Starling's Law reactivated

Cellular mechanisms for the slow phase of the Frank-Starling response

Following a step increase in sarcomere length, isometric cardiac muscle tension increases instantaneously by the Frank-Starling mechanism. In isolated papillary muscle and myocytes, there is an additional significant rise in developed tension over the following 15 min due to an unknown mechanism. This slow change in tension could not be explained by mechanical heterogeneity of the muscle preparations or by an increase in myofilament sensitivity to Ca2+. The slow change in tension was not dependent on sarcoplasmic reticulum Ca2+ loading assessed with rapid cooling contractures, and was not significantly altered by sarcoplasmic reticulum Ca2+ depletion (ryanodine) or inhibition of sarcoplasmic reticulum Ca2+ reuptake (cyclopiazonic acid). We used the Luo-Rudy ionic model of the ventricular myocyte together with a model of the length-dependent myofilament activation by Ca2+ to examine the effects of step changes in the parameters of sarcolemmal ion fluxes as possible mechanisms for the slow change in stress. The slow increase in tension was simulated by step changes in the Na+-K+ pump or Na+ leak currents, suggesting that the slow change in stress may be caused by length induced changes in Na+ fluxes. The model also predicted a slow increase in the magnitude of the initial repolarization during phase 1 of the action potential. The combination of experimental and computational models used in this investigation represents a valuable technique in elucidating the cellular mechanisms of fundamental processes in cardiac excitation-contraction coupling.

Cardiac muscle fiber force versus length determined by a cardiac muscle crossbridge model

A mathematical model incorporating Huxley's sliding filament crossbridge muscle model coupled with parallel and series elastic components was simulated to examine force-length relations under different external calcium concentrations. Several researchers have determined experimentally in both papillary muscle preparations and in situ heart experiments that the calcium concentration (or effective concentration from inotropic agents) will affect the strength and convexity of the cardiac muscle fiber force-length relations. Simulations were performed over a several-order-of-magnitude range of calcium concentrations in isometric contractions and these showed that the force-length curve convexity was changed. Simulation results demonstrated that increasing the stiffness in the model contractile element or series elasticity element did not change the force-length convexity. Increasing the series elasticity element stiffness did slightly change the shape of the force-length curve. The model predicts that the curve convexity changes as a result of the calcium-troponin interactions.

Length dependence of activation studied in the isovolumic blood-perfused dog heart

In studies utilizing the isolated isovolumic blood-perfused canine heart, left ventricular pressure was measured following a sudden expansion of ventricular volume. An increase in performance occurred in two phases: first, there was an instantaneous rise of developed pressure simultaneous with ventricular distension; in the second phase, developed pressure continued to increase for several minutes until a final steady state was reached. The immediate increase in developed pressure occurred with a prolongation of the time-to-peak pressure, and there was no further change of time-to-peak pressure during the time-dependent increase of developed pressure. In another series of experiments, systolic pressure was elevated without changing resting volume, and mechanical performance changed in a different manner: after an increase in systolic load, there was a modest and transient decrease of developed pressure; thereafter, ventricular pressure recovered only to original values. The influence of different degrees of ventricular expansion, calcium, and verapamil were studied. Under higher ventricular dilations the immediate as well as the slow increase of contraction were heightened and the time to reach half of the slow increase was shortened. When ventricular dilation was induced during an infusion of calcium chloride, higher values for the immediate pressure increase were observed, whereas the time-dependent increase and the time to reach half of the slow increase did not change in comparison with control studies. Verapamil decreased the immediate and the time-dependent enhancement of contraction. The time-dependent increase in developed pressure occurs more slowly with verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of cardiovascular performance: modification due to aging

With advancing age the ability of the cardiovascular system to respond to stress declines. The present discussion examines several mechanisms at both the organismal and cellular level that are known to determine cardiovascular performance, and analyses the current state of knowledge regarding the effect of aging on these processes. The conclusion reached is that aging does not result in a generalized decline in all parameters that regulate cardiovascular function, but rather a few specific functional deficits can be identified. Notably, these include a diminished responsiveness to catecholamines and altered vascular loading of the myocardium during high work load states.

