Digitalis enhances exercise-induced hyperkalaemia

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

Oral digoxin effects on exercise performance, K+ regulation and skeletal muscle Na+ ,K+ -ATPase in healthy humans

Abstract

We investigated whether digoxin lowered muscle Na⁺,K⁺‐ATPase (NKA), impaired muscle performance and exacerbated exercise K⁺ disturbances. Ten healthy adults ingested digoxin (0.25 mg; DIG) or placebo (CON) for 14 days and performed quadriceps strength and fatiguability, finger flexion (FF, 105%_{peak‐workrate}, 3 × 1 min, fourth bout to fatigue) and leg cycling (LC, 10 min at 33% VO2peak and 67% VO2peak, 90% VO2peak to fatigue) trials using a double‐blind, crossover, randomised, counter‐balanced design. Arterial (a) and antecubital venous (v) blood was sampled (FF, LC) and muscle biopsied (LC, rest, 67% VO2peak, fatigue, 3 h after exercise). In DIG, in resting muscle, [³H]‐ouabain binding site content (OB‐F_ab) was unchanged; however, bound‐digoxin removal with Digibind revealed total ouabain binding (OB+F_ab) increased (8.2%, P = 0.047), indicating 7.6% NKA–digoxin occupancy. Quadriceps muscle strength declined in DIG (−4.3%, P = 0.010) but fatiguability was unchanged. During LC, in DIG (main effects), time to fatigue and [K⁺]_a were unchanged, whilst [K⁺]_v was lower (P = 0.042) and [K⁺]_a‐v greater (P = 0.004) than in CON; with exercise (main effects), muscle OB‐F_ab was increased at 67% VO2peak (per wet‐weight, P = 0.005; per protein P = 0.001) and at fatigue (per protein, P = 0.003), whilst [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were each increased at fatigue (P = 0.001). During FF, in DIG (main effects), time to fatigue, [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were unchanged; with exercise (main effects), plasma [K⁺]_a, [K⁺]_v, [K⁺]_a‐v and muscle K⁺ efflux were all increased at fatigue (P = 0.001). Thus, muscle strength declined, but functional muscle NKA content was preserved during DIG, despite elevated plasma digoxin and muscle NKA–digoxin occupancy, with K⁺ disturbances and fatiguability unchanged.

Key points

The Na⁺,K⁺‐ATPase (NKA) is vital in regulating skeletal muscle extracellular potassium concentration ([K⁺]), excitability and plasma [K⁺] and thereby also in modulating fatigue during intense contractions.

NKA is inhibited by digoxin, which in cardiac patients lowers muscle functional NKA content ([³H]‐ouabain binding) and exacerbates K⁺ disturbances during exercise.

In healthy adults, we found that digoxin at clinical levels surprisingly did not reduce functional muscle NKA content, whilst digoxin removal by Digibind antibody revealed an ∼8% increased muscle total NKA content.

Accordingly, digoxin did not exacerbate arterial plasma [K⁺] disturbances or worsen fatigue during intense exercise, although quadriceps muscle strength was reduced.

Thus, digoxin treatment in healthy participants elevated serum digoxin, but muscle functional NKA content was preserved, whilst K⁺ disturbances and fatigue with intense exercise were unchanged. This resilience to digoxin NKA inhibition is consistent with the importance of NKA in preserving K⁺ regulation and muscle function.

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.

Hyperkalemia: a review

Potassium is the principal intracellular cation, and maintenance of the distribution of potassium between the intracellular and the extracellular compartments relies on several homeostatic mechanisms. When these mechanisms are perturbed, hypokalemia or hyperkalemia may occur. This review covers hyperkalemia, that is, a serum potassium concentration exceeding 5 mmol/L. The review includes a discussion of potassium homeostasis and the etiologies of hyperkalemia and focuses on the prompt recognition and treatment of hyperkalemia. This disorder should be of major concern to clinicians because of its propensity to cause fatal arrhythmias. Hyperkalemia is easily diagnosed, and rapid and effective treatments are readily available. Unfortunately, treatment of this life-threatening condition is often delayed or insufficiently attentive or aggressive.

Evaluation of cytochrome P450 probe substrates commonly used by the pharmaceutical industry to study in vitro drug interactions

Pharmaceutical industry investigators routinely evaluate the potential for a new drug to modify cytochrome p450 (p450) activities by determining the effect of the drug on in vitro probe reactions that represent activity of specific p450 enzymes. The in vitro findings obtained with one probe substrate are usually extrapolated to the compound's potential to affect all substrates of the same enzyme. Due to this practice, it is important to use the right probe substrate and to conduct the experiment under optimal conditions. Surveys conducted by reviewers in CDER indicated that the most common in vitro probe reactions used by industry investigators include the following: phenacetin O-deethylation for CYP1A2, coumarin 7-hydroxylation for CYP2A6, 7-ethoxy-4-trifluoromethyl coumarin O-dealkylation for CYP2B6, tolbutamide 4'-hydroxylation for CYP2C9, S-mephenytoin 4-hydroxylation for CYP2C19, bufuralol 1'-hydroxylation for CYP2D6, chlorzoxazone 6-hydroxylation for CYP2E1, and testosterone 6 beta-hydroxylation for CYP3A4. We reviewed the validation information in the literature on these reactions and other frequently used reactions, including caffeine N3-demethylation for CYP1A2, S-mephenytoin N-demethylation for CYP2B6, S-warfarin 7'-hydroxylation for CYP2C9, dextromethorphan O-demethylation for CYP2D6, and midazolam 1'-hydroxylation for CYP3A4. The available information indicates that we need to continue the search for better probe substrates for some enzymes. For CYP3A4-based drug interactions it may be necessary to evaluate two or more probe substrates. In many cases, the probe reaction represents a particular enzyme activity only under specific experimental conditions. Investigators must consider appropriateness of probe substrates and experimental conditions when conducting in vitro drug interaction studies and when extrapolating the results to in vivo situations.

