Transient activation of PKC results in long-lasting detrimental effects on systolic [Ca2+]i in cardiomyocytes by altering actin cytoskeletal dynamics and T-tubule integrity

Analysis of heroin effects on calcium channels in rat cardiomyocytes based on transcriptomics and metabolomics

Heroin can cause damage to many human organs, possibly leading to different types of arrhythmias and abnormal electrophysiological function of the heart muscle and the steady state of calcium-ion channels. We explored cardiomyocytes treated with heroin and the effect on calcium-ion channels. Transcriptomics and metabolomics were used to screen for differential genes and metabolite alterations after heroin administration to jointly analyze the effect of heroin on calcium channels in cardiomyocytes. Cardiomyocytes from primary neonatal rats were cultured in vitro and were treated with different concentrations of heroin to observe the changes in morphology and spontaneous beat frequency and rhythm by a patch clamp technique. Transcriptomic studies selected a total of 1,432 differentially expressed genes, 941 upregulated and 491 downregulated genes in rat cardiomyocytes from the control and drug intervention groups. Gene Ontology functional enrichment showed that 1,432 differential genes selected by the two groups were mainly involved in the regulation of the multicellular organismal process, response to external stimulus, myofibril, inflammatory response, muscle system process, cardiac muscle contraction, etc. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis indicated that these genes were mainly concentrated in cardiac muscle contraction, osteoclast differentiation, adrenergic signaling in cardiomyocytes, dilated cardiomyopathy, hypertrophic cardiomyopathy, and other important pathways. Metabolomic testing further suggested that cardiomyocyte metabolism was severely affected after heroin intervention. After the treatment with heroin, the L-type calcium channel current I-V curve was up-shifted, the peak value was significantly lower than that of the control group, action potential duration 90 was significantly increased in the action potential, resting potential negative value was lowered, and action potential amplitude was significantly decreased in cardiomyocytes. In this study, heroin could cause morphological changes in primary cardiomyocytes of neonatal rats and electrophysiological function. Heroin can cause myocardial contraction and calcium channel abnormalities, damage the myocardium, and change the action potential and L-type calcium channel.

Influence of verapamil on pressure overload-induced ventricular arrhythmias by regulating gene-expression profiles

BACKGROUND

Life-threatening ventricular arrhythmias can lead to sudden cardiac death in patients. This study aimed to investigate the changes in gene profiles involved when verapamil (VRP) affects increased wall stress (pressure overload)-induced ventricular arrhythmias, thus revealing the potential causative molecular mechanisms and therapeutic targets through gene-expression identification and functional analysis.

METHODS

Animal models with wall stress-induced ventricular arrhythmias were established. Low (0.5 mg/kg) and high (1 mg/kg) doses of VRP were administered intravenously 10 minutes before transverse aortic constriction, and average ventricular arrhythmia scores were calculated. Next, we evaluated the molecular role of VRP by characterising differential gene-expression profiles between VRP-pretreated (1 mg/kg) and control groups using RNA-sequencing technology. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were used to reveal molecular function. A protein-protein interaction (PPI) network was then developed.

RESULTS

VRP exerted its anti-arrhythmic effects in response to increases in left ventricular (LV) afterload. We detected differentially expressed genes (DEGs), of which 36 were upregulated and 1 397 downregulated, between the VRP-pretreated and model groups during acute increases in LV wall stress. GO analysis demonstrated that the DEGs were associated with cytoskeletal protein binding. KEGG analysis showed that enriched pathways were mainly distributed in adherens junctions, actin cytoskeleton regulation and the MAPK signalling pathway. Centralities analysis of the PPI identified Rac1, Grb2, Rbm8a and Mapk1 as hub genes.

CONCLUSIONS

VRP prevented acute pressure overload-induced ventricular arrhythmias, possibly through the hub genes Rac1, Grb2, Rbm8a and Mapk1 as potential targets of VRP.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Dyadic Plasticity in Cardiomyocytes

