Energy turnover in relation to slowing of contractile properties during fatiguing contractions of the human anterior tibialis muscle

Slowing and loss of muscle power are major factors limiting physical performance but little is known about the molecular mechanisms involved. The slowing might be a consequence of slow detachment of cross bridges and, if this were the case, then a reduction in the ATP cost of an isometric contraction would be expected as the muscle fatigued. The human anterior tibialis muscle was stimulated repeatedly under ischaemic conditions at 50 Hz for 1.6 s with a 50% duty cycle and muscle metabolites measured by ³¹P magnetic resonance spectroscopy. Over the course of 20 contractions the half-time of relaxation increased from 36.5 ± 0.09 ms (mean ± s.e.m. ) to 113 ± 17 ms and isometric force was reduced to 63 ± 3% of the initial value. ATP turnover was determined from the change in high energy phosphates and lactate production, the latter estimated from the change of intracellular pH. ATP turnover over the first three contractions was 2.45 ± 0.09 m m s⁻¹ and decreased to 1.8 ± 0.06 m m s⁻¹ over the last five tetani. However, when this latter value was normalised for the decrease in isometric force, it became 2.56 ± 0.3 m m s⁻¹, which is the same as the turnover of the fresh muscle. The data suggest that the rate of cross bridge detachment is unaffected by fatigue and are consistent the suggestion that it is the rate of attachment which is slowed rather than the rate of detachment. The present results focus attention on stages in the cross bridge cycle concerned with attachment and the transition from low to high force states that may be influenced by metabolic changes in the fatiguing muscle.     

Cardiac and vasomotor components of the carotid baroreflex control of arterial blood pressure during isometric exercise in humans

We sought to examine the importance of the cardiac component of the carotid baroreflex (CBR) in control of blood pressure during isometric exercise. Nine subjects performed 4 min of ischaemic isometric calf exercise at 20% of maximum voluntary contraction. Trials were repeated with β₁-adrenergic blockade (metoprolol, 0.15 ± 0.003 mg kg⁻¹) or parasympathetic blockade (glycopyrrolate, 13.6 ± 1.5 μg kg⁻¹). CBR function was determined using rapid pulses of neck pressure and neck suction from +40 to −80 mmHg, while heart rate (HR), mean arterial pressure (MAP) and changes in stroke volume (SV, Modelflow method) were measured. Metoprolol decreased and glycopyrrolate increased HR and cardiac output both at rest and during exercise ( P < 0.05), while resting and exercising blood pressure were unchanged. Glycopyrrolate reduced the maximal gain ( G _max) of the CBR-HR function curve (−0.58 ± 0.10 to −0.06 ± 0.01 beats min⁻¹ mmHg⁻¹, P < 0.05), but had no effect on the G _max of the CBR-MAP function curve. During isometric exercise the CBR-HR curve was shifted upward and rightward in the metoprolol and no drug conditions, while the control of HR was significantly attenuated with glycopyrrolate ( P < 0.05). Regardless of drug administration isometric exercise produced an upward and rightward resetting of the CBR control of MAP with no change in G _max. Thus, despite marked reductions in CBR control of HR following parasympathetic blockade, CBR control of blood pressure was well maintained. These data suggest that alterations in vasomotor tone are the primary mechanism by which the CBR modulates blood pressure during low intensity isometric exercise.     

