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PubMedDate not supplied

Ca entry and contraction as studied in isolated bovine ventricular myocytes.

Single bovine ventricular myocytes were superfused with Tyrode solution containing 1.8 mM CaCl2. The cells did not bear external load and contracted isotonically. Contraction and relaxation were characterized by the shortening and relengthening of the sarcomeres which resembled in their time course the isometric twitches of bovine papillary muscles. Resemblance was also found in regard to positive inotropic interventions as increase in the stimulation frequency, exposure to elevated [Ca]0 or to adrenaline. A two-microelectrode voltage-clamp technique was applied to the single myocyte. The transmembrane Ca inward current ICa was defined as difference current sensitive to 5 mM Ni or to 2 microM D600. During a voltage step from -45 to +5 mV, ICa peaked within 3 ms to -6 nA, afterwards it decayed to 15% of peak amplitude (incomplete inactivation with a 2 exponential time course). Experiments in Na-free media suggested that Na entry does not significantly contaminate ICa. Therefore, Ca entry could be calculated from ICa. The increment in total intracellular Ca concentration (delta[Ca]Ti) was estimated by referring Ca entry to the cell volume (50 pl). Within 100 ms delta[Ca]Ti came to 25 microM at control conditions, to 55 microM at [Ca]0 = 3.6 mM and to 88 microM when 0.1 microM adrenaline were present. The delta[Ca]Ti values were sufficient to activate contraction without the necessity of Ca-release from SR. Despite the new data, the relationship between Ca entry and activation of contraction was complex: during the "positive Herztreppe" ICa slightly attenuated but contractility doubled. Therefore, the old EC-model (M. Morad and Y. Goldman, Progr. Biophys. Mol. Biol. 27, 257 (1973)) was adapted. The Ca-entry's capability to load and to overload the intracellular Ca store (SR) is discussed.

Animals↗
PubMed1975

Feedback of isotonic muscle contraction on neuromuscular impulse transmission in the frog.

The effect of isotonic contraction on neuromuscular impulse transmission was studied in the in vivo M. gastrocnemius preparation of the frog. The N. ischiadicus was stimulated regularly with single pulses at different frequencies in the range of 1/8 to 8 Hz. When steady state conditions were reached, each half-minute a second stimulus was added to one of these pulses. The interval between the pulses of the pair was varied within the contraction cycle of the first stimulus. Compound extracellular or single intracellular action potentials were recorded from the muscle. At frequencies of 1/2-2 Hz a depression of the amplitude of the second compound muscle action potential of up to 50% of the first response was found when the muscle contracted isotonically. However, the second response was facilitated during an isometric contraction. The time course of the depression was equal to that of muscle shortening during the twitch, while the time course of facilitation corresponded roughly to that of facilitation of transmitter release. At lower frequencies the "isotonic depression" or "isometric facilitation" was not or only slightly present. However, at 1/8 Hz the depression could be evoked or increased by curarization. At frequencies higher than 2 Hz facilitation dominated over depression under isotonic conditions. With flexible intracellular micro-electrodes it was shown that the depression of the amplitude of the compound muscle action potential observed during the isotonic twitch was due to a reduction in neuromuscular impulse transmission. It is concluded that the isotonic depression is a negative feedback effect of the change of length of the contracting muscle on synaptic impulse transmission, probably due to an effect of length on transmitter release.

Action Potentials↗
PubMed1996

[Contribution of the vascular interstitial matrix to induced contraction by angiotensin II. In vitro studies on the rabbit aorta, mesenteric artery and vein].

This study examines the contribution of hyaluronan, a rich anionic glycoprotein, to angiotensin-II-induced contraction (AII: 10(-11) to 10(-8) M) of endothelium-free strips of aorta, mesenteric artery and vein obtained from normal rabbits. Tissues are treated with hyaluronidase (HYAL: 1 mg/ml) during 60 min before being mounted in organ baths superfused with normal Krebs solution for isotonic contraction. Isotonic contraction of the mesenteric artery to the four highest doses of AII is reduced by 50 to 60% following HYAL treatment, compared to the normal contraction curve (0.01 < p < 0.001). Isotonic contraction of the aorta and mesenteric vein to AII is not influenced by HYAL. Isometric contraction curves of the three tissues to AII are not modified by HYAL. In additional experiments, the Krebs solution was selectively enriched in calcium (3.8 mM/l) and in sodium (160 mEq/l) to verify if the effect of HYAL is associated with interstitial washing in the concentration of these cations, because of the hyaluronan digestion. In fact, the calcium-rich superfusion is associated with complete correction of the HYAL-induced reduction of the mesenteric artery isotonic contraction. The sodium-rich superfusion failed to normalize the depressed mesenteric artery contraction. Since HYAL only affected isotonic contraction of the resistance artery (mesenteric), it is likely that the interstitial space of this tissue contains more hyaluronan than the aortic or mesenteric vein matrix, or that HYAL only affected the smooth muscle cell population involved in the circular tonus of the resistance vessel. Correction of the abnormality by calcium enrichment of the Krebs solution suggests that a relative diminution and/or a redistribution of this important cation, obtained following the interstitial degradation of hyaluronan.

Angiotensin II↗
PubMed1986

Characterization of the mechanical behavior of isolated papillary muscle preparations of the ferret.

The characteristics of isometric and isotonic contraction were studied in isolated papillary muscle preparations from the right ventricle of the ferret and compared with those previously reported for comparable preparations from cat and rat. The effects of stimulation rate, temperature, extracellular calcium concentration, caffeine, verapamil, and ouabain were measured. Afterloaded contractions and load clamps were used to study the mechanics of relaxation. The contractile characteristics of ferret papillary muscle preparation appear to be intermediate in many respects between those reported for cat and rat preparations.

