Mechano-electric feedback in one-dimensional model of myocardium

We utilized our earlier developed 1D mathematical model of the heart muscle strand to study contribution of the bilateral interactions between excitation and contraction on the cellular and tissue levels to the local and global myocardium function. Numerical experiments on the model showed that an initially uniform strand, formed on the inherently identical cells, became functionally heterogeneous due to the asynchronous excitation via the electrical wave spread. Mechanical interactions between the cells and the mechano-electric feedback beat-to-beat affect the functional characteristics of coupled cardiomyocytes further, adjusting their electrical and mechanical heterogeneity to the activation timing. Model simulations showed that functional heterogeneity increases with an enlarged spatial extension of the myocardial strand (in terms of the longer slack length not a higher stretch of the strand), demonstrating a special role of the heart size in its function. Model analysis suggests that cooperative mechanisms of myofilament calcium activation contribute essentially to the generation of cellular functional heterogeneity in contracting cardiac tissue.

Pomolic acid reduces contractility and modulates excitation-contraction coupling in rat cardiomyocytes

Pomolic acid (PA) isolated from Licania pittieri has hypotensive effects in rats, inhibits human platelet aggregation and elicits endothelium-dependent relaxation in rat aortic rings. The present study was designed to investigate the effects of PA on cardiomyocytes. Trabeculae and enzymatically isolated cardiomyocytes from rats were used to evaluate the concentration-dependent effects of PA on cardiac muscle tension and excitation-contraction coupling (ECC) by recording Ca²⁺ transients reported with Fluo-3 and Fura-2, as well as L-type Ca²⁺ currents (LTCC). PA reduced the contractile force in rat cardiac trabeculae with an EC₅₀ = 14.3 ± 2.4 μM. PA also reduced the amplitude of Ca²⁺ transients in a concentration-dependent manner, with an EC₅₀ = 10.5 ± 1.3 μM, without reducing sarcoplasmic reticulum (SR) Ca²⁺ loading. PA decreased the half width of the Ca²⁺ transient by 31.7 ± 3.3% and increased the decay time and decay time constant (τ) by 7.6 ± 2.7% and 75.6 ± 3.7%, respectively, which was associated with increased phospholamban (PLN) phosphorylation. PA also reversibly reduced the macroscopic LTCC in the cardiomyocyte membrane, but did not demonstrate any effects on skeletal muscle ECC. In conclusion, PA reduces LTCC, Ca²⁺ transients and cardiomyocyte force, which along with its vasorelaxant effects explain its hypotensive properties. Increased PLN phosphorylation protected the SR from Ca²⁺ depletion. Considering the effects of PA on platelet aggregation and the cardiovascular system, we propose it as a new potential, multitarget cardiovascular agent with a demonstrated safety profile.

Dual loss of regulator of G protein signaling 2 and 5 exacerbates ventricular myocyte arrhythmias and disrupts the fine-tuning of Gi/o signaling

AIMS

Cardiac contractility, essential to maintaining proper cardiac output and circulation, is regulated by G protein-coupled receptor (GPCR) signaling. Previously, the absence of regulator of G protein signaling (RGS) 2 and 5, separately, was shown to cause G protein dysregulation, contributing to modest blood pressure elevation and exaggerated cardiac hypertrophic response to pressure-overload. Whether RGS2 and 5 redundantly control G protein signaling to maintain cardiovascular homeostasis is unknown. Here we examined how the dual absence of RGS2 and 5 (Rgs2/5 dbKO) affects blood pressure and cardiac structure and function.

METHODS AND RESULTS

We found that Rgs2/5 dbKO mice showed left ventricular dilatation at baseline by echocardiography. Cardiac contractile response to dobutamine stress test was sex-dependently reduced in male Rgs2/5 dbKO relative to WT mice. When subjected to surgery-induced stress, male Rgs2/5 dbKO mice had 75% mortality within 72-96 h after surgery, accompanied by elevated baseline blood pressure and decreased cardiac contractile function. At the cellular level, cardiomyocytes (CM) from Rgs2/5 dbKO mice showed augmented Ca²⁺ transients and increased incidence of arrhythmia without augmented contractile response to electrical field stimulation (EFS) and activation of β-adrenergic receptors (βAR) with isoproterenol. Dual loss of Rgs2 and 5 suppressed forskolin-induced cAMP production, which was restored by G_i/o inactivation with pertussis toxin that also reduced arrhythmogenesis during EFS or βAR stimulation. Cardiomyocyte NCX and PMCA mRNA expression was unaffected in Rgs2/5 dbKO male mice. However, there was an exaggerated elevation of EFS-induced cytoplasmic Ca²⁺ in the presence of SERCA blockade with thapsigargin.

CONCLUSIONS

We conclude that RGS2 and 5 promote normal ventricular rhythm by coordinating their regulatory activity towards G_i/o signaling and facilitating cardiomyocyte calcium handling.