Neurokinin-1 receptor desensitization to consecutive microdialysis infusions of substance P in human skin

Unloaded shortening velocity ( V ₀) of human triceps surae muscle was measured in vivo by applying the 'slack test', originally developed for determining V ₀ of single muscle fibres, to voluntary contractions at varied activation levels (ALs). V ₀ was measured from 10 subjects at five different ALs defined as a fraction (5, 10, 20, 40 and 60%) of the maximum voluntary contraction (MVC) torque. Although individual variability was apparent, V ₀ tended to increase with AL  ( R ²= 0.089; P = 0.035)  up to 60%MVC (8.6 ± 2.6 rad s⁻¹). This value of V ₀ at 60%MVC was comparable to the maximum shortening velocity of plantar flexors reported in the previous studies. Electromyographic analysis showed that the activities of soleus, medial gastrocnemius and lateral gastrocnemius muscles increased with AL during isometric contraction and after the application of quick release in a similar manner. Also, it showed that the activity of an antagonist, tibialis anterior muscle, was negligible, even though a slight increase took place after the quick release of agonist. Correlation analysis showed that there were no significant correlations between V ₀ and MVC torque normalized with respect to body mass, although the correlation coefficient was relatively high at low ALs. The results suggest that in human muscle, V ₀ represents the unloaded velocity of the fastest muscle fibres recruited, and increases with AL possibly because of progressive recruitment of faster fibres. Individual variability may be explained, at least partially, by the difference in fibre-type composition.     

The sources and sequestration of Ca2+ contributing to neuroeffector Ca2+ transients in the mouse vas deferens

The detection of focal Ca²⁺ transients (called neuroeffector Ca²⁺ transients, or NCTs) in smooth muscle of the mouse isolated vas deferens has been used to detect the packeted release of ATP from nerve terminal varicosities acting at postjunctional P2X receptors. The present study investigates the sources and sequestration of Ca²⁺ in NCTs. Smooth muscle cells in whole mouse deferens were loaded with the Ca²⁺ indicator Oregon Green 488 BAPTA-1 AM and viewed with a confocal microscope. Ryanodine (10 µ m ) decreased the amplitude of NCTs by 45 ± 6 %. Cyclopiazonic acid slowed the recovery of NCTs (from a time course of 200 ± 10 ms to 800 ± 100 ms). Caffeine (3 m m ) induced spontaneous focal smooth muscle Ca²⁺ transients (sparks). Neither of the T-type Ca²⁺ channel blockers NiCl₂ (50 µ m ) or mibefradil dihydrochloride (10 µ m ) affected the amplitude of excitatory junction potentials (2 ± 5 % and −3 ± 10 %) or NCTs (−20 ± 36 % and 3 ± 13 %). In about 20 % of cells, NCTs were associated with a local, subcellular twitch that remained in the presence of the α₁-adrenoceptor antagonist prazosin (100 n m ), showing that NCTs can initiate local contractions. Slow (5.8 ± 0.4 µm s⁻¹), spontaneous smooth muscle Ca²⁺ waves were occasionally observed. Thus, Ca²⁺ stores initially amplify and then sequester the Ca²⁺ that enters through P2X receptors and there is no amplification by local voltage-gated Ca²⁺ channels.     

Effects of old age on human skeletal muscle energetics during fatiguing contractions with and without blood flow

We recently reported lower glycolytic flux (ATP_GLY) and increased reliance on oxidative ATP synthesis (ATP_OX) in contracting muscle of older compared to young humans. To further investigate this age-related difference in the pathways of ATP synthesis, we used magnetic resonance spectroscopy to determine the rates of ATP_OX, ATP_GLY and net phosphocreatine hydrolysis in vivo during maximal muscle contractions under free-flow (FF) and ischaemic (ISC) conditions in the ankle dorsiflexors of 20 young (27 ± 3 years; 10 male, 10 female) and 18 older (70 ± 5 years; 10 male, 8 female) adults. We hypothesized that ATP_GLY would be higher in young compared to old during FF contractions, but that old would be unable to increase ATP_GLY during ISC to match that of the young, which would suggest impaired glycolytic ATP synthesis with old age. Peak glycolytic flux during FF was lower in older (0.8 ± 0.1 m m ATP s⁻¹) compared to young (1.4 ± 0.1 m m ATP s⁻¹, P < 0.001) subjects. During ISC, peak ATP_GLY increased in old to a level similar to that of young (1.4 ± 0.2 m m ATP s⁻¹, 1.3 ± 0.2 m m ATP s⁻¹, respectively; P = 0.86), suggesting that glycolytic function remains intact in aged muscle in vivo . Notably, older adults fatigued less than young during both FF and ISC ( P ≤ 0.004). These results provide novel evidence of unimpaired in vivo glycolytic function in the skeletal muscle of older adults during maximal isometric dorsiflexion, and suggest a potential role for differences in metabolic economy and as a result, metabolite accumulation, in the fatigue resistance of the old.     

