Effects of chronic mitral regurgitation on diastolic function in isolated cardiocytes

Mitral Annular Tissue Velocity Predicts Survival in Patients With Primary Mitral Regurgitation

BACKGROUND AND OBJECTIVES

Early diastolic mitral annular tissue (e') velocity is a commonly used marker of left ventricular (LV) diastolic function. This study aimed to investigate the prognostic implications of e' velocity in patients with mitral regurgitation (MR).

METHODS

This retrospective cohort study included 1,536 consecutive patients aged <65 years with moderate or severe chronic primary MR diagnosed between 2009 and 2018. The primary and secondary outcomes were all-cause and cardiovascular mortality, respectively. According to the current guidelines, the cut-off value of e' velocity was defined as 7 cm/s.

RESULTS

A total of 404 individuals were enrolled (median age, 51.0 years; 64.1% male; 47.8% severe MR). During a median 6.0-year follow-up, there were 40 all-cause mortality and 16 cardiovascular deaths. Multivariate analysis revealed a significant association between e' velocity and all-cause death (adjusted hazard ratio [aHR], 0.770; 95% confidence interval [CI], 0.634-0.935; p=0.008) and cardiovascular death (aHR, 0.690; 95% CI, 0.477-0.998; p=0.049). Abnormal e' velocity (≤7 cm/s) independently predicted all-cause death (aHR, 2.467; 95% CI, 1.170-5.200; p=0.018) and cardiovascular death (aHR, 5.021; 95% CI, 1.189-21.211; p=0.028), regardless of symptoms, LV dimension and ejection fraction. Subgroup analysis according to sex, MR severity, mitral valve replacement/repair, and symptoms, showed no significant interactions. Including e' velocity in the 10-year risk score improved reclassification for mortality (net reclassification improvement [NRI], 0.154; 95% CI, 0.308-0.910; p<0.001) and cardiovascular death (NRI, 1.018; 95% CI, 0.680-1.356; p<0.001).

CONCLUSIONS

In patients aged <65 years with primary MR, e' velocity served as an independent predictor of all-cause and cardiovascular deaths.

Interstitial Collagen Loss, Myocardial Remodeling, and Function in Primary Mitral Regurgitation

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The stretch of volume overload in PMR initiates interstitial collagen loss and decrease in LV sphericity index.

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LV chamber diastolic function is normal whereas LA function, LV twist/volume slope, early LV untwist, and myocardial circumferential strain are impaired.

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There is increased oxidative stress in the cardiomyocyte with cytoskeletal breakdown and myofibrillar loss in PMR.

Interstitial collagen loss and cardiomyocyte ultrastructural damage accounts for left ventricular (LV) sphericity and decrease in LV twist and circumferential strain. Normal LV diastolic function belies significantly abnormal left atrial (LA) function and early LV diastolic untwist rate. This underscores the complex interplay of LV and LA myocardial remodeling and function in the pathophysiology of primary mitral regurgitation. In this study, we connect LA function with LV systolic and diastolic myocardial remodeling and function using cardiac magnetic resonance tissue tagging in primary mitral regurgitation.

Large-Scale Contractility Measurements Reveal Large Atrioventricular and Subtle Interventricular Differences in Cultured Unloaded Rat Cardiomyocytes

The chambers of the heart fulfill different hemodynamic functions, which are reflected in their structural and contractile properties. While the atria are highly elastic to allow filling from the venous system, the ventricles need to be able to produce sufficiently high pressures to eject blood into the circulation. The right ventricle (RV) pumps into the low pressure pulmonary circulation, while the left ventricle (LV) needs to overcome the high pressure of the systemic circulation. It is incompletely understood whether these differences can be explained by the contractile differences at the level of the individual cardiomyocytes of the chambers. We addressed this by isolating cardiomyocytes from atria, RV, LV, and interventricular septum (IVS) of five healthy wild-type rats. Using a high-throughput contractility set-up, we measured contractile function of 2,043 cells after overnight culture. Compared to ventricular cardiomyocytes, atrial cells showed a twofold lower contraction amplitude and 1.4- to 1.7-fold slower kinetics of contraction and relaxation. The interventricular differences in contractile function were much smaller; RV cells displayed 12–13% less fractional shortening and 5–9% slower contraction and 3–15% slower relaxation kinetics relative to their LV and IVS counterparts. Aided by a large dataset, we established relationships between contractile parameters and found contraction velocity, fractional shortening and relaxation velocity to be highly correlated. In conclusion, our findings are in line with contractile differences observed at the atrioventricular level, but can only partly explain the interventricular differences that exist at the organ level.

