The influence of 'diastolic' length on the contractility of isolated cat papillary muscle

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

Regulation of cardiac Ca2+ and ion channels by shear mechanotransduction

Cardiac contraction is controlled by a Ca²⁺ signaling sequence that includes L-type Ca²⁺ current-gated opening of Ca²⁺ release channels (ryanodine receptors) in the sarcoplasmic reticulum (SR). Local Ca²⁺ signaling in the atrium differs from that in the ventricle because atrial myocytes lack transverse tubules and have more abundant corbular SR. Myocardium is subjected to a variety of forces with each contraction, such as stretch, shear stress, and afterload, and adapts to those mechanical stresses. These mechanical stimuli increase in heart failure, hypertension, and valvular heart diseases that are clinically implicated in atrial fibrillation and stroke. In the present review, we describe distinct responses of atrial and ventricular myocytes to shear stress and compare them with other mechanical responses in the context of local and global Ca²⁺ signaling and ion channel regulation. Recent evidence suggests that shear mechanotransduction in cardiac myocytes involves activation of gap junction hemichannels, purinergic signaling, and generation of mitochondrial reactive oxygen species. Significant alterations in Ca²⁺ signaling and ionic currents by shear stress may be implicated in the pathogenesis of cardiac arrhythmia and failure.

Mechanosensitivity of microdomain calcium signalling in the heart

In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.

Inhibition of carbonic anhydrase prevents the Na(+)/H(+) exchanger 1-dependent slow force response to rat myocardial stretch

Myocardial stretch is an established signal that leads to hypertrophy. Myocardial stretch induces a first immediate force increase followed by a slow force response (SFR), which is a consequence of an increased Ca(2+) transient that follows the NHE1 Na(+)/H(+) exchanger activation. Carbonic anhydrase II (CAII) binds to the extreme COOH terminus of NHE1 and regulates its transport activity. We aimed to test the role of CAII bound to NHE1 in the SFR. The SFR and changes in intracellular pH (pHi) were evaluated in rat papillary muscle bathed with CO2/HCO3(-) buffer and stretched from 92% to 98% of the muscle maximal force development length for 10 min in the presence of the CA inhibitor 6-ethoxzolamide (ETZ, 100 μM). SFR control was 120 ± 3% (n = 8) of the rapid initial phase and was fully blocked by ETZ (99 ± 4%, n = 6). The SFR corresponded to a maximal increase in pHi of 0.18 ± 0.02 pH units (n = 4), and pHi changes were blocked by ETZ (0.04 ± 0.04, n = 6), as monitored by epifluorescence. NHE1/CAII physical association was examined in the SFR by coimmunoprecipitation, using muscle lysates. CAII immunoprecipitated with an anti-NHE1 antibody and the CAII immunoprecipitated protein levels increased 58 ± 9% (n = 6) upon stretch of muscles, assessed by immunoblots. The p90(RSK) kinase inhibitor SL0101-1 (10 μM) blocked the SFR of heart muscles after stretch 102 ± 2% (n = 4) and reduced the binding of CAII to NHE1, suggesting that the stretch-induced phosphorylation of NHE1 increases its binding to CAII. CAII/NHE1 interaction constitutes a component of the SFR to heart muscle stretch, which potentiates NHE1-mediated H(+) transport in the myocardium.

Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation

In isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular dilation was promoted at three heart rate (HR) levels: low (L: 52 +/- 1.7 beats/min), intermediate (M: 82 +/- 2.2 beats/min), and high (H: 117 +/- 3.5 beats/min). DP increased instantaneously with chamber expansion (Delta(1)DP), and another continuous increase occurred for several minutes (Delta(2)DP). HR elevation did not alter Delta(1)DP (32.8 +/- 1.6, 33.6 +/- 1.5, and 34.3 +/- 1.2 mmHg for L, M, and H, respectively), even though it intensified Delta(2)DP (17.3 +/- 0.9, 20.7 +/- 1.0, and 26.8 +/- 1.2 mmHg for L, M, and H, respectively), meaning that the treppe phenomenon enhances the length dependence of the contraction component related to changes in intracellular Ca(2+) concentration. Frequency increments reduced the half time of the slow response (82 +/- 3.6, 67 +/- 2.6, and 53 +/- 2.0 s for L, M, and H, respectively), while the number of beats included in half time increased (72 +/- 2.9, 95 +/- 2.9, and 111 +/- 3.2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca(2+) channel currents and/or the Na(+)/Ca(2+) exchanger plays a relevant role in the stretch-triggered Ca(2+) gain when HR increases in the canine heart.

