Tension-time Integral Research Articles

Emerging Concepts of Mechanisms Controlling Cardiac Tension: Focus on Familial Dilated Cardiomyopathy (DCM) and Sarcomere-Directed Therapies.

Novel therapies for the treatment of familial dilated cardiomyopathy (DCM) are lacking. Shaping research directions to clinical needs is critical. Triggers for the progression of the disorder commonly occur due to specific gene variants that affect the production of sarcomeric/cytoskeletal proteins. Generally, these variants cause a decrease in tension by the myofilaments, resulting in signaling abnormalities within the micro-environment, which over time result in structural and functional maladaptations, leading to heart failure (HF). Current concepts support the hypothesis that the mutant sarcomere proteins induce a causal depression in the tension-time integral (TTI) of linear preparations of cardiac muscle. However, molecular mechanisms underlying tension generation particularly concerning mutant proteins and their impact on sarcomere molecular signaling are currently controversial. Thus, there is a need for clarification as to how mutant proteins affect sarcomere molecular signaling in the etiology and progression of DCM. A main topic in this controversy is the control of the number of tension-generating myosin heads reacting with the thin filament. One line of investigation proposes that this number is determined by changes in the ratio of myosin heads in a sequestered super-relaxed state (SRX) or in a disordered relaxed state (DRX) poised for force generation upon the Ca2+ activation of the thin filament. Contrasting evidence from nanometer-micrometer-scale X-ray diffraction in intact trabeculae indicates that the SRX/DRX states may have a lesser role. Instead, the proposal is that myosin heads are in a basal OFF state in relaxation then transfer to an ON state through a mechano-sensing mechanism induced during early thin filament activation and increasing thick filament strain. Recent evidence about the modulation of these mechanisms by protein phosphorylation has also introduced a need for reconsidering the control of tension. We discuss these mechanisms that lead to different ideas related to how tension is disturbed by levels of mutant sarcomere proteins linked to the expression of gene variants in the complex landscape of DCM. Resolving the various mechanisms and incorporating them into a unified concept is crucial for gaining a comprehensive understanding of DCM. This deeper understanding is not only important for diagnosis and treatment strategies with small molecules, but also for understanding the reciprocal signaling processes that occur between cardiac myocytes and their micro-environment. By unraveling these complexities, we can pave the way for improved therapeutic interventions for managing DCM.

Modulating the tension-time integral of the cardiac twitch prevents dilated cardiomyopathy in murine hearts.

Dilated cardiomyopathy (DCM) is often associated with sarcomere protein mutations that confer reduced myofilament tension–generating capacity. We demonstrated that cardiac twitch tension-time integrals can be targeted and tuned to prevent DCM remodeling in hearts with contractile dysfunction. We employed a transgenic murine model of DCM caused by the D230N-tropomyosin (Tm) mutation and designed a sarcomere-based intervention specifically targeting the twitch tension-time integral of D230N-Tm hearts using multiscale computational models of intramolecular and intermolecular interactions in the thin filament and cell-level contractile simulations. Our models predicted that increasing the calcium sensitivity of thin filament activation using the cardiac troponin C (cTnC) variant L48Q can sufficiently augment twitch tension-time integrals of D230N-Tm hearts. Indeed, cardiac muscle isolated from double-transgenic hearts expressing D230N-Tm and L48Q cTnC had increased calcium sensitivity of tension development and increased twitch tension-time integrals compared with preparations from hearts with D230N-Tm alone. Longitudinal echocardiographic measurements revealed that DTG hearts retained normal cardiac morphology and function, whereas D230N-Tm hearts developed progressive DCM. We present a computational and experimental framework for targeting molecular mechanisms governing the twitch tension of cardiomyopathic hearts to counteract putative mechanical drivers of adverse remodeling and open possibilities for tension-based treatments of genetic cardiomyopathies.

Impact of Fatiguing, Submaximal High- vs. Low-Torque Isometric Exercise on Acute Muscle Swelling, and Echo Intensity in Resistance-Trained Men.