Baroreflex-induced sympathetic activation does not alter cerebrovascular CO2 responsiveness in humans

We investigated the effect of baroreflex-induced sympathetic activation, produced by lower body negative pressure (LBNP) at −40 mmHg, on cerebrovascular responsiveness to hyper- and hypocapnia in healthy humans. Transcranial Doppler ultrasound was used to measure blood flow velocity (CFV) in the middle cerebral artery during variations in end-tidal carbon dioxide pressure ( P _ET,CO2) of +10, +5, 0, −5, and −10 mmHg relative to eupnoea. The slopes of the linear relationships between P _ET,CO2 and CFV were computed separately for hyper- and hypocapnia during the LBNP and no-LBNP conditions. LBNP decreased pulse pressure, but did not change mean arterial pressure. LBNP evoked an increase in ventilation that resulted in a 9 ± 2 mmHg decrease in P _ET,CO2, which was corrected by CO₂ supplementation of the inspired air. LBNP did not affect cerebrovascular CO₂ response slopes during steady-state hypercapnia (3.14 ± 0.24 vs. 2.96 ± 0.26 cm s⁻¹ mmHg⁻¹) or hypocapnia (1.31 ± 0.18 vs. 1.32 ± 0.19 cm s⁻¹ mmHg⁻¹), or the CFV responses to voluntary apnoea (+51 ± 19 vs. +50 ± 18 %). Thus, cerebrovascular CO₂ responsiveness was not altered by baroreflex-induced sympathetic activation. Our data challenge the concept that sympathetic activation restrains cerebrovascular responses to alterations in CO₂ pressure.     

A-Type potassium currents active at subthreshold potentials in mouse cerebellar purkinje cells

Voltage-dependent and calcium-independent K⁺ currents were whole-cell recorded from cerebellar Purkinje cells in slices. Tetraethylammonium (TEA, 4 m m ) application isolated an A-type K⁺ current ( I _{k ( a )}) with a peak amplitude, at +20 mV, of about one third of the total voltage-dependent and calcium-independent K⁺ current. The I _{k ( a )} activated at about −60 mV, had a V _0.5 of activation of −24.9 mV and a V _0.5 of inactivation of −69.2 mV. The deactivation time constant at −70 mV was 3.4 ± 0.4 ms, while the activation time constant at +20 mV was 0.9 ± 0.2 ms. The inactivation kinetics was weakly voltage dependent, with two time constants; those at +20 mV were 19.3 ± 3.1 and 97.6 ± 9.8 ms. The recovery from inactivation had two time constants of 60.8 ms (78.4%) and 962.3 ms (21.6%). The I _{k ( a )} was blocked by 4-aminopyridine with an IC₅₀ of 67.6 μM. Agitoxin-2 (2 n m ) blocked 17.4 ± 2.1% of the I _{k ( a )}. Flecainide completely blocked the I _{k ( a )} with a biphasic effect with IC₅₀ values of 4.4 and 183.2 μM. In current-clamp recordings the duration of evoked action potentials was affected neither by agitoxin-2 (2 n m ) nor by flecainide (3 μM), but action potentials that were already broadened by TEA were further prolonged by 4-aminopyridine (100 μM). The amplitude of the hyperpolarisation at the end of depolarising steps was reduced by all these blockers.     

