Skeletal muscle Na,K-ATPase concentration changes and intramuscular and extrarenal K homeostasis in animals and humans

Depressed Na+-K+-ATPase activity in skeletal muscle at fatigue is correlated with increased Na+-K+-ATPase mRNA expression following intense exercise

We investigated whether depressed muscle Na+-K+-ATPase activity with exercise reflected a loss of Na+-K+-ATPase units, the time course of its recovery postexercise, and whether this depressed activity was related to increased Na+-K+-ATPase isoform gene expression. Fifteen subjects performed fatiguing, knee extensor exercise at ∼40% maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue, 3 h, and 24 h postexercise and analyzed for maximal Na+-K+-ATPase activity via 3- O-methylfluorescein phosphatase (3- O-MFPase) activity, Na+-K+-ATPase content via [3H]ouabain binding sites, and Na+-K+-ATPase α1-, α2-, α3-, β1-, β2- and β3-isoform mRNA expression by real-time RT-PCR. Exercise [352 (SD 267) s] did not affect [3H]ouabain binding sites but decreased 3- O-MFPase activity by 10.7 (SD 8)% ( P < 0.05), which had recovered by 3 h postexercise, without further change at 24 h. Exercise elevated α1-isoform mRNA by 1.5-fold at fatigue ( P < 0.05). This increase was inversely correlated with the percent change in 3- O-MFPase activity from rest to fatigue (%Δ3- O-MFPaserest-fatigue) ( r = −0.60, P < 0.05). The average postexercise (fatigue, 3 h, 24 h) α1-isoform mRNA was increased 1.4-fold ( P < 0.05) and approached a significant inverse correlation with %Δ3- O-MFPaserest-fatigue( r = −0.56, P = 0.08). Exercise elevated α2-isoform mRNA at fatigue 2.5-fold ( P < 0.05), which was inversely correlated with %Δ3- O-MFPaserest-fatigue( r = −0.60, P = 0.05). The average postexercise α2-isoform mRNA was increased 2.2-fold ( P < 0.05) and was inversely correlated with the %Δ3- O-MFPaserest-fatigue( r = −0.68, P < 0.05). Nonsignificant correlations were found between %Δ3- O-MFPaserest-fatigueand other isoforms. Thus acute exercise transiently decreased Na+-K+-ATPase activity, which was correlated with increased Na+-K+-ATPase gene expression. This suggests a possible signal-transduction role for depressed muscle Na+-K+-ATPase activity with exercise.

Quantitative measurement of muscle fiber composition in a normal population

To obtain normative muscle morphology data on a healthy population recruited from a population survey, we examined vastus lateralis biopsies from 58 men and 33 women, aged 26-67 years. Biopsies were measured with automated, computer-aided techniques. Data were analyzed according to gender and age, and the influence of blood pressure, body mass index (BMI), and smoking habits was also examined. Men had larger muscle fibers (fiber area approximately 5400 microm(2)) than women ( approximately 4000 microm(2), P = 0.003). No gender differences were seen in fiber composition, fiber roundness, percentage of connective tissue, or capillary density. Blood pressure did not influence fiber size or composition, but was correlated with fiber roundness in men. BMI was associated with fiber area in men, but not in women. Variations in age, smoking habits, and physical activity did not influence muscle morphology data substantially. Thus, in a normal population, men have larger muscle fibers than women, but similar fiber composition. Variation in gender, BMI, blood pressure, and physical activity may influence morphological features to a minor degree.

Muscle strength, Na,K-pumps, magnesium and potassium in patients with alcoholic liver cirrhosis -- relation to spironolactone

OBJECTIVES

To evaluate the muscle strength in relation to muscle contents of magnesium (Mg), potassium (K) and sodium, potassium (Na,K)-pumps in patients with alcoholic cirrhosis.

DESIGN

An open cross-sectional study.

SETTING AND SUBJECTS

Fifty-one consecutive patients with liver cirrhosis admitted to the Department of Hepatology, Aarhus University Hospital, Denmark, and 28 age- and sex-matched healthy control subjects.

MAIN OUTCOME MEASURES

Biopsies of skeletal muscle were performed in patients and controls for measurements of Mg, K, and Na,K-pumps. Furthermore, maximum isokinetic knee extension and skeletal muscle mass were evaluated.

RESULTS

Muscle mass, muscle strength, muscle Mg and muscle K were substantially reduced in the patients (P < 0.01, all), and fell with increasing severity of the liver disease reflected in the Child-Pugh (C-P) class. Patients treated with spironolactone for 2 weeks or more, had increased muscle strength, muscle Mg and content of Na,K-pumps, compared with the rest of the patients (P < 0.05, all). In a multivariate analysis of the patients, skeletal muscle mass, muscle Mg and daily alcohol consumption (g) were independent predictors of isokinetic muscle strength (P < 0.05, all).

CONCLUSIONS

Patients with alcoholic liver cirrhosis showed considerably reduced muscle strength and muscle Mg was an independent predictor of muscle strength. Surprisingly, in the spironolactone treated patients, muscle weakness was less pronounced, possibly because of the action of spironolactone on muscle Mg, K and Na,K-pump content.

Exercise-induced hyperkalemia in hypothyroid dogs

We investigated the effect of hypothyroidism in dogs on (1) the Na+-, K+ -ATPase concentration in skeletal muscle, and (2) potassium (K+) homeostasis at rest and during exercise. Prior to and 1 year after induction of hypothyroidism by surgery and subsequent radiothyroidectomy, the Na+-, K+ -ATPase concentrations were quantified in biopsies of sternothyroid muscles of seven Beagle dogs by measuring [3H]ouabain binding capacity. In addition, plasma K+ concentrations were measured at rest and after treadmill exercise in six hypothyroid and seven euthyroid Beagle dogs. During hypothyroidism, the mean Na+ -, K+ -ATPase concentration in muscle biopsies was 41% lower than during euthyroidism. The mean resting plasma K+ value of the hypothyroid dogs was significantly (14%) higher than that of the euthyroid dogs. In the hypothyroid dogs, plasma K+ concentration increased significantly during exercise, whereas there was no rise in the euthyroid dogs. The rise in plasma K+ concentration could not be ascribed to muscle damage, as plasma creatine kinase concentrations remained within reference range. Also renal K+ retention was an unlikely explanation, as plasma aldosterone concentration and plasma renin activity rather increased than decreased during exercise. In conclusion, hypothyroid dogs tend to develop hyperkalemia during exercise, which for a large part can be explained by the severe reduction of the Na+ -, K+ -ATPase capacity in the skeletal muscle pool.