Mechanisms underlying the increase in force and Ca(2+) transient that follow stretch of cardiac muscle: a possible explanation of the Anrep effect

Myocardial stretch produces an increase in developed force (DF) that occurs in two phases: the first (rapidly occurring) is generally attributed to an increase in myofilament calcium responsiveness and the second (gradually developing) to an increase in [Ca(2+)](i). Rat ventricular trabeculae were stretched from approximately 88% to approximately 98% of L(max), and the second force phase was analyzed. Intracellular pH, [Na(+)](i), and Ca(2+) transients were measured by epifluorescence with BCECF-AM, SBFI-AM, and fura-2, respectively. After stretch, DF increased by 1.94+/-0.2 g/mm(2) (P<0.01, n = 4), with the second phase accounting for 28+/-2% of the total increase (P<0.001, n = 4). During this phase, SBFI(340/380) ratio increased from 0.73+/-0.01 to 0.76+/-0.01 (P<0.05, n = 5) with an estimated [Na(+)](i) rise of approximately 6 mmol/L. [Ca(2+)](i) transient, expressed as fura-2(340/380) ratio, increased by 9.2+/-3.6% (P<0.05, n = 5). The increase in [Na(+)](i) was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). The second phase in force and the increases in [Na(+)](i) and [Ca(2+)](i) transient were blunted by AT(1) or ET(A) blockade. Our data indicate that the second force phase and the increase in [Ca(2+)](i) transient after stretch result from activation of the Na(+)/H(+) exchanger (NHE) increasing [Na(+)](i) and leading to a secondary increase in [Ca(2+)](i) transient. This reflects an autocrine-paracrine mechanism whereby stretch triggers the release of angiotensin II, which in turn releases endothelin and activates the NHE through ET(A) receptors.

The role of calcium in the response of cardiac muscle to stretch

This review focuses on the complex interactions between two major regulators of cardiac function; Ca2+ and stretch. Initial consideration is given to the effect of stretch on myocardial contractility and details the rapid and slow increases in contractility. These are shown to be related to two diverse changes in Ca2+ handling (enhanced myofilament Ca2+ sensitivity and increased intracellular Ca2+ transient, respectively). Interaction between stretch and Ca2+ is also demonstrated with respect to the rhythm of cardiac contraction. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. A variety of Ca(2+)-dependent mechanisms including attenuation of Ca2+ extrusion via Na+/Ca2+ exchange, Ca2+ entry through stretch-activated channels (SACs) and mobilisation of intracellular Ca2+ stores have been proposed to account for the effect of stretch on rhythm. Finally, the interaction between stretch and Ca2+ in the secretion of natriuretic peptides and onset of hypertrophy is discussed. Evidence is presented that Ca2+ (entering through L-type Ca2+ channels or SACs, or released from sarcoplasmic reticular stores) influences secretion of both atrial and B-type natriuretic peptide; there is data to support both positive and negative modulation by Ca2+. Ca2+ also appears to be important in the pathway that leads to expression of precursors of hypertrophic protein synthesis. In conclusion, two of the major regulators of cardiac muscle function, Ca2+ and stretch, interact to produce effects on the heart; in general these effects appear to be additive.

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.

Mechanisms of stretch-induced changes in [Ca2+]i in rat atrial myocytes: role of increased troponin C affinity and stretch-activated ion channels

