The relationship between plasma potassium,muscle membrane excitability and force following quadriceps fatigue

To examine the simultaneous changes in plasma [K⁺], muscle excitability and force during fatigue, ten male adults (mean age = 22 ± 0.5 years) held an isometric contraction of their right quadriceps muscle at an intensity of 30% maximum voluntary contraction (MVC) for 3 min. Femoral venous and brachial arterial [K⁺] were determined from serial samples drawn before, during, and for 15 min following the 3-min contraction. Each blood sample was synchronized with a maximal stimulation of the right femoral nerve to evoke a twitch and compound muscle action potential (M-wave). Immediately post-exercise, twitch torque was only 42% of baseline and femoral venous plasma [K⁺] had increased significantly from 4.02 ± 0.08 mmol/l to 5.9 ± 0.22 mmol/l. Femoral venous plasma lactate rose to a peak level of 10.0 ± 0.8 mmol/l at 1 min post exercise. The recovery of the twitch torque was exponentially related to the recovery of femoral venous plasma [K⁺] (r ² = 0.93, P < 0.01). There was no evidence for any loss of muscle membrane excitability during the period of increased extracellular [K⁺], in fact, the M-waves tended to be potentiated in the early phases of the recovery period. These results suggest that muscle membrane excitability is maintained in spite of increased extracellular [K⁺] following fatigue induced by a sustained submaximal quadriceps contraction. However, the strong relationship between twitch torque and femoral venous plasma [K⁺] suggests that K⁺ may be exerting its effect distal to surface membrane action potential propagation, most likely in the T-tubular region. Received: 20 April 1995/Received after revision and accepted: 8 January 1996     

Remodelling of human atrial K<Superscript>+</Superscript> currents but not ion channel expression by chronic β-blockade

Chronic β-adrenoceptor antagonist (β-blocker) treatment in patients is associated with a potentially anti-arrhythmic prolongation of the atrial action potential duration (APD), which may involve remodelling of repolarising K⁺ currents. The aim of this study was to investigate the effects of chronic β-blockade on transient outward, sustained and inward rectifier K⁺ currents (I_TO, I_KSUS and I_K1) in human atrial myocytes and on the expression of underlying ion channel subunits. Ion currents were recorded from human right atrial isolated myocytes using the whole-cell-patch clamp technique. Tissue mRNA and protein levels were measured using real time RT-PCR and Western blotting. Chronic β-blockade was associated with a 41% reduction in I_TO density: 9.3 ± 0.8 (30 myocytes, 15 patients) vs 15.7 ± 1.1 pA/pF (32, 14), p < 0.05; without affecting its voltage-, time- or rate dependence. I_K1 was reduced by 34% at −120 mV (p < 0.05). Neither I_KSUS, nor its increase by acute β-stimulation with isoprenaline, was affected by chronic β-blockade. Mathematical modelling suggested that the combination of I_TO- and I_K1-decrease could result in a 28% increase in APD₉₀. Chronic β-blockade did not alter mRNA or protein expression of the I_TO pore-forming subunit, Kv4.3, or mRNA expression of the accessory subunits KChIP2, KChAP, Kvβ1, Kvβ2 or frequenin. There was no reduction in mRNA expression of Kir2.1 or TWIK to account for the reduction in I_K1. A reduction in atrial I_TO and I_K1 associated with chronic β-blocker treatment in patients may contribute to the associated action potential prolongation, and this cannot be explained by a reduction in expression of associated ion channel subunits.     

Cardiovascular reflexes during sustained handgrip exercise: role of muscle fibre composition,potassium and lactate

