Myocardial phenotype changes in heart failure: cellular and subcellular adaptations and their functional significance

The Loss of GSTM1 Associates with Kidney Failure and Heart Failure

Glutathione S-transferase mu 1 (GSTM1) encodes an enzyme that catalyzes the conjugation of electrophilic compounds with glutathione to facilitate their degradation or excretion. The loss of one or both copies of GSTM1 is common in many populations and has been associated with CKD progression. With the hypothesis that the loss of GSTM1 is also associated with incident kidney failure and heart failure, we estimated GSTM1 copy number using exome sequencing reads in the Atherosclerosis Risk in Communities (ARIC) Study, a community-based prospective cohort of white and black participants. Overall, 51.2% and 39.8% of white participants and 25.6% and 48.5% of black participants had zero or one copy of GSTM1, respectively. Over a median follow-up of 24.6 years, 256 kidney failure events occurred in 5715 participants without prevalent kidney failure, and 1028 heart failure events occurred in 5368 participants without prevalent heart failure. In analysis adjusted for demographics, diabetes, and hypertension, having zero or one copy of GSTM1 associated with higher risk of kidney failure and heart failure (adjusted hazard ratio [95% confidence interval] for zero or one versus two copies of GSTM1: kidney failure, 1.66 [1.27 to 2.17]; heart failure, 1.16 [1.04 to 1.29]). Risk did not differ significantly between participants with zero and one copy of GSTM1 (P>0.10). In summary, the loss of GSTM1 was significantly associated with incident kidney and heart failure, independent of traditional risk factors. These results suggest GSTM1 function is a potential treatment target for the prevention of kidney and heart failure.

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.

Cardiac hypertrophy, substrate utilization and metabolic remodelling: cause or effect?

1. Metabolic remodelling in the heart occurs in response to chronically altered workload and substrate availability. Recently, the importance of the metabolic remodelling processes inherent in the hypertrophic growth response (whether primary or secondary) has been recognized. 2. Altered energy demand, shifts in substrate utilization and increased oxidative stress are observed in the hypertrophic heart. Both a shift away from carbohydrate usage (i.e. insulin resistance) and a shift to carbohydrate usage (i.e. pressure loading) are associated with disturbed cardiomyocyte Ca(2+) homeostasis and the development of cardiac hypertrophy. 3. A change in the balance of myocardial usage of fatty acid and glucose substrates must entail a degree of cellular oxidative stress. Increased throughput of any substrate will necessarily involve a regional imbalance between reactive oxygen species (ROS) production and breakdown. 4. In addition to a number of enzyme generators of ROS at various intracellular locations, the heart also contains a number of endogenous anti-oxidants, to restrict steady state ROS levels. The balance between ROS generation and their elimination by endogenous anti-oxidant mechanisms plays a critical role in preserving cardiac function; inappropriate levels of myocardial ROS likely precipitate impairment of myocardial function and abnormalities in cardiac structure. 5. Although different metabolic adaptations are associated with hypertrophic responses of contrasting aetiology, there is accumulating evidence that the joint insults of increased production of ROS and disturbed Ca(2+) handling in the cardiomyocyte comprise the primary lesion. These molecular signals operate together in a feed-forward mode and have the capacity to inflict substantial functional and structural damage on the hypertrophic myocardium.

Differential regulation of Ca2+-dependent ATPase-activity in left ventricular myocardium during mechanical circulatory support

BACKGROUND

Myocardial recovery is observed in some end-stage heart failure patients after mechanical circulatory support. The sarcoplasmic reticulum Ca(2+)-adenosine triphosphatase (Ca2+-ATPase) activity is down-regulated in failing myocardium and contributes to heart failure-associated contraction/relaxation abnormalities. Regulation of Ca(2+)-ATPase after mechanical support was shown to be heterogeneous. Thus, we analyzed Ca(2+)-ATPase activity and protein expression in the paired myocardial samples of 21 patients supported by ventricular assist devices to identify factors that influence restoration of the Ca(2+)-transient after ventricular assist device support.

METHODS

We measured Ca(2+)-ATPase activity using a reduced nicotinamide-adenine dinucleotide-coupled reaction, determined sarcoplasmic reticulum Ca(2+)-dependent ATPase protein using Western blotting, and determined 4-hydroxyproline using amino-acid analysis.

