Hz Contraction Research Articles

Muscle hypertrophy following blood flow-restricted, low-force isometric electrical stimulation in rat tibialis anterior: role for muscle hypoxia.

Low-force exercise training with blood flow restriction (BFR) elicits muscle hypertrophy as seen typically after higher-force exercise. We investigated the effects of microvascular hypoxia [i.e., low microvascular O2 partial pressures (P mvO2)] during contractions on muscle hypertrophic signaling, growth response, and key muscle adaptations for increasing exercise capacity. Wistar rats were fitted with a cuff placed around the upper thigh and inflated to restrict limb blood flow. Low-force isometric contractions (30 Hz) were evoked via electrical stimulation of the tibialis anterior (TA) muscle. The P mvO2 was determined by phosphorescence quenching. Rats underwent acute and chronic stimulation protocols. Whereas P mvO2 decreased transiently with 30 Hz contractions, simultaneous BFR induced severe hypoxia, reducing P mvO2 lower than present for maximal (100 Hz) contractions. Low-force electrical stimulation (EXER) induced muscle hypertrophy (6.2%, P < 0.01), whereas control group conditions or BFR alone did not. EXER+BFR also induced an increase in muscle mass (11.0%, P < 0.01) and, unique among conditions studied, significantly increased fiber cross-sectional area in the superficial TA ( P < 0.05). Phosphorylation of ribosomal protein S6 was enhanced by EXER+BFR, as were peroxisome proliferator-activated receptor gamma coactivator-1α and glucose transporter 4 protein levels. Fibronectin type III domain-containing protein 5, cytochrome c oxidase subunit 4, monocarboxylate transporter 1 (MCT1), and cluster of differentiation 147 increased with EXER alone. EXER+BFR significantly increased MCT1 expression more than EXER alone. These data demonstrate that microvascular hypoxia during contractions is not essential for hypertrophy. However, hypoxia induced via BFR may potentiate the muscle hypertrophic response (as evidenced by the increased superficial fiber cross-sectional area) with increased glucose transporter and mitochondrial biogenesis, which contributes to the pleiotropic effects of exercise training with BFR that culminate in an improved capacity for sustained exercise. NEW & NOTEWORTHY We investigated the effects of low microvascular O2 partial pressures (P mvO2) during contractions on muscle hypertrophic signaling and key elements in the muscle adaptation for increasing exercise capacity. Although demonstrating that muscle hypoxia is not obligatory for the hypertrophic response to low-force, electrically induced muscle contractions, the reduced P mvO2 enhanced ribosomal protein S6 phosphorylation and potentiated the hypertrophic response. Furthermore, contractions with blood flow restriction increased oxidative capacity, glucose transporter, and mitochondrial biogenesis, which are key determinants of the pleiotropic effects of exercise training.

MEASUREMENT AND ESTIMATION OF MUSCLE CONTRACTION STRENGTH USING MECHANOMYOGRAPHY BASED ON ARTIFICIAL NEURAL NETWORK ALGORITHM

Muscle contraction strength estimation using mechanomyographic (MMG) signal is typically calculated by the root mean square (RMS) amplitude. Raw MMG signal is processed by rectification, low-pass filtering, and mapping. In this work, beside RMS amplitude, another significant parameter of MMG signal, i.e. frequency variance (VAR), is introduced and used for constructing an algorithm for estimating the muscle contraction strength. Seven participants produced isometric contractions about the elbow while MMG signal and generated torque (resultant of muscle contraction strength) of biceps brachii were recorded. We found that MMG RMS increased monotonously and VAR decreased under incremental voluntary contractions. Based on these results, a two-layer neural network was utilized for the model of estimating the muscle contraction strength from MMG RMS and VAR. Experimental evaluation was performed under constant posture and sinusoidal contractions at 0.5 Hz, 0.25 Hz, 0.125 Hz, and random frequency. The results of the proposed algorithm and MMG RMS linear mapping were also compared. The proposed algorithm has better accuracy than linear mapping for all contraction frequencies. The mean absolute error decreased 6% for the 0.5Hz contraction, 43% for 0.25 Hz contraction, 52% for 0.125 Hz contraction, and 30% for random frequency contraction.