Summary Six healthy men performed sustained static handgrip exercise for 2 min at 40% maximal voluntary contraction followed by a 6-min recovery period. Heart rate (f _c), arterial blood pressures, and forearm blood flow were measured during rest, exercise, and recovery. Potassium ([K⁺]) and lactate concentrations in blood from a deep forearm vein were analysed at rest and during recovery. Mean arterial pressure (MAP) andf _c declined immediately after exercise and had returned to control levels about 2 min into recovery. The time course of the changes in MAP observed during recovery closely paralleled the changes in [K⁺] (r=0.800,P<0.01), whereas the lactate concentration remained elevated throughout the recovery period. The close relationship between MAP and [K⁺] was also confirmed by experiments in which a 3-min arterial occlusion period was applied during recovery to the exercised arm by an upper arm cuff. The arterial occlusion affected MAP whilef _c recovered at almost the same rate as in the control experiment. Muscle biopsies were taken from the brachioradialis muscle and analysed for fibre composition and capillary supply. The MAP at the end of static contraction and the [K⁺] appearing in the effluent blood immediately after contraction were positively correlated to the relative content of fast twitch (% FT) fibres (r=0.886 for MAP vs %FT fibres,P<0.05 andr=0.878 for [K⁺] vs %FT fibres,P<0.05). Capillary to fibre ratio showed an inverse correlation to % FT fibres (r=–0.979,P<0.01). These results indicated that activation of FT rather than slow twitch fibres during static contraction induced a more marked arterial pressure reflex. It was concluded that the arterial pressure reflex would seem to be mediated through stimulation of unmyelinized free nerve endings in the contracted muscle. The [K⁺] would appear to be a more likely candidate than lactate as a mediator for this pressure reflex.     

High-intensity exercise decreases muscle buffer capacity via a decrease in protein buffering in human skeletal muscle

We have previously reported an acute decrease in muscle buffer capacity (βm_{in vitro}) following high-intensity exercise. The aim of this study was to identify which muscle buffers are affected by acute exercise and the effects of exercise type and a training intervention on these changes. Whole muscle and non-protein βm_{in vitro} were measured in male endurance athletes (VO_2max = 59.8 ± 5.8 mL kg⁻¹ min⁻¹), and before and after training in male, team-sport athletes (VO_2max = 55.6 ± 5.5 mL kg⁻¹ min⁻¹). Biopsies were obtained at rest and immediately after either time-to-fatigue at 120% VO_2max (endurance athletes) or repeated sprints (team-sport athletes). High-intensity exercise was associated with a significant decrease in βm_{in vitro} in endurance-trained males (146 ± 9 to 138 ± 7 mmol H⁺·kg d.w.⁻¹·pH⁻¹), and in male team-sport athletes both before (139 ± 9 to 131 ± 7 mmol H⁺·kg d.w.⁻¹·pH⁻¹) and after training (152 ± 11 to 142 ± 9 mmol H⁺·kg d.w.⁻¹·pH⁻¹). There were no acute changes in non-protein buffering capacity. There was a significant increase in βm_{in vitro} following training, but this did not alter the post-exercise decrease in βm_{in vitro}. In conclusion, high-intensity exercise decreased βm_{in vitro} independent of exercise type or an interval-training intervention; this was largely explained by a decrease in protein buffering. These findings have important implications when examining training-induced changes in βm_{in vitro}. Resting and post-exercise muscle samples cannot be used interchangeably to determine βm_{in vitro}, and researchers must ensure that post-training measurements of βm_{in vitro} are not influenced by an acute decrease caused by the final training bout.     

Restoration of cardiac contraction by angiotensin II during raised [K+]o in the rabbit

Catecholamines restore cardiac contraction depressed by hyperkalaemia (raised [K⁺]_o) and acidosis, yet in exercise hyperkalaemia and acidosis are tolerated during βadrenergic blockade. To test whether the negative effects of raised [K⁺]_o are offset by a non-adrenergic hormone, angiotensin II (AII) was given to rabbit papillary muscle (AII 75 nm , n=9) and rabbit isolated working hearts (AII 5 nm , n=8) perfused with 8 and 10 mm K⁺ Tyrode at 37 °C. A similar protocol was also performed in a further nine isolated hearts treated with propranolol (1 μm ) and prazosin (1 μm ). AII caused a significant (P<0.01) increases in contraction and aortic flow in normal Tyrode and maintained aortic flow during high [K⁺]_o. In the papillary muscle and isolated heart treated with adrenergic blockers, high [K⁺]_o reduced the stimulatory effects of AII, but contraction and aortic flow was still significantly greater (P<0.01) than in high [K⁺]_o alone. These results show that AII can ameliorate the depressive effects of high [K⁺]_o on the heart. The local release of AII in the heart during activation of the sympathetic nervous system and the rise in circulating AII during exercise could therefore play a role in protecting the heart from hyperkalaemia.     

