EFFECTS OF ADRENALINE, OUABAIN, AND SOME IONIC CONDITIONS ON TWITCH CONTRACTION OF BULLFROG VENTRICLE

Background: Cardiac hypertrophy accompanying pregnancy has generally been categorized as physiologic hypertrophy similar to that seen with exercise, however a reduction in cardiac function in late pregnancy has been suggested. Furthermore, the hemodynamic stress of pregnancy can induce a maladaptive, pathologic hypertrophy in a small number of women. This study seeks to characterize the contractile properties of late-pregnant myocardium. Methods and Results: Late Pregnancy (LP) Female Swiss-Webster mice were bred then studied at near term (Embryonic day 17-19) and compared to age-matched, non-pregnant (NP) controls. Individual cardiac myocytes were isolated using collagenase-based perfusion technique. Two-dimensional Surface Area measured in quiescent cells was elevated (p<.01) in LP myocytes (LPM) (3609± 132u 2 ) vs NP myocytes (NPM) (2736± 88u 2 ), and this increase was due to increases in both length (8.5%) and width (15.6%). Western Blot analysis showed a reduction in Ryanodine Receptor protein in LP, but no differences in L-type Ca Channel, SERCA or Phospholamban levels. Sarcomere length (light diffraction) and Ca 2+ transients (fluo-3) were measured at pacing rates of 1 Hz and at bath [Ca] of 2mM. Duration of twitch contraction was prolonged (p<.05) in LPM as measured by Time to 75% Recovery (.42 ± .02 vs .37 ±.01 sec in NPM) and Time to 90% recovery (.51 ± .02 vs .45 ± .02 sec in NPM). There were no differences in other contractile parameters measured or in the fluo-3 calcium transient properties. 10 -7 M Isoproterenol (ISO) was used to determine the responsiveness to adrenergic stimulation. ISO induced significantly enhanced contractility in both LPM and NPM, and the response was heightened in LPM such that the presence of ISO normalized the differences in the duration of twitch contraction between LPM and NPM. Conclusions: These results suggest that hypertrophied LPM have characteristics of both physiologic and pathologic hypertrophy including enhanced responsiveness to ISO and a prolonged relaxation phase. The prolongation of relaxation is not seen in physiologic hypertrophy induced by exercise and may contribute to the diastolic dysfunction reported in some pregnancies. Enhanced response to ISO suggests an increased cardiac reserve in LPM.

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Modulation of Ca2+ transient decay by tension and Ca2+removal in hyperthyroid myocardium

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1999.276.1.h289 ◽

1999 ◽

Vol 276 (1) ◽

pp. H289-H299 ◽

Cited By ~ 3

Author(s):

Tetsuya Ishikawa ◽

Hidetoshi Kajiwara ◽

Satoshi Kurihara

Keyword(s):

Sarcoplasmic Reticulum ◽

Decay Time ◽

Uptake Rate ◽

Muscle Length ◽

Length Change ◽

Troponin C ◽

Concentration Change ◽

Twitch Contraction ◽

Significant Dependence ◽

Dependent Change

We investigated the contribution of sarcoplasmic reticulum (SR) and Na+/Ca2+exchanger in the tension-dependent change in the decay of the Ca2+ transients (CaT) in euthyroid (Eu) and hyperthyroid (Hy) myocardium. Hy was induced by thyroxine treatment to enhance the rate of SR Ca2+ uptake. With the use of the aequorin method, CaT and tension in twitch contraction were simultaneously measured under various conditions (changing muscle length and Ca2+ concentration in solution). In both groups, the decay time of CaT (DT) showed a significant dependence on the developed tension, but the tension dependence of DT in Hy was significantly less than in Eu. In the presence of caffeine (3 mM), the tension dependence of DT in Hy became apparent as in Eu. Inhibition of Na+/Ca2+exchanger by replacing Na+ with Li+ did not affect the dependence in Hy. The normalized extra Ca2+, which is the Ca2+ concentration change in response to a quick length change, in Hy was similar to that in Eu. pCa-tension relations of skinned trabeculae measured at different lengths (1.9 and 2.3 μm) were nearly identical in both groups. These results indicate that the tension-dependent change in the affinity of troponin C for Ca2+works in both Eu and Hy myocardium and that the tension-dependent change in DT is influenced by the Ca2+ uptake rate of SR.

