Potassium fluxes in contracting human skeletal muscle and red blood cells

Impact of a 10 km running trial on eryptosis, red blood cell rheology, and electrophysiology in endurance trained athletes: a pilot study

PURPOSE

Blood rheology is a key determinant of blood flow and tissue perfusion. There are still large discrepancies regarding the effects of an acute running exercise on blood rheological properties and red blood cell (RBC) physiology. We investigated the effect of a 10 km running trial on markers of blood rheology and RBC physiology in endurance trained athletes.

METHODS

Blood was sampled before and after the exercise to measure lactate and glucose, hematological and hemorheological parameters (blood viscosity, RBC deformability, and aggregation), eryptosis markers (phosphatidylserine and CD47 exposure, RBC reactive oxygen species), RBC-derived microparticles (RBC-MPs), and RBC electrophysiological activity. Weight was measured before and after exercise. Peripheral oxygen saturation and heart rate were monitored before and during the trial.

RESULTS

Blood lactate and glucose levels increased after exercise and subjects significantly lost weight. All athletes experienced a significant fall in oxygen saturation. Mean corpuscular volume (MCV) was increased from 95.1 ± 3.2 to 96.0 ± 3.3 and mean corpuscular hemoglobin concentration (MCHC) decreased after exercise suggesting a slight RBC rehydration. Exercise increased RBC deformability from 0.344 ± 0.04 to 0.378 ± 0.07, decreased RBC aggregates strength and blood viscosity, while hematocrit (Hct) remained unaffected. While RBC electrophysiological recording suggested a modulation in RBC calcium content and/or chloride conductance, eryptosis markers and RBC-MPs were not modified by the exercise.

CONCLUSION

A 10 km acute running exercise had no effect on RBC senescence and membrane blebbing. In contrast, this exercise increased RBC deformability, probably through rehydration process which resulted in a decrease in blood viscosity.

Pulmonary Circulation Transvascular Fluid Fluxes Do Not Change during General Anesthesia in Dogs

General anesthesia (GA) can cause abnormal lung fluid redistribution. Pulmonary circulation transvascular fluid fluxes (J_VA) are attributed to changes in hydrostatic forces and erythrocyte volume (EV) regulation. Despite the very low hydraulic conductance of pulmonary microvasculature it is possible that GA may affect hydrostatic forces through changes in pulmonary vascular resistance (PVR), and EV through alteration of erythrocyte transmembrane ion fluxes (_ionJ_VA). Furosemide (Fur) was also used because of its potential to affect pulmonary hydrostatic forces and _ionJ_VA. A hypothesis was tested that J_VA, with or without furosemide treatment, will not change with time during GA. Twenty dogs that underwent castration/ovariectomy were randomly assigned to Fur (n = 10) (4 mg/kg IV) or placebo treated group (Con, n = 10). Baseline arterial (BL) and mixed venous blood were sampled during GA just before treatment with Fur or placebo and then at 15, 30 and 45 min post-treatment. Cardiac output (Q) and pulmonary artery pressure (P_AP) were measured. J_VA and _ionJ_VA were calculated from changes in plasma protein, hemoglobin, hematocrit, plasma and whole blood ions, and Q. Variables were analyzed using random intercept mixed model (P < 0.05). Data are expressed as means ± SE. Furosemide caused a significant volume depletion as evident from changes in plasma protein and hematocrit (P < 0.001). However; Q, P_AP, and J_VA were not affected by time or Fur, whereas erythrocyte fluid flux was affected by Fur (P = 0.03). Furosemide also affected erythrocyte transmembrane K⁺ and Cl⁻, and transvascular Cl⁻ metabolism (P ≤ 0.05). No other erythrocyte transmembrane or transvascular ion fluxes were affected by time of GA or Fur. Our hypothesis was verified as J_VA was not affected by GA or ion metabolism changes due to Fur treatment. Furosemide and 45 min of GA did not cause significant hydrostatic changes based on Q and P_AP. Inhibition of Na⁺/K⁺/2Cl⁻ cotransport caused by Fur treatment, which can alter EV regulation and J_VA, was offset by the Jacobs Stewart cycle. The results of this study indicate that the Jacobs Stewart cycle/erythrocyte Cl⁻ metabolism can also act as a safety factor for the stability of lung fluid redistribution preserving optimal diffusion distance across the blood gas barrier.

Salbutamol effects on systemic potassium dynamics during and following intense continuous and intermittent exercise

PURPOSE

Salbutamol inhalation is permissible by WADA in athletic competition for asthma management and affects potassium regulation, which is vital for muscle function. Salbutamol effects on arterial potassium concentration ([K⁺]_a) during and after high-intensity continuous exercise (HI_cont) and intermittent exercise comprising repeated, brief sprints (HI_int), and on performance during HI_int are unknown and were investigated.

METHODS

Seven recreationally active men participated in a double-blind, randomised, cross-over design, inhaling 1000 µg salbutamol or placebo. Participants cycled continuously for 5 min at 40 % [Formula: see text]O_2peak and 60 % [Formula: see text]O_2peak, then HI_cont (90 s at 130 % [Formula: see text]O_2peak), 20 min recovery, and then HI_int (3 sets, 5 × 4 s sprints), with 30 min recovery.