Potassium kinetics in human muscle interstitium during repeated intense exercise in relation to fatigue

Accumulation of K⁺ in skeletal muscle interstitium during intense exercise has been suggested to cause fatigue in humans. The present study examined interstitial K⁺ kinetics and fatigue during repeated, intense, exhaustive exercise in human skeletal muscle. Ten subjects performed three repeated, intense (61.6±4.1 W; mean±SEM), one-legged knee extension exercise bouts (EX1, EX2 and EX3) to exhaustion separated by 10-min recovery periods. Interstitial [K⁺] ([K⁺]_interst) in the vastus lateralis muscle were determined using microdialysis. Time-to-fatigue decreased progressively (P<0.05) during the protocol (5.1±0.4, 4.2±0.3 and 3.2±0.2 min for EX1, EX2 and EX3 respectively). Prior to these bouts, [K⁺]_interst was 4.1±0.2, 4.8±0.2 and 5.2±0.2 mM, respectively. During the initial 1.5 min of exercise the accumulation rate of interstitial K⁺ was 85% greater (P<0.05) in EX1 than in EX3. At exhaustion [K⁺]_interst was 11.4±0.8 mM in EX1, which was not different from that in EX2 (10.4±0.8 mM), but higher (P<0.05) than in EX3 (9.1±0.3 mM). The study demonstrated that the rate of accumulation of K⁺ in the muscle interstitium declines during intense repetitive exercise. Furthermore, whilst [K⁺]_interst at exhaustion reached levels high enough to impair performance, the concentration decreased with repeated exercise, suggesting that accumulation of interstitial K⁺ per se does not cause fatigue when intense exercise is repeated.     

Expression environment determines K+ current properties: Kv1 and Kv4 α-subunit-induced K+ currents in mammalian cell lines and cardiac myocytes

 Voltage-gated K⁺ channel (Kv) pore-forming (α) subunits of the Kv1 and Kv4 subfamilies have been cloned from heart cDNA libraries, and are thought to play roles in the generation of the transient outward K⁺ current, I _to. Heterologous expression of these subunits in Xenopus oocytes, however, reveals K⁺ currents that are quite distinct from I _to. In the experiments here, the detailed time- and voltage-dependent properties of the currents expressed in mammalian cell lines and in cardiac myocytes by Kv1.4 and Kv4.2 were examined and compared to previous findings in studies of oocytes, as well as to I _to characterized in various myocardial cells. As in oocytes, expression of Kv1.4 in HEK-293, Ltk ^– or neonatal rat ventricular cells reveals rapidly activating K⁺ currents. In contrast to the currents in oocytes, however, there are two components of inactivation of the Kv1.4-induced currents in mammalian cells, and both components are significantly slower in myocytes than in either HEK-293 or Ltk ^– cells. In addition, in all three cell types, recovery of Kv1.4 from steady-state inactivation is very slow, proceeding with mean time constants in the range of 6–8 s. The properties of Kv4.2-induced currents also vary with cell type and, importantly, the rates of activation, inactivation and recovery from inactivation are significantly faster in mammalian cells than in Xenopus oocytes. In HEK-293, Chinese hamster ovary (CHO) and neonatal rat ventricular cells, for example, the currents recover from steady-state inactivation with mean (±SD) time constants of 153±32 (n=12), 245±112 (n=10) and 86±38 (n=11) ms, respectively; therefore, recovery proceeds 5–10 times faster than observed for Kv4.2 in oocytes. These results emphasize the importance of the cellular expression environment in efforts to correlate endogenous K⁺ currents with heterologously expressed K⁺ channel subunits. In addition, the finding that Kv α subunits produce distinct K⁺ currents in different cells suggests that cell-type-specific associations with endogenous Kv α or accessory β subunits and/or post-translational processing play roles in determining the properties of functional K⁺ channels. Received: 14 August 1998 / Accepted: 19 October 1998     