To study the effects of stretch on the function of rat left atrium, we recorded contraction force, calcium transients, and intracellular action potentials (APs) during stretch manipulations. The stretch of the atrium was controlled by intra-atrial pressure. The Frank-Starling behavior of the atrium was manifested as a biphasic increase of the contraction force after increasing the stretch level. The development of the contraction force after step increase of the stretch (intra-atrial pressure from 1 to 3 mm Hg) was accompanied by the increase in the amplitude of the calcium transients (P<0.05, n=4) and decrease in the time constant of the Ca2+ transient decay. The APs of the individual myocytes were also affected by stretch; the duration of the AP was decreased at positive voltages (AP duration at 15% repolarization level, P<0.001; n=13) and increased at negative voltages (AP duration at 90% repolarization level, P<0. 01; n=13). To study the mechanisms causing these changes we developed a mathematical model describing [Ca2+]i and electrical behavior of single rat atrial myocytes. Stretch was simulated in the model by increasing the troponin (TnC) sensitivity and/or applying a stretch-activated (SA) calcium influx. We mimicked the Ca2+ influx by introducing a nonselective cationic conductance, the SA channels, into the membrane. Neither of the 2 plausible mechanosensors (TnC or SA channels) alone could produce similar changes in the Ca2+ transients or APs as seen in the experiments. The model simulated the effects of stretch seen in experiments best when both the TnC affinity and the SA conductance activation were applied simultaneously. The SA channel activation led to gradual augmentation of Ca2+ transients, which modulated the APs through increased Na+/Ca2+-exchanger inward current. The role of TnC affinity change was to modulate the Ca2+ transients, stabilize the diastolic [Ca2+]i, and presumably to produce the immediate increase of the contraction force after stretch seen in experiments. Furthermore, we found that the same mechanism that caused the normal physiological responses to stretch could also generate arrhythmogenic afterpotentials at high stretch levels in the model.

Mechanisms of length history-dependent tension in an ionic model of the cardiac myocyte

The ionic model of the ventricular myocyte developed by Luo and Rudy (Circ. Res. 74: 1071-1096, 1994) was used to investigate potential mechanisms of the slow changes in stress (SCS) that follow step changes in muscle length. A step change in myofilament sensitivity alone caused an immediate increase in active tension, but no SCS. The effects of additional step changes in the parameters of sarcolemmal ion fluxes were examined for each ion flux in the model. Changes in the coefficients of Ca2+ or K+ channels did not produce SCS. SCS was produced by step changes in parameters of the Na(+)-K+ pump or the Na+ leak current. This simulated mechanism was mediated through a slow increase in intracellular Na+ concentration and a resulting increase in systolic Ca2+ entry through the Na+/Ca2+ exchanger. The model reproduced the effects of several experimental interventions such as sarcoplasmic reticulum Ca2+ depletion, "diastolic" length changes, and changes in extracellular Ca2+. Thus SCS in cardiac muscle may be caused by length-induced changes in sarcolemmal Na+ fluxes.

Changes in force and cytosolic Ca2+ concentration after length changes in isolated rat ventricular trabeculae

1. Changes in cytosolic [Ca2+] ([Ca2+]i) were measured in isolated rat trabeculae that had been micro-injected with fura-2 salt, in order to investigate the mechanism by which twitch force changes following an alteration of muscle length. 2. A step increase in length of the muscle produced a rapid potentiation of twitch force but not of the Ca2+ transient. The rapid rise of force was unaffected by inhibiting the sarcoplasmic reticulum (SR) with ryanodine and cyclopiazonic acid. 3. The force-[Ca2+]i relationship of the myofibrils in situ, determined from twitches and tetanic contractions in SR-inhibited muscles, showed that the rapid rise of force was due primarily to an increase in myofibrillar Ca2+ sensitivity, with a contribution from an increase in the maximum force production of the myofibrils. 4. After stretch of the muscle there was a further, slow increase of twitch force which was due entirely to a slow increase of the Ca2+ transient, since there was no change in the myofibrillar force-[Ca2+]i relationship. SR inhibition slowed down, but did not alter the magnitude of, the slow force response. 5. During the slow rise of force there was no slow increase of diastolic [Ca2+]i, whether or not the SR was inhibited. The same was true in unstimulated muscles. 6. We conclude that the rapid increase in twitch force after muscle stretch is due to the length-dependent properties of the myofibrils. The slow force increase is not explained by length dependence of the myofibrils or the SR, or by a rise in diastolic [Ca2+]i. Evidence from tetani suggests the slow force responses result from increased Ca2+ loading of the cell during the action potential.