Contraction of cardiomyocytes is dependent on sub-cellular structures called dyads, where invaginations of the surface membrane (t-tubules) form functional junctions with the sarcoplasmic reticulum (SR). Within each dyad, Ca2+ entry through t-tubular L-type Ca2+ channels (LTCCs) elicits Ca2+ release from closely apposed Ryanodine Receptors (RyRs) in the SR membrane. The efficiency of this process is dependent on the density and macroscale arrangement of dyads, but also on the nanoscale organization of LTCCs and RyRs within them. We presently review accumulating data demonstrating the remarkable plasticity of these structures. Dyads are known to form gradually during development, with progressive assembly of both t-tubules and junctional SR terminals, and precise trafficking of LTCCs and RyRs. While dyads can exhibit compensatory remodeling when required, dyadic degradation is believed to promote impaired contractility and arrythmogenesis in cardiac disease. Recent data indicate that this plasticity of dyadic structure/function is dependent on the regulatory proteins junctophilin-2, amphiphysin-2 (BIN1), and caveolin-3, which critically arrange dyadic membranes while stabilizing the position and activity of LTCCs and RyRs. Indeed, emerging evidence indicates that clustering of both channels enables "coupled gating", implying that nanoscale localization and function are intimately linked, and may allow fine-tuning of LTCC-RyR crosstalk. We anticipate that improved understanding of dyadic plasticity will provide greater insight into the processes of cardiac compensation and decompensation, and new opportunities to target the basic mechanisms underlying heart disease.

Cholesterol Protects Against Acute Stress-Induced T-Tubule Remodeling in Mouse Ventricular Myocytes

Efficient excitation-contraction coupling in ventricular myocytes depends critically on the presence of the t-tubular network. It has been recently demonstrated that cholesterol, a major component of the lipid bilayer, plays an important role in long-term maintenance of the integrity of t-tubular system although mechanistic understanding of underlying processes is essentially lacking. Accordingly, in this study we investigated the contribution of membrane cholesterol to t-tubule remodeling in response to acute hyposmotic stress. Experiments were performed using isolated left ventricular cardiomyocytes from adult mice. Depletion and restoration of membrane cholesterol was achieved by applying methyl-β-cyclodextrin (MβCD) and water soluble cholesterol (WSC), respectively, and t-tubule remodeling in response to acute hyposmotic stress was assessed using fluorescent dextran trapping assay and by measuring t-tubule dependent I_K1 tail current (I_K1,tail). The amount of dextran trapped in t-tubules sealed in response to stress was significantly increased when compared to control cells, and reintroduction of cholesterol to cells treated with MβCD restored the amount of trapped dextran to control values. Alternatively, application of WSC to normal cells significantly reduced the amount of trapped dextran further suggesting the protective effect of cholesterol. Importantly, modulation of membrane cholesterol (without osmotic stress) led to significant changes in various parameters of I_K1, _tail strongly suggesting significant but essentially hidden remodeling of t-tubules prior to osmotic stress. Results of this study demonstrate that modulation of the level of membrane cholesterol has significant effects on the susceptibility of cardiac t-tubules to acute hyposmotic stress.

Excitation-Contraction Coupling Time is More Sensitive in Evaluating Cardiac Systolic Function

Background:

Pressure overload-induced myocardial hypertrophy is a key step leading to heart failure. Previous cellular and animal studies demonstrated that deteriorated excitation–contraction coupling occurs as early as the compensated stage of hypertrophy before the global decrease in left ventricular ejection fraction (LVEF). This study was to evaluate the cardiac electromechanical coupling time in evaluating cardiac systolic function in the early stage of heart failure.

Methods:

Twenty-six patients with Stage B heart failure (SBHF) and 31 healthy controls (CONs) were enrolled in this study. M-mode echocardiography was performed to measure LVEF. Tissue Doppler imaging (TDI) combined with electrocardiography (ECG) was used to measure cardiac electromechanical coupling time.

Results:

There was no significant difference in LVEF between SBHF patients and CONs (64.23 ± 8.91% vs. 64.52 ± 5.90%; P = 0.886). However, all four electromechanical coupling time courses (Qsb: onset of Q wave on ECG to beginning of S wave on TDI, Qst: onset of Q wave on ECG to top of S wave on TDI, Rsb: top of R wave on ECG to beginning of S wave on TDI, and Rst: top of R wave on ECG to top of S wave on TDI) of SBHF patients were significantly longer than those of CONs (Qsb: 119.19 ± 35.68 ms vs. 80.30 ± 14.81 ms, P < 0.001; Qst: 165.42 ± 60.93 ms vs. 129.04 ± 16.97 ms, P = 0.006; Rsb: 82.43 ± 33.66 ms vs. 48.30 ± 15.18 ms, P < 0.001; and Rst: 122.37 ± 36.66 ms vs. 93.25 ± 16.72 ms, P = 0.001), and the Qsb, Rsb, and Rst time showed a significantly higher sensitivity than LVEF (Rst: P =0.032; Rsb: P = 0.003; and Qsb: P = 0.004).

Conclusions:

The cardiac electromechanical coupling time is more sensitive than LVEF in evaluating cardiac systolic function.