Neural and muscular adjustments following repeated running sprints

This study aimed to reveal the neural and muscular adjustments following a repeated-sprint (RS) running exercise. Sixteen subjects performed a series of neuromuscular tests before, immediately after and 30 min (passive recovery) post-RS exercise (12 × 40 m sprints interspaced by 30 s of passive recovery). Sprint times significantly lengthened over repetitions (+17% from the first to the last sprint; P < 0.05). After RS running exercise, maximal voluntary contraction torque of the plantar flexors (−11 ± 7.3%), muscle activation (twitch interpolation) (−2.7 ± 3.4%) and soleus maximal M-wave amplitude (−20 ± 17%) were significantly (P < 0.05) reduced but returned close to baseline after 30 min. Both soleus EMG activity and maximal Hoffmann reflex normalized with respect to M-wave amplitude did not change during the whole experiment. From pre- to post-RS exercise, evoked twitch response was characterized by lower peak torque and maximal rate of torque development (−13 and −11%, respectively, P < 0.05), but was not different from baseline after recovery. Peak tetanus at 20 and 80 Hz were 17 and 8% lower (P < 0.05) in the fatigued state, respectively. Acute muscle fatigue induced by RS running exercise is mainly peripheral as the short-term (30 min) recovery pattern of plantar flexors contractile properties follows that of the voluntary force-generating capacity.     

Electrical characteristics of human ankle dorsi- and plantar-flexor muscles. Comparative responses during fatiguing stimulation and recovery

Changes in muscle excitability were investigated during fatigue and the recovery of human dorsi- and plantar-flexor isometric contractions. The indirectly evoked muscle compound action potentials [tibialis anterior (TA) and soleus (SOL) M-waves] were used as an index of excitability. Ten subjects successfully completed five experiments, spaced at least 1 week apart, in which intermittent tetanic trains at different frequencies of stimulation (0–30 Hz) were used to fatigue the ankle dorsi-flexors. Muscles were rendered ischaemic via a thigh cuff inflated above mean arterial pressure. The effects of ischaemia were examined by repeating the 20-Hz stimulation protocol under non-ischaemic conditions. Five of those subjects also participated in one further session in which the ischaemic plantar-flexors were also fatigued. It was hypothesized that muscle excitability would be preferentially retained in the SOL. Maintenance of excitability in both muscles was possible for 1 min regardless of stimulus frequency; thereafter, stimulation at the highest frequencies induced the greatest decline [30 Hz stimulation; 95.4 (0.5)%, P<0.01) in the amplitude of the M-wave. The decline in M-wave amplitude was always greater than the decline in M-wave area and occurred at firing rates not normally associated with neuromuscular blockade, implying propagation failure along the sarcolemma. The presence of ischaemia significantly accelerated the decline in both amplitude (78% versus 12%, P<0.01) and area (45% versus no decline, P<0.01) of the M-wave. Recovery was limited when tetanic stimulation ceased but progressed rapidly after circulation was restored. Twitch and tetanic torque declines were significantly different between SOL and TA (fall between rest and fatigue – SOL: 77%, 75.2%; TA: 95.5%, 96.9%, P<0.01, respectively). M-wave changes between the two muscles were not significantly different although the onset of the decline was delayed in the SOL. It is proposed that the observed delay in fatiguing decline was due to the early potentiation in muscle excitability observed in the SOL but not in the TA. Electronic Publication     

Potassium kinetics and its relationship with ventilation during repeated bouts of exercise in women

The purpose of this study was to determine the electrolyte concentration changes in arterial plasma from high-intensity repeated bouts of cycling exercise in well-trained females and to determine the relationships between arterial plasma lactate, potassium (K⁺), bicarbonate (HCO₃⁻), and pH with minute ventilation. Fourteen female subjects (mean age = 27 ± 4 years; mean height = 170 ± 7 cm; mean weight = 62 ± 7 kg; maximal oxygen uptake = 50 ± 6 ml/kg/min) were recruited to perform 3 × 5 min bouts of exercise at 236 ± 27 W with 10 min recovery between each set. Minute ventilation, arterial plasma lactate, potassium, calcium, chloride, and sodium ion concentrations were measured a minute 0, 1, 2, 3, 4, 5 of each set and midway through recovery (21 sampling points total per subject). The results showed that the strongest relationship was between arterial plasma K⁺ concentration and minute ventilation (r ² = 0.91), and, that arterial plasma lactate mirrored both arterial plasma HCO₃⁻ and pH. In conclusion, this study demonstrates that women exhibit similar electrolyte responses as reported elsewhere in men, and support the idea that K⁺ may partly contribute to controlling ventilation during high-intensity exercise and recovery.     