Myocardial length-force relationship in end stage dilated cardiomyopathy and normal human myocardium: analysis of intact and skinned left ventricular trabeculae obtained during 11 heart transplantations

The Frank-Starling-mechanism (FSM) was analyzed in isolated intact and skinned human left ventricular myocardium obtained from 11 heart transplantations (normal donor hearts (NDH), n = 8; dilated cardiomyopathy (DCM), n = 11). The new technique to utilize muscle strips from normal donor hearts which were actually implanted is described in detail.

METHODS

I) In electrically stimulated left ventricular trabeculae (37 degrees C, oxygenated Krebs-Henseleit solution, supramaximal electrical stimulation, frequency 1 Hz) force development was analyzed as a function of muscle length (NDH = 8; DCM = 11). II) In an additional series left ventricular myocardium was demembranized ("skinned") by Triton-X-100. At different sarcomere lengths and calcium concentrations corresponding to pCa values of 4.3, 5.5, and 8.0 force development was measured (DCM = 11; NDH = 9).

RESULTS

I) Developed force increased up to an optimum as a function of muscle length in intact NDH- and DCM-myocardium. However, the relative increment of developed force after any length step was smaller in DCM than in NDH. Near "Lmax" (muscle length associated with maximum developed force) passive resting tension was considerably elevated in DCM, indicating significantly increased diastolic stiffness. II) In skinned left ventricular DCM- and NDH-myocardium developed force depended on sarcomere length with an optimum near 2.2 microns. However, a reduction of activator calcium concentration from pCa 4.3 to pCa 5.5 produces a smaller percent decline in force at short sarcomere lengths in DCM than it does in NDH.

CONCLUSION

The present study shows that except for diastolic stiffness and a smaller relative force increment after any length step in DCM the Frank Starling mechanism is still present in isolated human left ventricular DCM- as in NDH-myocardium. The current study does not allow to decide whether in skinned myocardium the smaller percent decline in force after reduction of activator calcium concentrations in DCM is caused by an increased calcium sensitivity at short sarcomere lengths or decreased sensitivity at long sarcomere lengths.

Changes in [Ca2+]i, [Na+]i and Ca2+ current in isolated rat ventricular myocytes following an increase in cell length

Isolated rat ventricular myocytes were stretched using carbon fibres to investigate the mechanisms underlying the increase in contraction following stretch. 2. [Ca2+]i and [Na+]i were monitored using the fluorescent indicators fura-2 and sodium-binding benzofuran isophthalate, respectively. The L-type Ca2+ current was recorded simultaneously with contraction using the perforated patch-clamp technique. 3. Mechanical stretch caused an immediate increase in contraction, followed by a slow increase. Contraction was prolonged immediately after the stretch, but did not change during the slow phase. 4. The Ca2+ transient did not change immediately after the stretch. The slow increase in contraction was accompanied by an increase in the amplitude of the Ca2+ transient. However, diastolic [Ca2+]i did not change significantly following stretch. 5. [Na+]i did not change significantly either immediately, or during the slow increase in contraction, after the stretch. 6. The L-type Ca2+ current was not significantly altered either by mechanical loading of the cell with carbon fibres or by stretching the cell. 7. These results suggest that: (1) the rapid increase in contraction following a stretch is due to an increase in myofilament Ca2+ sensitivity rather than to changes in the L-type Ca2+ current or [Na+]i; and (2) a slow increase in the Ca2+ transient underlies the slow increase in contraction in isolated myocytes, but is not caused by either an increase in diastolic [Ca2+]i or a change in [Na+]i (and hence Ca2+ influx via Na(+)-Ca2+ exchange) or a change in myofilament Ca2+ sensitivity.