A Comparison of Phenomenologic Growth Laws for Myocardial Hypertrophy

The heart grows in response to changes in hemodynamic loading during normal development and in response to valve disease, hypertension, and other pathologies. In general, a left ventricle subjected to increased afterload (pressure overloading) exhibits concentric growth characterized by thickening of individual myocytes and the heart wall, while one experiencing increased preload (volume overloading) exhibits eccentric growth characterized by lengthening of myocytes and dilation of the cavity. Predictive models of cardiac growth could be important tools in evaluating treatments, guiding clinical decision making, and designing novel therapies for a range of diseases. Thus, in the past 20 years there has been considerable effort to simulate growth within the left ventricle. While a number of published equations or systems of equations (often termed "growth laws") can capture some aspects of experimentally observed growth patterns, no direct comparisons of the various published models have been performed. Here we examine eight of these laws and compare them in a simple test-bed in which we imposed stretches measured during in vivo pressure and volume overload. Laws were compared based on their ability to predict experimentally measured patterns of growth in the myocardial fiber and radial directions as well as the ratio of fiber-to-radial growth. Three of the eight laws were able to reproduce most key aspects of growth following both pressure and volume overload. Although these three growth laws utilized different approaches to predict hypertrophy, they all employed multiple inputs that were weakly correlated during in vivo overload and therefore provided independent information about mechanics.

Impaired contractile reserve in severe mitral valve regurgitation with a preserved ejection fraction

BACKGROUND

Impaired contractile reserve in chronic MR results from load-independent, myocyte contractile abnormalities.

AIMS

Investigate the mechanisms of contractile dysfunction in chronic mitral valve regurgitation (MR).

METHODS

Mild MR was produced in eight dogs followed by pacing induced left ventricular (LV) dilatation over eight months. In-vivo LV dP/dt was measured at several pacing rates. Contractile function was measured in isolated LV trabeculae and myocytes at several stimulation rates and during changes in extracellular [Ca2+]. Identical studies were performed with six control dogs.

RESULTS

Chronic MR resulted in a preserved ejection fraction with decreased dP/dt (p<0.01). LV trabeculae demonstrated significantly lower developed force and a negative force-frequency relation with chronic MR (p<0.05). Myocytes exhibited a negative shortening-frequency relationship in both groups with a greater decline with chronic MR (p<0.001) paralleled by decreases in peak [Ca2+](i) transients. Increases in extracellular [Ca2+] abrogated the defects in force generation in trabeculae from animals with chronic MR.

CONCLUSION

Even with a preserved EF, chronic severe MR results in a significant reduction in intrinsic contractile function and reserve. Functional impairment was load-independent reflecting a predominant defect in calcium cycling rather than impaired peak force generating capacity due to myofibrillar attenuation.

The extracellular matrix and the cytoskeleton in heart hypertrophy and failure

Cell characteristics and phenotype depend on the nature of the extracellular matrix, the type and organization of integrins and cytoskeleton. The interactions between these components are poorly known at the myocyte level and during cardiac remodeling associated with cardiac hypertrophy and heart failure. We analyze here the nature and organization of extracellular matrix (ECM) proteins, cytoskeleton and integrins and their regulation by growth factors, such as angiotensin II, in normal myocyte growth and in pathological growth (hypertrophy) of the myocardium and heart failure.

Cardiomyocytes from hearts with left ventricular dysfunction after ischemia-reperfusion do not manifest contractile abnormalities

OBJECTIVES

This study evaluated contractile function in cardiomyocytes isolated from hearts with global left ventricular dysfunction following ischemia-reperfusion.

BACKGROUND

Ischemia followed by reperfusion is associated with transient contractile dysfunction, termed "stunning." It is not clear whether this phenomenon is primarily due to intrinsic cardiomyocyte contractile dysfunction.