RESULTS

Plasma [K⁺]_a increased throughout exercise and subsequently declined below baseline (P < 0.001). Plasma [K⁺]_a was greater during HI_cont than HI_int (P < 0.001, HI_cont 5.94 ± 0.65 vs HI_int set 1, 4.71 ± 0.40 mM); the change in [K⁺]_a from baseline (Δ[K⁺]_a) was 2.6-fold greater during HI_cont than HI_int (P < 0.001). The Δ[K⁺] throughout the trial was less with salbutamol than placebo (P < 0.001, treatment main effect, 0.03 ± 0.67 vs 0.22 ± 0.69 mM, respectively); and remained less after correction for fluid shifts (P < 0.001). The Δ[K⁺] during HI_cont was less after salbutamol (P < 0.05), but not during HI_int. Blood lactate, plasma pH, and the work output during HI_int did not differ between trials.

CONCLUSIONS

Inhaled salbutamol modulated the [K⁺]_a rise across the trial, comprising intense continuous and intermittent exercise and recovery, lowering Δ[K⁺] during HI_cont. The limited [K⁺]_a changes during HI_int suggest that salbutamol is unlikely to influence systemic [K⁺] during periods of intense effort in intermittent sports.

Effect of intensified training on muscle ion kinetics, fatigue development, and repeated short-term performance in endurance-trained cyclists

The effects of intensified training in combination with a reduced training volume on muscle ion kinetics, transporters, and work capacity were examined. Eight well-trained cyclists replaced their regular training with speed-endurance training (12 × 30 s sprints) 2–3 times per week and aerobic high-intensity training (4–5 × 3–4 min at 90–100% of maximal heart rate) 1–2 times per week for 7 wk and reduced training volume by 70% (intervention period; IP). The duration of an intense exhaustive cycling bout (EX2; 368 ± 6 W), performed 2.5 min after a 2-min intense cycle bout (EX1), was longer ( P < 0.05) after than before IP (4:16 ± 0:34 vs. 3:37 ± 0:28 min:s), and mean and peak power during a repeated sprint test improved ( P < 0.05) by 4% and 3%, respectively. Femoral venous K+ concentration in recovery from EX1 and EX2 was lowered ( P < 0.05) after compared with before IP, whereas muscle interstitial K+ concentration and net muscle K+ release during exercise was unaltered. No changes in muscle lactate and H+ release during and after EX1 and EX2 were observed, but the in vivo buffer capacity was higher ( P < 0.05) after IP. Expression of the ATP-sensitive K+ (KATP) channel (Kir6.2) decreased by IP, with no change in the strong inward rectifying K+ channel (Kir2.1), muscle Na+-K+ pump subunits, monocarboxylate transporters 1 and 4 (MCT1 and MCT4), and Na+/H+ exchanger 1 (NHE1). In conclusion, 7 wk of intensified training with a reduced training volume improved performance during repeated intense exercise, which was associated with a greater muscle reuptake of K+ and muscle buffer capacity but not with the amount of muscle ion transporters.

Exercise-induced changes in plasma composition increase erythrocyte Na+,K+-ATPase, but not Na+-K+-2Cl- cotransporter, activity to stimulate net and unidirectional K+ transport in humans

We tested the hypothesis that exercise-induced changes in plasma composition result in peak stimulation of erythrocyte unidirectional K+ (JK,in) and net K+ (JK,net) transport within the first 120 s. In experimental series 1 (7 men; 2 women), plasma [K+] was continuously measured in vitro (37 degrees C) after the addition of red blood cells (RBCs) obtained from rested subjects (resting RBCs) into an exercise-simulated plasma (ESP; increased plasma osmolality, [K+], [H+], [lactate] and [adrenaline] (epinephrine)), and JK,net calculated. In experimental series 2 (7 men; 4 women), resting RBCs were incubated in true exercise plasma (TEP) obtained after two 30 s bouts of high intensity leg cycling exercise to determine JK,net and JK,in (via RBC 86Rb accumulation). JK,net of resting RBCs increased from 0.9 +/- 28.7 in resting plasma to 285 +/- 164 mmol (l RBCs)-1 h-1 in ESP and to 178 +/- 60 mmol (l RBCs)-1 h-1 after 10 s in TEP. Both JK,net and JK,in peaked within 10 s of incubation and decreased rapidly during the initial 120 s. The use of inhibitors for the Na+,K+-ATPase (ouabain) and the Na+-K+-2Cl- cotransporter (NKCC; bumetanide) indicated that rapid increases in JK,in and JK,net upon incubation of resting RBCs in TEP were due primarily to increased Na+,K+-ATPase activity; the NKCC appeared to be involved only when the Na+,K+-ATPase was blocked. It is concluded that RBCs rapidly increase JK,in and JK,net in response to exercise-induced changes in plasma composition.

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.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Interstitial K(+) in human skeletal muscle during and after dynamic graded exercise determined by microdialysis

Interstitial K(+) concentrations were measured during one-legged knee-extensor exercise by use of microdialysis with probes inserted in the vastus lateralis muscle of the subjects. K(+) in the dialysate was measured either by flame photometry or a K(+)-sensitive electrode placed in the perfusion outlet. The correction for fractional K(+) recovery was based on the assumption of identical fractional thallium loss. The interstitial K(+) was 4. 19 +/- 0.09 mM at rest and increased to 6.17 +/- 0.19, 7.48 +/- 1.18, and 9.04 +/- 0.74 mM at 10, 30, and 50 W exercise, respectively. The individual probes demonstrated large variations in interstitial K(+), and values >10 mM were obtained. The observed interstitial K(+) was markedly higher than previously found for venous K(+) concentrations at similar work intensities. The present data support a potential role for interstitial K(+) in regulation of blood flow and development of fatigue.