Microvascular blood flow responses to muscle contraction are not altered by high‐fat feeding in rats

Exercise and insulin each increase microvascular blood flow and enhance glucose disposal in skeletal muscle. We have reported that insulin-mediated microvascular recruitment in a diet-induced model of insulin resistance (high-fat feeding for 4 weeks) is markedly impaired; however, the effect of muscle contraction in this model has not been previously explored. We fed rats either normal (ND, 10% calories from fat) or high-fat (HFD, 60% calories from fat) diets ad libitum for 4-8 weeks. Animals were then anaesthetized and one hindlimb electrically stimulated to contract at 0.05, 0.1 and 2 Hz (field stimulation, 30 V, 0.1 ms duration) in 15 min stepwise increments. Femoral artery blood flow (Transonic flow probe), muscle microvascular blood flow (hindleg metabolism of 1-methylxanthine and contrast-enhanced ultrasound) and muscle glucose disposal (uptake of radiolabelled 2-deoxy-d-glucose and hindleg glucose disappearance) were measured. Both ND and HFD rats received the same voltage across the leg and consequently developed the same muscle tension. Femoral artery blood flow in the contracting leg increased during 2 Hz contraction, but not during the lower frequencies and these effects were similar between ND and HFD rats. Muscle microvascular blood flow significantly increased in a contraction frequency-dependent manner, and preceded increases in total limb blood flow and these effects were similar between ND and HFD rats. Muscle glucose disposal was markedly elevated during 2 Hz contraction and was comparable between ND and HFD rats. Contraction-mediated muscle microvascular recruitment and glucose uptake are not impaired in the HFD insulin resistant rat.

Force–frequency and force–length properties in skeletal muscle following unilateral focal ischaemic insult in a rat model

Our purpose was to quantify skeletal muscle properties following unilateral focal ischaemic insult (stroke) in a rat model. Male rats were divided into two groups: stroke and 2 weeks recovery (n = 8) and control group (n = 7). Stroke was induced in the area of the motor neocortex containing hind limb corticospinal neurones. Contractile properties of the medial gastrocnemius muscle were measured in situ in both limbs. Force-length and force-frequency properties were measured before and 35 min after 5 min fatiguing stimulation. Stroke resulted in bilateral tetanic fade during 200 Hz stimulation. When normalized to 100 Hz contractions, force at 200 Hz was 95.4 +/- 0.9% for the paretic muscles, 96.7 +/- 1.7% for non-paretic muscles and 102.2 +/- 1.0% for muscles of control rats (P = 0.006). Prior to fatiguing contractions, there was no difference in the length dependence of force. During repetitive contractions, active force fell significantly to 19 +/- 4 and 25 +/- 5% of initial force in paretic and non-paretic muscles of animals with a stroke respectively. In control animals active force fell to 37 +/- 5%. During repetitive contractions, fusion index increased in muscles of stroke animals to 1.0 +/- 0 but in control animals it was 0.95 +/- 0.02. There was selective force depression at short lengths for fatigued paretic muscle (significant difference at muscle lengths less than reference length -2 mm). The tetanic fade at high stimulation frequencies indicates that there may be activation failure following focal ischaemic insult. The greater magnitude of fatigue and selective depression at short lengths following repetitive contractions should be investigated further.

Muscle Contraction, but Not Insulin, Increases Microvascular Blood Volume in the Presence of Free Fatty Acid–Induced Insulin Resistance

OBJECTIVEInsulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia and whether free fatty acid (FFA)-induced insulin resistance differentially affects contraction- versus insulin-mediated increases in MBV.RESEARCH DESIGN AND METHODSRats were fasted overnight. Plasma FFAs were increased by intralipid/heparin infusion (3 h), insulin was increased with a euglycemic clamp (3 mU · min−1 · kg−1), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound. Insulin transport into muscle was measured using 125I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A.RESULTSSuperimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37 and 67% for 0.1 or 1 Hz, respectively (P < 0.01). FFA elevation alone did not affect MBV, whereas 0.1 Hz stimulation doubled MBV (P < 0.05) and increased muscle insulin uptake (P < 0.05) despite high FFA. Physiologic hyperinsulinemia during FFA elevation paradoxically decreased MBV (P < 0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET-1 receptor A antagonism, and the combination raised MBV above basal.CONCLUSIONSContraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin plus FFA may result from differential inhibition of insulin-stimulated nitric oxide–dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.

