namely prednisolone (6-9 microM), corticosterone (2 microM) and hydrocortisone (2 microM). The mineralocorticoid aldosterone (2 microM), and other types of steroids cause no increase in tissue ACh. The increases in hemidiaphragm ACh are not found in a Na+-depleted medium, or in a medium containing 20 mM Mg2+ extra. The increase in ACh due to Physo is Ca2+-dependent, even though an increase in ACh due to Dex plus Physo is found in the absence of Ca2+. No increase in ACh due to either Physo, or Dex plus Physo are found in the presence of the nicotinic antagonists (+)-tubocurarine (5 microM) or alpha-cobrotoxin (5 micrograms/ml), while the muscarinic ligands atropine or oxotremorine (10 microM) abolish the extra increase in ACh due to Dex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of glucocorticoid treatment on excitation-contraction coupling

We studied the myofibrillar calcium affinity and mechanical threshold of single muscle fibers from the extensor digitorum longus (EDL) and soleus muscles of male Sprague-Dawley rats treated with daily intramuscular injections of dexamethasone, 1.5 mg/kg, for 14 days. The strength-duration and pCa-tension relationships of EDL fibers were not altered by glucocorticoid treatment. However, in soleus fibers the mechanical threshold was increased and the myofibrillar calcium sensitivity was reduced by dexamethasone treatment. Glucocorticoid treatment did not change the maximum tension per cross-sectional area of single-skinned EDL or soleus fibers. The glucocorticoid-induced atrophy and impaired force-generating capacity of fast-twitch muscle are probably not caused by impaired excitation-contraction coupling.

Glucocorticoid-induced atrophy is not due to impaired excitability of rat muscle

We explored the possibility that glucocorticoid-induced muscle weakness and atrophy resulted from impaired muscle membrane excitability. Male Sprague-Dawley rats received intramuscular injections of dexamethasone, cortisone acetate (equivalent anti-inflammatory doses), or saline for up to 28 days. Temporal patterns of change in muscle mass, twitch and tetanic tension, and membrane potential, cable parameters, and excitability were studied in vitro in the extensor digitorum longus (EDL), soleus (SOL), omohyoid (OMO), caudofemoralis (CF), and the sternomastoid muscles (membrane potential only). the membrane properties of EDL fibers were also studied in vivo (pentobarbital anesthesia). The relative severity of atrophy was OMO greater than CF greater than EDL greater than SOL. Reduction in twitch or tetanic tension never preceded atrophy. The twitch and tetanic tension (per g muscle) increased with glucocorticoid treatment. There were no significant changes in the time course of the twitch or tetanus. Dexamethasone produced more severe atrophy and force reduction than did cortisone acetate. Glucocorticoid treatment produced a depolarization of EDL muscle fibers measured in vitro at 23 degrees C, but this did not appear to be physiologically significant because EDL fibers studied in vivo were not depolarized and had normal action potential amplitudes and thresholds. Glucocorticoid treatment did not change the membrane resistance or capacitance. We conclude that glucocorticoid treatment did not produce muscle weakness by impairing sarcolemmal excitability or excitation-contraction coupling, but that the weakness resulted from muscle atrophy.

Effect of glucocorticoid treatment on the excitability of rat skeletal muscle

Dexamethasone treatment in the rat produced depolarization of extensor digitorum longus (EDL) muscle fibers but not soleus (SOL) fibers studied in vitro at 23 degrees C. The depolarization of EDL fibers was most prominent after 1 day of treatment (treated -77.5 +/- 1.1 mV, control -87.2 +/- 0.8 mV; mean +/- S.E.), and was associated with elevation of the action potential threshold and reduction of the action potential overshoot. In vivo, or in vitro in chloride-free solution, the resting potential and action potential threshold and overshoot of EDL fibers from glucocorticoid-treated and control rats were similar. Sodium currents were studied with a patch voltage clamp. Glucocorticoid treatment did not alter the voltage dependence of sodium channel activation or inactivation in fast twitch muscle fibers. Maximal inward currents occurred at about -29 mV and half-maximal inward currents at about -50 mV. Sodium channels were half inactivated at about -71 mV. Glucocorticoid treatment did not alter the sarcolemmal resistance or capacitance. We conclude that glucocorticoid treatment does not produce muscle weakness or atrophy by altering the excitability of muscle fibers.

