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PubMed1980

The variation in tension output with active shortening in rat myocardium.

The effect of active shortening on tension development has been studied in rat papillary muscle preparations at 20 degrees C. Total tension at the peak of a contraction with a shortening phase is reduced below the value predicted by the isometric length-tension relationship. At the same instantaneous length, total tension is less in contractions with an isotonic phase than in isometric responses and the amount of the tension deficit increases with the extent of shortening. The influence of the time at which shortening takes place, on the amount of tension deficit, has been explored. The increase in [Ca2+]o does not affect the tension deficit. Conversely an increase in [Ca2+]o associated to addition of caffeine to the bathing solution almost abolished the effect of active shortening on tension development. Several possible explanations of the reduction of tension development by shortening are discussed.

Animals↗
PubMed1992

[The protective effect of carnosine in hypoxia and reoxygenation of the isolated rat heart].

The effect of carnosine (15 mM) on the contractile activity of isolated rat hearts contracting in an isotonic regime (37 degrees C at a 5 Hz stimulation frequency) has been studied. Carnosine added to the perfusing solution had no effect on the contractile activity either in hypoxia or during reoxygenation but decreased it with a simultaneous increase in the coronary flow during reoxygenation. Carnosine inhibited by 60% the lactate dehydrogenase release from cardiac cells. A conclusion is drawn that the protective effect of carnosine is due to its membrane-stabilizing action which is implemented during inhibition of peroxidation of membrane lipids.

Animals↗
PubMed1996

Mechanical and energetic effects of cromakalim on guinea pig left ventricular papillary muscle.

The mechanical and energetic effects of the adenosine triphosphate-sensitive potassium channel opener cromakalim at 50 microM and 100 microM were determined on guinea pig left ventricular papillary muscles and compared with those of low calcium concentration (n = 9 in each group). Both concentrations of cromakalim induced a negative inotropic effect (decrease in maximum shortening velocity at preload only and maximum extent of muscle shortening at preload only, P < .01 and P < .001 at 50 and 100 microM, respectively; maximum isometric active force normalized per cross-sectional area during the isometric twitch, P < .001; positive peak of the isometric force derivative normalized per cross-sectional area, P < .001) and a negative lusitropic effect (decrease in maximum lengthening velocity at preload only and negative peak of the isometric force derivative normalized per cross-sectional area, P < .01 and P < .001 at 50 and 100 microM, respectively). At 100 microM, the decrease in relaxation parameters was more marked than that in contraction parameters under isotonic conditions, as shown by the decrease in the slope of the relationship between peak lengthening velocity and maximum extent of muscle shortening (P < .01); this result reflects an intrinsic negative relaxant effect. No relaxant effect was found under isometric conditions at either concentration. The mechanical effects of low calcium were similar to those of cromakalim 100 microM, which suggests that the drug acted mainly by reducing the cellular calcium entry. Despite the negative mechanical effect of cromakalim, mechanical efficiency was preserved. This could partly explain the cardioprotective effect of cromakalim during ischemia.

Action Potentials↗
PubMed1989

Effects of age on mechanical and structural properties of myocardium of Fischer 344 rats.

To determine whether the mechanical and structural characteristics of the myocardium from the left and right ventricles are comparable in the adult rat and whether aging affects the two ventricles in a similar manner, the left and right posterior papillary muscles of rats at 10-11 and 19-20 mo of age were studied in terms of function and quantitative morphology. At 10-11 mo of age, the duration of isometric contraction was shorter and resting tension higher in the right muscle than in the left muscle. Isotonic velocity of shortening and relengthening were quicker in the left, but time-to-peak shortening was faster in the right. With aging (19-20 mo), velocity of shortening fell in the left ventricle while duration of contraction became prolonged. No such age-related effects were found in the right papillary muscle. On a structural basis, myocardial damage in the form of replacement scarring was present in the left and right muscles at 10-11 mo of age. However, the left muscle was more severely injured, and this difference persisted with age because a comparable increase in replacement fibrosis occurred in both muscles. It was concluded that cardiac performance is distinctly different in the left and right papillary muscles and aging exerts its detrimental impact on the left more than in the right myocardium, an observation that is further supported by the magnitude of tissue injury.

Aging↗
PubMed1978

Contraction transients of skinned muscle fibers: effects of calcium and ionic strength.

