Properties Of Cardiac Myocytes Research Articles

Mechanism of IL-6-related spontaneous atrial fibrillation after coronary artery grafting surgery: IL-6 knockout mouse study and human observation

We established a mouse model of cardiac surgery with nonpaced sPOAF. IL-6 knockout mice were protected from sPOAF compared with wild-type mice. At 48 hours after surgery, the heart was separated into 6 regions and cultured. IL-6 was expressed in all regions, with highest abundance in the left atrium (LA). In PHASE III, we demonstrated that IL-6 in the LA elicited early profibrotic properties in atria via the pSTAT3/STAT3 signaling pathway and contributed to sPOAF. In a translational prospective clinical study, we demonstrated that humans with POAF had a higher IL-6 concentration in pericardial drainage (PD). This study provides preliminary evidence of a causal relationship between IL-6 and POAF in a novel nonpaced sPOAF mouse model. IL-6 is a crucial prerequisite for eliciting profibrotic properties in cardiac myocytes via the pSTAT3 pathway during the early postoperative period, leading to an increased susceptibility to POAF. Measuring IL-6 in PD could be a new noninvasive biomarker for the clinical prediction of POAF.

The Effects of Serum Potassium and Magnesium Levels in a Patient with Gitelman's Syndrome on the Timing of Ventricular Wall Motion and the Pattern of Ventricular Strain and Torsion

Gitelman's syndrome is a primary renal tubular hypokalemic metabolic alkalosis. Hypokalemia and hypomagnesemia can cause cardiac tissue excitability and conduction. Global ventricular mechanical function is directly related to the contractile properties of cardiac myocytes, which are largely dependent on the flow of ions such as potassium and magnesium. Here, we show that increased levels of potassium, in addition to magnesium, in a patient with Gitelman's syndrome significantly impacts the timing of ventricular wall motion and the pattern of ventricular strain and torsion. Two-dimensional speckle tracking echocardiography was used for evaluation of the hypokalemic-hypomagnesemic period (first day) and third day after potassium chloride and magnesium replacement therapy. The transthoracic echocardiography showed that the percent ejection fraction was similar in hypokalemic-hypomagnesemic (63%) and normokalemic-normomagnesemic (after potassium and magnesium therapy, 67%) hearts. However, decreased left ventricular apical 4-chamber peak systolic longitudinal strain, left ventricle global peak systolic strain, and global torsion values increased after potassium chloride and magnesium replacement therapy.

Biophysical properties of slow potassium channels in human embryonic stem cell derived cardiomyocytes implicate subunit stoichiometry

Human embryonic stem cells (hESCs) are an important cellular model for studying ion channel function in the context of a human cardiac cell and will provide a wealth of information about both heritable arrhythmias and acquired electrophysiological disorders. However, detailed electrophysiological characterization of the important cardiac ion channels has been so far overlooked. Because mutations in the gene for the I(Ks) α subunit, KCNQ1, constitute the majority of long QT syndrome (LQT-1) cases, we have carried out a detailed biophysical analysis of this channel expressed in hESCs to establish baseline I(Ks) channel biophysical properties in cardiac myocytes derived from hESCs (hESC-CMs). I(Ks) channels are heteromultimeric proteins consisting of four identical α-subunits (KCNQ1) assembled with auxiliary β-subunits (KCNE1). We found that the half-maximal I(Ks) activation voltage in hESC-CMs and in myocytes derived from human induced pluripotent stems cells (hiPSC-CMs) falls between that of KCNQ1 channels expressed alone and with full complement of KCNE1, the major KCNE subunit expressed in hESC-CMs as shown by qPCR analysis. Overexpression of KCNE1 by transfection of hESC-CMs markedly shifted and slowed native I(Ks) activation implying assembly of additional KCNE1 subunits with endogenous channels. Our results in hESC-CMs, which indicate an I(Ks) subunit stoichiometry that can be altered by variable KCNE1 expression, suggest the possibility for variable I(Ks) function in the developing heart, in different tissues in the heart, and in disease. This establishes a new baseline for I(Ks) channel properties in myocytes derived from pluripotent stem cells and will guide future studies in patient-specific hiPSCs.

Norepinephrine and epinephrine induce distinct beta2 adrenoceptor signaling in cardiac myocytes

We have previously shown that different agonists can induce distinct conformational changes on beta2 adrenoceptor (AR), which is correlated to the different receptor signaling in HEK293 cells. In mouse neonatal cardiac myocytes, isoproterenol (Iso)-stimulated beta2AR sequentially couples to Gs and Gi to regulate myocyte contraction rate. The sequential G-protein coupling is depended on the translocation of the activated receptors through internalization and recycling. Here, we have tested whether norepinephrine (NE) and epinephrine (Epi), two endogenous agonists for betaARs in animal, can induce different signaling properties in cardiac myocytes. We have found that Epi, similar to Iso can induce receptor sequential coupling to Gs and Gi. In contrast, NE-stimulated receptors only couple to Gs. Moreover, while Epi-activated beta2ARs undergo internalization and recycling, NE-activated beta2ARs remain inside of cells after internalization. Further studies suggest that NE-activated receptors are co-localized with lysosomal marker lamp-1. We believe that NE and Epi induce different conformational changes on beta2AR in cardiac myocytes, which leads to distinct receptor signaling properties as well as the receptor stabilities. Further studies will help us understand the roles of NE and Epi in cardiac system in vivo.