Effects of a R133W β-tropomyosin mutation on regulation of muscle contraction in single human muscle fibres

A novel R133W β-tropomyosin (β-Tm) mutation, associated with muscle weakness and distal limb deformities, has recently been identified in a woman and her daughter. The muscle weakness was not accompanied by progressive muscle wasting or histopathological abnormalities in tibialis anterior muscle biopsy specimens. The aim of the present study was to explore the mechanisms underlying the impaired muscle function in patients with the β-Tm mutation. Maximum force normalized to fibre cross-sectional area (specific force, SF), maximum velocity of unloaded shortening ( V ₀), apparent rate constant of force redevelopment ( k _tr) and force–pCa relationship were evaluated in single chemically skinned muscle fibres from the two patients carrying the β-Tm mutation and from healthy control subjects. Significant differences in regulation of muscle contraction were observed in the type I fibres: a lower SF ( P < 0.05) and k _tr ( P < 0.01), and a faster V ₀ ( P < 0.05). The force–pCa relationship did not differ between patient and control fibres, indicating an unaltered Ca²⁺ activation of contractile proteins. Collectively, these results indicate a slower cross-bridge attachment rate and a faster detachment rate caused by the R133W β-Tm mutation. It is suggested that the R133W β-Tm mutation induces alteration in myosin–actin kinetics causing a reduced number of myosin molecules in the strong actin-binding state, resulting in overall muscle weakness in the absence of muscle wasting.     

Rapid Report

Sympathetic vasoconstriction is blunted in the vascular beds of contracting skeletal muscles. We sought to determine whether this blunted vasoconstriction is specific for post-junctional α₁- or α₂-adrenergic receptors. We measured forearm blood flow (Doppler ultrasound) and calculated the vascular conductance (FVC) responses to brachial artery infusions of tyramine (which evokes endogenous noradrenaline release), phenylephrine (an α₁ agonist) and clonidine (an α₂ agonist) in 10 healthy men during rhythmic handgrip exercise (10-15 % of maximum) and during a control non-exercise vasodilator condition (intra-arterial adenosine). Steady-state FVC during exercise and adenosine was similar in all trials (range: 243-272 and 234-263 ml min⁻¹ (100 mmHg)⁻¹, respectively; P > 0.5). During exercise the percentage reductions in FVC in response to tyramine (−24 ± 7 vs. −55 ± 6 %), phenylephrine (−12 ± 8 vs. −37 ± 8 %) and clonidine (−17 ± 6 vs. −49 ± 4 %) were significantly less compared with adenosine (all P < 0.05). The magnitude of the blunted vasoconstrictor responses was similar for both receptor subtypes. These findings are in contrast to those from studies in animals demonstrating that α₂-adrenergic receptor-mediated vasoconstrictor responses are much more sensitive to contraction-induced inhibition than α₁-mediated responses. We conclude that vasoconstrictor responses mediated via both post-junctional α₁- and α₂-adrenergic receptors are blunted in contracting human skeletal muscles.     

Arm Blood Flow and Oxygenation on the Transition from Arm to Combined Arm and Leg Exercise in Humans

The cardiovascular response to exercise with several groups of skeletal muscle implies that work with the legs may reduce arm blood flow. This study followed arm blood flow ( _arm) and oxygenation on the transition from arm cranking (A) to combined arm and leg exercise (A+L). Seven healthy male subjects performed A at ∼80 % of maximum work rate ( W _max) and A at ∼80 % W _max combined with L at ∼60 % W _max. A transition trial to volitional exhaustion was performed where L was added after 2 min of A. The _arm was determined by constant infusion thermodilution in the axillary vein and changes in biceps muscle oxygenation were measured with near-infrared spectroscopy. During A+L _arm was lowered by 0.38 ± 0.06 l min⁻¹ (10.4 ± 3.3 %,   P < 0.05  ) from 2.96 ± 1.54 l min⁻¹ during A. Total (HbT) and oxygenated haemoglobin (HbO₂) concentrations were also lower. During the transition from A to A+L _arm decreased by 0.22 ± 0.03 l min⁻¹ (7.9 ± 1.8 %,   P < 0.05  ) within 9.6 ± 0.2 s, while HbT and HbO₂ decreased similarly within 30 ± 2 s. At the same time mean arterial pressure and arm vascular conductance also decreased. The data demonstrate reduction in blood flow to active skeletal muscle during maximal whole body exercise to a degree that arm oxygen uptake and muscle tissue oxygenation are compromised.     