Diastolic scattered light fluctuation, resting force and twitch force in mammalian cardiac muscle

1. When coherent light was passed through isolated isometric cardiac muscles during the diastolic or resting period, intensity fluctuations were observed in the scattered field. The frequency of these intensity fluctuations (f((1/2))) varied with many experimental interventions known to enhance Ca(2+) flux into the cell.2. In rat muscles stimulated at low frequencies (0.1 +/- 2.0 min(-1)) stepwise increases (0.4-10 mm) of [Ca(2+)] in the bathing fluid ([Ca(2+)](e)), or addition of ouabain (10(-6)-6 x 10(-4)m) to the perfusate caused stepwise increases in f((1/2)). These were paralleled by increments in resting force (RF) such that the changes in f((1/2)) and RF were highly correlated. Substitution of K(+) for Na(+) in the perfusate resulted in parallel transients in RF and f((1/2)).3. In contrast to the rat, most cat muscles stimulated at low frequencies in the steady state exhibited neither diastolic intensity fluctuations nor Ca(2+)-dependent changes in RF in [Ca(2+)](e) of 10 mm or less; when [Ca(2+)](e) was increased to 12-32 mm, however, steady-state Ca(2+)-dependent f((1/2)) and RF were observed. In a given [Ca(2+)](e) reduction of [Na(+)](e) increased f((1/2)). In the transient state following cessation of regular stimulation at more rapid rates (12-96 min(-1)) intensity fluctuations were present in all [Ca(2+)](e) and decayed with time (seconds to minutes); the f((1/2)) and time course of the decay of the fluctuations were determined by the rate of prior stimulation and [Ca(2+)](e).4. Maximum potentiation of twitch force in response to the above inotropic interventions was associated with an optimal level of f((1/2)) which was similar in both species; when higher levels of f((1/2)) were produced by more intense inotropic intervention, twitch force declined. Over the range of inotropic intervention up to and including that at which maximum twitch potentiation occurred, the increase in diastolic f((1/2)) predicted the extent of twitch potentiation with a high degree of accuracy (r > 0.97) both in the transient and steady states.5. In contrast to the other inotropic interventions studied, catecholamines were unique in that neither f((1/2)) nor RF increased over a full range of concentrations that resulted in maximum potentiation of the twitch.6. It is concluded from these observations that f((1/2)) reflects diastolic Ca(2+)-dependent myofilament interaction; the increase in the extent of this interaction by inotropic interventions that do not alter the affinity of the myofilaments for Ca(2+) probably reflects an increase in diastolic myoplasmic [Ca(2+)], an optimal level of which is associated with maximal potentiation of twitch force; the difference in f((1/2)) in rat and cat muscles under a given set of in vitro conditions may be related to the marked species difference in the effectiveness of excitation-contraction coupling.

Force staircase kinetics in mammalian cardiac muscle: modulation by muscle length

1. Quiescent cat papillary muscles were stimulated to contract regularly at L(max), the length at which force production is optimal, and at 0.85 L(max). The resulting increase in force production (rate staircase) at each length was characterized as an exponential function.2. When stimulated from quiescence at 24 min(-1) in a bathing fluid [Ca(2+)] of 2.5 mM, sixteen of twenty-three muscles exhibited biexponential increases in force production at both lengths. The coefficients of the exponential function at L(max) were 1.5-2 times greater than their counterparts at the shorter length, and this length difference was highly significant. When the force staircases were normalized to the peak developed force attained at each length, the number of beats to attain 25, 50, 75, and 98% of peak force at 0.85 L(max) was approximately twice that required at L(max).3. At a given length the force staircase (1) exhibited a dependency on the number of beats rather than on stimulation frequency over a range of 12-60 min(-1), (2) was accelerated by increasing the bathing fluid [Ca(2+)] from 1.0 to 5.0 mM, (3) was accelerated in the presence of isoproterenol, and (4) was retarded in the presence of DL-verapamil. Over the entire range of bathing fluid [Ca(2+)] and at all stimulation frequencies 24 min(-1) and above, more beats were required to complete a given level of the normalized staircase at 0.85 L(max) than at L(max). There was no length difference in the presence of verapamil. These data suggest that transsarcolemmal Ca(2+) influx is an important determinant of the kinetics of the force staircase, and the length dependence of the latter indicates that muscle length is an important determinant of transsarcolemmal Ca(2+) influx.4. This conclusion was strengthened by the results of additional studies in which the [Ca(2+)] of the bathing fluid was abruptly increased from 1.0 to 5.0 mM with the muscle beating in the steady state. The resulting increase in force production (Ca(2+) staircase) was described by a monoexponential function with a greater coefficient at L(max) than 0.85 L(max); when normalized to the peak force difference in the two [Ca(2+)] at each length, a given level of the staircase was achieved in significantly fewer beats at L(max) than at the shorter length.5. The data provide a mechanism which, in part, explains the length dependence of excitation-contraction coupling in cardiac muscle with intact sarcolemmae.