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PubMedDate not supplied

[Age-related aspects of the reaction of isolated strips of rat rectum and colon to carbacholine, atropine and pentamine].

Experiments in vitro were made to record isotonic contractions of isolated strips of the rectum and colon of rats distributed into 4 age groups. It was shown that in one-week-old rats, the rectal and colonic strips reacted to carbacholine (1 x 10(-10--1 x 10(-5) M) to a far less extent than those in adult rats. In the course of postnatal development the reaction of the colonic strips to carbacholine reached the adult rat level more rapidly. Pretreatment of the colonic strips from one- and two-week-old rats with pentamine solutions (2 x 10(-5)--1 x 10(-4) M) led to the increased carbacholine response rather than to its lowering which was the case in the adult animals. Pretreatment of the strips in question with atropine solutions (1 x 10(-14)--1 x 10(-6) M) decreased the response to the cholinomimetic to a more significant extent than in the adult animals. High concentrations of the cholinomimetic did induce a considerable percentage of contractions.

Aging↗
PubMed1977

Numerical study of the behavior of an activation parameter in a sliding filament cat papillary muscle model.

A previously existing model of cross-bridge kinetics is modified to make it applicable to cardiac muscle and incorporated into a macroscopic model of isolated heart muscle. Equations are formulated which describe the responses of this model during simulated isometric and isotonic contractions. These equations allow one to calculate the time variation of the activation parameter in the model when given the contraction time history, i.e. the time course of length and tension changes. The activation parameter is defined as the instantaneous rate constant for cross bridge attachment. We calculate the time course of the activation parameter with the model's responses set equal to the measured responses of isolated cat papillary muscle and the model parameters chosen appropriately for cat papillary muscle. It is found that in order for the model and muscle response to match, the degree of activation of the muscle model must first increase and then decrease with shortening. The behavior of the activation parameter in the muscle model is consistent with physiological investigations which suggest that shortening in cardiac muscle is associated with both activation and deactivation.

Animals↗
PubMed1993

Ascending enteric reflex contraction: roles of acetylcholine and tachykinins in relation to distension and propagation of excitation.

The ascending reflex contraction of intestinal circular muscle involves both cholinergic and tachykininergic transmission, which are thought to be activated by different degrees of distension. Substance P, however, is colocalized with acetylcholine in myenteric neurons, and the present study examined the role of these transmitter substances in relation to low- and high-degree distension and with regard to ascending propagation of excitation. Reflex contractions in segments of the guinea pig isolated small intestine were evoked by inflation of an intraluminal balloon and recorded orally to the site of distension. Atropine (12.5 nM), hexamethonium (3 microM) and the neurokinin (NK)2-selective tachykinin antagonist MEN 10,376 (10 microM) inhibited contractions induced by low-degree distension to a larger extent than contractions induced by high-degree distension, whereas the receptor-nonselective tachykinin antagonist spantide (30 microM) did not differentiate in this way. Atropine, hexamethonium and spantide also depressed the propagation of excitation, i.e., the response recorded 2 cm away from the distension site was inhibited to a larger degree than the response recorded 1 cm away from the distension site. In contrast, MEN 10,376 did not interfere with the ascending propagation of excitation, and the NK1-selective tachykinin antagonist GR 82,334 (10 microM) was without effect on the ascending reflex contraction. These observations show that tachykinins and acetylcholine comediate ascending reflex contractions triggered by both low- and high-degree distension. When seen in context with the known projection of myenteric neurons, the findings relating to the ascending propagation of excitation indicate that NK2 receptors participate in neuromuscular transmission only, whereas neuroneuronal transmission involves both nicotinic and muscarinic acetylcholine receptors.

Acetylcholine↗
PubMed1994

The back compressive forces during maximal push-pull activities in the sagittal plane.

Ten normal young male and ten normal young female subjects (each group with a mean age of 21.1 years) performed isometric and isokinetic (50 cm per second) push and pull activity at 35 cm, 100 cm and 150 cm heights. The subjects were placed on a specially designed subject-stabilizing-platform to stabilize their lower extremities. Horizontal push-pull forces were exerted through a friction-reduced rod and sleeve assembly attached to the modified Static Dynamic Strength Tester. The strength measured by a SM 500 load cell was fed to an IBM XT through an A to D converter. The postural records were made on a videotape. The posture and strength were synchronized through an external light signal. The strength for pull activities was higher than the corresponding push activities (p < 0.01). The isometric strengths were significantly higher than the isokinetic strengths (p < 0.01). Though the push strengths were significantly lower than the pull strength, the low-back compressive forces for the push activities were 129% to 627% of the corresponding pull conditions. It is concluded that the push activities are more hazardous due to the higher magnitude of compressive load and their faster contribution to the threshold level of cumulative load leading to the precipitation of injuries.

