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Biomedical subjects

J Bobet

Publications and source records attributed to J Bobet.

12 recordsLinked to original sources
PubMed1995

The force-interval relation in aged hamster heart.

This study determined whether aging affects the force-interval relation in cardiac muscle. Isolated left atria from young (60-120 days) and old (> 535 days) adult hamsters were paced at 1 Hz, and effects of interposing test intervals (0.3-600 sec) on the force of a test beat following the interval (force-interval relation) were examined. To quantify differences, force-interval curves were fitted with an equation which used five parameters to define the shape. At test intervals < 10 sec, force-interval curves were similar in young and aged hearts. At longer intervals, however, force was greater in atria from old animals than in atria from young animals. The parameters E0, which reflects force amplitude at long intervals, was significantly increased in old atria while the parameter gamma, which reflects the time course of force decay at long intervals, was significantly reduced. Thus, at long test intervals the force-interval relation is markedly affected by the aging process.

Aging↗
PubMed1993

A linear time-varying model of force generation in skeletal muscle.

A model of isometric force production by skeletal muscle was developed in which the response to each stimulus in a train was described by a critically damped, linear second-order system. The parameters describing the system were constrained to be constant within an interstimulus interval, but were allowed to vary between interstimulus intervals. The ability of this model to match experimental data, and the time variation in the parameters (low-frequency gain and natural frequency) required to do so were examined in soleus and plantaris muscles of the cat stimulated by synchronous whole-nerve stimulation. The model produced good fits across firing rates from twitch to tetanus for slow and fast muscle, rested and fatigued muscle, and maximal submaximal stimulation. Both gain and natural frequency generally varied smoothly and predictably under all conditions. Gain increased at intermediate stimulation rates and in potentiated muscle, and decreased with fatigue and submaximal stimulation. Natural frequency was higher in fast muscle, and decreased with stimulation rate and fatigue. This modeling approach may provide a useful alternative to current models of skeletal muscle force, as its implementation is simple and it can describe force under conditions (fatigue, potentiation) where the muscle dynamics change with time.

Animals↗
PubMed1991

Force-interval relation in normal and cardiomyopathic hamster atria.

The purpose of this study was to determine how cardiomyopathy affects the beat-to-beat regulation of contractile force in cardiac muscle. Isometric force produced by left atria from 80- to 85-day-old normal and cardiomyopathic (CM) hamsters was measured in vitro at 29 degrees C in 2.5 and 6.0 mM Ca2+. During steady-state stimulation at 1 Hz, single test stimuli were interpolated at varying test intervals (0.3-600 s). The force-interval curves were fitted with an equation using five parameters to define the curve and were compared under different conditions; the recovery of force after long rest intervals was fitted with a single exponential curve. Results showed that the force-interval curves were similar in normal and CM atria except that force was depressed at all intervals in 2.5 mM external Ca2+ concentration ([Ca2+]e) and that the parameter U(0), reflecting force produced at short test intervals, tended to be lower in CM muscles. At high [Ca2+]e (6.0 mM) the force-interval curves were similar, but recovery of steady-state force after long test intervals was much slower in CM atria (tau = 77.3 +/- 8.5 s, n = 11) than in normal atria (tau = 30.5 +/- 3.9 s, n = 11). Recovery was also slower at 2.5 mM [Ca2+]e. These findings suggest that, on a beat-to-beat basis, there is less Ca2+ available in intracellular compartments in the CM heart.

Animals↗
PubMed1991

Possible mechanisms underlying differences in force production between normal and cardiomyopathic hamster atria.