Muddle, TWD, Magrini, MA, Colquhoun, RJ, Luera, MJ, Tomko, PM, and Jenkins, NDM. Impact of fatiguing, submaximal high- vs. low-torque isometric exercise on acute muscle swelling, and echo intensity in resistance-trained men. J Strength Cond Res 33(4): 1007-1019, 2019-To evaluate the effects of repeated, fatiguing isometric knee extension contractions performed at high-torque (HT) vs. low-torque (LT) maximal voluntary isometric contraction (MVIC) on acute muscle swelling, and echo intensity (EI) of the rectus femoris (RF) and vastus lateralis (VL) muscles in resistance-trained males. Twenty-two resistance-trained men performed either HT (70% MVIC) or LT (30% MVIC) isometric contractions to fatigue on separate days. Before and after exercise, muscle cross-sectional area (mCSA) and EI of the RF and VL were assessed through ultrasound. Muscle cross-sectional area and EI (collapsed across muscle) increased similarly after HT (mean ± 95% confidence interval: mCSAΔ: 3.52 ± 0.52 cm; EIΔ: 4.58 ± 1.65 au) and LT (mCSAΔ: 3.61 ± 0.59 cm; EIΔ: 3.30 ± 1.36 au) exercise. No relationships between mCSAΔ and time to task failure (TTF), normalized MVIC, or the tension-time integral were observed during either HT or LT exercise. During both conditions, EIΔ was related to TTF (HT: r = 0.44; LT: r = 0.66) and normalized MVIC (HT: r = -0.60; LT: r = -0.57). These results suggest that acute muscle swelling and increased EI observed in response to intermittent, submaximal, fatiguing isometric exercise are similar for the RF and VL and are not dependent on the torque-level or tension-time integral when exercise is performed to failure. Factors such as the duration of time performing exercise and relative muscle strength may influence the magnitude of change in EI, but not acute muscle swelling, during intermittent fatiguing isometric exercise.

Tension-Time Integrals and Genetic Cardiomyopathy: The Force Is with You

Hundreds of different mutations in genes encoding a few dozen sarcomeric proteins cause two reciprocal human disease phenotypes, hypertrophic or dilated cardiomyopathy. How molecular dysfunction evokes different patterns of cardiac remodeling is unclear. Davis etal. describe a biophysical metric of cardiomyocyte function, the force-time integral, which predicts disease phenotype.

Slowed muscle oxygen uptake kinetics with raised metabolism are not dependent on blood flow or recruitment dynamics

Oxygen uptake kinetics (τVO2) are slowed when exercise is initiated from a raised metabolic rate. Whether this reflects the recruitment of muscle fibres differing in oxidative capacity, or slowed blood flow (Q) kinetics is unclear. This study determined τVO2 in canine muscle in situ, with experimental control over muscle activation and Q during contractions initiated from rest and a raised metabolic rate. The gastrocnemius complex of nine anaesthetised, ventilated dogs was isolated and attached to a force transducer. Isometric tetanic contractions (50 Hz; 200 ms duration) via supramaximal sciatic nerve stimulation were used to manipulate metabolic rate: 3 min stimulation at 0.33 Hz (S1), followed by 3 min at 0.67 Hz (S2). Circulation was initially intact (SPON), and subsequently isolated for pump-perfusion (PUMP) above the greatest value in SPON. Muscle VO2 was determined contraction-by-contraction using an ultrasonic flowmeter and venous oximeter, and normalised to tension-time integral (TTI). τVO2/TTI and τQ were less in S1SPON (mean ± s.d.: 13 ± 3 s and 12 ± 4 s, respectively) than in S2SPON (29 ± 19 s and 31 ± 13 s, respectively; P < 0.05). τVO2/TTI was unchanged by pump-perfusion (S1PUMP, 12 ± 4 s; S2PUMP, 24 ± 6 s; P < 0.001) despite increased O2 delivery; at S2 onset, venous O2 saturation was 21 ± 4% and 65 ± 5% in SPON and PUMP, respectively. VO2 kinetics remained slowed when contractions were initiated from a raised metabolic rate despite uniform muscle stimulation and increased O2 delivery. The intracellular mechanism may relate to a falling energy state, approaching saturating ADP concentration, and/or slowed mitochondrial activation; but further study is required. These data add to the evidence that muscle VO2 control is more complex than previously suggested.