Extracellular bicarbonate and non-bicarbonate buffering against lactic acid during and after exercise

Defense of extracellular pH constancy against lactic acidosis can be estimated from changes (Δ) in lactic acid ([La]), [HCO₃⁻], pH and PCO₂ in blood plasma because it is equilibrated with the interstitial fluid. These quantities were measured in earlobe blood during and after incremental bicycle exercise in 13 untrained (UT) and 21 endurance-trained (TR) males to find out if acute and chronic exercise influence the defense. During exercise the capacity of non-bicarbonate buffers (β_nbi = −Δ[La] · ΔpH⁻¹ − Δ[HCO₃⁻] · ΔpH⁻¹) available for the extracellular fluid (mainly hemoglobin, dissolved proteins and phosphates) amounted to 32 ± 2(SEM) and 20 ± 2 mmol l⁻¹ in UT and TR, respectively (P < 0.02). During recovery β_nbi decreased to 14 (UT) and 12 (TR) mmol l⁻¹ (both P < 0.001) corresponding to values previously found at rest by in vivo CO₂ titration. Bicarbonate buffering (β_bi) amounted to 44–48 mmol l⁻¹ during and after exercise. The large exercise β_nbi seems to be mainly caused by an increasing concentration of all buffers due to shrinking of the extracellular volume, exchange of small amounts of HCO₃⁻ or H⁺ with cells and delayed HCO₃⁻ equilibration between plasma and interstitial fluid. Increase of [HCO₃⁻] during titration by these mechanisms augments total β and thus the calculated β_nbi more than β_bi because it reduces ΔpH and Δ[HCO₃⁻] at constant Δ[La]. The smaller rise in exercise β_nbi in TR than UT may be caused by an increased extracellular volume and an improved exchange of La⁻, HCO₃⁻ and H⁺ between trained muscles and blood.     

Contribution of pH,diprotonated phosphate and potassium for the reflex increase in blood pressure during handgrip

The relative importance of pH, diprotonated phosphate (H₂PO₄^?) and potassium (K⁺) for the reflex increase in mean arterial pressure (MAP) during exercise was evaluated in seven subjects during rhythmic handgrip at 15 and 30% maximal voluntary contraction (MVC), followed by post-exercise muscle ischaemia (PEMI). During 15% MVC, MAP rose from 92 ± 1 to 103 ± 2 mmHg, [K⁺] from 4.1 ± 0.1 to 5.1 ± 0.1 mmol L^?1, while the intracellular (7.00 ± 0.01 to 6.80 ± 0.06) and venous pH fell (7.39 ± 0.01 to 7.30 ± 0.01) (P < 0.05). The intracellular [H₂PO₄^?] increased 8.4 ± 2 mmol kg^?1 and the venous [H₂PO₄^?] from 0.14 ± 0.01 to 0.16 ± 0.01 mmol L^?1 (P < 0.05). During PEMI, MAP remained elevated along with the intracellular [H₂PO₄^?] as well as a low intracellular and venous pH. However, venous [K⁺] and [H₂PO₄^?] returned to the level at rest. During 30% MVC handgrip, MAP rose to 130 ± 3 mmHg, [K⁺] to 5.8 ± 0.2 mmol L^?1, the intracellular and extracellular [H₂PO₄^?] by 20 ± 5 mmol kg^?1 and to 0.20 ± 0.02 mmol L^?1, respectively, while the intracellular (6.33 ± 0.06) and venous pH fell (7.23 ± 0.02) (P < 0.05). During post-exercise muscle ischaemia all variables remained close to the exercise levels. Analysis of each variable as a predictor of blood pressure indicated that only the intracellular pH and diprotonated phosphate were linked to the reflex elevation of blood pressure during handgrip.     