The rate of recovery of force after a long rest interval is lower than normal in left atria from 80- to 85-day-old cardiomyopathic (CM) golden Syrian hamsters. To determine whether this difference was due to a reduced amount of Ca2+ available for release with each beat, we manipulated the amount of Ca2+ entering excised atria using the Ca2+ agonist BAY K 8644 and the antagonist nifedipine. We also simulated altered Ca2+ influx in a recent model of cardiac excitation-contraction coupling (V. J. A. Schouten, J. K. Van Deen, P. de Tombe, and A. A. Verveen Biophys. J. 51: 13-26, 1987) by varying the parameter representing Ca2+ influx and observing the effect on the force it predicted. Steady-state force of normal and CM atria was recorded in response to 1-Hz stimulation and recovery of steady-state force was monitored after a 600-s rest interval. Ca2+ fluxes were manipulated by raising external Ca2+ or by the presence and absence of drug. The recovery of force after a 600-s rest interval was digitized and fitted with an exponential function, and the time constant and steady-state force to which the muscle recovered after the pause were compared. Inclusion of the Ca2+ agonist BAY K 8644 (0.25 or 2.0 microM) made the response of CM atria similar to that of normal, while inclusion of the Ca2+ antagonist nifedipine (0.8 microM) made the response of normal atria similar to that of the CM. Similarly, decreasing the simulated Ca2+ influx in the model produced all of the differences observed between normal and CM muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1990

Mechanisms relating force and high-frequency stiffness in skeletal muscle.

Muscle stiffness increases faster than muscle force during the rising phase of a tetanic contraction, and decreases more slowly during the falling phase. Different models of the stiffness arising from series, parallels, and crossbridge elasticity were compared to determine whether they could account quantitatively for the observed time course of force and stiffness. Data for slow and fast twitch mouse muscles at temperatures from 6 to 37 degrees C (Stein and Gordon, Can. J. Physiol. Pharmacol. 64, 1236-1244, 1986) and for single frog muscle fibers (Cecchi et al., Contractile Mechanisms in Muscle, pp. 641-655. Plenum, New York, 1984) were compared. The results showed that a good fit to the data for mouse muscles could be obtained with a model in which: (1) a nonlinear series elasticity contributed significantly to stiffness; (2) the attached crossbridges went from a stiff, force-generating state to a stiff, non-force-generating state; and (3) the rate of transition between these two states increased abruptly at the onset of relaxation. The increased transition rate probably arises from the internal rearrangement in which some sarcomeres shorten at the expense of other sarcomeres, once the muscle begins to relax. A significant series elasticity was not required for the frog data, but a pre-tension state was then needed to obtain a good fit.

Animals↗
PubMed1988

The kinetics relating calcium and force in skeletal muscle.

The kinetics relating Ca2+ transients and muscle force were examined using data obtained with the photoprotein aequorin in skeletal muscles of the rat, barnacle, and frog. These data were fitted by various models using nonlinear methods for minimizing the least mean square errors. Models in which Ca2+ binding to troponin was rate limiting for force production did not produce good agreement with the observed data, except for a small twitch of the barnacle muscle. Models in which cross-bridge kinetics were rate limiting also did not produce good agreement with the observed data, unless the detachment rate constant was allowed to increase sharply on the falling phase of tension production. Increasing the number of cross-bridge states did not dramatically improve the agreement between predicted and observed force. We conclude that the dynamic relationship between Ca2+ transients and force production in intact muscle fibers under physiological conditions can be approximated by a model in which (a) two Ca2+ ions bind rapidly to each troponin molecule, (b) force production is limited by the rate of formation of tightly bound cross-bridges, and (c) the rate of cross-bridge detachment increases rapidly once tension begins to decline and free Ca2+ levels have fallen to low values after the last stimulus. Such a model can account not only for the pattern of force production during a twitch and tetanus, but also the complex, nonlinear pattern of summation which is observed during an unfused tetanus at intermediate rates of stimulation.

Actins↗
PubMed1984

Effects of load placement on back muscle activity in load carriage.

The effect of two different load placements (just below mid-back or just above shoulder level) on erector spinae EMG, trapezius EMG, and heart rate were investigated during load carriage. The EMG and heart rates were telemetered from 11 subjects while they walked on a smooth level surface at an average velocity of 5.6 km . h-1 carrying a load of 19.5 kg in a specially designed backpack. The average rectified EMG amplitude was calculated digitally for both load placements. The high load placement resulted in significantly higher levels of muscle activity than did the lower placement. Heart rate was not significantly different between the two placements. A qualitative biomechanical analysis suggests that the EMG differences are primarily due to differences in the moments and forces arising from the angular and linear accelerations of the load and trunk. The results indicate that metabolic measures alone are not sufficient to adequately assess tasks which evoke primarily local muscle demands.

Acceleration↗