Selective inhibition of ATPase activity during contraction alters the activation of p38 MAP kinase isoforms in skeletal muscle

Muscle contractions strongly activate p38 MAP kinases, but the precise contraction-associated sarcoplasmic event(s) (e.g., force production, energetic demands, and/or calcium cycling) that activate these kinases are still unclear. We tested the hypothesis that during contraction the phosphorylation of p38 isoforms is sensitive to the increase in ATP demand relative to ATP supply. Energetic demands were inhibited using N-benzyl-p-toluene sulphonamide (BTS, type II actomyosin) and cyclopiazonic acid (CPA, SERCA). Extensor digitorum longus muscles from Swiss Webster mice were incubated in Ringer's solution (37°C) with or without inhibitors and then stimulated at 10 Hz for 15 min. Muscles were immediately freeze-clamped for metabolite and Western blot analysis. BTS and BTS + CPA treatment decreased force production by 85%, as measured by the tension time integral, while CPA alone potentiated force by 310%. In control muscles, contractions resulted in a 73% loss of ATP content and a concomitant sevenfold increase in IMP content, a measure of sustained energetic imbalance. BTS or CPA treatment lessened the loss of ATP, but BTS + CPA treatment completely eliminated the energetic imbalance since ATP and IMP levels were nearly equal to those of non-stimulated muscles. The independent inhibition of cytosolic ATPase activities had no effect on contraction-induced p38 MAPK phosphorylation, but combined treatment prevented the increase in phosphorylation of the γ isoform while the α/β isoforms unaffected. These observations suggest that an energetic signal may trigger phosphorylation of the p38γ isoform and also may explain how contractions differentially activate signaling pathways.

Aortic reservoir function, estimated myocardial demand and coronary perfusion pressure following steady‐state and interval exercise

Aortic reservoir function is a measure of the aorta's ability to distribute blood during diastole, attenuating the pulsatility of blood flow, and is important in balancing cardiac flow. Effects of acute high versus moderate exercise intensity on reservoir function and cardiac energetics is unknown. Eighteen athletes completed a interval (INT) and steady-state (SS) cycling bout at 60% of VO(2) peak. Reservoir function was calculated as the ratio of diastolic run-off to stroke volume and expressed as a percentage. Coronary perfusion pressure was derived from tissue Doppler imaging and echocardiography. Systolic tension-time integral (TTI) from the aortic pressure waveform served as a measure of myocardial oxygen consumption. All measures were made at rest, 30-min postexercise and 60-min postexercise. Average reservoir function before SS was 76%, which was reduced to 62% 30-min post-SS and 67% 60-min post-SS (P<0.05). Significantly greater reductions in reservoir function were seen following INT (from 71% pre-INT to 45% 30-min post-INT and 53% 60-min INT, P<0.05). Estimated coronary perfusion pressure was reduced 30min following INT but not SS; both bouts reduced coronary perfusion pressure at 60-min postexercise (P<0.05). TTI increased following both INT and SS at 30- and 60-min postexercise with greater increases following INT (P<0.05). Following exercise, reservoir function was associated with TTI (P<0.05), but not coronary perfusion pressure (P>0.05). We conclude that reservoir function is attenuated following acute SS and INT, but these reductions were greater post-INT, suggesting that exercise intensity affects reservoir function. Reduction of reservoir function following exercise is related to TTI, a reflection of myocardial oxygen consumption but apparently not associated with coronary perfusion pressure.

Inter‐individual differences in rapid vasodilation in older males with and without type 2 diabetes

PURPOSETo identify if rapid vasodilatory responses differ between individuals with or without type 2 diabetes.METHODS13 males (7 type II diabetics (T2D), 6 healthy controls (CON)) lay supine with the arm at heart level and completed 6 trials of single handgripping contractions (3 trials/day/contraction intensity, on 2 separate days, 1 s contraction duration) at 10, 20, and 40 % of their maximal voluntary contraction (randomly ordered). Forearm brachial artery blood flow (FBF, echo and Doppler ultrasound), mean arterial pressure (MAP, finger photoplethysmography) were measured continuously.RESULTStension‐time integral of single contraction force production (kg·s) vs. FBF relationships were constructed for each individual. There was no difference in the slope (4.7 ± 2.9 vs. 4.8 ± 2.5) or y‐intercept (y‐int) (35.2 ± 32.5 vs. 23.3 ± 15.2) between T2D and CON. However, there was considerable variation in both the slope and y‐int between subjects in each group. T2D coefficient of variation (COV) of slope was 64% and y‐int was 93%. COV for the slope in CON was 52% and y‐int 65%. Despite the variation, the r2 value for each individual was strong, ranging from 0.34–0.7 (σ = 0.51 ± 0.14) in T2D and 0.44–0.97 (σ = 0.76 ± 0.22) in CON.CONCLUSIONOlder males exhibit substantial inter‐individual rapid vasodilatory responses to muscle contraction. These individual differences do not appear to be influenced by type 2 diabetes.