Bi-modal recovery of quadriceps femoris muscle function after sustained maximum voluntary contraction at different muscle length

The aim of this study was to test the hypothesis that contractility of quadriceps femoris muscle during a 15-min period after a sustained maximum voluntary contraction for 1 min is determined by the interaction of posttetanic potentiation, metabolic fatigue, and nonmetabolic fatigue. Eleven healthy untrained men (age, 22.9+/-1.8 years; body weight, 77.5+/-5.2 kg) performed isometric 1-min maximum voluntary contraction at long (90 degrees in knee joint) and short (135 degrees in knee joint) muscle length at two different occasions. Contractility of quadriceps femoris muscle was monitored via the evoked contractions at 1, 10, 20, and 50 Hz and maximum voluntary contraction at short and long muscle length on both occasions. Force generating capacity was reduced immediately after 1-min maximum voluntary contraction at short and long muscle length, and then a bi-modal time-course of recovery was observed which consisted of (1) rapid recovery of all measured indexes at 3 min and (2) divergence in the changes of forces at low and high stimulation frequencies, as well as maximal voluntary contraction force at 7 and 15 min after exercising. The decline in force immediately after 1-min isometric load was caused by metabolic and nonmetabolic fatigue; however, factors related to the metabolic fatigue were prevalent. As the effect of metabolic fatigue was diminishing and posttetanic potentiation was still present, force generation capacity recovered at 3 minutes after exercising. Further dynamics of contractility can be explained by the fading influence of posttetanic potentiation and dominant effect of nonmetabolic fatigue.

Effects of Antioxidants on Contracting Spinotrapezius Muscle Force Production and Oxygen Consumption in Aged (26–30 Month Old) Rats

Aged rats exhibit a decreased muscle microvascular O2 partial pressure (PO2mv) at rest and during contractions compared to young rats. Age‐related reductions in nitric oxide bioavailability due, in part, to elevated reactive O2 species (ROS) constrain muscle blood flow (Q̇ m). Therefore antioxidants may restore NO bioavailability, Q̇ m and ameliorate the reduction in PO2mv.PURPOSE:To test the hypothesis that antioxidants would elevate Q̇ m and therefore PO2mv.METHODS:PO2mv and Q̇ m were measured in the spinotrapezius of 20 anesthetized male F344xBN hybrid rats at rest and during 1 Hz contractions before and after antioxidant intravenous infusion (76mg/kg vitamin C and 52mg/kg tempol).RESULTS:Antioxidants did not change Q̇ m at rest but during contractions Q̇ m was reduced from 157 ± 28 to 91 ± 15 ml/min/100g (P&lt;0.05). In the control condition, contractions invoked a biphasic PO2mv that fell from 30.6 ± 0.9 mmHg to a nadir of 16.8 ± 1.2 mmHg with an ‘undershoot’ of 2.8 ± 0.7 mmHg (P&lt;0.05) below the subsequent steady‐state (19.7 ± 1.2 mmHg). Antioxidants elevated baseline PO2mv to 35.7 ± 0.8 mmHg (P&lt;0.05) and reduced the ‘undershoot’ (P&lt;0.05) without changing the steady‐state contracting PO2mv.CONCLUSION:At rest, antioxidants increased PO2mv without changing O2 delivery. During contractions antioxidants transformed the aged PO2mv profile to that of young rats.Supported by AHA Heartland Affiliate Grant #0750090Z.