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Anatomic and physiological characteristics of the ferret lateral rectus muscle and abducens nucleus

Journal of Applied Physiology ◽

10.1152/japplphysiol.00580.2007 ◽

2007 ◽

Vol 103 (5) ◽

pp. 1706-1714 ◽

Cited By ~ 5

Author(s):

Keith N. Bishop ◽

J. Ross McClung ◽

Stephen J. Goldberg ◽

Mary S. Shall

Keyword(s):

Motor Units ◽

Lateral Rectus ◽

Contraction Time ◽

Fatigue Index ◽

Muscle Twitch ◽

Abducens Nucleus ◽

Single Motor Unit ◽

Twitch Contraction ◽

Whole Muscle ◽

Individual Motor

The ferret has become a popular model for physiological and neurodevelopmental research in the visual system. We believed it important, therefore, to study extraocular whole muscle as well as single motor unit physiology in the ferret. Using extracellular stimulation, 62 individual motor units in the ferret abducens nucleus were evaluated for their contractile characteristics. Of these motor units, 56 innervated the lateral rectus (LR) muscle alone, while 6 were split between the LR and retractor bulbi (RB) muscle slips. In addition to individual motor units, the whole LR muscle was evaluated for twitch, tetanic peak force, and fatigue. The abducens nucleus motor units showed a twitch contraction time of 15.4 ms, a mean twitch tension of 30.2 mg, and an average fusion frequency of 154 Hz. Single-unit fatigue index averaged 0.634. Whole muscle twitch contraction time was 16.7 ms with a mean twitch tension of 3.32 g. The average fatigue index of whole muscle was 0.408. The abducens nucleus was examined with horseradish peroxidase conjugated with the subunit B of cholera toxin histochemistry and found to contain an average of 183 motoneurons. Samples of LR were found to contain an average of 4,687 fibers, indicating an LR innervation ratio of 25.6:1. Compared with cat and squirrel monkeys, the ferret LR motor units contract more slowly yet more powerfully. The functional visual requirements of the ferret may explain these fundamental differences.

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Adaptations in physiological properties of rat motor units following 5 weeks of whole-body vibration

Applied Physiology Nutrition and Metabolism ◽

10.1139/apnm-2012-0478 ◽

2013 ◽

Vol 38 (9) ◽

pp. 913-921 ◽

Cited By ~ 2

Author(s):

Dawid Łochyński ◽

Marcin Bączyk ◽

Dominik Kaczmarek ◽

Maria Jolanta Rędowicz ◽

Jan Celichowski ◽

...

Keyword(s):

Myosin Heavy Chain ◽

Heavy Chain ◽

Whole Body Vibration ◽

Motor Units ◽

Medial Gastrocnemius ◽

Whole Body ◽

Force Frequency ◽

Body Vibration ◽

Myosin Heavy Chain Isoform ◽

Twitch Contraction

The purpose of the study was to determine the effects of 5-week whole-body vibration (WBV) on contractile parameters and force–frequency relationship of functionally isolated motor units of the rat medial gastrocnemius muscle: fast fatigable (FF), fast fatigue-resistant (FR), and slow (S). Moreover, myosin heavy chain isoform content was quantified. Following WBV, the maximum tetanic force of FF units was increased by ∼25%. The twitch half-relaxation time in all types of motor units and the twitch contraction time in FR units were shortened. The twitch-to-tetanus force ratio was decreased and the force–frequency curves were shifted rightwards in S and FR units. Myosin heavy chain distribution was not changed. These findings suggest modifications of the excitation–contraction coupling towards shortening of a twitch contraction. The observed increase in force of FF units may contribute to gains in muscle dynamic strength reported following WBV treatment.

