Time Course Of Contraction Research Articles

The relationship of agonist muscle single motor unit firing rates and elbow extension limb movement kinematics.

This study explored the relationship between single motor unit (MU) firing rates (FRs) and limb movement velocity during voluntary shortening contractions when accounting for the effects of time course variability between different kinematic comparisons. Single MU trains recorded by intramuscular electromyography in agonist muscles of the anconeus (n = 15 participants) and lateral head of the triceps brachii (n = 6) were measured during each voluntary shortening contraction. Elbow extension movements consisted of a targeted velocity occurring along the sagittal plane at 25, 50, 75 and 100% of maximum velocity. To account for the effect of differences in contraction time course between parameters, each MU potential was time locked throughout the shortening muscle contraction and linked with separated kinematic parameters of the elbow joint. Across targeted movement velocities, instantaneous FRs were significantly correlated with elbow extension rate of torque development (r = 0.45) and torque (r = 0.40), but FRs were not correlated with velocity (r = 0.03, p = n.s.). Instead, FRs had a weak indirect relationship with limb movement velocity and position assessed through multiple correlation of the stepwise kinematic progression. Results show that voluntary descending synaptic inputs correspond to a more direct relationship between agonist muscle FRs and torque during shortening contractions, but not velocity. Instead, FRs were indirectly correlated to preparing the magnitude of imminent movement velocity of the lagging limb through torque.

Myosin filament-based regulation of the dynamics of contraction in heart muscle

Myosin-based mechanisms are increasingly recognized as supplementing their better-known actin-based counterparts to control the strength and time course of contraction in both skeletal and heart muscle. Here we use synchrotron small-angle X-ray diffraction to determine the structural dynamics of local domains of the myosin filament during contraction of heart muscle. We show that, although myosin motors throughout the filament contribute to force development, only about 10% of the motors in each filament bear the peak force, and these are confined to the filament domain containing myosin binding protein-C, the "C-zone." Myosin motors in domains further from the filament midpoint are likely to be activated and inactivated first in each contraction. Inactivated myosin motors are folded against the filament core, and a subset of folded motors lie on the helical tracks described previously. These helically ordered motors are also likely to be confined to the C-zone, and the associated motor conformation reforms only slowly during relaxation. Myosin filament stress-sensing determines the strength and time course of contraction in conjunction with actin-based regulation. These results establish the fundamental roles of myosin filament domains and the associated motor conformations in controlling the strength and dynamics of contraction in heart muscle, enabling those structures to be targeted to develop new therapies for heart disease.

The Effects of Mechanical Preload on Transmural Differences in Mechano-Calcium-Electric Feedback in Single Cardiomyocytes: Experiments and Mathematical Models.

Transmural differences in ventricular myocardium are maintained by electromechanical coupling and mechano-calcium/mechano-electric feedback. In the present study, we experimentally investigated the influence of preload on the force characteristics of subendocardial (Endo) and subepicardial (Epi) single ventricular cardiomyocytes stretched by up to 20% from slack sarcomere length (SL) and analyzed the results with the help of mathematical modeling. Mathematical models of Endo and Epi cells, which accounted for regional heterogeneity in ionic currents, Ca2+ handling, and myofilament contractile mechanisms, showed that a greater slope of the active tension–length relationship observed experimentally in Endo cardiomyocytes could be explained by greater length-dependent Ca2+ activation in Endo cells compared with Epi ones. The models also predicted that greater length dependence of Ca2+ activation in Endo cells compared to Epi ones underlies, via mechano-calcium-electric feedback, the reduction in the transmural gradient in action potential duration (APD) at a higher preload. However, the models were unable to reproduce the experimental data on a decrease of the transmural gradient in the time to peak contraction between Endo and Epi cells at longer end-diastolic SL. We hypothesize that preload-dependent changes in viscosity should be involved alongside the Frank–Starling effects to regulate the transmural gradient in length-dependent changes in the time course of contraction of Endo and Epi cardiomyocytes. Our experimental data and their analysis based on mathematical modeling give reason to believe that mechano-calcium-electric feedback plays a critical role in the modulation of electrophysiological and contractile properties of myocytes across the ventricular wall.

The Huxley crossbridge model as the basic mechanism for airway smooth muscle contraction.