Left ventricular torsion and untwisting during exercise in heart transplant recipients

Left ventricular (LV) rotation is the dominant deformation during relaxation and links systole with early diastolic recoil. LV torsion and untwisting rates during submaximal exercise were compared between heart transplant recipients (HTRs), young adults and healthy older individuals to better understand impaired diastolic function in HTRs. Two dimensional and colour M-mode echocardiography with speckle-tracking analysis were completed in eight HTRs (age: 61 ± 9 years), six recipient age-matched (RM, age: 60 ± 11 years), and five donor age-matched (DM, age: 35 ± 8 years) individuals (all males) at rest and during submaximal cycle exercise. LV peak torsion, peak rate of untwisting and peak intraventricular pressure gradients (IVPGs) were examined. LV torsion increased with exercise in DMs (6.5 ± 5.6 deg, P < 0.05), but not in RMs (−2.6 ± 7.0 deg) or HTRs (−0.9 ± 4.4 deg). The change from rest to exercise in the peak rate of untwisting was significantly greater for DMs (−2.1 ± 0.5 rads s⁻¹, P < 0.05) compared to RMs (−0.7 ± 1.3 rads s⁻¹) and HTRs (−0.2 ± 0.9 rads s⁻¹). The amount of untwisting occurring prior to mitral valve opening substantially declined with exercise in RMs and HTRs only. The change in IVPGs was 1.3-fold greater in DMs versus HTRs or RMs ( P > 0.05). Peak LV torsion and untwisting are blunted during exercise in HTRs and RMs compared to DMs. These factors may contribute to the impaired diastolic filling found in HTRs during exercise. Similarities between HTRs and RMs during exercise suggest functional accelerated ageing of the cardiac allograft.     

Ether-à-go-go-related gene K+ channels contribute to threshold excitability of mouse auditory brainstem neurons

The ionic basis of excitability requires identification and characterisation of expressed channels and their specific roles in native neurons. We have exploited principal neurons of the medial nucleus of the trapezoid body (MNTB) as a model system for examining voltage-gated K⁺ channels, because of their known function and simple morphology. Here we show that channels of the ether-à-go-go -related gene family (ERG, Kv11; encoded by kcnh ) complement Kv1 channels in regulating neuronal excitability around threshold voltages. Using whole-cell patch clamp from brainstem slices, the selective ERG antagonist E-4031 reduced action potential (AP) threshold and increased firing on depolarisation. In P12 mice, under voltage-clamp with elevated [K⁺]_o (20 m m ), a slowly deactivating current was blocked by E-4031 or terfenadine ( V _0.5,act=−58.4 ± 0.9 mV, V _0.5,inact=−76.1 ± 3.6 mV). Deactivation followed a double exponential time course (τ_slow= 113.8 ± 6.9 ms, τ_fast= 33.2 ± 3.8 ms at −110 mV, τ_fast 46% peak amplitude). In P25 mice, deactivation was best fitted by a single exponential (τ_fast= 46.8 ± 5.8 ms at −110 mV). Quantitative RT-PCR showed that ERG1 and ERG3 were the predominant mRNAs and immunohistochemistry showed expression as somatic plasma membrane puncta on principal neurons. We conclude that ERG currents complement Kv1 currents in limiting AP firing at around threshold; ERG may have a particular role during periods of high activity when [K⁺]_o is elevated. These ERG currents suggest a potential link between auditory hyperexcitability and acoustic startle triggering of cardiac events in familial LQT2.     