Glibenclamide increases post-fatigue tension in slow skeletal muscle fibers of the chicken

In contrast to fast-twitch skeletal muscle fibers of the chicken, slow-twitch fibers are fatigue-resistant. In fast fibers, the fatigue process has been related to KATP channels. In the present study, we investigated the action of glibenclamide (an anti-diabetic sulphonylurea that acts on KATP channels) on fatigued slow skeletal muscle, studying twitch and tetanus tension after inducing the muscle to fatigue by continuous electrical stimulation. Our results showed that glibenclamide (150 μM) increased post-fatigue twitch tension by about 25% with respect to the fatigued condition (P < 0.05). In addition, glibenclamide (150 μM) increased post-fatigue tetanic tension (83.61 ± 15.7% in peak tension, and 85.0 ± 19.0% in tension-time integral, P = 0.02, and 0.04, respectively; n = 3). Moreover, after exposing the muscle to a condition that inhibits mitochondrial ATP formation in order to activate KATP channels with cyanide (10 mM), tension also diminished, but in the presence of glibenclamide the effect produced by cyanide was abolished. To determine a possible increase in intracellular calcium concentration, the effects of glibenclamide on caffeine-evoked contractures were explored. After muscle pre-incubation with glibenclamide (150 μM), tension of caffeine-evoked contractures increased (6.5 ± 1.5% in maximal tension, and 5.9 ± 3.8% in tension-time integral, P < 0.05). These results suggest a possible role of KATP channels in the fatigue process, since glibenclamide increases twitch and tetanus tension in fatigued slow muscle of the chicken and during metabolic inhibition, possibly by increasing intracellular calcium.

The marine polyether gambierol enhances muscle contraction and blocks a transient K + current in skeletal muscle cells

Gambierol is a complex marine toxin first isolated with ciguatoxins from cell cultures of the toxic dinoflagellate Gambierdiscus toxicus. Despite the chemical complexity of the polycyclic ether toxin, the total successful synthesis of gambierol has been achieved by different chemical strategies. In the present work the effects of synthetic gambierol on mouse and frog skeletal neuromuscular preparations and Xenopus skeletal myocytes have been studied. Gambierol (0.1–5 μM) significantly increased isometric twitch tension in neuromuscular preparations stimulated through the motor nerve. Less twitch augmentation was observed in directly stimulated muscles when comparing twitch tension–time integrals obtained by nerve stimulation. Also, gambierol induced small spontaneous muscle contraction originating from presynaptic activity that was completely inhibited by d-tubocurarine. Gambierol slowed the rate of muscle action potential repolarization, triggered spontaneous and/or repetitive action potentials, and neither affected action potential amplitude nor overshoot in skeletal muscle fibers. These results suggest that gambierol through an action on voltage-gated K + channels prolongs the duration of action potentials, enhances the extent and time course of Ca 2+ release from the sarcoplasmic reticulum, and increases twitch tension generation. Further evidence is provided that gambierol at sub-micromolar concentrations blocks a fast inactivating outward K + current that is responsible for action potential prolongation in Xenopus skeletal myocytes.