The cyclic interaction between myosin crossbridges and actin filaments underlies smooth muscle contraction. Phosphorylation of the 20-kDa myosin light chain (MLC20) is a crucial step in activating the crossbridge cycle. Our current understanding of smooth muscle contraction is based on observed correlations among MLC20 phosphorylation, maximal shortening velocity (Vmax), and isometric force over the time course of contraction. However, during contraction there are changes in the extent of phosphorylation of many additional proteins as well as changes in activation of enzymes associated with the signaling pathways. As a consequence, the mechanical manifestation of muscle contraction is likely to change with time. To simplify the study of these relationships, we measured the mechanical properties of airway smooth muscle at different levels of MLC20 phosphorylation at a fixed time during contraction. A simple correlation emerged when time-dependent variables were fixed. MLC20 phosphorylation was found to be directly and linearly correlated with the active stress, stiffness, and power of the muscle; the observed weak dependence of Vmax on MLC20 phosphorylation could be explained by the presence of an internal load in the muscle preparation. These results can be entirely explained by the Huxley crossbridge model. We conclude that when the influence of time-dependent events during contraction is held constant, the basic crossbridge mechanism in smooth muscle is the same as that in striated muscle.

The effects of load on transmural differences in contraction of isolated mouse ventricular cardiomyocytes

Mechanical properties of cardiomyocytes from different transmural regions are heterogeneous in the left ventricular wall. The cardiomyocyte mechanical environment affects this heterogeneity because of mechano-electric feedback mechanisms. In the present study, we investigated the effects of the mechanical load (preload and afterload) on transmural differences in contraction of subendocardial (ENDO) and subepicardial (EPI) single cells isolated from the murine left ventricle. Various preloads imposed via axial stretch and afterloads (unloaded and heavy loaded conditions) were applied to the cells using carbon fiber techniques for single myocytes. To simulate experimentally obtained results and to predict mechanisms underlying the cellular response to change in load, our mathematical models of the ENDO and EPI cells were used.Our major findings are the following. Our results show that ENDO and EPI cardiomyocytes have different mechanical responses to changes in preload to the cells. Under auxotonic contractions at low preload (unstretched cells), time to peak contraction (Tmax) and the time constant of [Ca2+]i transient decay were significantly longer in ENDO cells than in EPI cells. An increase in preload (stretched cells) prolonged Tmax in both cell types; however, the prolongation was greater in EPI cells, resulting in a decrease in the transmural gradient in Tmax at high preload. Comparing unloaded and heavy loaded (isometric) contractions of the cells we found that transmural gradient in the time course of contraction is independent of the loading conditions. Our mathematical cell models were able to reproduce the experimental results on the distinct cellular responses to changes in the mechanical load when we accounted for an ENDO/EPI difference in the parameters of cooperativity of calcium activation of myofilaments.

Effects of simulated ischemia on the transmural differences in the Frank–Starling relationship in isolated mouse ventricular cardiomyocytes

The electrical and mechanical functions of cardiomyocytes differ in relation to the spatial locations of cells in the ventricular wall. This physiological heterogeneity may change under pathophysiological conditions, providing substrates for arrhythmia and contractile dysfunctions. Previous studies have reported distinctions in the electrophysiological and mechanical responses to ischemia of unloaded subendocardial (ENDO) and subepicardial (EPI) single cardiomyocytes. In this paper, we briefly recapitulated the available experimental data on the ischemia effects on the transmural cellular gradient in the heart ventricles and for the first time evaluated the preload-dependent changes in passive and active forces in ENDO and EPI cardiomyocytes isolated from mouse hearts subjected to simulated ischemia. Combining the results obtained in mechanically loaded contracting cardiomyocytes with data from previous studies, we showed that left ventricular ENDO and EPI cardiomyocytes are different in their mechanical responses to metabolic inhibition. Simulated ischemia showed opposite effects on the stiffness of ENDO and EPI cells and greatly prolonged the time course of contraction in EPI cells than in ENDO cells, thereby changing the normal transmural gradient in the cellular mechanics.