Heterologous expression of wild-type and mutant β-cardiac myosin changes the contractile kinetics of cultured mouse myotubes

The properties of myosin expressed in muscle are a major determinant of muscle performance. In this study we used a novel approach to examine the functional impact of changes in myosin heavy chain (MHC) isoform expression, as well as the consequences of expressing the mutant MHC implicated in familial hypertrophic cardiomyopathy (FHC). Cultured mouse myoblasts that normally express fast embryonic myosin were untransfected, or stably transfected with a plasmid expressing either wild-type (cWT) or mutant (D778G or G741R) β-cardiac myosin. After differentiation for 5–7 days, cWT or mutant β-cardiac myosin was expressed at 25 % of total myosin in the myotube. We measured time-to-peak shortening (ttp), time for half-relaxation ( t _0.5), the maximum velocity of shortening ( V _max) at 1 Hz stimulation, and the tetanic fusion frequency. Expression of cWT β-cardiac myosin significantly increased ttp and t _0.5 and decreased the fusion frequency compared with untransfected myotubes. However, when we compared myotubes expressing mutant β-cardiac myosin with those expressing cWT β-cardiac myosin, we found that ttp and t _0.5 were significantly decreased, and V _max was increased for the D778G mutant, whereas ttp, t _0.5 and V _max were unchanged for the G741R mutant. The fusion frequency was increased for both mutant myosins. Our data support the conclusion that the impact of the slower myosin isoform dominates when both slow and fast isoforms are present. This work suggests that FHC associated with either D778G or G741R mutation in MHC is an 'energy cost' disease, but that the phenotype of D778G is more severe than that of G741R.     

Effects of Thyroxine on Myosin Isoform Expression and Mechanical Properties in Guinea-Pig Smooth Muscle

Information on the effects of thyroid hormone on smooth muscle contractile protein expression and mechanical properties is sparse. We have addressed the following questions. (1) Can thyroxine hormone alter myosin isoform composition in smooth muscle? (2) Can a change in myosin isoform composition lead to altered mechanical properties in smooth muscle? (3) Are alterations, if occurring, equal in fast and slow smooth muscle types? Guinea-pigs were treated with thyroxine (T₄) for 12 days. Control animals were given physiological saline solution. Maximal unloaded shortening velocity ( V _max) was measured in chemically skinned, maximally activated muscle preparations from the aorta and the taenia coli. V _max increased following thyroxine treatment, by approximately 20 % in the taenia coli. In the aorta, no significant increase in V _max could be detected. The sensitivity of isometric force to inorganic phosphate (P_i) was increased in the taenia coli following thyroxine treatment. The expression of mRNA (determined with RT-PCR) for the myosin heavy chain with the seven amino acid insert increased by approximately 70 % in the aorta and about 25 % in the taenia coli following thyroxine treatment. Western blot analysis showed an increase in the inserted myosin heavy chain form in the taenia coli. Expression of mRNA for the myosin essential light chains and the corresponding proteins did not change significantly in either muscle type. No alterations in non-muscle myosin heavy chain isoforms could be detected after thyroxine treatment. In conclusion, thyroxine treatment alters the isoform composition of myosin in fast and slow smooth muscles in vivo . This change is sufficient to increase shortening velocity and sensitivity of isometric force to P_i in the fast, but not in the slow, smooth muscle type.     

