Effects of volatile anesthetics on stiffness of mammalian ventricular muscle.

To assess the effects of halothane, isoflurane, and sevoflurane on cross bridges in intact cardiac muscle, electrically stimulated (0.25 Hz, 25 degrees C) right ventricular ferret papillary muscles (n = 14) were subjected to sinusoidal load oscillations (37-182 Hz, 0.2-0.5 mN peak to peak) at the instantaneous self-resonant frequency of the muscle-lever system. At resonance, stiffness is proportional to m * omega(2) (where m is equivalent moving mass and omega is angular frequency). Dynamic stiffness was derived by relating total stiffness to values of passive stiffness at each length during shortening and lengthening. Shortening amplitude and dynamic stiffness were decreased by halothane > isoflurane > or = sevoflurane. At equal peak shortening, dynamic stiffness was higher in halothane or isoflurane in high extracellular Ca(2+) concentration than in control. Halothane and isoflurane increased passive stiffness. The decrease in dynamic stiffness and shortening results in part from direct effects of volatile anesthetics at the level of cross bridges. The increase in passive stiffness caused by halothane and isoflurane may reflect an effect on weakly bound cross bridges and/or an effect on passive elastic elements.

Mechanics of K(+)-induced isotonic and isometric contractions in isolated canine coronary microarteries.

The effects of shortening in isotonic contractions on the mechanics of microvascular smooth muscle were investigated. Intramyocardial canine coronary microarteries (in situ diameter 60 +/- 3 microns) were mounted as rings, connected to a newly developed photoelectromagnetic force-length transducer, and activated with 125 mM K+. Shortening during isotonic contractions depressed the length-force relation (shortening deactivation) compared with the length-force relation obtained from isometric contractions; the effect was present at the earliest moments after activation, suggesting that a fundamental mechanism associated with the actual sliding of contractile filaments delayed onset of contractile activity in isotonic contractions compared with isometric contractions. Force-velocity relations were obtained by isotonic quick releases from isotonic and isometric contractions at various times. Isotonic shortening before the quick releases reduced the constants of the apparent hyperbolic force-velocity relations and maximal velocity of shortening (Vmax) compared with isometric contractions released at the same time. Increasing contraction duration reduced Vmax but more so in isotonic than in isometric contractions. Vmax also decreased with decreasing instantaneous length. A possible effect of force development on Vmax before the isotonic quick release was also described. Quick increments of load during isotonic contractions were sustained during active shortening in the phasic part, but during the tonic part loading resulted in a pronounced transient relaxation. Thus, in microvascular preparations, active isotonic shortening altered the length-force, force-velocity, and velocity-time relations and uncovered a time-dependent sensitivity to loading conditions. These experiments suggested that the mechanics of smooth muscle contraction may contribute significantly to the mechanisms of the physiological control of coronary microvascular diameter.

Post-reextension force decay of relaxing cardiac muscle.

Residual active cardiac muscle force during ventricular filling causes deviations of the pressure-volume and pressure-segment length relations from passive left ventricular compliance curves. A possible interaction at the myocardial level between muscle reextension and subsequent active force decay has not yet been investigated. We therefore studied the relation between isolated cat papillary muscle reextension, load during reextension, and isometric force decay after isotonic reextension. Both timing and extent of the isotonic muscle reextension phase were altered while load during reextension was lowered, subsequent residual isometric force was decreased. The extent of reextension or the final muscle length did not alter residual active isometric force after isotonic reextension at an identical load. Moreover, irrespective of the loading history of the shortening phase of the contraction, equal loads during reextension resulted in superimposable subsequent isometric force decay traces. From these results it therefore appears that residual isometric force after isotonic reextension is determined by the load during reextension. Extrapolation of these results to the filling ventricle implies the existence of a dynamic interaction between instantaneous extent of filling, wall stress, and residual force development.

Uniform sarcomere behaviour during twitch of intact single cardiac cells.