The O2cost of the tension-time integral in isolated single myocytes during fatigue

One proposed explanation for the Vo(2) slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O(2), thereby resulting in a dissociation of cellular respiration from force generation. The present study used intact isolated single myocytes with differing fatigue resistance profiles to investigate the relationship between fatigue, tension development, and aerobic metabolism. Single Xenopus skeletal muscle myofibers were allocated to a fast-fatiguing (FF) or a slow-fatiguing (SF) group, based on the contraction frequency required to elicit a fall in tension to 60% of peak. Phosphorescence quenching of a porphyrin compound was used to determine Delta intracellular Po(2) (Pi(O(2)); a proxy for Vo(2)), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak DeltaPi(O(2)) was not different between groups (P = 0.36), peak tension was lower (P < 0.05) in SF vs. FF (1.97 +/- 0. 17 V vs. 2. 73 +/- 0.30 V, respectively) and time to 60% of peak tension was significantly longer in SF vs. FF (242 +/- 10 s vs. 203 +/- 10 s, respectively). Before fatigue, both DeltaPi(O(2)) and TxT rose proportionally with contraction frequency in SF and FF, resulting in DeltaPi(O(2))/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but DeltaPi(O(2)) decreased proportionately only in the FF group, resulting in an increase in DeltaPi(O(2))/TxT in the SF group relative to the prefatigue condition. These data show that more fatigue-resistant fibers maintain aerobic metabolism as they fatigue, resulting in an increased O(2) cost of contractions that could contribute to the Vo(2) slow component seen in whole body exercise.

Effects of Cannabinoids on Caffeine Contractures in Slow and Fast Skeletal Muscle Fibers of the Frog

The effect of cannabinoids on caffeine contractures was investigated in slow and fast skeletal muscle fibers using isometric tension recording. In slow muscle fibers, WIN 55,212-2 (10 and 5 μM) caused a decrease in tension. These doses reduced maximum tension to 67.43 ± 8.07% (P = 0.02, n = 5) and 79.4 ± 14.11% (P = 0.007, n = 5) compared to control, respectively. Tension-time integral was reduced to 58.37 ± 7.17% and 75.10 ± 3.60% (P = 0.002, n = 5), respectively. Using the CB1 cannabinoid receptor agonist ACPA (1 μM) reduced the maximum tension of caffeine contractures by 68.70 ± 11.63% (P = 0.01, n = 5); tension-time integral was reduced by 66.82 ± 6.89% (P = 0.02, n = 5) compared to controls. When the CB1 receptor antagonist AM281 was coapplied with ACPA, it reversed the effect of ACPA on caffeine-evoked tension. In slow and fast muscle fibers incubated with the pertussis toxin, ACPA had no effect on tension evoked by caffeine. In fast muscle fibers, ACPA (1 μM) also decreased tension; the maximum tension was reduced by 56.48 ± 3.4% (P = 0.001, n = 4), and tension-time integral was reduced by 57.81 ± 2.6% (P = 0.006, n = 4). This ACPA effect was not statistically significant with respect to the reduction in tension in slow muscle fibers. Moreover, we detected the presence of mRNA for the cannabinoid CB1 receptor on fast and slow skeletal muscle fibers, which was significantly higher in fast compared to slow muscle fiber expression. In conclusion, our results suggest that in the slow and fast muscle fibers of the frog cannabinoids diminish caffeine-evoked tension through a receptor-mediated mechanism.

Temperature affects post‐tetanic potentiation in frog skeletal muscle

Brief tetanic contractions cause increases in twitch tension, i.e. post‐tetanic potentiation (PTP). Reducing incubation temperature produces a similar effect. The purpose of this investigation was to determine what interaction might exist between these two, powerful potentiating influences. Sartorii muscles (60‐70 mg) from male R. pipiens were dissected‐free and vertically‐mounted in water‐jacketed chambers containing oxygenated (100%), Ringer solution (pH 7.2). To determine peak twitch tension (PT, g) at different temperatures (10°‐30°C), muscles were stimulated via supramaximal square wave pulses (0.2 ms) every 10 s for 2 min before and after a 2.5 s tetanic contraction (80 Hz). Averaged PT immediately preceding tetanic stimulation was 25.2 ± 2.7, 18.1 ± 2.2 and 14.1 ± 0.9 at 10°C, 20°C and 30°C, respectively. Corresponding thermal ratios (R10) were 0.72 (10°‐20°C) and 0.78 (20°‐30°C). Peak tetanic tensions (PTT, g) and tension‐time integrals (TTI, g·s) were 47.3 ± 4.7, 60.1 ± 6.5, and 49.4 ± 5.8, and 100.9 ± 6.8, 140.4 ± 17.0 and 107.4 ±16.1 at 10°C, 20°C and 30°C, respectively. Averaged PT immediately following tetanic stimulation was 27.2 ± 2.1, 25.1 ± 0.7, and 15.4 ± 0.6 at 10°C, 20°C and 30°C, respectively. R10 values were 0.92 (10°‐20°C) and 0.61 (20°‐30°C). Results suggest that variations in temperature may affect the neural regulation of force output in poikilothermic animals.