Transmural Differences in Preload-Dependency of CA2+ Transients in Isolated Cardiomyocytes

Motivation: We have shown previously that response of the cellular mechanical activity to the mechanical preload differ in isolated subepicardial (EPI) and subendocardial (ENDO) cardiomyocytes (Khokhova et al., 2015). In this study, to examine transmural differences in excitation-contraction (EC) coupling at different preloads, we investigate regional differences in cellular [Ca2+]i transient.Methods: To change the preload, axial stretch to the cells was applied with using carbon fiber technique (Iribe et al., 2009). For measurement of [Ca2+]i cardiomyocytes were loaded with the Ca2+ indicator Fura-4F AM. Our cellular electromechanical EPI and ENDO models were used to simulate effects of the mechanical preload and to predict underlying cellular mechanisms.Results: We found that [Ca2+]i amplitude in ENDO myocytes tended to be higher compared to EPI myocytes during auxotonic contractions at any preload. Stretch did not affect the [Ca2+]i amplitude neither in EPI or ENDO cells. At low preload, the time constant of decay of [Ca2+]i transient (τ) was greater in ENDO then in EPI cells. At high preload, τ delayed in EPI cells but not in ENDO cells, which is consistent with the previously observed greater preload-dependency in time course of contraction in EPI cells.Conclusions: Our results suggest that regional differences in [Ca2+]i are inherent feature of the ventricular myocardium and may reflect transmural differences in the EC coupling. Our simulation study suggests that differences in the cooperative interactions between regulatory proteins and cross-bridges assumed in EPI and ENDO models may essentially contribute to the differences in the preload-dependency between the cells. Supported by The Russian Science Foundation (#14-35-00005) and JSPS KAKENHI 2628212.

CHANGING OF ISCHEMIC M. SOLEUS TETANIC CONTRACTION PARAMETERS IN RATS WITH CHRONIC ALCOHOL INTOXICATION

This article deals with the changes of isolated ischemic m. soleus tetanus parameters in rats with chronic alcohol intoxication. The experiments were carried out on 15 male Wistar rats that were divided into three groups for 5 animals in each: group I (control) and two groups in which was induced hind limbs acute muscles ischemia: group II - rats without alcoholic intoxication, group III - rats with chronic alcoholic intoxication. Strain measurement muscle mechanical activity were conducted in isometric mode under conditions of direct electrical muscular preparation stimulation. It is proved that ischemic m. soleus tetanic force in rats with chronic alcoholic intoxication in comparison with rats without alcoholic intoxication does not significant changes. But signifycantly increases the reaching tetanus peak time. It is shown that in rats without alcoholic intoxication and with chronic alcoholic intoxication in comparison with intact animals, significantly decreases the duration of ischemic m. soleus stabile force level. It is shoved significant changes of individual muscles contraction time course of ischemic m. soleus tetanus in this rats group in comparison to intact animal. It is shown that these changes influence on successive muscular contraction efficiency of frequency summation in ischemic m. soleus tetanus and their speed-power characteristics.

The temperature challenges on cardiac performance in winter-quiescent and migration-stage eels Anguilla anguilla

The present study was undertaken to examine cardiac responses to some of the temperature challenges that eels encounter in their natural environment. The contractile properties of ventricular muscle was studied on electrically paced tissue strips after long term acclimation at 0°C, 10°C, or 20°C, and following acute ±10°C temperature changes. The time-course of contraction, and thus maximal attainable heart rates, was greatly influenced by working temperature, but was independent of acclimation history. The absolute force of contraction and power production (i.e. the product of force and stimulation frequency) was significantly influenced by acute temperature decrease from 20°C to 10°C. The role of adrenaline as a modulator of contraction force, power production, rates of contraction and relaxation, and minimum time in contraction was assessed. Increased adrenergic tonus elicited a positive inotropic, temperature-dependent response, but did not influence twitch duration. This suggests that adrenaline acts as an agent in maintaining an adequate contractile force following temperature challenges. A significant increased relative ventricular mass was observed in 0°C and 10°C-acclimated eels compared to 20°C-acclimated, which suggests that at low temperatures, eels secure cardiac output by heart enlargement. Inhibition of specific sarcolemmal Ca2+ channels by selective drug treatment revealed that, depending on temperature, L-type channels is the major entry site, but also that reverse-mode Na+/Ca2+-exchange and store operated calcium entry contribute to the pool of activator Ca2+.