Long-term versus intrabeat history of ejection as determinants of canine ventricular end-systolic pressure

We studied the effect of ejection on end-systolic pressure in isolated heart preparations. Ejecting beats were compared with isovolumic beats having the same volume as at end systole. While holding end-systolic volume constant, various stroke volumes, including negative stroke volumes (volume injected during systole), were imposed using a predetermined volume command. After switching contraction mode between ejecting and isovolumic, we measured the immediate and steady changes in end-systolic pressure. In the first isovolumic beat after switching from steady-state ejecting beats, the change in end-systolic pressure was variable, depending on the stroke volume. The end-systolic pressure of the ejecting beat exceeded that of the isovolumic beat on average by up to 18 mm Hg with small stroke volume, but the ejecting end-systolic pressure became lower than isovolumic with either large stroke volume (stroke volume/end-systolic volume less than 0.96) or with negative stroke volume. During the transient phase following a switch from ejecting to isovolumic, the end-systolic pressure gradually decreased to a steady state. Consequently, even in steady state, ejecting end-systolic pressure exceeded isovolumic pressure over a significant range of stroke volume (stroke volume/end-systolic volume less than 1.18). After returning contraction mode from isovolumic back to ejecting, we observed responses that were a mirror image. These results indicated that in addition to negative uncoupling effect, ejection exerts positive effects on ventricular end-systolic pressure that are manifest both quickly and gradually. We hypothesized that the mechanism responsible for the positive effect is length-dependent activation via the larger volume (both at the initiation of contraction and averaged over a cardiac cycle) of a beat that ejects compared to one held isovolumic at end-systolic volume. The results with volume injection were consonant with this concept.

Influence of contractile state on curvilinearity of in situ end-systolic pressure-volume relations

Although in situ end-systolic pressure-volume relations (ESPVRs) are approximately linear throughout a limited load range, they often yield seemingly "negative" volume axis intercepts (V0) and V0 shifts with inotropic interventions. We tested whether or not these findings could stem from in situ ESPVR nonlinearity, and we examined the physiologic meaning and limitations of linearized ESPVR variables frequently used for assessing contractile state. Continuous left ventricular pressures and volumes were obtained by micromanometer and conductance (volume) catheters in six open-chest dogs. Left ventricular loading was varied throughout a wide range by rapid left atrial hemorrhage into a reservoir. Propranolol and verapamil were administered to reduce inotropic state, with heart rate maintained by atrioventricular sequential pacing. ESPVRs were fit to nonlinear [Pes = a(Ves-V'0)2 + b(Ves-V'0)] and linear (Pes = Ees (Ves-V0)] models. Contractile state was assessed by the slope of the ESPVR at V'0 (b, of nonlinear model) and by two other ESPVR model-independent measures: the slope of the dP/dtmax and end-diastolic volume relation, and the slope of the stroke work and end-diastolic volume relation. ESPVR was frequently curvilinear, and a significant correlation existed between the extent of nonlinearity (a) and contractile state. Volume intercepts derived from linear fits to the high load ESPVR range were mostly negative and were dependent on changes in Ees. V0 estimates derived from the low load portion were positive and relatively insensitive to Ees. Thus, in situ ESPVR displays contractility-dependent curvilinearity. The contractility range throughout which ESPVRs are essentially linear is typical for isolated hearts, but the range represents low values for in situ ventricles. Despite curvilinearity, Ees determined in situ throughout limited load ranges can accurately assess inotropic state; however, comparisons between ESPVRs should consider potential nonlinearity, and if possible, they should be made within similar end-systolic pressure ranges.

The effects of changes in muscle length during diastole on the calcium transient in ferret ventricular muscle

1. Ferret papillary muscles were isolated and injected with aequorin to measure intracellular Ca2+ concentration [( Ca2+]i). Developed tension and [Ca2+]i were measured in response to length changes. 2. A maintained reduction in muscle length produced an immediate decrease in developed tension followed by slow decline over 10-20 min. This slow decline in tension was accompanied by a slow decline in the amplitude of the systolic [Ca2+]i rise (the Ca2+ transient). The immediate decrease in tension was accompanied by a prolongation of the Ca2+ transient and an abbreviation of the twitch. 3. Repeated reductions in muscle length timed to occur only during the period of contraction (systolic shortening) produced an immediate decrease of developed tension but the subsequent slow decline was substantially smaller. The slow decline in the amplitude of the Ca2+ transients was also smaller. The prolongation of the Ca2+ transient and abbreviation of the twitch were similar to those observed with a maintained reduction of length. 4. Repeated reductions in muscle length during the period between contractions (diastolic shortening) did not produce the immediate decrease of tension but the slow decline of tension was present. The slow decline in the amplitude of the Ca2+ transients was also present. However no change in the duration of the Ca2+ transient or the twitch was present under these conditions. 5. These results suggest that diastolic muscle length can influence the amplitude of the Ca2+ transients achieved during systole. This conclusion was confirmed by experiments in which the recovery of tension and Ca2+ transients was observed after periods of rest. Both developed tension and Ca2+ transients on recovery from a rest were reduced when the rest occurred at a short length in comparison with a long length. 6. We suggest that muscle length influences resting [Ca2+]i and this in turn affects the Ca2+ transients and developed tension.