Impact of length during repetitive contractions on fatigue in rat skeletal muscle

The magnitude of fatigue resulting from repeated contractions at a short length has been reported to be less than that which occurs with contractions at a long length. However, there have been what appear to be contradictory reports; the rate of fatigue is greater at a short length. The purpose of this study was to evaluate the impact of length on the magnitude and the rate of fatigue resulting from a series of repetitive stimulations. Experiments were done with anesthetized rats and the medial gastrocnemius muscle was stimulated via the sciatic nerve. Submaximal force-length relationships were obtained prior to and 45 min after repeated contractions (50 Hz, 300 ms) at short or long length. Stimulation was applied at 1 Hz or 0.5 Hz for 5 min at the long length or 1 Hz at the short length (difference = 3.6mm). This approach permitted evaluation of the impact of rate of muscle activation as well as length on subsequent contractile response. Repetitive stimulation at a short length resulted in more potentiation and a greater (relative) rate of fatigue but after 5 min the depression of relative active force was similar between the series at long and short length. The submaximal force-length relationship obtained after 45 min of recovery revealed that depression of force was greater after 1 Hz contractions at the long length. These results are consistent with both sides of the apparent contradiction in the literature; rate of fatigue is greater at a short length and magnitude of fatigue is greater at a long length.

Interaction of Femoral Artery Ligation and TVaining on Microvascular PO2 and Blood Flow in Rat Soleus Muscle

The microvascular PO2 (PmO2) reflects the dynamic balance between oxygen delivery and oxygen uptake, and is the driving force for oxygen flux into muscle cells. Femoral artery ligation reduces hindlimb muscle blood flow in rats which is partially restored with the development of collateral circulation, and is enhanced with exercise training (Am J Physiol 287: H2434, 2004). It is not known how effectively this collateral circulation restores oxygen delivery. PURPOSE: To test the hypothesis that increased collateral flow would restore PO2m and that this would be enhanced with exercise training. METHODS: The left femoral artery of female Sprague-Dawley rats (∼275 g) was either left intact (sham) or double-ligated. Sham and ligated rats were divided into four experimental groups; sedentary sham (SED-SH; n=10), sedentary ligated (SED-LIG; n=12), exercise trained sham (ET-SH; n=8), and exercise trained ligated (ET-LIG; n=10). Daily treadmill training began 48 hrs after surgery. The exercise intensity and duration were increased progressively to 17 m/min (at 20 degrees) for up to 60 min/day, as tolerated. Experiments were performed 21–28 days later. PmO2 (phosphorescence quenching) and blood flow (radiolabeled microspheres) were measured in the left soleus muscle at rest and during 1 Hz contractions. RESULTS: Soleus PmO2 in the SED-LIG rats was lower than in the SED-SH rats (33 ± 3 vs. 19 ± 3 mmHg). Training did not alter resting PmO2 (ET-SH, 33 ± 2; ET-LIG, 19 ± 2 mmHg). During contractions, PmO2 was lower in the SED-LIG (19 ± 3 mmHg) compared to SED-SH (28 ± 3 mmHg). In contrast, PmO2 during contractions in the trained groups was higher than the sedentary groups (ET-SH, 35 ± 4; ET-LIG, 26 ± 3 mmHg). Soleus blood flow during contractions was less in the SED-LIG rats (27 ±13 vs. 64 ± 14 ml/min/100 g). Soleus flow was doubled in ET-SH rats, but was unchanged in ET-LIG rats (140 ± 18 and 34 ± 15 ml/min/100 g). CONCLUSIONS: Even after chronic femoral artery ligation, there were marked deficits in blood flow and PmO2 in the soleus muscle. Low intensity training increased PmO2 during contractions in the femoral artery ligated rats. This was not associated with an increase in blood flow, and suggests there was a redistribution of flow to support oxygen delivery to the working muscle.

Dantrolene, like fatigue, has a length‐dependent effect on submaximal force–length relationships of rat gastrocnemius muscle

Fatigue is length-dependent; relative active force depression is greater when measured at short lengths than at long lengths. Several unsatisfactory mechanisms have been proposed to explain this length dependence of fatigue, including: damaged myofilaments, stretch of 'in-series' structures, impaired t-tubule conduction and reduced intensity of activation. Dantrolene targets the ryanodine receptors, inhibiting stimulation-induced release of Ca2+. The purpose of this study was to determine if the force depression caused by dantrolene treatment also has a length dependence. Submaximal (single-pulse, double-pulse and 50 Hz stimulation) active force-length relationships were obtained from the medial gastrocnemius muscle of anaesthetized rats, before and after intravenous injection with dantrolene dissolved in propylene glycol. Dantrolene treatment was sufficient to reduce twitch amplitude by 25%. Similar to the consequences of repetitive stimulation, dantrolene treatment caused the same decrease in absolute active force across a broad range of test lengths, for twitch, double-pulse and 50 Hz contractions. Considering that active force is smaller at short lengths than at long lengths, this similar absolute force decrease represents a greater relative decrease at short lengths. Clearly, there is a length-dependent impact of attenuated Ca2+ release by dantrolene on relative active force. This study demonstrates that there is a length dependence of force depression associated with decreased Ca2+ release due to dantrolene treatment; therefore, if fatigue is due to decreased Ca2+ release, then additional length-dependent mechanisms are not required to explain the reported length dependence of force depression.