Enhanced Propagation of Calcium-Induced Calcium-Release (CICR) from the Cell Periphery to the Core Increases Contractility of Detubulated Myocardium

Mechanics and Excitation-Contraction-Coupling were studied in rat ventricular trabeculae and myocytes before and after formamide-induced detubulation. Detubulation was checked using FM4-64 membrane staining and dual-photon microscopy. Electrical uncoupling of t-tubules from sarcolemma was confirmed by simultaneous action potential recordings at surface and t-tubular membranes (Sacconi et al., this meeting). Global and regional (periphery/core) Ca2+-transients were measured either under confocal microscopy or in wide-field with a high-speed CCD-camera from Fluo4-loaded myocytes. At low [Ca2+]o (1mM,2Hz,30°C) twitch tension of detubulated trabeculae was reduced to ∼40% of controls and the time course of contraction was significantly slower. Under the same conditions, the global Ca2+-transients were depressed and slowed down in detubulated cells vs. controls. Regional Ca2+-transients were synchronous and homogeneous in control myocytes while in detubulated cells they were mostly restricted to a subsarcolemmal ring with the cell core displaying lower and delayed Ca2+-signals. The results are consistent with detubulation inducing a shift from synchronous CICR to a propagated form of CICR that is slower and may not recruit deep myofibril layers. At high [Ca2+]o (>8mM), or following post-rest potentiation, twitch tension of detubulated trabeculae was unchanged compared to controls, though contractions were prolonged. At high [Ca2+]o, or at resting stimulation frequency, the Ca2+ gradient between cell periphery and core was markedly reduced in detubulated myocytes while the time-lag between the two Ca2+-transients was still present. The results suggest that CICR propagation from cell periphery to the core can be enhanced and contractility improved in detubulated myocardium by increasing the Ca2+-trigger and the SR Ca2+-load. Interventions that increase RyR2 open probability may also enhance CICR propagation in detubulated myocytes. This has been confirmed by comparing the effects of caffeine (200µM) on control and detubulated myocytes.

Changing pattern of gene expression is associated with ventricular myocyte dysfunction and altered mechanisms of Ca2+signalling in young type 2 Zucker diabetic fatty rat heart

The association between type 2 diabetes and obesity is very strong, and cardiovascular complications are the major cause of morbidity and mortality in diabetic patients. The aim of this study was to investigate early changes in the pattern of genes encoding cardiac muscle regulatory proteins and associated changes in ventricular myocyte contraction and Ca(2+) transport in young (9- to 13-week-old) type 2 Zucker diabetic fatty (ZDF) rats. The amplitude of myocyte shortening was unaltered; however, time-to-peak shortening and time to half-relaxation of shortening were prolonged in ZDF myocytes (163 ± 5 and 127 ± 7 ms, respectively) compared with age-matched control rats (136 ± 5 and 103 ± 4 ms, respectively). The amplitude of the Ca(2+) transient was unaltered; however, time-to-peak Ca(2+) transient was prolonged in ZDF myocytes (66.9 ± 2.6 ms) compared with control myocytes (57.6 ± 2.3 ms). The L-type Ca(2+) current was reduced, and inactivation was prolonged over a range of test potentials in ZDF myocytes. At 0 mV, the density of L-type Ca(2+) current was 1.19 ± 0.28 pA pF(-1) in ZDF myocytes compared with 2.42 ± 0.40 pA pF(-1) in control myocytes. Sarcoplasmic reticulum Ca(2+) content, release and uptake and myofilament sensitivity to Ca(2+) were unaltered in ZDF myocytes compared with control myocytes. Expression of genes encoding various L-type Ca(2+) channel proteins (Cacna1c, Cacna1g, Cacna1h and Cacna2d1) and cardiac muscle proteins (Myh7) were upregulated, and genes encoding intracellular Ca(2+) transport regulatory proteins (Atp2a2 and Calm1) and some cardiac muscle proteins (Myh6, Myl2, Actc1, Tnni3, Tnn2, and Tnnc1) were downregulated in ZDF heart compared with control heart. A change in the expression of genes encoding myosin heavy chain and L-type Ca(2+) channel proteins might partly underlie alterations in the time course of contraction and Ca(2+) transients in ventricular myocytes from ZDF rats.