Reduced ventricular flow propagation velocity in elite athletes is augmented with the resumption of exercise training

Chronic exercise induces physiological enlargement of the left ventricle ('athlete's heart'), but the effects of current and long-term exercise training on diastolic function have not been investigated. Echocardiography and Doppler imaging were used to assess left ventricular (LV) dimensions and indices of diastolic filling in 22 elite athletes at the end of their 'off-season' (baseline) and, subsequently, following 3 and 6 months of training. Twelve matched controls were also studied at baseline, 3 and 6 months. Compared to controls at baseline, athletes exhibited significantly higher LV mass (235.7 ± 7.1 g versus 178.1 ± 14.5 g, P < 0.01) and reduced flow propagation velocity ( V _P: 50.21 ± 1.7 versus 72.2 ± 3.6 cm s⁻¹, P < 0.01), a measure of diastolic function. Three months of training further increased LV mass in athletes (253.2 ± 7.1 g; P < 0.01 versus baseline), and significantly increased their V _P (66.7 ± 2.5 cm s⁻¹; P < 0.05 versus baseline). These trends for increased mass and diastolic filling persisted following 6 months of training (LV mass 249.0 ± 8.7 g P < 0.05 versus baseline; V _P 75.7 ± 3.0 cm s⁻¹; P < 0.01 versus baseline, and P = 0.01 versus 3 months). This study suggests that following a period of relative inactivity the rate of ventricular relaxation during early diastole may be slowed in athletes who exhibit ventricular hypertrophy, whilst resumption of training increases the speed of ventricular relaxation in the presence of further hypertrophy of the left ventricle.     

In vivo ATP production during free-flow and ischaemic muscle contractions in humans

The aim of this study was to determine how ATP synthesis and contractility in vivo are altered by ischaemia in working human skeletal muscle. The hypotheses were: (1) glycolytic flux would be higher during ischaemic (ISC) compared to free-flow (FF) muscle contractions, in compensation for reduced oxidative ATP synthesis, and (2) ischaemic muscle fatigue would be related to the accumulation of inhibitory metabolic by-products rather than to the phosphorylation potential ([ATP]/[ADP][P_i]) of the muscle. Twelve healthy adults (6 men, 6 women) performed six intermittent maximal isometric contractions of the ankle dorsiflexors (12 s contract, 12 s relax), once with intact blood flow and once with local ischaemia by thigh cuff inflation to 220 Torr. Intracellular phosphorous metabolites and pH were measured non-invasively with magnetic resonance spectroscopy, and rates of ATP synthesis through oxidative phosphorylation, anaerobic glycolysis, and the creatine kinase reaction were determined. The force–time integral declined more during ISC (66 ± 3% initial) than FF (75 ± 2% initial, P = 0.002), indicating greater fatigue in ISC. [ATP] was preserved in both protocols, indicating matching of ATP production and use under both conditions. Glycolytic flux (m m s⁻¹) was similar during FF and ISC ( P = 0.16). Total ATP synthesis rate was lower during ISC, despite adjustment for the greater muscle fatigue in this condition ( P < 0.001). Fatigue was linearly associated with diprotonated inorganic phosphate (FF r = 0.94 ± 0.01, ISC r = 0.92 ± 0.02), but not phosphorylation potential. These data provide novel evidence that ATP supply and demand in vivo are balanced in human skeletal muscle during ischaemic work, not through higher glycolytic flux, but rather through increased metabolic economy and decreased rates of ATP consumption as fatigue ensues.     

Non-selective β-adrenergic blockade prevents reduction of the cerebral metabolic ratio during exhaustive exercise in humans