METHODS

Global contractile dysfunction was induced in isolated perfused rat hearts (n = 8) using a model of transient global ischemia (20 min) followed by reperfusion (20 min). Hearts perfused uninterrupted for 40 min were used as controls (n = 8). Cardiomyocytes were isolated using enzymatic digestion and were studied under varying degrees of inotropy (using increasing extracellular calcium [Ca2+]o) and loading conditions (varying extracellular perfusate viscosity). Mechanical function was studied with video edge detection and intracellular calcium ([Ca2+]i) kinetics using fura-2 AM.

RESULTS

Global ischemia-reperfusion increased left ventricle (LV) end diastolic pressure (450% vs. 33%, p < 0.01) and reduced LV developed pressure (9% vs. 33%, p < 0.01), LV positive (3% vs. 26%, p < 0.01) and negative (5% vs. 33%, p < 0.01) dP/dt. However, cells isolated from these hearts did not manifest contractile dysfunction. In fact, cell shortening (p < 0.0001) and peak rate of cell shortening (p < 0.05) and increase in [Ca2+]i with each contraction (p < 0.024) were higher in these cells during stimulation with [Ca2+]o of 1 to 10 mmol/liter. The EC50 values for calcium dose response and the slope of the relation between change in [Ca2+]i and change in cell length were no different between the groups. Cell loading (with increasing superfusate viscosity from 1 cp to 300 cp) also did not reveal any abnormalities in cells from the hearts subjected to ischemia-reperfusion.

CONCLUSIONS

Cardiomyocytes isolated from hearts with ischemia-reperfusion-induced LV dysfunction or "stunning" have normal contractile function and normal [Ca2+]i transients, when studied both in the unloaded and loaded state. Our data suggest that nonmyocyte factors such as abnormalities in extracellular matrix or abnormal myocyte-interstitial tissue coupling may be important for the genesis of cardiac contractile failure in the stunned heart.

Beneficial Effects of Aminophylline on Ischemia-Reperfusion in Isolated Rabbit Heart

Eighteen rabbit hearts were arrested for 3 hours with cardioplegic solution at 4°C, followed by reperfusion with oxygenated perfusion solution at 37°C for 2 hours. Six control hearts received no drug during arrest or reperfusion (group 1). Six hearts received 3 mg·L−1 aminophylline during the arrest period (group 2). Six hearts received 3 mg·L−1 aminophylline during the reperfusion period (group 3). Effects of aminophylline were evaluated in terms of the pressure-volume relationship, coronary flow, myocardial oxygen extraction, and lactate release before cardioplegic arrest and after 1 and 2 hours of reperfusion. End-diastolic pressure at constant volume after 2 hours of reperfusion was 19 ± 2.63 mm Hg in group 1, 14 ± 1.7 mm Hg in group 2, and 19 ± 2.55 mm Hg in group 3 (p < 0.05 for group 2 versus groups 1 and 3). End-systolic pressure at constant volume after 2 hours of reperfusion was 81 ± 3.55 mm Hg in group 1, 90 ± 2.95 mm Hg in group 2, and 84 ± 3.47 mm Hg in group 3 (p < 0.05 for group 2 versus groups 1 and 3). Oxygen extraction was significantly higher and release of lactate was significantly lower in group 2 compared to groups 1 and 3. The results indicate that aminophylline administration during cardioplegic arrest improved systolic and diastolic function and had a beneficial effect on metabolic recovery.

Role of SR Ca2+-ATPase in contractile dysfunction of myocytes in tachycardia-induced heart failure