Dietary salt reduces hydrogen peroxide‐dependent dilation in contracting muscle

Skeletal muscle fibers can release H2O2 during contraction. We assessed the contribution of H2O2 to the arteriolar dilation during muscle contraction, the effect of dietary salt on the magnitude of this contribution, and the mechanism(s) by which H2O2 relaxes vascular smooth muscle. Arterioles were studied in the spinotrapezius muscle of rats fed a normal or high salt (0.45%, NS; 7%, HS). Resting diameters were identical in both groups, but dilation during 2 Hz and 4 Hz contraction was significantly less in HS rats than in NS rats. Exogenous catalase, at a concentration that effectively scavenges extracellular H2O2, did not affect resting diameters in either group, but reduced the dilation during 2 Hz and 4 HZ contraction by 55±11% and 60±10%, respectively, in NS rats. In HS rats, catalase reduced dilation during 2 Hz contraction by 36±12%, and had no effect on dilation during 4 Hz contraction. Dilation in response to exogenous H2O2 was significantly greater in HS rats than NS rats, but these responses were similarly reduced in both groups by inhibition of endogenous catalase with 3-AT, or inhibition of guanylyl cyclase with ODQ. We conclude that arteriolar dilation in contracting skeletal muscle is due in part to locally produced H2O2, and that the effect of H2O2 on arteriolar tone depends in part on its metabolism by intracellular catalase and activation of guanylyl cyclase. Increased dietary salt reduces the contribution of H2O2 to this dilation, but may not change the mechanism through which it relaxes vascular smooth muscle. (NIH HL44012).

Cisatracurium: myographical and electrophysiological studies in the isolated rat muscle

Myographical and electrophysiological studies of cisatracurium were performed, in vitro, in the isolated sciatic nerve-extensor digitorum longus muscle preparation of the rat. Indirect twitches were generated at 0.1 Hz and tetanic contractions at 50 Hz. endplate potentials (epps) were generated in trains of 50 Hz. The electrophysiological variables used in the analysis of the epps were: amplitude of the first epp in the train, average amplitude of the 30 degrees to the 59 degrees epp in the train (epps-plateau), tetanic rundown (percent loss in amplitude of epps-plateau relative to the first epp in the train), quantal size and quantal content. The myographical results showed that the inhibitory concentration 50% (IC(50)) of cisatracurium for the blockade of twitches (0.48 microm) is 12 times its IC(50) for the induction of tetanic fade (0.04 microm). The electrophysiological results showed a concentration dependent decrease in the amplitudes of first epps in the trains and of epps-plateau in the two used concentrations (0.13 microm and 0.38 microm). The tetanic rundown was intensified only in the presence of the higher (0.38 microm) concentration of cisatracurium. In cisatracurium 0.13 microm (a concentration which affects only tetanic contractions, inducing their fade, while leaving the twitch unaffected) there was a decrease in the quantal content of the first epp and of epps-plateau in the train. In cisatracurium (0.38 microm), a concentration, which affects the twitch, there was a decrease of the quantal size and of quantal content of epps-plateau, but not of the quantal content of the first epp in the train. The results indicate that the fade of the tetanic contraction induced by cisatracurium at the concentration of 0.13 microm is entirely because of a pre-synaptic blocking effect while the decrease in the twitch induced by cisatracurium at the concentration of 0.38 microm is due to a post-synaptic blocking effect.