Subcontracture depolarization alters doublet potentiation

Doublet potentiation occurs when a brief, high‐frequency burst of pulses is added to a subtetanic train of pulses. The purpose of this investigation was to determine the effect of subcontracture K+‐induced depolarization on doublet potentiation. Sartorii muscles (60‐70 mg) from R. pipiens were excised and mounted in water‐jacketed (20°C) chambers containing oxygenated (100%) Ringer's solution (R; pH 7.2) containing 2.5 mM KCl (R+KCl2.5). Muscles were exposed to an 800 ms train (20 Hz) of supramaximal square‐wave pulses (.2 ms) via field stimulation. An additional pulse was inserted between the 9th and 10th pulses (ISI = 10 ms). Peak tensions (PT, g) for contractions (PTA‐E) between the eighth and twelveth pulses were determined. Tension‐time integrals (TTI, g·s) were calculated. This protocol was then repeated with the [KCl] of R increased incrementally up to 15 mM/L.Subcontracture depolarization lessens the effect of transient high‐frequency paired stimulation and may mitigate its potential as a motor control strategy.

Deletion Of Triadin Results In Marked Alterations In Tetanic Contraction And Global Calcium Handling

In our recent work (Shen et al., JBC, 282(52), 2007), we examined the role of triadin in skeletal muscle EC coupling in a pan-triadin null mouse. No contractile dysfunction was evident during single twitches in vitro, however a reduction in Ca2+ transient magnitude was observed. Analysis of single myofibers revealed a decrease in SR Ca2+ content and an increase in sarcoplasmic [Ca2+], suggesting that triadin is not critical to EC coupling, yet might play a modulatory role at the RyR. In this investigation, we test our hypothesis that an increase in functional demands will unmask an EC coupling phenotype in muscle null for triadin (Tdn). We used tetanic stimulation to examine the contractile characteristics of EDL muscles in vitro and TA muscles in situ, as well as global calcium transients. The tetanic force vs. stimulation frequency (FF) relationship (250msec train; 0.5ms pulse @ 1-300Hz) between WT and Tdn EDLs was evaluated in vitro. Tdn EDLs were unable sustain the initial peak achieved during each 250msec train. The magnitude of the tetanic fade was progressive with increasing stimulation frequency. At peak tetanic stimulation (300 Hz) this tetanic fade resulted in a ∼23% decrease in the tension-time integral; a finding that was replicated with nerve evoked tetanic stimulation of the tibialis anterior muscle in-situ. Similarly, our assay of myoplasmic [Ca2+] in FDB myofibers (MagFluo4AM) demonstrated that the plateau of the tetanic [Ca2+] was not sustained in triadin null FDBs, even though the peak [Ca2+] amplitude during a 250msec tetanic train (80Hz; field stim.) was not different between genotypes. We believe that the contractility deficits in the Tdn muscles are due to a decrease in SR Ca2+ release. Funded by grants from NIH-NIAMS to C.W.W, R.M.L, P.D.A, and C.P.