Time-dependent increase in left ventricular contractility following acute volume loading in the dog

Acute volume alterations were produced in eight anesthetized dogs to determine if the contractility of the left ventricle is partially volume-dependent. Pressures were measured in the left ventricle with a micromanometer and regional ventricular function was measured with sonomicrometers implanted in the midwall of the anterior, lateral, and posterior left ventricle. Regional end-systolic pressure-length relations (ESPLR) were determined with transient venae cavae occlusions. The ESPLR data were fitted to a quadratic equation, then end-systolic lengths were compared at matched end-systolic pressures at multiple time periods after the acute load alterations. Bilateral vagotomy, carotid sinus denervation, and stellate ganglion denervation were performed to prevent reflex alterations in contractility. The heart was paced at a constant rate. In seven animals, dextran was infused intravenously over 77 +/- 23 seconds (+/- SD) to increase left ventricular end-diastolic pressure from 5 +/- 2 to 13 +/- 3 mm Hg and peak pressures from 113 +/- 11 to 147 +/- 18 mm Hg. The end-diastolic lengths increased 14 +/- 6% in the anterior, 9 +/- 2% in the lateral, and 12 +/- 4% in the posterior segments. The ESPLR was measured immediately (77 +/- 23 seconds), early (3 +/- 1 minutes), and late (10 +/- 2 minutes) after initiation of the volume load. In all three regions, there was a significant time-dependent leftward shift in the ESPLR. From 1 to 10 minutes after the volume load, the regional end-systolic lengths decreased by a mean of 4-6% when compared at a matched end-systolic pressure of 96 +/- 10 mm Hg, and decreased by 7-9% when compared at a matched pressure of 139 +/- 20 mm Hg. The end-systolic lengths immediately after the volume load were not significantly different from the control value (compared at a matched end-systolic pressure) but were significantly shorter 10 minutes after the volume load. The leftward shift of the ESPLR represented a true increase in contractility and not merely a recovery from a transient myocardial depression. A similar leftward shift in the ESPLR occurred in four animals after release of a partial venae cavae occlusion, producing an acute volume load without acute hemodilution. Acute volume loads in four animals treated with propranolol also produced a leftward shift in the ESPLR, ruling out the possibility that a time-dependent increase in circulating catecholamines was responsible for the alterations in contractility. In five animals, the ESPLR shifted to the right after a transient (1-2 minutes) decrease in venous return.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemomechanics of altered perfusion pressure in rat hearts

In an apex-ejecting isolated perfused working rat heart, as well as isovolumic preparations of rat hearts, perfusion pressure was studied independent of afterload. A decrease in perfusion pressure caused an immediate decrease in developed pressure (10s). There was a significant increase in free Pi and the phosphorylation potential after 20-30 min of perfusion at a reduced coronary flow induced by a reduction in perfusion pressure. Developed pressure decreased prior to the phosphorylation potential and inorganic phosphate; however, the phosphorylation set a limit to maximum work performance. At a perfusion pressure of 140 cm H2O and an afterload of 140 cm H2O, work imposed on the heart was maximum; there was no further increase in work.

Starling's law of the heart is explained by an intimate interaction of muscle length and myofilament calcium activation

The results of several different types of investigations over the last decade clearly indicate that muscle length modulates the extent of myofilament calcium ion (Ca2+) activation. Similarly, the fiber length during a contraction, which is determined in part by the load encountered during shortening, also determines the extent of myofilament Ca2+ activation. Thus, "contractile" or "inotropic" state as it refers to the extent of myofilament activation can, in theory, no longer be considered independent of the muscle length, as was formerly thought to be the case. Accordingly, terms such as preload, afterload and myocardial contractile state as they pertain to cardiac muscle properties lose part of their significance in light of current knowledge.