Reduced Arteriolar Responses to Skeletal Muscle Contraction after Ingestion of a High Salt Diet

We previously reported that in skeletal muscle arterioles of rats fed a very high salt (HS; 7%) diet, the bioavailability of endothelium-derived nitric oxide (NO) is reduced through scavenging by reactive oxygen species. Because arteriolar NO can play an important role in local blood flow control, we investigated whether arteriolar responses to increased tissue metabolism become compromised in skeletal muscle of salt-fed rats. Consumption of a HS (4%) diet for 4 weeks had no effect on arteriolar diameters, volume flow or shear stress in resting spinotrapezius muscle. Arteriolar responses to a modest elevation in metabolic demand (0.5 Hz contraction) were not different from those in rats fed a normal diet, but diameter responses to a greater elevation in metabolic demand (4 Hz contraction) were significantly less in HS rats than in rats fed a normal diet. In both groups, the NO synthase inhibitor N^G-monomethyl-L-arginine reduced resting arteriolar diameters and flow by a similar amount and had little or no effect on arteriolar diameter or flow responses to muscle contraction. Arterioles in HS rats exhibited an increase in overall oxidant activity (tetranitroblue tetrazolium reduction) but not in superoxide activity (dihydroethidine oxidation). Reactive oxygen species scavengers (2,2,6,6-tetramethylpiperidine-N-oxyl and catalase) did not normalize the reduced arteriolar responses to muscle contraction in HS rats. These findings suggest that increased oxidant activity in the arteriolar network of salt-fed rats is not due to accumulation of superoxide anion and that neither this oxidant activity nor reduced NO availability can account for the blunted active arteriolar dilation in rats fed a 4% salt diet.

Potentiation of shortening and velocity of shortening during repeated isotonic tetanic contractions in mammalian skeletal muscle.

The purpose of this study was to investigate the enhancement of shortening and of the velocity of shortening during repeated incompletely fused isotonic tetanic contractions. The medial gastrocnemius muscle of anesthetized rats was isolated in situ and the motor nerve stimulated with supramaximal pulses, 50 micros duration, at optimal length. Estimated maximal velocity of shortening (V(max)) was 210 +/- 6 mm x s(-1) (mean +/- SEM). Repeated incompletely fused tetanic contractions (three pulses at 80 Hz) resulted in initial shortening which was 1.5 +/- 0.1 mm, and this increased to 2.7 +/- 0.2 mm after 7 s of 4 s(-1) contractions. Peak velocity of shortening for intermittent 80 Hz stimulation increased from 60.5 +/- 4 mm x s(-1) to 91.8 +/- plus minus 6 mm x s(-1). The initial velocity of shortening for 80 Hz stimulation was substantially less than the velocity of shortening observed with 200 Hz stimulation, but increased to 72 +/- 3% of the load-specific value for 200 Hz stimulation. Myosin regulatory light chain phosphorylation increased from 11.1 +/- 1.5% at rest to 32.9 +/- 5.4% after 4 s of intermittent 80 Hz stimulation and 50.4 +/- 8.8% after 7 s ( P<0.01). The ascending limb of the length-force relationship was determined with tetanic contractions, 200 Hz for 100 ms. At the length corresponding to peak shortening after 7 s of repeated 80 Hz contractions, the maximal isometric force was five times greater than the isotonic load. The rate of phosphorylation was sustained from 4 to 7 s, but the rate of increase in shortening and velocity decreased. The slower rate of change in shortening and velocity may be due to approaching maximal velocity for this short duration of contraction, but is not due to slowing of the rate of phosphorylation of the myosin regulatory light chains.

Effects of inhibition of calcium and potassium currents in guinea-pig cardiac contraction: comparison of beta-caryophyllene oxide, eugenol, and nifedipine.