Moment-Angle Relations in the Initial Time of Contraction

Standard moment-angle relations are typically obtained for values at maximum of contraction. Muscle properties for contraction times, similar to the short times of powerful athletic activities, may be more important than muscle properties that are obtained at maximum (TMAX) of contraction. The research question was whether moment-angle relations obtained during the initial time course of contraction are linearly scaled reflections of moment-angle relations obtained at maximum of contraction. The isometric moment of the knee extensor and the electromyographic activity (EMG) of the knee extensor and flexor muscles were measured at 9 knee angles in 22 elite female track and field jumpers. Absolute moments (Nm), relative moments (%TMAX) and EMG integrals were determined at 30 ms, 50 ms, 150 ms and 200 ms from the onset of contraction and at maximum. For moment-angle relations obtained during the initial time course of contraction, there was a shift of the optimum knee angle to a more extended knee. During the initial time course of the contraction, %TMAX moment was greater near full knee extension. The EMG activity was the same across knee angles. The results may have important practical implications for muscular force evaluation for activities where maximum force production is coupled to a short activation time.

The Concentration-Dependent Contractile Effect of Methylene Blue in the Human Internal Mammary Artery: A Quantitative Approach to Its Use in the Vasoplegic Syndrome

To quantitate the contractile effect of methylene blue on isolated human internal mammary artery (IMA) as used in the vasoplegic syndrome. An in vitro experimental study. Cardiovascular Pharmacology Laboratory, Department of Medical Pharmacology. IMA segments were used from 24 patients undergoing coronary artery bypass surgery. The responses to methylene blue, norepinephrine, and acetylcholine were recorded isometrically by a force-displacement transducer in an isolated organ bath. Methylene blue (10 nmol/L-100 micromol/L) produced concentration-dependent contraction in the arteries. The maximal contraction to methylene blue was 44.2% +/- 3.8% of KCl (68 mmol/L) maximum contraction; the pEC(50) (-log(10) of 50% effective concentration) value was 5.5 +/- 0.1. Methylene blue caused an insignificant leftward shift of the concentration-response curve of norepinephrine. Acetylcholine-induced relaxation in submaximal contracted rings with phenylephrine recovered nearly 6 hours after the methylene blue challenge. Methylene blue caused concentration-dependent contraction in human IMAs. Furthermore, the inhibition of ACh-induced relaxation for 6 hours after the methylene blue challenge points out an additional mechanism (ie, receptor occupation). The concentration-dependent contractile effect of methylene blue justifies its use in the vasoplegic syndrome. The findings also suggest that the time course of contraction is longer than the exposure to methylene blue.

β1‐ and β2‐adrenoceptor responses in cardiomyocytes derived from human embryonic stem cells: comparison with failing and non‐failing adult human heart

Characterization of human embryonic stem cell-derived cardiomyocytes (hESC-CM) in relation to adult myocytes is essential for their future use in transplantation or as a model system. The beta-adrenoceptor pathways, which are known to be effective early in hESC-CM development, are of major importance because of their control of rate and force of beating, arrhythmia generation and apoptosis/necrosis. We have therefore performed detailed pharmacological analysis of the beta-adrenoceptor responses in developing hESC-CM. hESC-CMs were differentiated from H7 ESCs and studied up to 79 days of differentiation. Rate of beating and time course of contraction and relaxation were measured in superfused preparations. Responses to the mixed beta(1)- and beta(2)-adrenoceptor agonist isoprenaline were evident from day 10 to day 79. Stability of the responses during an application, for repeated applications on the same experimental day and over the time of development, was determined. Concentrations for half-maximal response (12.9 nM) were similar to those from adult human heart, but closer to those obtained from failing rather than normal ventricle. Acceleration of both contraction and relaxation was quantitatively similar to that in adult ventricular myocytes, as was sensitivity to muscarinic inhibition. Use of specific antagonists showed that both beta(1)- and beta(2)-adrenoceptors contributed to contractile responses, as seen with adult myocytes. These data show the compatibility of hESC-CM with adult human myocardium in terms of beta-adrenoceptor response. The experiments described here also confirm the utility of the hESC-CM preparation for detailed pharmacological analysis.