Calcium and ionic strength are both known to modify the force developed by skinned frog muscle fibers. To determine how these parameters affect the cross-bridge contraction mechanism, the isotonic velocity transients following step changes in load were studied in solutions in which calcium concentration and ionic strength were varied. Analysis of the motion showed that calcium has no effect on either the null time or the amplitude of the transients. In contrast, the transient amplitude was increased in high ionic strength and was suppressed in low ionic strength. These results are consistent with the idea that calcium affects force in skeletal muscle by modulating the number of force generators in a simple switchlike "on-off" manner and that the steady force at a given calcium level is proportional to cross-bridge number. On the other hand, the effect of ionic strength on force is associated with changes in the kinetic properties of the cross-bridge mechanism.

Animals↗
PubMed1976

Work-induced potassium changes in skeletal muscle and effluent venous blood assessed by liquid ion-exchanger microelectrodes.

Using liquid ion-exchanger semimicroelectrodes with a side pore, we measured changes of extracellular potassium concentration (Ke+) in adult rabbit and cat gastrocnemius muscles and in venous effluent blood flowing from the cat gastrocnemius muscle during various bouts of activity induced by sciatic nerve stimulation. 1. Isometric tetanic contractions (at 50 Hz) of various durations caused transient accumulation of Ke+ which was non-linearly related to the duration of muscle activity. The peak values of Ke+ in response to muscle stimulation were analogous in rabbits and cats, attaining values, e.g. after a 20-sisometric tetanus, between 8-9 mEq/1K+ in both species. 2. Potassium concentration in venous effleunt blood (K+ven) was transiently increased after isometric tetani. Since blood flow was measured at the same time, it was possible to calculate the amount of K+ lost by the muscle after tetani of various durations. A 32 g gastrocnemius muscle of the cat, for example, loses 9.36 +/- 1.52 muEqK+ after a 20-s isometric tetanus, which corresponds roughly to 0.5% of the total muscle potassium content. The loss of K+ in this muscle was 29.3 pEq K+ /impulse/100 g fresh muscle tissue. 3. There was no evident difference between the amount of K+ released during isometric tetani, or tetanic contractions performed under isotonic conditions. Single twitches evoked by indirect stimulation at 1 HZ for several minutes also induced a small rise in K+ven. 4. If the loss of K+ from the muscle into the blood stream is transiently prevented by arterio-venous occlusion installed immediately before a 10-s isometric tetanus, most K+ is released subsequently when blood flow is renewed, if the occlusion lasts for 20-25 s. It is not until blood flow is occuded for 40-60 s that most K+ is apparently resorbed and only a minor portion is released and is to be found in the venous blood. 5. The transient accumulation of muscle extra-cellular potassium may locally affect nerve endings, skeletal and smooth muscle cells.

Animals↗
PubMed2000

Myocardial function during chronic food restriction in isolated hypertrophied cardiac muscle.

BACKGROUND: The effect of food restriction (FR) on myocardial performance has been studied in normal hearts. Few experiments analyzed the effects of undernutrition on hearts subjected to cardiac overload. The aim of this study was to determine whether chronic FR promotes more significant changes in hypertrophied hearts than in normal hearts. METHODS: Myocardial performance was studied in isolated left ventricular papillary muscle from young male spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY) submitted to FR or to control diet. The animals subjected to FR were fed 50% of the amount of food consumed by control groups for 60 days. Isolated muscles were studied while contracting isometrically and isotonically. RESULTS: FR decreased the body weight and the left ventricular weight in both groups. FR increased the left ventricular weight-to-body weight ratio in the WKY rats and tended to decrease this ratio in SHR (P = 0.055). The arterial systolic pressure was greater in SHR than in WKY groups and did not change with FR. In the animals with normal diet, myocardial performance was better in SHR than in WKY. FR increased time to tension to fall from peak to 50% of peak tension and time to peak tension in the WKY rats and time to peak tension in the SHR. CONCLUSIONS: FR for 60 days has a trend to attenuate the development of cardiac hypertrophy and does not promote more mechanical functional changes in the hypertrophied myocardium than in the normal cardiac muscle.

Animals↗
PubMed1992

Actin-binding proteins regulate the work performed by myosin II motors on single actin filaments.

