Increased skeletal muscle Na/K-ratio in obese men, but not in women, with glucose intolerance

Hormonal regulation of Na+-K+-ATPase from the evolutionary perspective

Na⁺-K⁺-ATPase, an α/β heterodimer, is an ancient enzyme that maintains Na⁺ and K⁺ gradients, thus preserving cellular ion homeostasis. In multicellular organisms, this basic housekeeping function is integrated to fulfill the needs of specialized organs and preserve whole-body homeostasis. In vertebrates, Na⁺-K⁺-ATPase is essential for many fundamental physiological processes, such as nerve conduction, muscle contraction, nutrient absorption, and urine excretion. During vertebrate evolution, three key developments contributed to diversification and integration of Na⁺-K⁺-ATPase functions. Generation of novel α- and β-subunits led to formation of multiple Na⁺-K⁺-ATPase isoenyzmes with distinct functional characteristics. Development of a complex endocrine system enabled efficient coordination of diverse Na⁺-K⁺-ATPase functions. Emergence of FXYDs, small transmembrane proteins that regulate Na⁺-K⁺-ATPase, opened new ways to modulate its function. FXYDs are a vertebrate innovation and an important site of hormonal action, suggesting they played an especially prominent role in evolving interaction between Na⁺-K⁺-ATPase and the endocrine system in vertebrates.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Association between maximal oxygen uptake and the heart rate corrected-QT interval in postmenopausal overweight women

BACKGROUND

Increased aerobic capacity can reduce the incidence of cardiovascular disease and the mortality rate. On the other hand, a prolonged heart rate corrected-QT (QTc) interval is associated with an increased risk of arrhythmias, cardiac sudden death and coronary artery disease.

AIMS

The association of the aerobic capacity and coronary risk factors with QTc interval was investigated in postmenopausal overweight women.

SUBJECTS AND METHODS

The subjects included 84 postmenopausal overweight women [age: 58.7+/-6.4 years, body mass index (BMI): 27.9+/-3.3] with coronary risk factors. Electrocardiogram (ECG) was recorded with a standard resting 12-lead ECG after more than 5 minutes of rest. The QTc interval was automatically calculated according to Bazett's formula. A multistage graded submaximal exercise test was performed on an electric bicycle ergometer to determine the estimated maximal oxygen uptake (VO(2)max).

RESULTS

Single correlation analysis showed the QTc interval to be positively associated with hemoglobin A(1)c (HbA(1)c), fasting glucose, fasting insulin, BMI, waist circumference, serum potassium and the number of coronary risk factors, while negatively correlated with VO(2)max. Stepwise multiple regression analysis demonstrated the strong association of the QTc interval with HbA(1)c and VO(2)max (r(2)=0.244, p<0.0001). In both patients with and without metabolic syndrome (n=15, n=69, respectively), the QTc interval was independently associated with the HbA(1)c (r(2)=0.318, p<0.05, r(2)= 0.115, p<0.05, respectively).

CONCLUSIONS

These results suggest that decreased aerobic capacity and glucose intolerance may be independent risk factors for a prolonged QTc interval, while demonstrating no relationship with metabolic syndrome.

Altered expression and insulin-induced trafficking of Na+-K+-ATPase in rat skeletal muscle: effects of high-fat diet and exercise

Skeletal muscle Na(+)-K(+)-ATPase plays a central role in the clearance of K(+) from the extracellular fluid, therefore maintaining blood [K(+)]. Na(+)-K(+)-ATPase activity in peripheral tissue is impaired in insulin resistant states. We determined effects of high-fat diet (HFD) and exercise training (ET) on skeletal muscle Na(+)-K(+)-ATPase subunit expression and insulin-stimulated translocation. Skeletal muscle expression of Na(+)-K(+)-ATPase isoforms and transcription factor DNA binding was determined before or after 5 days of swim training in Wistar rats fed chow or HFD for 4 or 12 wk. Skeletal muscle insulin resistance was observed after 12 wk of HFD. Na(+)-K(+)-ATPase alpha(1)-subunit protein expression was increased 1.6-fold (P < 0.05), whereas alpha(2)- and beta(1)-subunits and protein expression were decreased twofold (P < 0.01) in parallel with decrease in plasma membrane Na(+)-K(+)-ATPase activity after 4 wk of HFD. Exercise training restored alpha(1)-, alpha(2)-, and beta(1)-subunit expression and Na(+)-K(+)-ATPase activity to control levels and reduced beta(2)-subunit expression 2.2-fold (P < 0.05). DNA binding activity of the alpha(1)-subunit-regulating transcription factor ZEB (AREB6) and alpha(1) mRNA expression were increased after HFD and restored by ET. DNA binding activity of Sp-1, a transcription factor involved in the regulation of alpha(2)- and beta(1)-subunit expression, was decreased after HFD. ET increased phosphorylation of the Na(+)-K(+)-ATPase regulatory protein phospholemman. Phospholemman mRNA and protein expression were increased after HFD and restored to control levels after ET. Insulin-stimulated translocation of the alpha(2)-subunit to plasma membrane was impaired by HFD, whereas alpha(1)-subunit translocation remained unchanged. Alterations in sodium pump function precede the development of skeletal muscle insulin resistance. Disturbances in skeletal muscle Na(+)-K(+)-ATPase regulation, particularly the alpha(2)-subunit, may contribute to impaired ion homeostasis in insulin-resistant states such as obesity and type 2 diabetes.