Ca 2+ -Dependent Rapid Ca 2+ Sensitization of Contraction in Arterial Smooth Muscle

Ca(2+) ion is a universal intracellular messenger that regulates numerous biological functions. In smooth muscle, Ca(2+) with calmodulin activates myosin light chain (MLC) kinase to initiate a rapid MLC phosphorylation and contraction. To test the hypothesis that regulation of MLC phosphatase is involved in the rapid development of MLC phosphorylation and contraction during Ca(2+) transient, we compared Ca(2+) signal, MLC phosphorylation, and 2 modes of inhibition of MLC phosphatase, phosphorylation of CPI-17 Thr38 and MYPT1 Thr853, during alpha(1) agonist-induced contraction with/without various inhibitors in intact rabbit femoral artery. Phenylephrine rapidly induced CPI-17 phosphorylation from a negligible amount to a peak value of 0.38+/-0.04 mol of Pi/mol within 7 seconds following stimulation, similar to the rapid time course of Ca(2+) rise and MLC phosphorylation. This rapid CPI-17 phosphorylation was dramatically inhibited by either blocking Ca(2+) release from the sarcoplasmic reticulum or by pretreatment with protein kinase C inhibitors, suggesting an involvement of Ca(2+)-dependent protein kinase C. This was followed by a slow Ca(2+)-independent and Rho-kinase/protein kinase C-dependent phosphorylation of CPI-17. In contrast, MYPT1 phosphorylation had only a slow component that increased from 0.29+/-0.09 at rest to the peak of 0.68+/-0.14 mol of Pi/mol at 1 minute, similar to the time course of contraction. Thus, there are 2 components of the Ca(2+) sensitization through inhibition of MLC phosphatase. Our results support the hypothesis that the initial rapid Ca(2+) rise induces a rapid inhibition of MLC phosphatase coincident with the Ca(2+)-induced MLC kinase activation to synergistically initiate a rapid MLC phosphorylation and contraction in arteries with abundant CPI-17 content.

Mechanism of the Frank–Starling law—A simulation study with a novel cardiac muscle contraction model that includes titin and troponin I

A stretch-induced increase of active tension is one of the most important properties of the heart, known as the Frank–Starling law. Although a variation of myofilament Ca 2+ sensitivity with sarcomere length (SL) change was found to be involved, the underlying molecular mechanisms are not fully clarified. Some recent experimental studies indicate that a reduction of the lattice spacing between thin and thick filaments, through the increase of passive tension caused by the sarcomeric protein titin with an increase in SL within the physiological range, promotes formation of force-generating crossbridges (Xbs). However, the mechanism by which the Xb concentration determines the degree of cooperativity for a given SL has so far evaded experimental elucidation. In this simulation study, a novel, rather simple molecular-based cardiac contraction model, appropriate for integration into a ventricular cell model, was designed, being the first model to introduce experimental data on titin-based radial tension to account for the SL-dependent modulation of the interfilament lattice spacing and to include a conformational change of troponin I (TnI). Simulation results for the isometric twitch contraction time course, the length-tension and the force–[Ca 2+] relationships are comparable to experimental data. A complete potential Frank–Starling mechanism was analyzed by this simulation study. The SL-dependent modulation of the myosin binding rate through titin's passive tension determines the Xb concentration which then alters the degree of positive cooperativity affecting the rate of the TnI conformation change and causing the Hill coefficient to be SL-dependent.

Cardiomyocyte dysfunction in insulin-resistant rats: a female advantage

The goal of this investigation was to determine whether there are sex-related differences in the development of cardiomyocyte dysfunction in prediabetic, insulin-resistant animals. Male and female rats were maintained on a high-sucrose diet for 5-11 weeks, and mechanical properties of isolated ventricular myocytes were measured by high-speed video edge detection. Several in vitro interventions were used to manipulate intracellular Ca(2+) in order to determine whether altered Ca(2+) availability contributes to the cardiomyocyte dysfunction. Myocyte shortening and relengthening were significantly slower in sucrose-fed (insulin-resistant) males than in starch-fed (normal) male rats, whereas only relengthening was slower in sucrose-fed females when compared with normal females. Areas under the contraction and relaxation phases for sucrose-fed males were also significantly larger than in diet-matched females, and the slowed cardiomyocyte mechanics appeared earlier in males (7 vs 10 weeks). Prolonged relaxation was ameliorated in myocytes from sucrose-fed female rats by all interventions (i.e. 10(-8) mol/l isoprenaline, elevated extracellular Ca(2+), and higher rates of stimulation). Twice as much extracellular Ca(2+) (4 mmol/l) was required to restore normal time courses of contraction and relaxation in sucrose-fed males than in females, and mechanical responses to higher frequency stimulation remained impaired (slower) in some myocytes from sucrose-fed male rats. These data suggest that in myocytes from insulin-resistant rats altered Ca(2+) handling occurs, contributing to abnormal excitation-contraction coupling; female rats seem to have some cardioprotection during early stages in the progression towards type 2 diabetes. Females show delayed onset and milder abnormalities in metabolic status and cardiomyocyte function, but with a much tighter temporal coupling of these dysfunctions.

