Myotonic dystrophy: effect of testosterone on total body potassium and on creatinine excretion

Hypogonadism is common in men with myopathies

BACKGROUND

Hypogonadism has been described in patients with myotonic muscular dystrophy type 1 but has not been evaluated in other myopathies.

METHODS

We measured total and free serum testosterone levels in 59 men with myotonic muscular dystrophy type 1 (N = 12), facioscapulohumeral muscular dystrophy (N = 11), dystrophinopathy (N = 12), metabolic myopathy (N = 7), and inclusion body myositis (N = 17) and compared these with the normal reference interval.

RESULTS

Thirty-two of the 59 (54%) participants had low total testosterone, 23 (39%) had low total and free values, and 5 (8%) had low free with normal total levels. There were no significant differences in the prevalence of hypogonadism between those with myotonic muscular dystrophy type 1 and the other groups even after considering age as a confounder.

CONCLUSIONS

Hypogonadism is common in men with myopathies, and with the importance of testosterone in the maintenance of muscle mass, treatment of hypogonadism should be considered.

Specific binding and effects of dehydroepiandrosterone sulfate (DHEA-S) on skeletal muscle cells: possible implication for DHEA-S replacement therapy in patients with myotonic dystrophy

Dehydroepiandrosterone (DHEA) and its sulfate (DHEA-S) are the most abundant steroidal products and major circulating steroids in humans. The serum concentrations of DHEA-S are lower in patients with myotonic dystrophy (DM) than normal controls, and possible improvement of myotonia and muscle weakness was recently reported following DHEA-S replacement therapy. However, the molecular mechanism of action of DHEA-S remains unknown. To understand the reported anti-DM action of DHEA-S, we investigated DHEA-S binding in skeletal muscle cells in vitro. We identified two populations of DHEA-S binding sites (Kd = 5-9 microM and 35-40 microM) in C2C12 myocytes. Similar binding sites were also identified in human skeletal muscles. The Kd value of the high-affinity site was within the range of serum concentrations of DHEA-S in adult humans. Our results suggest that DHEA-S might act directly on skeletal muscles under normal physiological conditions in humans.

Forearm 3-methylhistidine efflux in myotonic dystrophy

Myotonic dystrophy is associated with progressive muscular atrophy. To define the mechanism of muscle wasting in this disease, we studied myofibrillar proteolysis in vivo in 8 men moderately affected with myotonic dystrophy, and compared the results with those of 10 normal men. Myofibrillar proteolysis was estimated by measuring the 3-methylhistidine arteriovenous difference (A-V) and efflux (Q) across the forearm in the postabsorptive state. Plasma 3-methylhistidine concentrations were determined by high-performance liquid chromatography with postcolumn o-phthalaldehyde derivatization and fluorescence detection. Plasma flow to the forearm muscles (F) was estimated to represent 85% of total forearm plasma flow as determined by the indicator-dilution technique. Forearm 3-methylhistidine efflux was calculated as: Q = F(A-V). Mean muscle mass (24-hour creatinine excretion), lean body mass, and forearm volume were decreased in the patients with myotonic dystrophy, confirming the presence of muscle atrophy. Mean forearm 3-methylhistidine arteriovenous difference and efflux were not significantly different in the two groups. We conclude that myofibrillar protein degradation is not increased in myotonic dystrophy, even when measured in a muscle compartment selectively affected by wasting. Muscle atrophy in myotonic dystrophy is probably the result of defective anabolism rather than accelerated catabolism.

The role of lipid peroxidation in McArdle's disease: applications for treatment of other myopathies

Experimental therapies for McArdle's disease have been directed toward increasing substrate availability to exercising muscle. Such therapies to date have proven largely unsuccessful. These include administration of isoproterenol to increase blood flow, glucagon treatment to elevate serum glucose and increased dietary fat intake. Each of these therapies also results in greater levels of unesterified fatty acids in blood. More recently, a high protein diet is suggested to provide increased amounts of amino acids which would be available as fuel sources. We hypothesize that the absence of myophosphorylase in McArdle's disease creates an imbalance between the enzymes of the redox systems that control the generation, propagation and inactivation of free radicals. This occurs because muscle cells are forced to rely more heavily on fatty acid oxidation. The resulting free radical damage to cellular components disrupts metabolic control and increases the permeability of membranes. Elevated levels of Ca2+ in the sarcoplasm activate proteases, phospholipases and other catabolic enzymes initiating muscle fatigue and cramping. Lipid peroxidation is a consequence of normal muscle activity and may occur unchecked in individuals with McArdle's disease. Continued muscle activity in the absence of a favorable nutritional environment may promote the progression of the disease by increasing susceptibility to oxidative stress.

