Insights into length-dependent regulation of cardiac cross-bridge cycling kinetics in human myocardium

Maternal Body Mass Index, Myometrium Contractility and Uterotonic Receptor Expression in Pregnancy

Pregnant individuals with obesity (body mass index, BMI ≥ 30 kg/m2) are more likely to experience prolonged labor and have double the risk of cesarean compared with individuals with normal weight (BMI < 25 kg/m2). The aim of this study was to evaluate whether obesity in pregnancy is associated with reduced spontaneous and oxytocin-stimulated myometrial contractile activity using ex vivo preparations. We also assessed the relationship between maternal BMI and the expression of oxytocin (OXTR) and prostaglandin (FP) receptors in the myometrial tissue. We enrolled 73 individuals with a singleton gestation undergoing scheduled cesarean delivery at term in a prospective cohort study. This included 49 individuals with a pre-pregnancy BMI ≥ 30 kg/m2 and 24 with BMI < 25.0 kg/m2. After delivery, a small strip of myometrium was excised from the upper edge of the hysterotomy. Baseline spontaneous and oxytocin stimulated myometrial contractile activity was measured using ex vivo preparations. Additionally, expression of oxytocin and prostaglandin receptors from myometrial samples were compared using qRT-PCR and western blot techniques. Spontaneous and oxytocin-stimulated contraction frequency, duration, and force were not significantly different in myometrial samples from the obese and normal-weight individuals. Myometrial OXTR gene and protein expression was also similar in the two groups. While FP gene expression was lower in the myometrial samples from the obese group, protein expression did not differ. These data help to address an important knowledge gap related to the biological mechanisms underlying the association between maternal obesity and dysfunctional labor.

Effect of hypothyroidism on contractile performance of isolated end-stage failing human myocardium

The relationship between hypothyroidism and the occurrence and progression of heart failure (HF) has had increased interest over the past years. The low T3 syndrome, a reduced T3 in the presence of normal thyroid stimulating hormone (TSH), and free T4 concentration, is a strong predictor of all-cause mortality in HF patients. Still, the impact of hypothyroidism on the contractile properties of failing human myocardium is unknown. Our study aimed to investigate that impact using ex-vivo assessment of force and kinetics of contraction/relaxation in left ventricular intact human myocardial muscle preparations. Trabeculae were dissected from non-failing (NF; n = 9), failing with no hypothyroidism (FNH; n = 9), and failing with hypothyroidism (FH; n = 9) hearts. Isolated muscle preparations were transferred into a custom-made setup where baseline conditions as well as the three main physiological modulators that regulate the contractile strength, length-dependent and frequency-dependent activation, as well as β-adrenergic stimulation, were assessed under near-physiological conditions. Hypothyroidism did not show any additional significant impact on the contractile properties different from the recognized alterations usually detected in such parameters in any end-stage failing heart without thyroid dysfunction. Clinical information for FH patients in our study revealed they were all receiving levothyroxine. Absence of any difference between failing hearts with or without hypothyroidism, may possibly be due to the profound effects of the advanced stage of heart failure that concealed any changes between the groups. Still, we cannot exclude the possibility of differences that may have been present at earlier stages. The effects of THs supplementation such as levothyroxine on contractile force and kinetic parameters of failing human myocardium require further investigation to explore its full potential in improving cardiovascular performance and cardiovascular outcomes of HF associated with hypothyroidism.

The Case for, and Challenges of, Human Cardiac Tissue in Advancing Phosphoprotein Research

Cardiovascular disease (CVD) and stroke affect over 92 million Americans and account for nearly 1 out of 3 deaths in the US. The use of animal models in cardiovascular research has led to considerable advances in treatment and in our understanding of the pathophysiology of many CVDs. Still, animals may not fully recapitulate human disease states; species differences have long been postulated to be one of the main reasons for a failure of translation between animals and humans in drug discovery and development. Indeed, it has become increasingly clear over the past few decades that to answer certain biomedical questions, like the physiological mechanisms that go awry in many human CVDs, animal tissues may not always be the best option to use. While human cardiac tissue has long been used for laboratory research, published findings often contradict each other, leading to difficulties in interpretation. Current difficulties in utilizing human cardiac tissue include differences in acquisition time, varying tissue procurement protocols, and the struggle to define a human “control” sample. With the tremendous emphasis on translational research that continues to grow, research studies using human tissues are becoming more common. This mini review will discuss advantages, disadvantages, and considerations of using human cardiac tissue in the study of CVDs, paying specific attention to the study of phosphoproteins.