The potential benefits of aldosterone antagonism in Type 2 diabetes mellitus

Interest in the renin-angiotensin-aldosterone system (RAAS) has increased since the development of angiotensin-converting enzyme (ACE) inhibitors. It has been discovered that the potential uses of this class of treatment extend far beyond their initial developmental role as antihypertensives, and they are now used routinely in the treatment of heart failure, nephropathy, myocardial infarction and diabetes. However, there is more to RAAS blockade than just inhibition of angiotensin II, and inhibition of aldosterone is becoming recognised as an additional therapeutic manoeuvre in chronic heart failure. Since inhibition of the RAAS at the level of ACE is now seen to be an important therapy in diabetes; the purpose of this article is to explore the potential benefits of additional aldosterone inhibition in Type 2 diabetes mellitus.

Insulin and isoproterenol differentially regulate mitogen-activated protein kinase-dependent Na(+)-K(+)-2Cl(-) cotransporter activity in skeletal muscle

Recent studies have demonstrated that p44/42(MAPK) extracellular signal-regulated kinase (ERK)1 and -2-dependent Na(+)-K(+)-2Cl(-) co-transporter (NKCC) activity may contribute to total potassium uptake by skeletal muscle. To study the precise mechanisms regulating NKCC activity, rat soleus and plantaris muscles were stimulated ex vivo by insulin or isoproterenol (ISO). Both hormones stimulated total uptake of the potassium congener (86)Rb by 25--70%. However, only ISO stimulated the NKCC-mediated (86)Rb uptake. Insulin inhibited the ISO-stimulated NKCC activity, and this counteraction was sensitive to the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 in the predominantly slow-twitch soleus muscle. Pretreatment of the soleus muscle with the phosphatidylinositol (PI) 3-kinase inhibitors wortmannin and LY294002 or with SB203580 uncovered an insulin-stimulated NKCC activity and also increased the insulin-stimulated phosphorylation of ERK. In the predominantly fast-twitch plantaris muscle, insulin-stimulated NKCC activity became apparent only after inhibition of PI 3-kinase activity, accompanied by an increase in ERK phosphorylation. PI 3-kinase inhibitors also abolished insulin-stimulated p38 MAPK phosphorylation in the plantaris muscle and Akt phosphorylation in both muscles. These data demonstrated that insulin inhibits NKCC-mediated transport in skeletal muscle through PI 3-kinase-sensitive and SB203580-sensitive mechanisms. Furthermore, differential activation of signaling cascade elements after hormonal stimulation may contribute to fiber-type specificity in the control of potassium transport by skeletal muscle.

Skeletal muscle magnesium and potassium by gender and hypertensive status

The relation between blood pressure and skeletal muscle magnesium and potassium, and the relation between these electrolytes and body mass index, blood lipids, blood glucose and plasma insulin concentrations were studied in 29 hypertensive and 21 normotensive men. In addition, a comparison was made between the normotensive men and 37 normotensive women regarding the concentrations of muscle potassium and magnesium. Mean skeletal muscle potassium concentration was lower and plasma insulin higher in hypertensive compared to normotensives. Systolic and diastolic blood pressures were inversely correlated to muscle potassium and positively correlated to insulin. Muscle magnesium was positively correlated to muscle potassium but not to blood pressure. Muscle magnesium was significantly higher in normotensive women, compared to normotensive men. Muscle potassium did not differ between the genders.