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Constancy and variability of contractile efficiency as a function of calcium and cross-bridge kinetics: simulation

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1996.270.4.h1501 ◽

1996 ◽

Vol 270 (4) ◽

pp. H1501-H1508 ◽

Cited By ~ 1

Author(s):

H. Yamaguchi ◽

M. Takaki ◽

H. Matsubara ◽

S. Yasuhara ◽

H. Suga

Keyword(s):

Rate Constants ◽

Mechanical Energy ◽

Muscle Length ◽

Dissociation Rate ◽

Twitch Contraction ◽

Cross Bridge ◽

Cardiac Contractile ◽

Total Mechanical Energy ◽

Isometric Twitch ◽

Dissociation Rate Constants

We simulated myocardial Ca2+ (Ca) and cross-bridge (CB) kinetics to get insight into the experimentally observed constancy and variability of cardiac contractile efficiency in generating total mechanical energy under various inotropic and pathological conditions. The simulation consisted of a Ca transient, Ca association and dissociation rate constants of troponin C, and CB on and off rate constants. We evaluated sarcomere isometric twitch contractions at a constant muscle length. We assumed that each CB cycle hydrolyzes one ATP and that the force-length area (FLA) quantifies the total mechanical energy generated by CB cycles in a twitch contraction. FLA is a linear version of pressure-volume area, which quantifies the total mechanical energy of cardiac twitch contraction and correlates linearly with cardiac oxygen consumption (H. Suga, Physiol. Rev. 70: 247-277, 1990). The simulation shows that the contractile efficiency varies with changes in the Ca transient and Ca and CB kinetics except when they simultaneously speed up or slow down proportionally. These results point to possible mechanisms underlying the constancy and variability of cardiac contractile efficiency.

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Effects of thyroidectomy on development of skeletal muscle in fetal sheep

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1991.261.5.r1300 ◽

1991 ◽

Vol 261 (5) ◽

pp. R1300-R1306 ◽

Cited By ~ 7

Author(s):

D. I. Finkelstein ◽

P. Andrianakis ◽

A. R. Luff ◽

D. Walker

Keyword(s):

Skeletal Muscle ◽

Thyroid Gland ◽

Extensor Digitorum Longus ◽

Relaxation Times ◽

Medial Gastrocnemius ◽

Fetal Sheep ◽

Twitch Contraction ◽

Slow Twitch ◽

Contraction And Relaxation ◽

Fast Twitch

The influence of the thyroid gland on the functional and histochemical development of fast- and slow-twitch skeletal muscle of fetal sheep has been studied in euthyroid fetal sheep (n = 6) and athyroid fetuses (n = 4) surgically thyroid-ectomized at 70-75 days of gestation. Two fast-twitch muscles, the medial gastrocnemius and extensor digitorum longus, and the slow-twitch soleus muscle were studied at the fetal age of 140 days gestation. The athyroid fetuses had significantly slower twitch contraction and relaxation times in both the medial gastrocnemius and extensor digitorum longus muscles compared with the euthyroid fetuses. Twitch contraction and relaxation times of the soleus were not different in the two groups. Thyroidectomy resulted in an increase in the proportion of fast (type II) muscle fibers and myosin, as shown histochemically and by gel electrophoresis of heavy-chain myosins. These results indicate that the functional maturation of the fast-twitch muscles of sheep is influenced by the presence of an intact thyroid gland from at least 70 days of gestation. In contrast, the slow-twitch soleus muscle fiber diameter and twitch contraction and relaxation times were not different in the two groups.

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Neural and muscular determinants of maximal rate of force development

Journal of Neurophysiology ◽

10.1152/jn.00330.2019 ◽

2020 ◽

Vol 123 (1) ◽

pp. 149-157 ◽

Cited By ~ 4

Author(s):

Jakob L. Dideriksen ◽

Alessandro Del Vecchio ◽

Dario Farina

Keyword(s):

Motor Unit ◽

Maximum Rate ◽

Rapid Development ◽

Motor Units ◽

Rate Of Force Development ◽

Time Range ◽

Limiting Factor ◽

Force Development ◽

Twitch Contraction ◽

Discharge Rates

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals. NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

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