RESULTS

The mean Ca(2+)-ATPase activity decreased at assist-device implantation and slightly increased at transplantation, but remained significantly lower than in non-failing donor hearts. However, individual responses were heterogeneous. Patients with older age, increased left ventricular diameter, and increased 4-hydroxyproline content showed down-regulation of Ca(2+)-ATPase activity, whereas we found up-regulation in patients with low values for these parameters after assist-device support.

CONCLUSIONS

Sarcoplasmic reticulum Ca(2+)-ATPase activity, which influences the myocardial Ca(2+)-transient, generally is not restored to normal values in assist-device-supported hearts, but depends on a combined score of the left ventricular end-diastolic diameter, degree of ventricular fibrosis, and age of the patient at the time of assist-device implantation.

Quantitation and distribution of beta-tubulin in human cardiac myocytes

Increasing evidence suggests that derangements of cytoskeletal proteins contribute to alterations in intracellular signaling, myocyte function, and the coupling of myocytes to the extracellular matrix during cardiac hypertrophy and failure. Data from animal studies have shown an increased density of beta-tubulin protein in the right or left ventricle subjected to pressure overload, and have demonstrated that interfering with excess polymerization of beta-tubulin improves contractility. We tested the hypothesis that beta-tubulin is increased in human left ventricular hypertrophy and end-stage heart failure. Confocal microscopy of fluorescently labeled beta-tubulin protein revealed an increased density of the beta-tubulin network in cardiomyocytes from both hypertrophied and failing human hearts as compared to cells from nonfailing hearts. Western blot analysis on total heart homogenate showed no change in beta-tubulin when data were normalized to either actin or calsequestrin, although there was a significant increase in failing human hearts when data were normalized only for a constant amount of protein per heart. The mRNA for beta-tubulin was not changed in hypertrophied hearts, but was significantly decreased in failing human hearts. Thus, similar to animal models, we have shown that the density of the microtubular network within the cardiomyocyte is increased in end-stage failing human hearts. We have also shown for the first time that beta-tubulin density is increased in cells from hypertrophied human hearts. Although the functional implications of this finding in the human heart remain to be explored, data from animal studies suggest that increased beta-tubulin protein contributes to cardiac dysfunction.

Chronic stretch of engineered heart tissue induces hypertrophy and functional improvement

To examine the influence of chronic mechanical stretch on functional behavior of cardiac myocytes, we reconstituted embryonic chick or neonatal rat cardiac myocytes to a 3-dimensional engineered heart tissue (EHT) by mixing freshly isolated cells with neutralized collagen I and culturing them between two Velcro-coated silicone tubes, held at a fixed distance with a metal spacer. After 4 days, EHTs were subjected to a phasic unidirectional stretch for 6 days in serum-containing medium. Compared to unstretched controls, RNA/DNA and protein/cell ratios increased by 100% and 50%, respectively. ANF mRNA and alpha-sarcomeric actin increased by 98% and 40%, respectively. Morphologically, stretched EHTs exhibited improved organization of cardiac myocytes into parallel arrays of rod-shaped cells, increased cell length and width, longer myofilaments, and increased mitochondrial density. Thus, stretch induced phenotypic changes, generally referred to as hypertrophy. Concomitantly, force of contraction was two- to fourfold higher both under basal conditions and after stimulation with calcium or the beta-adrenergic agonist isoprenaline. Contraction kinetics were accelerated with a 14-44% decrease in twitch duration under all those conditions. In summary, we have developed a new in vitro model that allows morphological, molecular, and functional consequences of stretch to be studied under defined conditions. The main finding was that stretch of EHTs induced cardiac myocyte hypertrophy, which was accompanied by marked improvement of contractile function.

Myocyte recovery after mechanical circulatory support in humans with end-stage heart failure

BACKGROUND

The failing myocardium is characterized by decreased force production, slowed relaxation, and depressed responses to beta-adrenergic stimulation. In some heart failure patients, heart function is so poor that a left ventricular assist device (LVAD) is inserted as a bridge to transplantation. In the present research, we investigated whether circulatory support with an LVAD influenced the functional properties of myocytes from the failing heart.