Prefrontal cortex oxygenation and neuromuscular responses to exhaustive exercise

Near-infrared spectroscopy (NIRS) allows non-invasive monitoring of central and peripheral changes in oxygenation during exercise and may provide valuable insight into the factors affecting fatigue. This study aimed to explore the changes in oxygenation of prefrontal cortex and active muscle tissue as limiting factors of incremental exercise performance in trained cyclists. Thirteen trained healthy subjects (mean ± SE: age 24.9 ± 1.5 years, body mass 70.1 ± 1.2 kg, training 6.1 ± 0.9 h week⁻¹) performed a progressive maximal exercise to exhaustion on a cycling ergometer. Prefrontal cortex (Cox) and vastus lateralis muscle (Mox) oxygenation were measured simultaneously by NIRS throughout the exercise. Maximal voluntary isometric knee torques and quadriceps neuromuscular fatigue (M-wave properties and voluntary activation ratio) were evaluated before and after exercise. Maximal power output and oxygen consumption were 380.8 ± 7.9 W and 75.0 ± 2.2 ml min⁻¹ kg⁻¹, respectively. Mox decreased significantly throughout exercise while Cox increased in the first minutes of exercise but decreased markedly from the workload corresponding to the second ventilatory threshold up to exhaustion (P < 0.05). No significant difference was noted 6 min after maximal exercise in either the voluntary activation ratio or the M-wave properties. These findings are compatible with the notion that supraspinal modulation of motor output precedes exhaustion. An erratum to this article can be found at     

Environmental heat stress, hyperammonemia and nucleotide metabolism during intermittent exercise

This study investigated the influence of environmental heat stress on ammonia (NH₃) accumulation in relation to nucleotide metabolism and fatigue during intermittent exercise. Eight males performed 40 min of intermittent exercise (15 s at 306±22 W alternating with 15 s of unloaded cycling) followed by five 15 s all-out sprints. Control trials were conducted in a 20°C environment while heat stress trials were performed at an ambient temperature of 40°C. Muscle biopsies and venous blood samples were obtained at rest, after 40 min of exercise and following the maximal sprints. Following exercise with heat stress, the core and muscle temperatures peaked at 39.5±0.2 and 40.2±0.2°C to be ~ 1°C higher (P<0.05) than the corresponding control values. Mean power output during the five maximal sprints was reduced from 618±12 W in control to 558±14 W during the heat stress trial (P<0.05). During the hot trial, plasma NH₃ increased from 31±2 μM at rest to 93±6 at 40 min and 151±15 μM after the maximal sprints to be 34% higher than control (P<0.05). In contrast, plasma K⁺ and muscle H⁺ accumulation were lower (P<0.05) following the maximal sprints with heat stress compared to control, while muscle glycogen, CP, ATP and IMP levels were similar across trials. In conclusion, altered levels of “classical peripheral fatiguing agents” does apparently not explain the reduced capacity for performing repeated sprints following intermittent exercise in the heat, whereas the augmented systemic NH₃ response may be a factor influencing fatigue during exercise with superimposed heat stress.     

Effects of digoxin on muscle reflexes in normal humans

Blockade of the skeletal muscle Na⁺–K⁺-ATPase pump by digoxin could result in a more marked hyperkaliema during a forearm exercise, which in turn could stimulate the mechano- and metaboreceptors. In a randomized, double-blinded, placebo-controlled, and cross-over-design study, we measured mean blood pressure (MBP), heart rate (HR), ventilation (V _E), oxygen saturation (SpO₂), muscle sympathetic nerve activity (MSNA), venous plasma potassium and lactic acid during dynamic handgrip exercises, and local circulatory arrest in 11 healthy subjects. Digoxin enhanced MBP during exercise but not during the post-handgrip ischemia and had no effect on HR, V _E, SpO₂, and MSNA. Venous plasma potassium and lactic acid were also not affected by digoxin-induced skeletal muscle Na⁺–K⁺-ATPase blockade. We conclude that digoxin increased MBP during dynamic exercise in healthy humans, independently of changes in potassium and lactic acid. A modest direct sensitization of the muscle mechanoreceptors is unlikely and other mechanisms, independent of muscle reflexes and related to the inotropic effects of digoxin, might be implicated.     