Contraction-relaxation coupling is unaltered by exercise training and infarction in isolated canine myocardium

The two main phases of the mammalian cardiac cycle are contraction and relaxation; however, whether there is a connection between them in humans is not well understood. Routine exercise has been shown to improve cardiac function, morphology, and molecular signatures. Likewise, the acute and chronic changes that occur in the heart in response to injury, disease, and stress are well characterized, albeit not fully understood. In this study, we investigated how exercise and myocardial injury affect contraction–relaxation coupling. We retrospectively analyzed the correlation between the maximal speed of contraction and the maximal speed of relaxation of canine myocardium after receiving surgically induced myocardial infarction, followed by either sedentary recovery or exercise training for 10–12 wk. We used isolated right ventricular trabeculae, which were electrically paced at different lengths, frequencies, and with increasing β-adrenoceptor stimulation. In all conditions, contraction and relaxation were linearly correlated, irrespective of injury or training history. Based on these results and the available literature, we posit that contraction–relaxation coupling is a fundamental myocardial property that resides in the structural arrangement of proteins at the level of the sarcomere and that this may be regulated by the actions of cardiac myosin binding protein C (cMyBP-C) on actin and myosin.

The role of pacing rate in the modulation of mechano-induced immediate and delayed changes in the force and Ca-transient of cardiac muscle

Myocardial function is tuned by dynamic changes in length and load via mechano-calcium feedback. This regulation may be significantly affected by heart rhythm. We evaluated the mechano-induced modulation of contractility and Ca-transient (CaT) in the rat myocardium subjected to twitch-by-twitch shortening-re-lengthening (↓-↑) trains of different lengths (N = 1 … 720 cycles) at low (1 Hz) and near-physiological (3.5 Hz) pacing rates. Force/CaT characteristics were evaluated in the first post-train isometric twitch (immediate effect) and during slow changes (delayed maximal elevation/decrease) and compared with those of the pre-train twitch. The immediate inotropic effect was positive for N = 30 … 720 and negative for N = 1 … 20, while the delayed effect was always positive. The immediate and delayed inotropic effects were significantly higher at 3.5-Hz vs 1-Hz (P < 0.05). The prominent inotropism was accompanied by much smaller changes in the CaT diastolic level/amplitude. The shortening-re-lengthening train induced oscillations of the slow change in force at 3.5-Hz (always) and at 1-Hz (∼50% of muscles), which were dependent of the train length and independent of the pacing rate. We suggest that twitch-by-twitch shortening-re-lengthening of cardiac muscle decreases Ca²⁺ buffering by troponin C and elevates Ca²⁺ loading of the sarcoplasmic reticulum (SR); the latter cumulatively depends on the train length. A high pacing rate intensifies the cumulative transient shift in the SR Ca²⁺ loading, augmenting the post-train inotropic response and prolonging its recovery to the pre-train level. The pacing-dependent mechano-induced inotropic effects remain to be elucidated in the myocardium with impaired Ca handling.

Assessment of PKA and PKC inhibitors on force and kinetics of non-failing and failing human myocardium

AIMS

Heart failure (HF) is a prevalent disease that is considered the foremost reason for hospitalization in the United States. Most protein kinases (PK) are activated in heart disease and their inhibition has been shown to improve cardiac function in both animal and human studies. However, little is known about the direct impact of PKA and PKC inhibitors on human cardiac contractile function.

MATERIAL AND METHODS

We investigated the ex vivo effect of such inhibitors on force as well as on kinetics of left ventricular (LV) trabeculae dissected from non-failing and failing human hearts. In these experiments, we applied 0.5 μM of H-89 and GF109203X, which are PKA and PKC inhibitors, respectively, in comparison to their vehicle DMSO (0.05%).

KEY FINDINGS AND CONCLUSION

Statistical analyses revealed no significant effect for H-89 and GF109203X on either contractile force or kinetics parameters of both non-failing and failing muscles even though they were used at a concentration higher than the reported IC₅₀s and K_is. Therefore, several factors such as selectivity, concentration, and treatment time, which are related to these PK inhibitors according to previous studies require further exploration.

Protein Kinase A as a Promising Target for Heart Failure Drug Development

Heart failure (HF) is a clinical syndrome characterized by impaired ability of the heart to fill or eject blood. HF is rather prevalent and it represents the foremost reason of hospitalization in the United States. The costs linked to HF overrun those of all other causes of disabilities, and death in the United States and all over the developed as well as the developing countries which amplify the supreme significance of its prevention. Protein kinase (PK) A plays multiple roles in heart functions including, contraction, metabolism, ion fluxes, and gene transcription. Altered PKA activity is likely to cause the progression to cardiomyopathy and HF. Thus, this review is intended to focus on the roles of PKA and PKA-mediated signal transduction in the healthy heart as well as during the development of HF. Furthermore, the impact of cardiac PKA inhibition/activation will be highlighted to identify PKA as a potential target for the HF drug development.