Sarcoplasmic reticulum (SR) Ca2+-ATPase gene expression is reduced in the failing myocardium. However, the functional relevance of these changes to myocardial contractility is not yet established. We assessed myocardial contractile function by analyzing sarcomere motion of isolated myocytes and also quantified SR Ca2+ regulatory protein gene expression by Northern blot analysis in the same hearts obtained from 10 dogs with pacing-induced heart failure (HF; 240 beats/min, 4 wk) and 7 control dogs. Sarcomere-shortening velocity was depressed in HF myocytes, accompanied by the prolongation of intracellular Ca2+ concentration ([Ca2+]i) transient measured by indo 1 fluorescence ratio. SR Ca2+-ATPase mRNA levels (normalized to glyceraldehyde-3-phosphate dehydrogenase mRNA) were significantly depressed in HF, and calsequestrin mRNA was increased. For control and HF dogs, sarcomere-shortening velocity correlated positively with Ca2+-ATPase mRNA levels (r = 0.73, n = 17, P < 0.01) but not with calsequestrin mRNA. Ca2+-ATPase mRNA levels were correlated with 45Ca2+ uptake rate by SR, which was also reduced in HF. Moreover, the inhibition of SR Ca2+-ATPase with thapsigargin or cyclopiazonic acid reproduced in normal myocytes the abnormalities observed in HF myocytes, such as depressed contractility and the prolonged [Ca2+]i transient duration. A downregulation of Ca2+-ATPase gene expression and a resultant decrease in Ca2+ uptake by SR may be responsible for the contractile dysfunction and the alterations of [Ca2+]i transient in HF.

Effects of left ventricular systolic function on left ventricular diastolic filling patterns in severe mitral regurgitation

Significant mitral regurgitation (MR) may alter the normal pattern of Doppler detected left ventricular (LV) filling by causing a prominent early filling (E) wave velocity. The manner and extent to which the typical filling pattern of uncomplicated MR is affected by concomitant impaired LV systolic function has not been characterized. Twenty patients with severe LV systolic dysfunction (2-dimensional echocardiographic estimation of ejection fraction < or = 30%) and 21 age- and sex-matched case controls with normal systolic function (ejection fraction > or = 55%) were selected. In addition, 20 subjects with normal LV systolic function and no MR were analyzed as a reference group. Maximal E-wave velocity was increased and highest among MR patients with preserved LV systolic function (124 +/- 37 cm/s) than among those with LV systolic dysfunction (101 +/- 25 cm/s; p <0.05) and normal controls (74 +/- 18 cm/s; p <0.001). Concurrently, A-wave velocity was lowest in patients with systolic dysfunction and MR (47 +/- 23 cm/s; p <0.001) than in patients with normal systolic function and MR (79 +/- 33 cm/s) and normal controls (74 +/- 20 cm/s). Deceleration time of the E wave was longest among those with normal systolic function and MR (203 +/- 41 ms) than among those with systolic dysfunction and MR (152 +/- 35 ms; p <0.001) and normal controls (167 +/- 53 ms; p <0.05). Thus, systolic LV dysfunction in patients with severe MR, compared to patients with MR and normal LV systolic function, is associated with important changes in diastolic inflow velocities, including reduction of the maximal A-wave velocity to a greater extent than the E wave, resulting in an increased E/A ratio and shortening of deceleration time of the E wave.

Effects of protamine on myocyte contractile function and beta-adrenergic responsiveness

The use of protamine sulfate in patients has been associated with severe circulatory collapse and myocardial failure. However, the exact mechanisms responsible for these reactions to protamine remain unclear. Accordingly, we examined the effect of protamine on isolated myocyte contractile function. Indexes of isolated myocyte contractile function, percent shortening, and velocity of shortening were examined using videomicroscopy. Porcine cardiocytes (n = 75) were studied at baseline and in the presence of 80 micrograms/mL protamine. In addition, myocyte function was examined sequentially, first during treatment with 8 IU/mL heparin and then after the addition of a protamine dose sufficient to completely bind the heparin. The binding of heparin and protamine resulted in the formation of a heparin-protamine complex. The protamine concentration of 80 micrograms/mL is approximately equal to the serum concentration of protamine obtained in patients when administered in a dose of 5 mg/kg. In the presence of 80 micrograms/mL protamine, both percent shortening and velocity of shortening fell by more than 32% from baseline values (p < 0.05). The presence of either heparin alone or the heparin-protamine complex resulted in no change in baseline myocyte contractile measurements. Furthermore, to examine the effect of protamine on myocyte beta-adrenergic responsiveness a second series of experiments were performed. Myocyte contractile function was measured when 25 nmol/L isoproterenol was added to each of the protocols above. The presence of 80 micrograms/mL protamine resulted in a significant blunting of myocyte beta-adrenergic responsiveness. The presence of either heparin alone or the heparin-protamine complex resulted in no change in myocyte beta-adrenergic responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)