Adolescent↗
PubMed1995

Compatibility of adaptive responses with combining strength and endurance training.

Impairment in strength development has been demonstrated with combined strength and endurance training as compared with strength training alone. The purpose of this study was to examine the effects of combining conventional 3 d.wk-1 strength and endurance training on the compatibility of improving both VO2peak and strength performance simultaneously. Sedentary adult males, randomly assigned to one of three groups (N = 10 each), completed 10 wk of training. A strength-only (S) group performed eight weight-training exercises (4 sets/exercise, 5-7 repetitions/set), an endurance-only (E) group performed continuous cycle exercise (50 min at 70% heart rate reserve), and a combined (C) group performed the same S and E exercise in a single session. S and C groups demonstrated similar increases (P < 0.0167) in 1RM squat (23% and 22%) and bench press (18% for both groups), in maximal isometric knee extension torque (12% and 7%), in maximal vertical jump (6% and 9%), and in fat-free mass (3% and 5%). E training did not induce changes in any of these variables. VO2peak (ml.kg-1.min-1) increased (P < 0.01) similarly in both E (18%) and C (16%) groups. Results indicate 3 d.wk-1 combined training can induce substantial concurrent and compatible increases in VO2peak and strength performance.

Adaptation, Physiological↗
PubMed1995

Blood flow and pressure relationships which determine VO2max.

The role of O2 delivery in regulating VO2max has been studied in an isolated gastrocnemius-plantaris muscle preparation contracting in situ; recent data addressing this issue are presented. VO2 increases nonlinearly with stimulation frequency reaching a peak at 5 twitches.s-1 or 1 tet.s-1 (200 ms trains, 50 imp.s-1). Further increases in stimulation frequency result in a lower VO2. Measured VO2 maxima are less than predicted VO2 capacity, and peak VO2 during tetanic contractions is greater than that during twitches. Above 150 imp.min-1, VO2 is directly related to the level of blood flow attained as VO2/Q (arterial-venous O2 difference) is fixed by some unknown mechanism. Increasing blood flow, with a pump, during 1.s-1 tetanic contractions increases O2 diffusive conductance and peak VO2. When O2 delivery is reduced, ischemic hypoxia appears to result in more rapid reductions in muscle performance than hypoxic hypoxia because of decreases in perfusion pressure and Q. 31P-NMR studies reveal that reductions in creatine phosphate and energy charge are similar between ischemia and hypoxia suggesting a common regulator, O2. We conclude that VO2max is limited by O2 delivery as a result of a limited and uneven distribution of muscle blood flow. These limitations appear secondary to mechanical restraints imposed by contraction duty cycle and vascular compression.

Animals↗
PubMed1996

Hemodynamic effects of aerobic vs resistance exercise.

Previous studies suggest that aerobic exercise lowers blood pressure (BP), while isometric exercise increases BP, at least transiently. The purpose of this study was to examine the hemodynamic effect of a 6-week training period of aerobic exercise or weight training. Twenty deconditioned healthy males ages 18-36, self-selected a training regimen. The aerobic group exercised 30 min/day, 4 times each week to achieve 60-80% maximal heart rate. The resistance group lifted weights at 65-80% maximal voluntary contraction; 3-4 sets of 8-12 repetitions; 3 day/week using large muscle groups. Hemodynamic measurements of heart rate, BP, venous capacitance, forearm blood flow, and vascular resistance were made at baseline and week 6 by plethysmography and analyzed by 2-way ANOVA. The groups showed no differences in baseline characteristics. A training effect was confirmed by a decrease in resting heart rate in the aerobic group (71.5 +/- 4.4 to 64.5 +/- 3.7, beats per minute, P = 0.004), and an increase in total work capacity in the weight lifting group (6231 vs 7508, P = 0.01). Forearm blood flow increased similarly in both groups, averaging 17% (3.5 +/- 0.2 vs 4.2 +/- 0.2 ml 100 g/min, P = 0.03), while forearm vascular resistance fell 19% (28.8 +/- 1.7 vs 24.3 +/- 1.7 mm Hg/ml/min 100 g, P = 0.08). The main differences between the groups after training was found in their response to isometric stress (1/3 maximal handgrip). The weight-lifting group had a greater increase of forearm blood flow and venous capacitance, less increase in systolic BP (SBP) and a greater fall of forearm vascular resistance, (P < 0.05) while the aerobic group had less increase in SBP and heart rate (P < 0.04) but no significant change of forearm hemodynamics. We conclude that both aerobic and repetitive weight programs have short term favorable effects on resting forearm BP and resistance. The exercise programs differ in altering the individual's physiologic response to subsequent isometric stress. However, exercise training of longer duration or greater intensity or frequency could alter these results.