Regulation of actin/myosin II force generation by calcium [Kamm and Stull, Annu. Rev. Physiol. 51:299-313, 1989] and phosphorylation of myosin II light chains [Sellers and Adelstein, "The Enzymes," Vol. 18, Orlando, FL: Academic Pres, 1987, pp. 381-418] is well established. However, additional regulation of actin/myosin II force generation/contraction may result from actin-binding proteins [Stossel et al., Ann. Rev. Cell Biol. 1:353-402, 1985; Pollard and Cooper, Ann. Rev. Biochem. 55:987-1035, 1986] as they affect the gel state of the actin cytomatrix [reviewed in Taylor and Condeelis, Int. Rev. Cytol., 56:57-143, 1979]. Regulation of the gel state of actin may determine whether an isotonic or isometric contraction results from the interaction between myosin and actin. We have extended the single actin filament motility assay of Kron and Spudich [Proc. Natl. Acad. Sci. U.S.A. 83:6272-6276, 1986] by including filamin or alpha-actinin on the substrate with myosin II to examine how actin-crosslinking proteins regulate the movements of single actin filaments. Increasing amounts of actin-crosslinking proteins inhibit filament velocity and decrease the number of filaments moving. Reversal of crosslinking yields increased velocities and numbers of moving filaments. These results support the solation-contraction coupling hypothesis [see Taylor and Fechheimer, Phil. Trans. Soc. London B 299:185-197, 1982] which proposes that increased crosslinking of actin inhibits myosin-based contraction. This study also illustrates the potentially varied roles of different actin-crosslinking proteins and offers a novel method to examine actin-binding protein activity and their regulation of motility at the single molecule level.

Actins↗
PubMedDate not supplied

Relaxation in atrial and ventricular myocardium: activation decay and different load sensitivity.

Isolated atrial and ventricular preparations from rat heart have been compared. In atrial specimens relaxation is faster than in papillary muscles both in isometric and isotonic conditions. In papillary muscles the tension decay occurs earlier in isotonic than isometric contractions and a stretch applied at or after the peak of isometric twitches promotes a faster relaxation: this load dependence of relaxation is less pronounced in atrial specimens. The decay of activation, evaluated from the decline of the muscle shortening ability, is faster in atrium than in ventricle. These findings suggest that the sensitivity of relaxation to the loading conditions might be determined by both the activation decay rate and the cross bridge kinetics.

Animals↗
PubMed1981

Effects and interactions of prostaglandins E1 and E2 on human chorionic plate arteries.

Helically cut strips of human chorionic plate arteries were maintained in a physiologic salt solution in vitro while isotonic measurements of contraction were made. The arterial strips were exposed to two prostaglandins, PGE1 and PGE2, added cumulatively to the tissue baths. The effects and interactions of these two prostaglandins were examined. PGE1 had a dose-dependent relaxatory effect on the strips at low concentrations, but contracted them at higher concentrations. PGE2, on the other hand, only induced contraction. The addition of a low concentration of PGE2 inhibited the relaxatory response to PGE1 in approximately half the tissue strips. The contractile response was enhanced in all strips. Conversely, a low concentration of PGE2. The suggestion is made that, in the human term placenta, PGE1 and PGE2 may act in concert as part of the regulatory mechanism for fetoplacental.

Adolescent↗
PubMed1983

The propagation of motor unit action potential and the location of neuromuscular junction investigated by surface electrode arrays.

A myoelectric signal was recorded from the biceps brachii muscle using surface electrode arrays which consisted of 15 pieces of stainless steel wire placed parallel to each other with 5 mm interelectrode spacing. The electrode assembly was positioned parallel to the underlying muscle fibers and amplifiers were connected bipolarly to 11 pairs of adjacent electrodes. In the chart of recorded signals a propagation of the action potential was observed, originating from a neuromuscular junction and traveling both in the proximal and distal directions. In one recording under an isotonic and isometric contraction condition, two neuromuscular junctions were found to be active. One was located approximately at the center of the muscle and the other was 15 mm distal from it. It was found that the amplitude of the myoelectric potential originating from the distal neuromuscular junction increased gradually during a 1 min heavy muscular loading (50% maximum voluntary contraction).

Action Potentials↗
PubMed1984

Force-velocity relationships of guinea pig ureter.

Longitudinal force-velocity relationships of the guinea pig ureter were studied in vitro in 3 age groups: 2 to 3 weeks, 2 to 3 months and 2 to 3 years. Velocity of shortening depends on the load lifted and not on the initial length of the muscle. Force-velocity relationships did not change with age, but the isometric force when the velocity is zero and no shortening occurs, increased with age. The shortening of the muscle, expressed as per cent of in vivo length, was independent of age and of preload. Therefore, work performed by a unit length of ureter which is the product of shortening and load lifted, and power which is the product of velocity and load lifted, are unaffected by age or by initial length. Thus there appears to be no significant age-related change in isotonic parameters of contraction such as velocity, shortening, work and power.

Aging↗
PubMed1997

Regulation of filtration rate by glomerular mesangial cells in health and diabetic renal disease.