Simple Mechanism for Stabilizing Motor Output. Focus on “Temperature Compensation of Neuromuscular Modulation in Aplysia”

The production of distinct and appropriate behaviors under different circumstances requires that an organism’s neural circuitry be flexible and, at times, multifunctional. One means by which neural circuits or networks are accorded flexibility is through the release of neuromodulatory substances

Augmentation of α1-Adrenoceptor-Mediated Contraction by Warming Without Increased Phosphorylation of Myosin in Rat Caudal Arterial Smooth Muscle

We previously reported the relationship between α1-adrenoceptor-mediated contraction and phosphorylation of 20-kDa myosin light chain (LC20) in de-endothelialized rat caudal arterial smooth muscle at room temperature (Mita M, Walsh MP. Biochem J. 1997;327:669–674). We now describe the effect of increasing the temperature to 37°C on this relationship. The EC50 value (76.6 ± 18.2 nM) for cirazoline (α1-adrenergic agonist)-induced contraction of the strips at room temperature (23°C) was significantly greater than that (14.5 ± 1.9 nM) at 37°C. The initial rate of the contraction to a sub-maximal concentration of cirazoline (0.3 µM) was similar at the two temperatures. However, cirazoline-induced maximal force at 37°C was approximately 1.8 times that at room temperature. LC20 phosphorylation in response to cirazoline at room temperature and 37°C closely matched the time courses of contraction, but values were not significantly different at the two temperatures: resting phosphorylation levels were 0.09 ± 0.04 mol Pi/mol LC20 at 37°C and 0.22 ± 0.06 mol Pi/mol LC20 at room temperature; maximal cirazoline-stimulated LC20 phosphorylation levels were 0.58 ± 0.08 mol Pi/mol LC20 at room temperature and 0.49 ± 0.05 mol Pi/mol LC20 at 37°C. We conclude, therefore, that the enhanced cirazoline-induced contraction at 37°C is not due to increased LC20 phosphorylation.

The effects of alfentanil on cytosolic Ca(2+) and contraction in rat ventricular myocytes.

Previous investigations of the effects of potent opioid analgesics on the heart have concentrated on effects on contraction magnitude and time course, but little is known about their effects on cytosolic Ca(2+) regulation in cardiac tissue. In this study, we sought to assess the effects of alfentanil on contractility and the cytosolic Ca(2+) transient in ventricular myocytes isolated from the rat ventricle by enzymatic dispersion. Cells were loaded with fura-2 and electrically stimulated at 1 Hz, and Ca(2+) transients and contractions were recorded optically at 30 degrees C. Alfentanil 10(-8) and 10(-7) M had no effect on the magnitude or time course of contraction or the cytosolic Ca(2+) transient. In contrast, 10(-6) M alfentanil induced a significant (P < 0.001) positive inotropic effect, increasing the mean (+/-SEM) unloaded shortening from 7.3 +/- 1.3 microm to 8.7 +/- 1.4 microm (an increase of 20%), with no change in the cytosolic Ca(2+) transient. Myofilament Ca(2+) sensitivity was significantly (P = 0.027) increased by 10(-6) M alfentanil but unaffected at 10(-7) M alfentanil. These data show that 10(-6) M alfentanil, a concentration close to the maximum clinical free plasma concentration, induced a positive inotropic effect due to sensitization of the myofilaments to Ca(2+) rather than to modified cytosolic Ca(2+) regulation. Alfentanil, at concentrations achieved in clinical practice, increased contraction in ventricular cells by a mechanism involving an increase in the sensitivity of the contractile apparatus to Ca(2+).