1. To investigate the effects of the clove oil constituents beta-caryophyllene oxide and eugenol on the heart muscle, experiments were performed on isolated papillary muscles and on ventricular myocytes of the guinea-pig. The results were compared with those obtained with the dihydropyridine, nifedipine. 2. All three substances exerted negative inotropic effects in heart muscle although with different potencies and different influences on the time course of the contraction curve. 3. They all reduced rested-state contractions (RSCs) in the presence of isoprenaline which are thought to be due to the Ca(2+) current (I(Ca)). 4. beta-Caryophyllene oxide, eugenol and nifedipine inhibited the I(Ca) in single cells from the guinea-pig ventricle in a concentration-dependent, reversible way, but with different potencies. 5. In addition to the I(Ca)-inhibiting effect, beta-caryophyllene oxide strongly inhibited and eugenol slightly inhibited the potassium current. 6. The action potential of papillary muscles at a 1 Hz contraction frequency was greatly shortened by nifedipine, slightly shortened by eugenol, but not changed by beta-caryophyllene oxide. 7. The inhibition of the potassium current by beta-caryophyllene oxide obviously prevents a shortening of the action potential due to the diminution of the I(Ca). Accordingly, the negative inotropic effect of beta-caryophyllene oxide is closely related to the inhibition of I(Ca). In contrast, eugenol and nifedipine, which shorten the action potential, exert stronger negative inotropic effects than expected from their influence on I(Ca). 8. The results show that the negative inotropic effect of a calcium channel blocker can be attenuated by an additional inhibition of potassium channels.

The effect of nitric oxide and endothelin on skeletal muscle contractility changes when stimulation is altered

To investigate the effect of endothelial-derived products on the force of contraction of blood-perfused skeletal muscle, we infused S-nitroso-N-acetylpenicillamine (SNAP, 10(-4) M) (a nitric oxide (NO) donor), endothelin-1 (ET-1, 10(-8) M), N-acetylpenicillamine (NAP, 10(-4) M), or saline at a constant vascular concentration into the vascular bed of pump-perfused dog gastrocnemius-plantaris muscles in situ (n = 17). Muscles performed isometric twitch contractions at 0.5, 1.5, and 4 Hz and isometric tetanic contractions (150 ms, 50 Hz) at 12 and 40 contractions/min. We perfused the muscle at a constant pressure at rest and for the first 3 min of the 6-min contraction period, and then switched to constant flow perfusion and infused the substance over the remaining 3 min. Neither NAP nor saline had a significant effect on force of contraction or perfusion pressure. SNAP significantly attenuated developed force as compared with NAP at 40 contractions/min and at 1.5 and 4 Hz. The effect of SNAP on developed force was greater during twitch than tetanic contractions. ET-1 had no significant effect on twitch or tetanic developed force. To test for these results on another mammalian skeletal muscle preparation, we stimulated curarized trimmed mouse soleus in vitro for 500 ms at 50 Hz at either 2 or 4 contractions/min in the presence of SNAP (10(-4) M) or ET-1 (10(-9) M). SNAP, which increased force at 1 contraction/90 s, had no significant effect at the higher contraction frequencies. The inhibition by ET-1 at 2 contractions/min disappeared at 4 contractions/min. Therefore the effect of both NO and ET-1 on mammalian skeletal muscle appears to be dependent upon contraction pattern and frequency.

Effects of the creatine analogue β-guanidinopropionic acid on skeletal muscles of mice deficient in muscle creatine kinase

To evaluate the effects of phosphocreatine (PCr) and creatine (Cr) depletion on skeletal muscles of mice deficient in muscle creatine kinase (M-CK), we have fed mutant mice a diet containing the creatine analogue β-guanidinopropionic acid (βGPA). After 8–10 weeks of feeding, accumulation of the creatine analogue in M-CK-deficient muscles was comparable to that observed in muscles of wild-type mice. Strikingly, and unlike wild types, mutants did not accumulate phosphorylated βGPA, indicating that MM-CK is the only muscle CK isoform which can phosphorylate βGPA. In M-CK-deficient muscles there was respective depletion of PCr, Cr and ATP levels to 31, 41 and 83% of normal. The average cross-sectional area of type 2B fibres in gastrocnemius muscles was very much reduced and was similar to type 1 and type 2A fibres which maintained their normal size. The maximal isometric twitch force developed by gastrocnemius-plantaris-soleus (GPS) muscle complexes of βGPA-treated mutants was reduced by about 30%, but these muscles showed an increased fatigue resistance during 1 and 5 Hz contraction. Mitochondrial enzyme activities in the upper hind limb musculature of null mutants were 20–35% increased by the βGPA diet. Altogether, these results provide evidence that certain functions of the creatine kinase/phosphocreatine (CK/PCr) system are not eliminated solely by the loss of M-CK.