Animals↗
PubMed1985

Force-velocity properties of fast-twitch and slow-twitch muscles of the kitten.

The characteristics of the isometric and force-velocity properties of the fast-twitch flexor digitorum longus (f.d.l.) and the slow-twitch soleus muscles of the developing kitten were determined at post-natal ages of 2 days and 1,2,4 and 6 weeks. The contractile properties of the isometric twitch and tetanus were similar to those previously reported by other authors. The minimum stimulation frequency required for isotonic contractions to achieve the maximum speed of shortening at any given load was 200 Hz for f.d.l. and 150 Hz for soleus at all ages. The extrapolated maximum speed of sarcomere (sarc.) shortening (Vmax) was significantly different between the f.d.l. (22.3 microns s-1 sarc.-1) and soleus (13.1 microns s-1 sarc.-1) at 2 days of age. The f.d.l. Vmax achieved adult values by 4 weeks of age, while there was little or no change in the soleus Vmax. The force-velocity properties are discussed and compared to the changes which have been observed in other contractile properties of the cat f.d.l. and soleus muscles.

Animals↗
PubMed1986

Efferent discharges recorded from single skeletomotor and fusimotor fibres in man.

Experiments were performed on awake human subjects in which single nerve fibre activity was recorded in the lateral peroneal nerve using tungsten micro-electrodes as described by Hagbarth & Vallbo (1967, 1968a). The discharge of twelve single efferent fibres innervating the tibialis anterior muscle (t.a.) or the extensor digitorum longus muscle (e.d.l.) was recorded. On the basis of their functional activity, six fibres were identified as skeletomotor and six as fusimotor fibres. Skeletomotor fibres, which were completely silent in relaxed subjects, discharged when subjects performed voluntary isometric or isotonic contractions, they also fired during Jendrassik's manoeuvre and tonic vibration reflex (t.v.r.) induced by mechanical vibration applied to the distal muscle tendon. Units considered as fusimotor fibres were generally spontaneously active with some fluctuation in the discharge frequency. Various tests used to identify afferent fibres elicited no response of these fibres (nor of the skeletomotor fibres). Efferent fibres were considered as fusimotor because their discharges were uncorrelated with any activation of extrafusal muscle fibres. Several means were used to detect activation of extrafusal fibres: surface electromyogram (e.m.g.) electrodes, tungsten electrodes deeply implanted in the muscle and especially the use of a high-sensitivity tension transducer (0 X 1 mN) placed on muscle tendons. The activity in fusimotor fibres could be either elicited or modulated under the following conditions: clenching of the fists, pinna twisting, mental computation, voluntary isometric contraction, passive phasic stretch of the muscle, environmental disturbances, subject laughing, the sound of hand clapping, and subject listening to manoeuvre instructions. Moreover, during spontaneous fusimotor fibre activity the subject was able to voluntarily stop the unit discharge. The results are compared to those obtained in animal studies and discussed with reference to the notion of alpha-gamma linkage, static and dynamic gamma-motoneurone activities, and to other available data concerning the effects of various stimulations on muscle spindle afferent activities in man.

Action Potentials↗
PubMed1995

Papillary muscles split in the presence of 2,3-butanedione monoxime have normal energetic and mechanical properties.

A number of studies have used 2,3-butanedione monoxime (BDM) to avoid myocardial damage when small muscle preparations were cut from large hearts. The present study investigates the mechanical and energetic effects of varying muscle cross-sectional area (CSA) by dissection in physiological saline containing BDM. By use of adult rat hearts, three muscle groups were obtained: whole left ventricular papillary muscles (Whole) and left ventricular papillary muscles split longitudinally in the presence of 30 mM BDM, with removal of approximately 10% (BDMSP1) or 40-50% (BDMSP2) of the muscle (5 animals in each group). The isolated muscle preparations were studied at 27 degrees C and stimulated at 0.167 Hz. The Whole and BDMSP1 preparations had comparable CSAs; in isotonically contracting muscles working against a range of afterloads, work, enthalpy (energy use), and mechanical efficiency (work/enthalpy x 100%) were similar for the two groups. In addition, isometric performance [e.g., developed stress (force/CSA), length-tension relationship, and contraction time course] was also similar for the two groups. The thinner BDMSP2 preparations showed an enhanced mechanical performance compared with the Whole and BDMSP1 groups. This outcome was in accordance with data in the literature documenting a negative correlation between stress and CSA. The results suggest that BDM-split and intact papillary muscles of similar CSA have comparable energetic and mechanical properties.

Animals↗
PubMed2001

Modification of soft tissue vibrations in the leg by muscular activity.

Vibration characteristics were recorded for the soft tissues of the triceps surae, tibialis anterior, and quadriceps muscles. The frequency and damping of free vibrations in these tissues were measured while isometric and isotonic contractions of the leg were performed. Soft tissue vibration frequency and damping increased with both the force produced by and the shortening velocity of the underlying muscle. Both frequency and damping were greater in a direction normal to the skin surface than in a direction parallel to the major axis of each leg segment. Vibration characteristics further changed with the muscle length and between the individuals tested. The range of the measured vibration frequencies coincided with typical frequencies of impact forces during running. However, observations suggest that soft tissue vibrations are minimal during running. These results support the strategy that increases in muscular activity may be used by some individuals to move the frequency and damping characteristics of the soft tissues away from those of the impact force and thus minimize vibrations during walking and running.

Adult↗