Mechanism of muscle wasting in myotonic dystrophy

Myotonic dystrophy is associated with progressive muscular atrophy. In order to determine the mechanism of muscle wasting in this condition, we measured fractional mixed skeletal muscle protein synthesis in the postabsorptive state in 8 patients with myotonic dystrophy, and compared the results with those of 10 normal subjects. Fractional muscle protein synthesis was determined by measuring the increment of 13C leucine in mixed skeletal muscle protein obtained by needle biopsy from the quadriceps muscle during a primed-continuous infusion of L-(1-13C) leucine. We used plasma 13C alpha-ketoisocaproate (representing intracellular leucine labeling) as the precursor pool for the calculation of fractional muscle protein synthesis and leucine kinetics. Fractional muscle protein synthesis was depressed in the patients with myotonic dystrophy (28% decrease, p less than 0.02). Leucine flux, leucine oxidation, and the nonoxidative portion of leucine flux were not different between the patients with myotonic dystrophy and the normal control subjects. Muscle atrophy in myotonic dystrophy reflects a selective decrease in muscle protein synthesis without any similar decrease in nonmuscle protein synthesis. This decrease may result from an impaired end-organ response to anabolic hormones or substrates.

Quantitation of muscle mass and muscle protein synthesis rate: documenting a response to myoblast transfer

Initial trials of myoblast transfer into individual muscles should include studies of regional muscle mass. Magnetic resonance imaging is a safe and accurate technique that should be studied in the context of these trials. Creatinine excretion should also be measured in order to provide baseline information for future studies of myoblast injections into a larger number of muscles. In vivo assessment of muscle protein synthesis can be combined with histologic study to determine if the treatment alters the rate of protein synthesis.

Treatment of muscular dystrophies

1. Specific therapies to cure the muscular dystrophies are not yet available. Therapeutic trials designed on the basis of our understanding of the pathophysiology of these disorders have had only limited success. 2. However, recent investigations in Duchenne muscular dystrophy have identified the abnormal gene and the missing or defective gene product, dystrophin. 3. These discoveries provide information which will lead to more rational and specific therapeutic approaches. 4. The advances in genetic research have led to more effective preventive therapy. Gene mapping has been applied successfully in carrier detection and antenatal diagnosis, and specific gene probes will soon become available for carrier testing for the two most common forms of muscular dystrophy, Duchenne muscular dystrophy and myotonic dystrophy. 5. Supportive therapies for muscular dystrophy patients now include respiratory support for selected patients with chronic respiratory insufficiency. 6. This review will focus on the two most common muscular dystrophies, Duchenne muscular dystrophy and myotonic dystrophy.

Effect of testosterone on muscle protein synthesis in myotonic dystrophy

Muscle wasting in myotonic dystrophy may result from decreased muscle anabolic processes rather than from increased catabolism. Male patients with myotonic dystrophy often have low levels of circulating androgens, and androgen administration has been shown to increase their muscle mass. We have studied the effect of testosterone enanthate administration (3 mg/kg weekly for 3 months) on muscle and whole body protein synthesis in 6 male patients with myotonic dystrophy. Muscle protein synthesis was estimated from the rate of isotope incorporation into muscle protein obtained by quadriceps muscle biopsy during a primed continuous infusion of L-[1-13C]leucine. Testosterone administration resulted in a significant increase in muscle protein synthesis in all patients. Whole body protein synthesis did not increase, indicating that protein synthesis in other tissues may have declined. Muscle ribonucleic acid content rose significantly in response to testosterone administration, suggesting that testosterone initiated its effect by hormone receptor interaction with muscle nuclei.

Effect of testosterone on whole body amino acid utilization in myotonic dystrophy

Patients with myotonic dystrophy are markedly insulin resistant and have an associated abnormality in the regulation of arterialized amino acid concentrations during euglycemic insulin infusions. We studied the effect of testosterone treatment on whole body amino acid balance in myotonic dystrophy, since it increases muscle mass and muscle protein synthesis rate. Six patients with myotonic dystrophy underwent studies of glucose disposal and amino acid regulation during low dose insulin infusions with maintenance of euglycemia, prior to and after 10 to 13 weeks of testosterone (3 mg/kg/wk). Testosterone increased the insulin-stimulated decline of certain amino acids, but did not improve whole body glucose uptake. The anabolic effect of testosterone is separate from the anabolic effect of insulin.