Effects of immobilization on electromyogram power spectrum changes during fatigue

The maximal force and median frequency (MF) of the electromyogram (EMG) power density spectrum (PDS) have been compared in disused (6 weeks' immobilization) and control (contralateral) human adductor pollicis muscles during fatigue induced by voluntary or electrically-triggered (30 Hz) contractions. The results indicated that after 6 weeks' immobilization, MF was not significantly different in disused and control muscles although the force and integrated EMG were drastically reduced during a maximal voluntary contraction (MVC; by 55% and 45%, respectively, n = 8). During sustained 60 s MVC, the force decreased at the same rate in immobilized and control muscles, but the shift of MF towards lower frequency values was smaller (P less than 0.05) in disused muscle as compared to control by (14% vs 28%, respectively). In electrically-induced fatigue, the force decrease and the MF shift were larger after inactivity (41% and 43% in one subject, and 50% and 54% in the other subject, respectively) as compared to control (29% and 34% in one subject, and 37% and 38% in the other subject, respectively). These results emphasize the caution that should be exercised when EMG signals are quantified by computing the power density spectrum. The different effects of fatigue during voluntary and electrically-imposed contractions in disused and control muscles indicated that immobilization induced changes in the neural command for the contraction which compensated, at least in part, for its decreased contractile efficiency and resistance to fatigue.

Differential Depression of Myocardial Contractility by Volatile Anesthetics In Vitro

Depression of rested state contractions (RSCs) and 0.1-0.25 Hz contractions by equianesthetic concentrations of isoflurane (2.5%), halothane (1.5%), and enflurane (3.5%) was studied in guinea pig papillary muscles in which tension development was enhanced by 0.1 microM isoproterenol. In a second series of experiments, an RSC was elicited, followed by a second contraction elicited with stimulus intervals of 300-600 ms. In both types of experiments, the results were similar. Halothane and enflurane depressed rapid initial tension development more than isoflurane. This initial tension development was also selectively depressed by 0.1 microM ryanodine, which specifically decreases Ca2+ release from the sarcoplasmic reticulum (SR). Isoflurane and also enflurane depressed a delayed and late peaking component of tension development, which was very prominent after rest and was depressed by 200 microM procaine or 500 microM benzocaine. Although isoflurane and enflurane were similar to the local anesthetics in depressing late tension, unlike the local anesthetics they prolonged the late phase of tension development as well. The late tension of the RSC is associated with Ca2+, which enters the rested myocyte on depolarization and may be transiently sequestered in the SR before release. Both early initial and late tension development are depressed to a similar degree by application of 10-20 nM nifedipine. These results emphasize the multiple differing actions of the volatile anesthetics on myocardial contractions, with halothane and isoflurane possessing distinct depressant characteristics.

The negative inotropic effect of neuropeptide Y on the ventricular cardiomyocyte

The effect of neuropeptide Y (NPY) on cell contractions of ventricular myocytes isolated from the adult rat heart was investigated. Maximum changes in cell length (dL) during stimulated (0.5 Hz) contractions were determined in presence of the phosphodiesterase inhibitor Ro 20-1724 (0.5 mM) and adenosine deaminase (5 U/ml). Under these basal conditions NPY (10(-6) M) reduced dL by 39% of control. Isoproterenol (10(-6) M) increased dL by 105% of control; the EC50 was 2 x 10(-9) M. NPY reduced the increase in dL achieved by isoproterenol in a dose dependent manner. The IC50 value was 1 x 10(-9) M and NPY (10(-6) M) produced complete inhibition. In the absence of the phosphodiesterase inhibitor the IC50 was 4 x 10(-9) M. The EC50 of isoproterenol and IC50 of NPY producing accumulation of cAMP in myocytes (Millar et al. 1988) exceeded the respective values of dL by one order of magnitude. Prior treatment of the myocytes with pertussis toxin abolished the potency of NPY to antagonize the increase in dL by isoproterenol while not interfering with the response to the beta-agonist. These results demonstrate a negative inotropic effect of NPY on the ventricular myocardial cell. Complete abolition of the effect of NPY by pertussis toxin indicate that this effect is mediated by a sarcolemmal receptor for NPY linked to adenylate cyclase via an inhibitory guanine nucleotide binding protein.