The rate of renal filtration is in large part responsible for volume and electrolyte balance in an organism. Integral components of the renal glomerulus are the mesangial cells (MCs), excitable renal pericytes that regulate the glomerular filtration rate by modulating the surface area of the capillaries. Similar to vascular smooth muscle, the signal transduction pathways and ion selective channels regulating isotonic and isometric contraction of MCs are dependent on the voltage-gated Ca influx. During the response to contractile agonists, both Cl and nonselective cation channels play critical roles to depolarize the membrane potential and activate Ca channels. The relaxation pathways involve a negative-feedback mechanism that counteracts mesangial contraction by regulating voltage-dependent Ca signaling. Part of the feedback response involves the activation of plasmalemmal K channels, which hyperpolarize the membrane potential and inhibit voltage-gated Ca entry. This calcium- and voltage-activated feedback K (BKCa) channel shares biophysical, pharmacologic, and molecular properties with the BKCa channels identified in brain and muscle, and with the sio gene product as expressed in Xenopus laevis oocytes. Systemic hormones, such as atrial natriuretic peptide, and paracrine factors, such as nitric oxide (NO), use guanosine 3',5'-cyclic monophosphate (GMP) as a second messenger and enhance the gain in this feedback system by decreasing the voltage and Ca activation thresholds for BKCa. Diabetes mellitus is often associated with high rates of glomerular filtration, mesangial expansion, and secretory abnormalities of the basement membrane. NO-mediated increases in negative-feedback regulation of mesangial tone may attribute, in part, to the pathology of hyperfiltration. Stimulation of inducible nitric oxide synthetase in glomerular MCs by inflammatory cytokines is a possible positive-feedback pathway that contributes to further glomerular destruction. In addition, high ambient glucose, through modulation of BKCa activity, facilitates MC relaxation and thus propagates hyperfiltration. Since cellular arachidonic acid is metabolically linked to extracellular glucose, this fatty acid is a possible mediator of the pathologic actions of hyperglycemia. Clarification of the signal transduction pathways and ionic mechanisms regulating the normal and dysfunctional tones of MCs is essential for rational clinical management of glomerular disease and critical to understanding fluid and electrolyte homeostasis.

Animals↗
PubMed1993

Age related changes in contractility and alpha 1 adrenergic responsiveness of myocardium from normal hamsters and hamsters with hereditary cardiomyopathy.

OBJECTIVE: The aim was to evaluate age related changes in myocardial contractility and alpha 1 adrenergic responsiveness in myocardium from normal and cardiomyopathic hamsters. METHODS: 50 male cardiomyopathic hamsters (CHF 147) and 45 golden Syrian hamsters were killed at 30, 50, 100, 200, or 300 days of age. Left ventricular papillary muscles were isolated and mounted in an isolated bath and isometric, isotonic, and unloaded contractions were recorded during a phenylephrine dose-response curve. The effects of increasing stimulation rate from 6 to 45 x min-1 and of increasing temperature from 29 degrees C to 35 degrees C before and after the addition of phenylephrine were verified in 100 day old hamsters. RESULTS: At 30 days of age, tension generation and shortening indices were similar in both strains. Thereafter, contractility increased in normal hamsters, to reach a peak at 50 d which was maintained until 300 d, when a slight decrease occurred. In cardiomyopathic hamsters, contractility increased more slowly, such that by 50 d tension was lower than in normals, at 15.8(SEM 1.9) v 26.5(2.2) mN.mm-2 (p < 0.01). This decrease persisted until 200 d, when it normalised and then decreased again by 300 d. The response of normal and cardiomyopathic muscles to phenylephrine was similar at 30 days of age. Thereafter, the response of cardiomyopathic muscles decreased more than in normals. This decrease was accompanied by a rightward shift in the dose-response curve. Increasing stimulation rate had no effect under basal conditions; however, once phenylephrine was added it caused tension to increase in cardiomyopathic muscles instead of decreasing, thereby rendering the relative effects of phenylephrine at 45 stimuli.min-1 similar in both groups. CONCLUSIONS: Force generating and shortening indices in normal and cardiomyopathic hamster myocardium are similarly immature at 30 days of age. Normal myocardium reaches its full potential by 50 d, but cardiomyopathic myocardium does not reach it until 200 d, and it decreases thereafter. A decrease in alpha 1 adrenergic responsiveness in cardiomyopathic myocardium at lower stimulation rates can be reversed by increasing the stimulation rate.

Aging↗
PubMed1989

Inotropic effect of ketamine on rat cardiac papillary muscle.