Adult↗
PubMed1997

Effects of methylene blue on the development of intrinsic contractile responses of the guinea pig tracheal smooth muscle.

The aim of the present study was to investigate the effects of methylene blue, a guanylyl cyclase inhibitor, on the development of intrinsic contractile responses of the guinea pig tracheal smooth muscle. Paired tracheal chains were mounted for isotonic contractions under 500 mg of tension in Krebs-Henseleit solution. The intrinsic contractile tone modulated carbachol-induced contractions and was inhibited by indomethacin, suggesting the involvement of cyclooxygenase products on this tone. Methylene blue (5 x 10(-5)M) irreversibly inhibited the intrinsic contractile responses. Considering that methylene blue prevents any endogenous production of cGMP, it would be expected to enhance the contractions. However, since methylene blue has effects over nitric oxide synthase and nitric oxide itself, we suggest that guanylyl cyclase activation is not important for the development of the intrinsic tone.

Animals↗
PubMed1998

Effects of methylene blue and indomethacin on sodium nitroprusside-induced relaxations of the guinea pig tracheal smooth muscle.

The aim of the present investigation was to evaluate the potential modulatory effect of the guinea pig tracheal smooth muscle intrinsic tone on the relaxant responses to sodium nitroprusside and the effects of methylene blue on this response. Paired tracheal chains were mounted for isotonic contractions under 500 mg of tension in Krebs-Henseleit solution. The intrinsic modulatory tone was inhibited by indomethacin, in a concentration that did not have any effect over carbachol induced contractions. Sodium nitroprusside-induced relaxations were the same in the absence or presence of the modulatory tone. Methylene blue inhibited 50% sodium nitroprusside-induced relaxations, in the presence or absence of the intrinsic system. This suggests that sodium nitroprusside-induced relaxations are mediated through guanylyl-cyclase activation and that these are not under the modulation of the intrinsic prostaglandinergic tone.

Animals↗
PubMed1999

Exercise-induced altitude decompression sickness.

BACKGROUND: It has been known since World War II that exercise at altitude increases incidence of decompression sickness (DCS). However, data on the effects of specific exercise types at altitude are lacking. This research focused on the relative hazards of exercise without motion (isometric, straining) vs. dynamic exercise involving motion. The study also compared arm vs. leg exercise. METHODS: There were 32 healthy male subjects exposed, while resting, to 29,500 ft (8992 m) for 4 h or until DCS occurred, at which time they were brought to ground level. If the subject developed DCS on this exposure, he was exposed in successive months to lower altitudes, using the same procedure, until the subject was free of symptoms for the 4-h exposure. At this symptom-free altitude, as low as 20,000 ft (6096 m), the subject performed isometric arm, isometric leg, dynamic arm and dynamic leg exercises at less than 10% of maximal oxygen consumption, each during separate exposure months. Precordial venous gas emboli (VGE) were monitored every 20 min during each exposure with a Hewlett-Packard SONOS 1000 Echo Imaging System. RESULTS: Dynamic arm, dynamic leg, isometric arm, and isometric leg exercise induced DCS in 50%, 38%, 41% and 31% of the subjects, respectively. VGE incidence varied from 47-66%. No significant differences in DCS or VGE were found. CONCLUSIONS: Under our test conditions, there was no difference between dynamic and isometric exercise in eliciting DCS. Exercise during exposure to the symptom-free altitude for 4 h produced a 40% incidence DCS.

Adult↗
PubMed1975

Cardiac muscle models for both isotonic and isometric contractions.

The choice of a suitable muscle model consistent with the mechanical behaviour of Rat papillary muscle at rest or during isotonic or isometric contraction has been considered. Three different preload levels within the ascending limb of the Frank-Starling curve have been used in five papillary muscles. Series elastic (SE) and parallel elastic (PE) length-tension relationships have been evaluated according to five mechanical muscle models using data from systolic and diastolic quick-release manoeuvres. For each preload level and muscle model, the time course of the force-velocity relationship of the contractile element (CE) has been calculated using a PDP 8/L digital computer. Several mechanical and biological characteristics are considered which exclude four of the five selected muscle models. The Maxwell model with a damper seems to be the most suitable mechanical model for Rat papillary muscle. According to the selected muscle model it seems that VCEmax is dependent on muscle length variation; the reliability of this parameter as an index of myocardial contractility is discussed.

Animals↗
PubMed1988

Efficiency of work performance and contraction velocity in isotonic tetani of frog sartorius.