Behaviour of sarcomere length was analysed in different regions of single cardiac cells (n = 249) of the ventricle, both at rest (n = 144) and during twitch contractions (n = 57). At rest, regional distribution of sarcomere length proved to be uniform. In the leaky cell (n = 48), resting sarcomere length was not affected over longer periods of time (up to 2 h), nor by lowering the ATP concentration (from 5 mM to 2.5 mM and 500 microM), nor by increasing free calcium within subactivating ranges (5, 20, 60 microM). No statistical differences could be detected between resting cell dimensions and sarcomere length between cells isolated from left and right ventricle (n = 64), nor between cells from epicardial or endocardial layers (n = 80). During twitch contraction in the intact unloaded cardiac cell (n = 32), sarcomere lengths in different regions were analysed every 20 ms and behaved synchronously, presenting arguments for uniformity during the myocardial contraction-relaxation cycle in the free-lying intact cardiac cell.

Nonuniformity of contraction and relaxation of mammalian cardiac muscle.

Nonuniformity of contraction and relaxation in cardiac muscle results from relaxation asynchronies between segments in the central region and their interaction with the prevailing loading conditions, and this may reflect regional differences in activation or force potential at a given time during a twitch.

Relaxation of tetanised smooth muscle of canine saphenous vein.

Since vascular filling depends on the cross-sectional area of the venous bed and since this is determined by the state of relaxation of the mural smooth muscle, we have studied load bearing capacity during relaxation of the smooth muscle of canine saphenous vein. The effect of load on the time course of relaxation was analysed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load (load clamps). Unlike mammalian cardiac muscle in which relaxation was reported sensitive to loading conditions, relaxation in the smooth muscle of the saphenous vein was largely independent of loading conditions. In this it resembled frog heart muscle. This type of relaxation, which is not influenced by manipulation of loading conditions, has been termed "inactivation-dependent" relaxation. It appears to operate in muscle tissue in which the calcium sequestering apparatus is poorly developed and sequestration or some process downstream to it appears to be the rate limiting step during relaxation.

Sarcomere distribution patterns in single cardiac cells.

Sarcomeres of single cardiac cells isolated either by microdissection or by enzymatic dissociation were visualized on a television screen, through the objective (63 X) of an inverted microscope and a television camera. A distinct line of the television picture was positioned on the preparation and the frequency content, corresponding to the dark and light areas of the striations was tracked by a phase-locked loop. This technique permitted the measurement of the length of successive sarcomeres and hence the sarcomere distribution pattern over the entire preparation.

Effect of temperature on the mechanical behaviour of single skinned cardiac cells.

The effect of changes in temperature (16-35 degrees C) on the contractile behaviour of single, skinned cardiac cells was analysed. Lowering the temperature decreased force development, extent of shortening and velocity of shortening. The effects of altering temperature on the calcium sensitivity of velocity of shortening were more pronounced than the temperature effect upon the calcium dependence of force and extent of shortening. At maximal activation force-velocity curves showed a marked shift in peak velocity, with hardly any effect on peak isometric force.

Relaxation of tetanized canine tracheal smooth muscle.

As is the case for striated muscle, relaxation in smooth has been little studied and is less understood. We report studies of load bearing capacity during relaxation of airway smooth muscle. The model employed was the canine tracheal smooth muscle (TSM). The effect of load on the time course of relaxation was analyzed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load (load clamps). Unlike mammalian cardiac muscle in which relaxation was reported sensitive to loading conditions, relaxation in TSM was largely independent of loading conditions. In this it resembled frog heart muscle and mammalian cardiac muscle cells without functioning calcium sequestering systems. This type of relaxation which is not influenced by manipulation of loading conditions, has been termed "inactivation-dependent' relaxation. It appears to operate in muscle tissue in which the calcium sequestering apparatus is poorly developed and the dissipation of activation (removal of activating calcium, detachment of force generating sites, etc.) appears to be the rate limiting step during relaxation.

Myocardial stiffness during hypoxic and reoxygenation contracture.

Isolated rat papillary muscle preparations were used to study hypoxic contracture, and cat papillary muscle preparations with ouabain to study reoxygenation contracture. Electronic analysis of the response to rapid small sinusoidal perturbations gave a continuous measurement of the elastic and viscous components of total stiffness. Increased resting force during hypoxic contracture was characterised by an increase in resting elastic and viscous stiffness relative to the control stiffness-active force relationships. During reoxygenation contracture the stiffness-force relationships followed those of active force development. The linear active force-elastic stiffness relationship (dt/dl=kT+c) was also reversibly altered during hypoxic contracture, predominantly by an increase in intercept c. These data imply that hypoxic contracture unlike reoxygenation contracture is not due solely to a rise in intracellular calcium, but is associated with a component of stiffness not participating an active force development, for example rigor.