The direct effect of ketamine on cardiac muscle was studied using rat left ventricular papillary muscle. At an extracellular calcium concentration [( Ca++]0) of 2.5 mM, rat myocardial contractility is nearly maximum, and a positive inotropic effect was demonstrated by an increase in maximum shortening velocity (Vmax) with ketamine at 10(-5) M but not 10(-4) M. At a [Ca++]0 of 0.5 mM, ketamine 10(-5) and 10(-4) M had a positive inotropic effect as shown by an increase in Vmax (135% +/- 22% and 147% +/- 33%, respectively) and in isometric active force (AF/s) (120% +/- 10% and 152% +/- 44%, respectively). The positive inotropic effect of ketamine was not related to catecholamine uptake inhibition and/or alpha/beta receptor stimulation because it persisted after phentolamine and propranolol and because ketamine had no relaxing effect. Ketamine 10(-5) and 10(-4) M impaired isotonic relaxation, contraction-relaxation coupling under low loading conditions, and the load sensitivity of relaxation, which suggests impairment of the calcium sequestering systems, especially the sarcoplasmic reticulum (SR). Ketamine modified postrest recovery: the first beat (B1) after a 1-min rest period was decreased by ketamine 10(-4) M but not ketamine 10(-5) M. Moreover, the beat-to-beat postrest recovery has been demonstrated to be exponential, and tau, the time constant of the decay was increased by ketamine 10(-4) M (5.4 +/- 0.3 vs. 3.9 +/- 0.2 beats) but not by ketamine 10(-5) M (3.4 +/- 0.4 vs. 3.7 +/- 0.2 beats). These effects on postrest recovery suggest that ketamine impairs SR function. The authors suggest that ketamine had a dual action on rat myocardium: a positive inotropic effect without any relaxing effect, probably related to an increase in calcium influx, and an impairment of SR function. Nevertheless, impairment of SR is only significant at high concentration (10(-4) M) and might overcome the positive inotropic effect only at supratherapeutic concentration.

Animals↗
PubMed1976

Afterload-induced homeometric autoregulation in isolated cardiac muscle.

Pressure-induced homeometric autoregulation (HAR) has been demonstrated by many investigators in the mammalian ventricle; In isolated cardiac muscle, however, several investigators have reported the opposite effect (anti-HAR); namely, that the first beat after a transition from isotonic to isometric contraction is the most forceful, with a decline over several beats to a steady state. In the present study we find that trabeculae from the canine right ventricle demonstrate either HAR or anti-HAR, depending on the rate of stimulation, the calcium level, and the temperature. Higher calcium, higher temperature, and lower rate of stimulation produce either less HAR or more anti-HAR. When only temperature and rate of stimulation are varied, in each muscle there is a unique rate for each temperature above which HAR occurs and below which anti-HAR occurs.

Animals↗
PubMed1996

Bovine tracheal responsiveness in vitro: role of the epithelium and nitric oxide.

Airway epithelium releases inhibitory factors, such as nitric oxide (NO) and prostaglandin E2 (PGE2), which may counteract bronchoconstriction. We investigated whether epithelium-derived inhibitory substances exert a crucial influence on bovine tracheal responsiveness in vitro. Isotonic and isometric contractions in response to histamine of intact and epithelium-denuded tracheal smooth muscle strips were compared. In addition, the effects of L-arginine (L-arg), N(G)-nitro-L-arginine methyl esther (L-NAME), and N(G)-monomethyl L-arginine (L-NMMA) on histamine responsiveness were investigated. The release of NO and PGE2 from tracheal epithelium was measured. Removal of the epithelium from tracheal smooth muscle strips did not change the negative log of the concentration of histamine producing half the maximal effect (pD2) or the maximal effect (Emax). Incubation of the tissues for 25 min with L-arg or L-NAME did not influence basal tone or the contractions induced by histamine. However, incubation with L-NMMA increased the basal tone and caused a slight hyporesponsiveness to histamine. S-nitroso-N-acetyl-penicillamine (SNAP, a direct NO donor) reversed the contraction induced by histamine in a concentration-dependent manner. Stimulation of the epithelial layer by 0.1 microM histamine increased the release of NO 3-4 fold compared to basal levels; this effect was completely inhibited in the presence of L-NMMA. In addition, 1 mM histamine caused a significant increase in the release of PGE2 from the epithelial tissue. In conclusion, no functional inhibitory influence of the epithelium can be identified in bovine airways. The S-nitroso-N-acetyl-penicillamine-induced relaxation demonstrates the presence of a nitric oxide sensitive pathway in bovine airways. However, the amounts of nitric oxide and prostaglandin E2 released from bovine tracheal epithelium are probably too low to exert a significant effect on the histamine-induced contractions.

Animals↗