METHODS AND RESULTS

Myocytes were isolated from human explanted failing hearts (HF-myocytes) and failing hearts with antecedent LVAD support (HF-LVAD-myocytes). Studies of myocyte function indicated that the magnitude of contraction was greater (9.6+/-0.7% versus 6.9+/-0.5% shortening), the time to peak contraction was significantly abbreviated (0.37+/-0.01 versus 0.75+/-0.04 seconds), and the time to 50% relaxation was reduced (0.55+/-0.02 versus 1.45+/-0.11 seconds) in the HF-LVAD-myocytes compared with the HF-myocytes (P<0.05). The HF-LVAD-myocytes had larger contractions than the HF-myocytes at all frequencies of stimulation tested. The negative force-frequency relationship of the HF-myocytes was improved in HF-LVAD-myocytes but was not reversed. Responses to beta-adrenergic stimulation (by isoproterenol) were greater in HF-LVAD-myocytes versus HF-myocytes.

CONCLUSIONS

The results of the study strongly support the idea that circulatory support with an LVAD improves myocyte contractile properties and increases beta-adrenergic responsiveness.

Differential effects of tert-butyl-benzohydroquinone, a putative SR Ca2+ pump inhibitor, on isometric relaxation during the staircase in the rabbit and rat ventricle

1. The effects of 2,5 di-(tert-butyl)-1,4-benzohydroquinone (TBQ), a putative inhibitor of the sarcoplasmic reticulum Ca2+ pump, on mechanical relaxation and contraction-relaxation coupling have been studied at different frequencies (0.5-3 Hz) in isometrically contracting isolated right ventricular preparations of rabbit and rat at 37 degrees C. Two types of mechanical responses have been studied: the twitch tension and the force transient (rewarming spike, RSp) following a rapid cooling contracture (RCC, an index of sarcoplasmic reticulum Ca2+ content) on return to 37 degrees C. 2. The coupling between contraction and relaxation was assessed by two methods: (a) by evaluation of the variation of the slope relating the maximal rate of tension fall to twitch peak tension; (b) by modelling the twitch according to the following equation: TwT (t) = C x (t/A)B x exp(1-(t/AB) where TwT(t) is the time course of isometric tension, t is time, C and A are an inotropic and a chronotropic index respectively and B, a contraction-relaxation coupling index (Nwasokwa, 1993). 3. In the rabbit ventricle, 30 microM TBQ did not prevent the frequency-induced shortening of the twitch time to half-relaxation (t1/2) and of the time constant (tau) describing the final part of the RSp relaxation (tau decreased from 140 ms (0.5 Hz) to 133 ms (3 Hz) in control and from 253 ms (0.5 Hz) to 197 ms (3 Hz) after exposure to TBQ). By contrast, at a given frequency, the prolongation of relaxation induced by TBQ was proportional to its inotropic effect (unchanged slopes and B values) but TBQ did not prevent the acceleration of relaxation observed at high frequencies: B increased from 2.02 (0.5 Hz) to a peak value of 2.18 (1 Hz) in control and from 1.88 (0.5 Hz) to a maximum of 2.48 (2 Hz) after TBQ exposure. TBQ significantly attenuated the decay of RCCs elicited after increasingly longer periods of muscle quiescence as normally observed in control conditions. 4. In the rat ventricle, TBQ depressed relaxation more than expected on the basis of its negative inotropic effects (B decreased from 2.16 to 1.84 at 0.5 Hz and from 2.15 to 1.66 at 3 Hz). TBQ also slowed the rate of RSp relaxation (tau increased from 95 ms to 168 ms at 0.5 Hz, and from 109 ms to 149 ms at 3 Hz) and increased twitch t1/2. By contrast with the results obtained in the rabbit ventricle, B, tau and t1/2 were frequency-insensitive whether or not TBQ was present. 5. TBQ exerts negative inotropic effects consistent with inhibition of the SR Ca2+ pump. In the rabbit ventricle, the TBQ-induced potentiation of relaxation acceleration at high pacing frequencies suggests the involvement of counteracting Ca(2+)-mediated mechanisms probably via Ca(2+)-calmodulin-activated kinases. In the rat ventricle, TBQ did not have any differential effect on relaxation depending on the frequency, probably because the extent of the negative staircase was small in the present experimental conditions.