Intense exercise decreases the cerebral metabolic ratio of oxygen to carbohydrates [O₂/(glucose +½lactate)], but whether this ratio is influenced by adrenergic stimulation is not known. In eight males, incremental cycle ergometry increased arterial lactate to 15.3 ± 4.2 m m (mean ± s.d. ) and the arterial–jugular venous (a–v) difference from −0.02 ± 0.03 m m at rest to 1.0 ± 0.5 m m ( P < 0.05). The a–v difference for glucose increased from 0.7 ± 0.3 to 0.9 ± 0.1 m m ( P < 0.05) at exhaustion and the cerebral metabolic ratio decreased from 5.5 ± 1.4 to 3.0 ± 0.3 ( P < 0.01). Administration of a non-selective β-adrenergic (β₁+β₂) receptor antagonist (propranolol) reduced heart rate (69 ± 8 to 58 ± 6 beats min⁻¹) and exercise capacity (239 ± 42 to 209 ± 31 W; P < 0.05) with arterial lactate reaching 9.4 ± 3.6 m m . During exercise with propranolol, the increase in a–v lactate difference (to 0.5 ± 0.5 m m ; P < 0.05) was attenuated and the a–v glucose difference and the cerebral metabolic ratio remained at levels similar to those at rest. Together with the previous finding that the cerebral metabolic ratio is unaffected during exercise with administration of the β₁-receptor antagonist metropolol, the present results suggest that the cerebral metabolic ratio decreases in response to a β₂-receptor mechanism.     

Myofilament-based relaxant effect of isoprenaline revealed during work-loop contractions in rat cardiac trabeculae

In cardiac muscle, β-adrenergic stimulation increases contractile force and accelerates relaxation. The relaxant effect is thought to be due primarily to stimulation of Ca²⁺ uptake into the sarcoplasmic reticulum (SR), although changes in myofilament properties may also contribute. The present study investigated the contribution of the myofilaments to the β-adrenergic response in isolated rat cardiac trabeculae undergoing either isometric or work-loop contractions (involving simultaneous force generation and shortening) at different stimulation frequencies (range 0.25-4.5 Hz). SR-dependent effects were eliminated by treatment with ryanodine (1 μM) and cyclopiazonic acid (30 μM). In isometric contractions during SR inhibition, isoprenaline increased the force but did not alter the time course of the twitch. In contrast, in work-loop contractions, the positive inotropic effect was accompanied by a reduced diastolic force between beats, most apparent at higher frequencies (e.g. diastolic stress fell from 58.6 ± 5.5 to 28.8 ± 5.8 mN mm⁻² at 1.5 Hz). This relaxant effect contributed to a β-adrenoceptor-mediated increase in net work and power output at higher frequencies, by reducing the amount of work required to re-lengthen the muscle. Consequently, the frequency for maximum power output increased from 1.1 ± 0.1 to 1.6 ± 0.1 Hz. We conclude that the contribution of myofilament properties to the relaxant effect of β-stimulation may be of greater significance when force and length are changing simultaneously (as occurs in the heart) than during force development under isometric conditions.     

Force enhancement following muscle stretch of electrically stimulated and voluntarily activated human adductor pollicis

For electrically stimulated muscles, it has been observed that maximal muscle force during and after stretch is substantially greater than the corresponding isometric force. However, this observation has not been made for human voluntary contractions. We investigated the effects of active muscle stretch on muscle force production for in vivo human adductor pollicis ( n = 12) during maximal voluntary contractions and electrically induced contractions. Peak forces during stretch, steady-state isometric forces following stretch, and passive forces following muscle deactivation were compared to the corresponding isometric forces obtained at optimal muscle length. Contractions with different stretch magnitudes (10, 20, and 30 deg at a constant speed of 10 deg s⁻¹) and different speeds (10, 20, and 60 deg s⁻¹ over a range of 30 deg) were performed in triplicate in a random order, balanced design. We found three novel results: (i) there was steady-state force enhancement following stretch in voluntarily contracted muscles; (ii) some force enhancement persisted following relaxation of the muscle and (iii) force enhancement, for some stretch conditions, exceeded the maximum isometric force at optimal muscle length. We conclude from these results that voluntary muscle contraction produces similar force enhancement to that observed in the past with electrically stimulated preparations. Therefore, steady-state force enhancement may play a role in everyday movements. Furthermore, these results suggest that non-uniformities in sarcomere length do not, at least not exclusively, account for the force enhancement following active muscle stretch, and that the stretch magnitude-dependent passive force enhancement observed here may be responsible for the enhancement of force above the isometric reference force at optimal muscle length.