Mechanical work, ATP, ADP, PC, free creatine and lactate concentrations were determined on IAA poisoned frog sartorii tetanically stimulated in humidified N2 at 10 degrees C in isotonic conditions (0.25 or 0.45 Po). Tetanus duration was 0.35 s, number of tetani was varied from 0 (rest) to 25 (exhaustion). The mechanical work performed per mole ATP + PC split (W*P) amounted on the average to 16.7 kJ/mol. It was observed, however, that W*P increased from about 13 to about 24 kJ/mol with decreasing ATP concentration from about 2 (resting value) to about 1 mumol/g and that this decrease in ATP was associated with a decrease of the shortening (and relaxation) speed of the muscle to about 30% of the values observed on the first tetanus. It is concluded that the thermodynamic efficiency of muscle contraction, calculated from the ratio of WP* (measured) to the thermodynamic affinity (free energy change) of ATP hydrolysis (estimated) increases from about 0.3 to about 0.5 with decreasing ATP concentration and shortening speed.

Adenosine Triphosphate↗
PubMed1990

The role of series elastic structures in prestretch-induced work enhancement during isotonic and isokinetic contractions.

The influence of series elastic structures on the dynamics of the contractile machinery was examined in the gastrocnemius medialis (GM) of five male Wistar rats, with respect to enhancement of work of a muscle-tendon complex after active stretch. Imposed isotonic and isokinetic contractions were preceded by either an isometric phase (PI) or an active stretch (PS). The effects of fibre length differences at the onset of shortening, due to differences of extension of tendinous structures, were studied. For the isotonic experiments fibre length and shortening velocity were estimated 30 ms after release and compared with the PI length-velocity curve determined at the same force level. For shortening above the optimum length, about half of the enhanced shortening found after prestretch could be explained by PS-PI fibre length differences. Below the optimum length, PS shortening velocity was somewhat lower than expected on the basis of length-velocity characteristics. Enhancement of work output due to stretch was different for isokinetic and isotonic shortening. In isokinetic shortening, following prestretch, fibre work was limited because of enhanced shortening of the tendinous structures. In stretch-shortening cycles imposed on a muscle-tendon complex, the length of the complex affected all prestretch effects, i.e. potentiation of the contractile element, contractile element interaction with the tendinous structures, and elastic energy release. It is concluded that, besides potentiation effects and enhanced elastic energy release, the influence of series elastic structures on fibre dynamics determines active stretch-induced work enhancement. The contribution by these mechanisms to this work enhancement depends largely on the type of stretch-shortening cycle.

Animals↗
PubMed1989

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

We have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two force-grading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.

Humans↗
PubMed1995

Skeletal muscle contraction in healthy volunteers: assessment with Doppler tissue imaging.

PURPOSE: To evaluate Doppler tissue imaging (DTI) in the measurement and characterization of skeletal muscle movement and contraction. MATERIALS AND METHODS: Five healthy male subjects were examined with a commercially available ultrasound imaging unit modified for DTI. Velocities of the rectus femoris and gastrocnemius-soleus muscle motion were measured during the patellar and ankle reflexes, respectively. Velocity profiles were analyzed after digitization of the DTI images. Time to peak velocity, velocity range, and relaxation time were examined in each case over three contractions. For reflex contractions, reproducibility was examined by determining the latency period, area under the velocity curve, and duration of contraction for each of five consecutive reflexes. RESULTS: Isotonic muscle contraction could be distinguished from passive movement by a higher tissue velocity range, by a more rapid initial acceleration slope, and by the velocity differential across the muscle. The intensity and duration of the contraction and plateau and relaxation phases of isotonic contraction could be measured in all muscle groups. CONCLUSION: DTI provides detailed information about the dynamics of skeletal muscle contraction.

Adult↗
PubMedDate not supplied

Experimental hyperthyroidism II. Mechanics of contraction and relaxation of isolated ventricular myocardium.

The effect of experimental hyperthyroidism on myocardial mechanics was examined on isolated ventricular myocardium (right ventricular papillary muscle) of cats. 1. Isotonic muscle contraction and isometric tension development were largely unchanged compared with euthyroidism. 2. Isotonic contraction velocity and maximal isometric tension rise velocity showed considerable rises of between 41 and 78%. 3. Force-velocity relations of contraction showed changes with increases of contraction velocity with every degree of load. 4. Force-velocity relations of isotonic relaxation showed increases of isotonic relaxation maxima with comparable loads without any alteration of the typical course of the relaxation curves compared with euthyroidism. 5. Lowering the temperature (from 34 to 24 degrees C) produced a considerable fall of the raised contraction and relaxation velocity in hyperthyroidism whereas the values of muscle contraction and tension development important for the pump function were largely unchanged. It is concluded that the myocardium in experimental hyperthyroidism is characterised by a primarily velocity-related increase of inotropy. Lowering the temperature produces an effective fall of the raised velocity values. The mechanisms of the increase of inotropy and significance of the findings are discussed.

Animals↗