Temperature and Doublet Potentiation: Effects of Varied Interspike Interval

The addition of two closely spaced pulses (doublets) at the beginning of alow‐frequency train of stimuli has been shown to increase peak isometric tension (PT) and tension‐time integral (TTI). This study examined the effects of cooling on potentiation associated with paired pulses having varied (10–100 ms) interspike intervals (ISIs). Sartorii muscles (60–70 mg) from male R. pipiens were dissected free and mounted vertically in water‐jacketed chambers containing oxygenated (100%) Ringer solution (pH 7.2). Muscles were exposed serially to single (SES) and paired (i.e., doublet; DES) electrical stimulation. DES was adjusted to produce ISIs ranging from 10–100 ms. PT (g/g) was measured via field stimulation with supramaximal square wave pulses (1 ms). At 20°C, PT in response to DES (ISI = 100 ms; PTD) was elevated when compared to SES (PTS): 105.7 ± 21.5 v. 79.5 ± 10.0, respectively. Decreasing ISI produced further increases in PTD, with maximum PTD occurring at an ISI of 10 ms: 178.0 ± 34.0 v. 79.5 ± 10.0, respectively. At 10°C, PTS at was increased when compared to 20°C: 120.7 ± 8.1 (10°C) v. 79.5 ± 10.0, respectively. Furthermore, PTD (10°C; ISI = 100 ms) was also elevated when compared to SES (10°C); 196.7 ± 22.5 v. 120.7 ± 8.1, respectively. Decreasing ISI produced additional increases in PTD. However, maximum PTD was developed when paired pulses were actually than 10 ms (i.e., ISI = 40 ms). ISIs &lt; 40 ms failed to produce further increases in PTD. Calculated values for TTI (g·s) were similarly affected. These results suggest that: 1) the potentiating effect of paired stimulation is temperature‐sensitive, and 2) the efficacy of paired stimulation in vivo, as a potential neural control strategy to augment force production, may be mediated by environmental factors.

Subcontracture Depolarization Alters Doublet Potentiation in Isolated Skeletal Muscle

The addition of two closely spaced stimulating impulses (doublets) at the beginning of a low‐frequency train of stimuli has been shown to increase isometrictension and maximize tension‐time integral (TTI). The purpose of this investigation was to determine the combined effects of subcontracture K+‐induced depolarization, itself a powerful potentiating influence, and doublet potentiation on peak muscle tension development. Sartorii muscles (60–70 mg) from small male R. pipiens were dissected free and mounted vertically in water‐jacketed (20°C) chambers containing oxygenated (100%) amphibian Ringer's solution (pH 7.2) containing 2.5 mM KCl (R+KCl2.5). After equilibrium was achieved, muscles were exposed serially to single and paired (ISI = 20ms) supramaximal square wave pulses (1 ms) via field stimulation. Peak tension (PT) in response to paired stimulation (PTD, g/g) was greater when compared to single pulse stimulation (PTS, g/g): 202.2 ± 18.4 v. 104.8 ± 6.7, respectively. Following complete recovery, the stimulation protocol was repeated after substituting Ringer's containing 15 mM KCl (R+KCl15) for R+KCl2.5. PTS increased from 101.0 ± 4.9 (R+KCl2.5) to 152.7 ± 14.9 (R+KCl15). PTD increased from 101.0 ± 4.9 (R+KCl2.5) to 184.5 ± 18.61 (R+KCl15). Corresponding values for the TTI (g·s) mimicked results for PT. Subcontracture K+‐induced depolarization lessened the effect of subsequent paired stimulation: (+ 92.9% (R+KCl2.5) v. + 82.7% (R+KCl15)). These results suggest that alterations in extracellular environment may mitigate the potentiating effects of paired stimulation and alter it's efficacy as a motor control strategy.

Acute INDUCTION OF Transcription Factors in Fast-Twitch Muscle DURING CHRONIC SCIATIC NERVE STIMULATION