Evaluation of the beta 2 adrenoceptor agonist/antagonist activity of formoterol and salmeterol

BACKGROUND

Salmeterol and formoterol have a lower intrinsic activity at beta 2 receptors than isoprenaline in human bronchus in vitro. The aim of the present study was to evaluate in vivo the beta 2 agonist/antagonist activity of salmeterol and formoterol at rest with low endogenous adrenergic tone, on exercise with raised endogenous adrenergic tone, and in the presence of fenoterol, an exogenous full beta 2 receptor agonist.

METHODS

Eight normal subjects were randomised to receive single doses of placebo, salmeterol 300 micrograms, formoterol 72 micrograms, or propranolol 80 mg at weekly intervals. beta 2 adrenoceptor responses were evaluated at rest, at peak exercise, and after treatment with fenoterol 2.4 mg.

RESULTS

At rest salmeterol and formoterol exhibited equivalent beta 2 agonist activity with regard to decrease in serum potassium levels and increase in finger tremor, with propranolol having no effect. Salmeterol and formoterol, like propranolol, potentiated the hyperkalaemic delta response to exercise compared with placebo, consistent with beta 2 antagonism: (mean difference and 95% confidence interval (CI) compared with placebo) salmeterol 0.20 (0.02 to 0.38) mmol/l, formoterol 0.17 (0.00 to 0.34) mmol/l, propranolol 0.45 (0.08 to 0.82) mmol/l. Propranolol blunted the heart rate delta response to exercise, consistent with beta 1 blockade, whilst salmeterol and formoterol had no effect. Salmeterol and formoterol, like propranolol, attenuated the hypokalaemic, tremor, and heart rate delta responses to fenoterol compared with placebo, in keeping with beta 2 blockade: potassium, salmeterol 0.18 (0.0 to 0.36) mmol/l, formoterol 0.17 (-0.03 to 0.37) mmol/l, propranolol 0.80 (0.54 to 1.06) mmol/l; tremor, salmeterol -0.69 (-1.26 to -0.12) log units, formoterol -0.71 (-1.53 to 0.11) log units, propranolol -0.85 (-1.66 to -0.04) log units; heart rate, salmeterol -6 (-13 to 1) beats/min, formoterol -10 (-19 to -1) beats/min, propranolol -18 (-29 to -7) beats/min.

CONCLUSIONS

At rest with low endogenous adrenergic tone salmeterol and formoterol showed equivalent beta 2 mediated agonist activity in terms of serum potassium and finger tremor responses. In the presence of raised endogenous adrenergic tone at peak exercise and in the presence of fenoterol (an exogenous full beta 2 receptor agonist), salmeterol and formoterol, like propranolol, exhibited beta 2 receptor antagonism as evidenced by their attenuation of beta 2 receptor mediated responses. The degree of beta 2 blockade with formoterol and salmeterol was comparable but less than with propranolol. The relevance of these findings at extrapulmonary beta 2 receptors with regard to airway beta 2 responses remains unclear and warrants further investigation.

Expression of the beta 2 adrenoceptor partial agonist/antagonist activity of salbutamol in states of low and high adrenergic tone

BACKGROUND

Salbutamol exhibits partial agonist/antagonist activity at airway beta 2 receptors in vitro in that it attenuates the bronchorelaxant effect of the full agonist isoprenaline. The aim of the present study was to characterise the partial beta 2 agonist/antagonist activity of salbutamol in vivo during supine rest and exercise, in states of low and high adrenergic tone.

METHODS

Eight normal subjects were randomised to receive single oral doses of salbutamol 2 mg, 4 mg, 8 mg (S2, S4, S8), placebo (PL), or propranolol 80 mg (PR). The beta 2 adrenoceptor responses were evaluated after supine rest and subsequently in response to maximal exercise.

RESULTS

Salbutamol demonstrated a dose-related increase in resting heart rate and tremor and a fall in serum potassium level consistent with beta 2 agonism. On exercise, the hyperkalaemic response was augmented by propranolol compared with placebo consistent with beta 2 blockade: mean difference for delta response (95% CI) PR v PL was 0.60 (0.02 to 1.27) mmol/l. This effect also occurred with salbutamol in a dose-related fashion: S8 v PL 0.33 (0.01 to 0.71) mmol/l, S8 v S2 0.31 (-0.02 to 0.61) mmol/l. Whilst propranolol blunted exercise heart rate in keeping with beta 1 blockade, salbutamol had no effect. Exercise produced an increase in lymphocyte beta 2 receptor binding density (Bmax) which was not affected by salbutamol. Plasma levels of adrenaline and noradrenaline at peak exercise were also unaltered by salbutamol in comparison with placebo.

CONCLUSIONS

In a state of low adrenergic tone at rest salbutamol produces effects consistent with beta 2 agonism. In contrast, in a state of increased adrenergic tone during exercise salbutamol produced beta 2 selective antagonism as evidenced by its effects on exercise-induced hyperkalaemia (beta 2) but not on exercise-induced tachycardia (beta 1). The effects of salbutamol on beta 2 receptor density do not explain its effects on exercise-induced hyperkalaemia since upregulation rather than downregulation was observed. This in vivo phenomenon of partial beta 2 agonist/antagonist activity of salbutamol may be of relevance in the setting of acute asthma if adrenergic tone is increased.