Human Myocardium Has a Robust α1A-Subtype Adrenergic Receptor Inotropic Response

Recent studies report that a single subtype of α1-adrenergic receptor (α1-AR), the α1A-subtype, mediates robust cardioprotective effects in multiple experimental models of heart failure, suggesting that the α1A-subtype is a potential therapeutic target for an agonist to treat heart failure. Moreover, we recently found that the α1A-subtype is present in human heart. The goal of this study was to assess the inotropic response mediated by the α1A-subtype in human myocardium, and to determine whether the response is downregulated in myocardium from failing human heart. We measured in vitro contractile responses of cardiac muscle preparations (trabeculae) isolated from the right ventricle from nonfailing and failing human hearts. Addition of the α1A-subtype agonist A61603 (100 nM) resulted in a large positive inotropic response (force increased ≈ 2-fold). This response represented ≈70% of the response mediated by the β-adrenergic receptor agonist isoproterenol (1 μM). Moreover, in myocardium from failing hearts, α1A-subtype responses remained robust, and only slightly reduced relative to nonfailing hearts. We conclude that α1A-subtype-mediated inotropy could represent a significant source of inotropic support in the human heart. Furthermore, the α1A-subtype remains functional in myocardium from failing human hearts and thus, might be a therapeutic target to support cardioprotective effects in patients with heart failure.

Etiology-dependent impairment of relaxation kinetics in right ventricular end-stage failing human myocardium

BACKGROUND

In patients with end-stage heart failure, the primary etiology often originates in the left ventricle, and eventually the contractile function of the right ventricle (RV) also becomes compromised. RV tissue-level deficits in contractile force and/or kinetics need quantification to understand involvement in ischemic and non-ischemic failing human myocardium.

METHODS AND RESULTS

The human population suffering from heart failure is diverse, requiring many subjects to be studied in order to perform an adequately powered statistical analysis. From 2009-present we assessed live tissue-level contractile force and kinetics in isolated myocardial RV trabeculae from 44 non-failing and 41 failing human hearts. At 1 Hz stimulation rate (in vivo resting state) the developed active force was not different in non-failing compared to failing ischemic nor non-ischemic failing trabeculae. In sharp contrast, the kinetics of relaxation were significantly impacted by disease, with 50% relaxation time being significantly shorter in non-failing vs. non-ischemic failing, while the latter was still significantly shorter than ischemic failing. Gender did not significantly impact kinetics. Length-dependent activation was not impacted. Although baseline force was not impacted, contractile reserve was critically blunted. The force-frequency relation was positive in non-failing myocardium, but negative in both ischemic and non-ischemic myocardium, while the β-adrenergic response to isoproterenol was depressed in both pathologies.

CONCLUSIONS

Force development at resting heart rate is not impacted by cardiac pathology, but kinetics are impaired and the magnitude of the impairment depends on the underlying etiology. Focusing on restoration of myocardial kinetics will likely have greater therapeutic potential than targeting force of contraction.

Increased cross-bridge recruitment contributes to transient increase in force generation beyond maximal capacity in human myocardium

Cross-bridge attachment allows force generation to occur, and rate of tension redevelopment (k_tr) is a commonly used index of cross-bridge cycling rate. Tension overshoots have been observed briefly after a slack-restretch k_tr maneuver in various species of animal models and humans. In this study, we set out to determine the properties of these overshoots and their possible underlying mechanism. Utilizing human cardiac trabeculae, we have found that tension overshoots are temperature-dependent and that they do not occur at resting states. In addition, we have found that myosin cross-bridge cycle is vital to these overshoots as inhibition of the cycle results in the blunting of the overshoots and the magnitude of the overshoots are dependent on the level of myofilament activation. Lastly, we show that the number of cross-bridges transiently increase during tension overshoots. These findings lead us to conclude that tension overshoots are likely due to a transient enhancement of the recruitment of myosin heads into the cross-bridge cycling, regulated by the myocardium, and with potential physiological significance in determining cardiac output.

NEWS AND NOTEWORTHY

We show that isolated human myocardium is capable of transiently increasing its maximal force generation capability by increasing cross-bridge recruitment following slack-restretch maneuver. This process can potentially have important implications and significance in cardiac contraction in vivo.