Effects of glucocorticoid treatment and food restriction on rat hindlimb muscles

Various metabolic, histochemical, and contractile measurements were made on soleus and gastrocnemius muscles of adult rats treated with triamcinolone acetonide (1 mg . kg-1 . day-1 for 6 wk), and compared to similar measurements in pair-fed and control animals. Contractile measurements were performed in situ under pentobarbital anesthesia. Gastrocnemius muscles from steroid-treated rats showed higher twitch and tetanic tensions when expressed in grams/gram of muscle, compared to control rats. Similar, although less marked, trends occurred in the less atrophied gastrocnemius muscles from pair-fed rats. Soleus muscles from steroid-treated rats exhibited significantly higher (139% of control) twitch tension (grams/gram of muscle), and elevated twitch/tetanus ratios, compared to control and pair-fed groups. No alterations occurred in muscle contractile speed in either muscle. The greater atrophy of fast compared to slow fiber types in gastrocnemius muscles under both experimental conditions did not occur in the soleus, a predominantly slow-twitch muscle. The atrophic and contractile responses of fast and slow muscles to these two conditions were quantitatively as well as qualitatively different.

Contractile responses of rat fast-twitch and slow-twitch muscles to glucocorticoid treatment

In an attempt to determine whether chronic glucocorticoid treatment affects speed- and strength-related properties of fast-twitch muscle and slow-twitch muscle, in-situ contractile properties of soleus and gastrocnemius muscles were measured in rats after 14 days on a regiment of triamcinolone acetonide injected at a dosage of 2 mg/kg/day. No effects on contractile speed or strength were noted for either muscle. The time to peak tension in soleus muscle was significantly longer in the treated group (43.0 +/- 1.0 msec) than in the control group (38.7 +/- 0.7 msec); however, this was attributed to a prolongation of "active state" duration, as suggested by the fact that treated muscles also showed significantly elevated twitch-tetanus ratios. The treatment-induced enhancement of the specific twitch tension was more pronounced in the gastrocnemius muscles (198% +/- 19% of control) than in the soleus muscles (153% +/- 9% of control). The difference in response to steroid treatment may reflect structural and functional differences in fast- and slow-muscle membrane systems.

Experimental corticosteroid myopathy: effect on myofibrillar ATPase activity and protein degradation

Corticosteroid myopathy was studied in young, mature New Zealand white rabbits given daily injections of betamethasone (0.3 mg/kg body weight/day) for two weeks. Control rabbits were pair-fed and received saline injections. Bethamethasone treatment caused significant wasting of type 2 gluteus medius and psoas muscles but did not cause any atrophy of type 1 soleus and gluteus minimus muscles. The Mg2+- and Ca2+-activated myofibrillar ATPase activities of the corticosteroid-treated rabbits did not differ from controls despite a 30% reduction in muscle wet weight and pronounced reduction in cross-sectional area of fibers. SDS-polyacrylamide gel electrophoresis profiles of myofibrillar proteins did not differ quantitatively or qualitatively between experimental and control rabbits. Studies of net muscle protein degradation (using 3H-leucine) in betamethasone-treated and control rabbits indicate that both type 1 and type 2 muscle fiber proteins are degraded several times faster in the corticosteroid-treated group. This suggests that a compensatory mechanism exists for those type 1 and mixed fiber type muscles which have increased degradation but do not undergo wasting.

Short-term effects of prednisolone on neuromuscular transmission in normal rats and those with experimental autoimmune myasthenia gravis

Electrophysiological investigations of the effects of bath-applied prednisolone at the neuromuscular junction were performed in muscles from normal rats and rats with experimental autoimmune myasthenia gravis (EAMG). In muscles from both groups, prednisolone reversible and significantly depressed the amplitudes of minature end-plate potentials (MEPPs), end-plate potentials (EPPs) and indirectly elicited action potentials (APs) without affecting resting membrane potentials. Prednisolone also caused a significant reduction in EPP rise time to peak and half-decay time while markedly increasing MEPP frequency and AP rise time to peak and duration. These effects were shown to be dose-dependent. The percentage decrease in amplitude after prednisolone perfusion was similar for EPPs and MEPPs, indicating that the depressive effect of prednisolone at the junction is postsynaptic. In all of the parameters studied, the percentage effect of prednisolone was the same in EAMG and normal preparations. No stimulus-linked repetitive EPPs or APs were observed after prednisolone. It is concluded that prednisolone has a depressive effect on neuromuscular transmission, but that this occurs only at high concentrations of the drug which are not achieved during the treatment of myasthenia gravis.