The molecular signals whereby contractile activity induces physiologic and biochemical adaptations in muscles are not fully understood. PURPOSE: To identify transcription factors affected in fast twitch muscle (extensor digitorum longus; EDL) subjected to extrinsic slow motor unit stimulation. METHODS: Adult male Swiss-Webster mice (n=4) were anesthetized and the sciatic nerve of one leg exposed and stimulated electrically at 10 Hz for up to 4 hours through implanted Pt electrodes. The non-stimulated leg served as a paired control. At the end of each experiment, both muscles were flash-frozen for RNA extraction and microarray analysis (GE: CodeLink Mouse 10k Uniset). Paired comparisons corrected for multiple testing were employed to detect differentially expressed genes which were classified into functional groups using NIAID:DAVID. RESULTS: A 50% decrease in peak force and tension time integral occurred within 10 min of stimulation onset and persisted for the entire time course, however time to peak force and half relaxation times remained constant. Stimulation increased gene expression in 78% of the differentially expressed genes detected in the stimulus vs. contralateral control EDL. Significant changes in transcript levels associated with fast to slow phenotype transformation were present including increased troponin C-slow, laminin-beta 3, and tropomyosin 3-γ and decreased myosin heavy chain 2A fast, creatine kinase, myosin light chain-fast, and troponin I fast 2. Transcripts associated with induction and regulation of transcription were statistically overrepresented (vizXlabs: GeneSifter) above other functional classes with 65 of 70 (625 assayed) increased in the stimulated EDL. Specifically, PPARGC1-α (8.4-fold), PPARGC1-β (2.8-fold), PPAR-γ (2.5-fold) and HIF1-α (3.5-fold) were significantly elevated after stimulation although present in the unstimulated leg. Twenty-nine additional transcription factors detected at or below threshold levels in the unstimulated EDL were induced to significant levels in the stimulated EDL. CONCLUSIONS: Our results suggest that 10 Hz short-term stimulation activates both latent and constitutive transcription factors as part of a broad molecular response associated with phenotype modification.

A rapid algorithm for processing digital physiologic signals: Application to skeletal muscle contractions

Quantifying mechanical output is fundamental to understanding metabolism that fuels muscle contraction and more recent attempts to understand signal transduction and gene regulation. The latter requires long-term application of exercise protocols that result in large amounts of data on muscle performance. The purpose of this study was to develop software for automated quantification of skeletal muscle contractions. An in situ mouse sciatic nerve stimulation model was used to produce contractions over a broad range of frequencies and recorded as both digital and analog signals using a PC analog to digital converter board and chart recorder, respectively. Spectral analysis of the noise components formed the basis for designing a smoothing Chebyshev filter. Algorithms implemented in custom software identified twitches and estimated baseline levels from the smoothed signal. The time to peak force, peak force, tension-time integral, and half-relaxation time were determined for each twitch after baseline correction. The automated results were compared to those obtained from manual measurements of the analog signal. Bland–Altman analysis of the parameters computed from digital signals compared with the corresponding measurements by manual planometry demonstrates the agreement of the digital processing algorithm with planometry over a wide range of twitch characteristics. This program may also be used to study the mechanics of other preparations from isolated muscles, human proximal limb performance, and other digital physiologic signals. Adaptation of the filter function is required to apply the analysis to another experimental apparatus with differing noise characteristics. A full version of the program and instructions for its use are available for download at www.rad.msu.edu.

Temperature‐effects on motor unit doublet potentiation in isolated skeletal muscle in vitro

The additionof an extra stimulating pulse, or doublet, at the beginning of a low-frequency train of stimuli has been shown to substantially increase isometricforce. This study examined the effects of temperature on doublet potentiation. Sartorii muscles (60–70 mg) from small male R. pipiens were dissected free and mounted vertically in water-jacketed chambers containing oxygenated (100%) Ringer solution (pH 7.2). After recovery, muscles were exposed serially to three different incubation temperatures: 10°, 20°, and 30° C. Twitch characteristics were examined via field stimulation with supramaximal square wave pulses (1 ms). At ambient temperature (20°), peak tension in response to paired stimulation (PTD, g/g) was greater when compared to single (PTS, g/g) stimulation (308.4 ± 23.4 v. 188.3 ± 16.8, respectively, mean ± SE). Peak tension in response to both single and paired stimulation increased with decreasing temperature (PTS = 159.0 ± 20.7 (30°) v. 188.3 ± 16.8 (20°) v. 248.7 ± 16.3 (10°); PTD =302.2 ± 32.7(30°) v. 308.4 ± 23.4 (20°) v. 322.9 ± 21.3 (10°)). Furthermore, the tension-time integral (TTI, g·s) was also found to be increased with paired stimulation and decreased temperature. However, the doublet-to-singlet tension ratio (PTD/ PTS) was observed to decrease with decreasing temperature (PTD /PTS = 1.95 ± 0.09 (30°) v. 1.66 ± 0.08 (20°) v. 1.30 ± 0.03 (10°). The results presented here indicate that doublet potentiation is temperature-sensitive and suggest that the efficacy of paired stimulation in vivo, as a potential neural control strategy to augment force production, may be somewhat mediated by changes in muscle temperature.