Relaxation of mammalian single cardiac cells after pretreatment with the detergent Brij-58.

1. The influence of load and activation on relaxation of heart muscle has been studied. 2. Cardiac cells devoid of functioning sarcolemma were isolated from rat and cat ventricular myocardium. Pretreatment with the detergent Brij-58 destroyed residual sarcoplasmic reticulum function. In order to analyse the mechanical properties of relaxation in these cells, a new miniature transducer was designed which could measure force by feedback sensing (resolution of 1 microgram). Contraction was induced by ionophoretically released calcium ions. Activation, sequestration of calcium and loading conditions could be controlled independently. 3. The time course of relaxation was shown to be governed by the amount of calcium released, and unlike intact preparations from rat or cat heart (but like those from frog), to be independent of load and of alterations in load. 4. We conclude that relaxation of the cardiac contractile system is determined basically by an activation-dependent mechanism, which is masked by load dependence in intact muscle preparations with a well developed calcium sequestering membraneous system.

Force velocity relations of single cardiac muscle cells: calcium dependency.

Cellular cardiac preparations in which spontaneous activity was suppressed by EGTA buffering were isolated by microdissection. Uniform and reproducible contractions were induced by iontophoretically released calcium ions. No effects of a diffusional barrier to calcium ions between the micropipette and the contractile system were detected since the sensitivity of the mechanical performance for calcium was the same regardless of whether a constant amount of calcium ions was released from a single micropipette or from two micropipettes positioned at different sites along the longitudinal axis of the preparation. Force development, muscle length, and shortening velocity of eitherisometric or isotopic contractions were measured simultaneously. Initial length, and hence preload of the preparation were established by means of an electronic stop and any additional load was sensed as afterload. Mechanical performance was derived from force velocity relations and from the interrelationship between simultaneously measured force, length, and shortening velocity. From phase plane analysis of shortening velocity vs, instantaneous length during shortening and from load clamp experiments, the interrelationship between force, shortening, and velocity was shown to be independent of time during the major portion of shortening. Moreover, peak force, shortening, and velocity of shortening depended on the amount of calcium ions in the medium at low and high ionic strength.

Physiological loading of isolated mammalian cardiac muscle.

Cat papillary muscles were subjected to a complex loading function resulting from an analysis of the heart as a pump. The papillary muscle was assumed to be a hypothetical bundle of circumferential muscle fibers in the wall of a simplified cylindrical ventricle. The loading included inertial, resistive, and capacitive components of the cardiovascular system. Changes of ventricular dimensions were taken into account by application of a Laplace relationship. When this complex dynamic loading function was imposed on a shortening muscle by means of an electromagnetic feedback system, the developed force continuously changed with time. The time course of this changing force corresponded to the time course of calculated stress in the intact ejecting heart. Directly displayed force-velocity loops also were similar to loops obtained for the intact heart. Loads proportional to velocity of shortening (damping), acceleration of shortening (inertia), and to the square of shortening velocity (Bernoulli) were investigated separately. Cardiac muscle appeared rather insensitive to inertial loads, and the contribution of inertial loads in the early phase of a contraction under physiological pump loading was minimal. Moreover, during all these dynamic loadings, as long as loading was dynamically increasing or decreasing, velocity of shortening was respectively lower or higher at any muscle length and total load, when compared to velocity at the same length and load under static (constant preload and afterload only) loading.

Physiological pump loading of isolated cardiac muscle.

Cat papillary muscles were subjected to a continuously changing load, resulting from an analysis of the left ventricle as a muscle pump system. The papillary muscle was assumed to be part of a circumferential bundle of muscle fibers of a simplified ejecting ventricle. The load included the pressure--stress relationship of this ventricle and the peripheral vascular load with its inertial, resistive and capacitive components. When this loading function was imposed on a shortening muscle through an electronic feedback circuit, the time course of force development and the velocity versus force plots closely resembled data obtained in the intact heart. Analysis of mechanical work (delta 1 X f) and power (V X f) and their respective time course permitted distinction between changes of contractile performance due to (1) positive or negative inotropic interventions, (2) altered hypothetical ventricular dimensions and changed preload, and (3) the long-term load-dependent memory of cardiac muscle.