Unchanged muscle fiber conduction velocity relates to mild acidosis during exhaustive bicycling

Muscle fiber conduction velocity (MFCV) has often been shown to decrease during standardized fatiguing isometric contractions. However, several studies have indicated that the MFCV may remain constant during fatiguing dynamic exercise. It was investigated if these observations can be related to the absence of a large decrease in pH and if MFCV can be considered as a good indicator of acidosis, also during dynamic bicycle exercise. High-density surface electromyography (HDsEMG) was combined with read-outs of muscle energetics recorded by in vivo ³¹P magnetic resonance spectroscopy (MRS). Measurements were performed during serial exhausting bouts of bicycle exercise at three different workloads. The HDsEMG recordings revealed a small and incoherent variation of MFCV during all high-intensity exercise bouts. ³¹P MRS spectra revealed a moderate decrease in pH at the end of exercise (~0.3 units down to 6.8) and a rapid ancillary drop to pH 6.5 during recovery 30 s post-exercise. This additional degree of acidification caused a significant decrease in MFCV during cycling immediately after the rest period. From the data a significant correlation between MFCV and [H⁺] ([H⁺] = 10^−pH) was calculated (p < 0.001, Pearson’s R = −0.87). Our results confirmed the previous observations of MFCV remaining constant during fatiguing dynamic exercise. A constant MFCV is in line with a low degree of acidification, considering the presence of a correlation between pH and MFCV after further increasing acidification.     

Passive properties of neostriatal neurons during potassium conductance blockade

Voltage recordings from neostriatal projection neurons were obtained using in vitro intracellular techniques before and during K⁺-conductance blockade. Neurons were stained with the biocytin technique. Somatic surface area (A _S) was determined by both whole-cell recordings in isolated somata and by measuring stained somata recorded in slices. Dendritic measurements were done in reconstructed neurons. Average determinations of dendritic (A _D) and neuronal (A _N) surface areas coincided with previously reported anatomical data. Thus: A _S≈ 6.5 × 10^–6 cm²; A _D≈ 1.9 × 10^–4 cm²; A _N≈A _D + A _S≈ 2 × 10^–4 cm²; A _D/A _S≈ 30. Measurements were done before and after superfusion with K⁺-conductance blockers (K⁺-blockers). Cells whose neuronal morphology was not obviously distorted by K⁺-blockade were chosen for the present study. Electrotonic transients were matched to a somatic shunt equivalent cylinder model adjusted with the generalized correction factor (F _dga) that constrains the parameters for neuronal anatomy. Neuronal input resistance (R _N; mean ± SEM) increased when it was corrected for somatic shunt, from 49 ± 2 MΩ (n = 80) to 179 ± 7 MΩ (n = 32). A difference was also obtained between the slowest time constant, τ₀ = 16 ± 0.9 ms (n = 49), and the dendritic membrane time constant, τ_mD = 33 ± 1.6 ms (n = 36). When these electrophysiological measurements were used to calculate A _N, the value obtained was similar to the anatomical measurements. Combining anatomical and electrophysiological data, somatic and dendritic input resistances were determined: R _D = 182 ± 7 MΩ; R _S (with shunt) = 74 ± 4 MΩ (n = 32). The generalized correction factor, F _dga = 0.91 ± 0.007 (n = 10), implied a short effective electrotonic length for dendrites: L _D = 0.46 ± 0.014 (n = 32). Saturating concentrations of the K⁺-blockers tetraethylammonium, Cs⁺, and Ba²⁺ increased R _N and induced charging curves well fitted by single exponential functions in 56% of neostriatal neurons. Ba²⁺ greatly decreased the somatic shunt (n = 5): (R _N = 216 ± 21 MΩ, τ₀ = 46 ± 2 ms, R _D = 239 ± 25 MΩ, and R _S = 3.2 ± 0.5 GΩ), rendering values similar to those obtained with whole-cell recordings (e.g., R _N≈ 198 MΩ, R_S≈ 2.62 GΩ) (n = 52). Cs⁺ (n = 5) had less effect on the somatic shunt (R _N = 115 ± 19 MΩ, τ₀ = 49 ± 13 ms, R _S = 161 ± 8 MΩ), although dendritic conductance was equally blocked (R _D = 261 ± 16 MΩ). The Cs⁺-sensitive conductance exhibited inward rectifying properties not displayed by the Ba²⁺-sensitive conductance, suggesting that Cs⁺ preferentially acted upon inward rectifier conductances. In contrast, Ba²⁺ significantly acted upon linear conductances making up the somatic shunt. This suggests a differential action of different K⁺-blockers on the somato-dendritic membrane, implying a differential distribution of membrane conductances. Another action of K⁺-blockers, in about 40% of the cells, was to induce dye and probably electrical coupling between neighboring neurons. Received: 30 April 1997 / Accepted: 14 October 1997     

Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pH_i) changers such as sodium acetate (SA) and NH₄Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH₄Cl. Intracellular Ca²⁺ concentration ([Ca²⁺]_i) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R _340/380). SA (20 mM) increased and NH₄Cl (20 mM) decreased R _340/380 by 0.2 ± 0.03 and 0.1 ± 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R _340/380 by 0.9 ± 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH₄Cl, implying that SA and NH₄Cl may affect [Ca²⁺]_i (p < 0.05). Accordingly, we studied the effects of SA and NH₄Cl on Ca²⁺-activated K⁺ current (IK_Ca) under patch clamp technique. Caffeine produced transient outward current at holding potential (V _h) of 0 mV, caffeine induced transient outward K⁺ current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH₄Cl. In addition, IK_Ca was significantly increased by acidotic condition when pH_i was lowered by altering the NH₄Cl gradient across the cell membrane. Finally, the effects of SA and NH₄Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was −28 ± 2.3 mV in a single cell level and was depolarized by 13 ± 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH₄Cl depolarized RMP by 10 ± 1.9 and 16 ± 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH₄Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca²⁺]_i and Ca²⁺-activated K channels in rabbit basilar artery.     

Differential changes in long-interval intracortical inhibition and silent period duration during fatiguing hand exercise

During fatiguing exercise corticomotor excitability increases as force declines, which may serve to increase motor output to the exercising muscle, but paradoxically at the same time there is an increase in silent period (SP) duration which is thought to represent a build-up of intracortical inhibition. Paired-pulse TMS at long interstimulus intervals can also be used to derive an index of long-interval cortical inhibition (LICI), however this has not yet been investigated in fatigue. Our aim was to measure LICI during and after a fatiguing exercise and determine if the changes in the index of LICI parallel the changes in SP duration. To do this, we used single and paired-pulse TMS to measure motor evoked potential (MEP) amplitude, LICI and SP duration during, and for 10 min after, a 10-min intermittent maximal fatiguing exercise of the index finger, designed to fatigue the first dorsal interosseous (FDI) muscle (force after 10-min of exercise 64 ± 7% of baseline, P < 0.05). Single-pulse MEP amplitude and SP duration were increased during fatiguing exercise (minute 10; 179 ± 24% and 128 ± 9% of baseline, respectively, P < 0.05), in contrast the measure of LICI was reduced compared to baseline (minute 10; 0.45 ± 0.17 vs. baseline; 0.70 ± 0.10, P < 0.05). These results suggest that SP duration and LICI may reflect processes occurring in different neuronal populations. The increased SP duration may correspond to processes of central fatigue in centres ‘upstream’ of primary motor cortex (M1), whereas the decrease in LICI, together with increased MEP amplitude, are consistent with an increase in M1 output during fatigue that may serve to compensate for reduced central drive.     

Contractions of the filariidAcanthocheilonema viteae induced by potassium chloride

The effects of K⁺ on muscle contractility were explored in the filarial nematodeAcanthocheilonema viteae (Dipetalonema viteae). The parasite was slit open longitudinally and mounted in a smooth muscle chamber that was filled with aerated (95% N₂–5% CO₂) physiological solution at 37°C. KCl at concentrations ranging from 20 to 100 mM induced a rapid isotonic contraction of the filarial muscle. The maximal response from KCl was similar to the maximal response to acetylcholine chloride (ACh). When KCl was applied for several minutes, tolerance frequently occurred. Contractions were also induced by K₂SO₄ but not by NaCl, Na₂SO₄ or sucrose. Nifedipine was more than 10 times as potent in reducing the KCl-induced contraction as in reducing that caused by ACh. The KCl-induced contraction was abolished in a Ca-free physiological medium containing ethyleneglycol-bis-(-aminoethyl ether)N,N,N,N-tetraacetic acid (EGTA, 10^–4 M). Low [Ca²⁺]/[Mg²⁺] solutions blocked the spontaneous activity, the KCl-induced contractions, and the ACh-induced contractions. KCl also induced contractions in denervated muscle strips, supporting the hypothesis that K⁺ acts directly on the muscle cells. These results indicate that K⁺ can depolarize the muscle membrane and induce a muscle contraction that is dependent on extracellular calcium ions.