SR33805, a Ca2+ antagonist with length-dependent Ca2+ -sensitizing properties in cardiac myocytes.

1. This study examined the effects of SR33805, a fantofarone derivative with reported strong Ca(2+) -antagonistic properties, on the contractile properties of intact and skinned rat ventricular myocytes. 2. On intact cells loaded with the Ca(2+)-fluorescent indicator Indo-1, the application of low concentrations of SR33805 enhanced the amplitude of unloaded cell shortening and decreased the duration of cell shortening. Amplitude of the Ca(2+) transient was also decreased. 3. These effects were accompanied with a shortening of the action potential and a dose-dependent blockade of L-type calcium current (IC(50)=2.4 x 10(-8) M). 4. On skinned cardiac cells, the application of a low SR33805 concentration (10(-8) M) induced a significant increase in maximal Ca(2+)-activated force at the two-tested sarcomere lengths (SLs), 1.9 and 2.3 microm. 5. The application of a larger dose of SR33805 (10(-6)-10(-5) M) induced a significant leftward shift of the tension-pCa relation that accounts for Ca(2+)-sensitization of the myofilaments, particularly at 2.3 microm SL. 6. In conclusion, despite its strong Ca(2+)-antagonistic properties SR33805 increases cardiac cell contractile activity as a consequence of its Ca(2+)-sensitizing effects. These effects are attributable to both an increase in the maximal Ca(2+)-activated force and a length-dependent Ca(2+)-sensitization.

Influence of Gender on Intrinsic Contractile Properties of Isolated Ventricular Myocytes from Calmodulin‐Induced Diabetic Transgenic Mice

Diabetes mellitus impairs ventricular function, which itself may be disparately influenced by gender. This study compared the impact of gender on cardiac contractile response in ventricular myocytes from wild‐type FVB and calmodulin‐induced diabetic transgenic (OVE26) mice at young (2 month) and older (11 month) age. Mechanical and intracellular Ca2+ properties of cardiac myocytes were evaluated using an IonOptix MyoCam® system. Diabetic mice of both genders exhibited significantly elevated blood glucose regardless of age. OVE26 myocytes displayed reduced peak shortening (PS) and maximal velocity of shortening/relengthening (±dL/dt), and prolonged time‐to‐PS (TPS) and time‐to‐90% relengthening (TR90), associated with higher resting intracellular Ca2+ levels and attenuated Ca2+‐induced intracellular Ca2+ release compared with the FVB myocytes. Peak shortening and ± dL/dt were smaller in female FVB groups when compared to the age‐matched male counterparts. However, these gender differences were ablated by the diabetic state. No significant gender‐related differences in intracellular Ca2+ handling were noted in either FVB or OVE26 myocytes, with the exception of overt gender differences in OVE26 mice when age was taken into account. Young female OVE26 mice exhibited better‐preserved mechanical function while older female OVE26 mice displayed the worst mechanical function among all four OVE26 groups. In conclusion, our data confirmed impaired cardiac contractile function in diabetes, partially due to altered intracellular Ca2+ handling, in both genders. Mechanical differences existed between genders but were “cancelled off” by diabetic state. Nevertheless, a “female advantage” in ventricular function may still persist in young female diabetic subjects.

M1Muscarinic Receptors Heterologously Expressed in Cardiac Myocytes Mediate Ras-dependent Changes in Gene Expression

Stimulation of alpha 1-adrenergic receptors in neonatal ventricular cardiomyocytes induces hypertrophic changes including activation of the atrial natriuretic factor (ANF) gene. This receptor couples to Gq to activate phospholipase C (PLC) and protein kinase C, which have been implicated as mediators of the hypertrophic response. To directly determine whether receptor coupling to Gq/PLC is sufficient to induce ANF expression, we expressed wild-type and chimeric muscarinic cholinergic receptors (mAChRs) with altered G-protein coupling properties in cardiac myocytes and examined their ability to activate an ANF promoter/luciferase reporter gene. The cholinergic agonist carbachol failed to induce transcriptional activation of the ANF reporter gene through endogenous Gi-linked M2mAChRs or in cells transfected with M2mAChRs. In contrast, in cells transfected with M1mAChRs, which effectively couple to Gq/PLC, carbachol increased ANF reporter gene expression 10-fold and also increased ANF protein, as determined by immunofluorescence. Carbachol-mediated ANF gene expression was inhibited by the mAChR antagonist pirenzepine with a Ki value characteristic of an M1mAChR. Studies using chimeric M1- and M2mAChRs demonstrated that the N-terminal 21 amino acids of the third intracellular loop of the M1mAChR were required for receptor coupling to ANF gene expression. This region, previously shown to specify receptor coupling to Gq/PLC, also conferred partial activity to a chimeric M2 receptor. We further demonstrated that M1mAChR coupling to ANF gene expression was Ras-dependent since co-expression of dominant-interfering Ala-15 Ras inhibited M1mAChR-induced ANF expression by 60%. In contrast, ANF expression induced by the chimeric M2 receptor was not blocked by dominant-interfering Ras. We suggest that receptor coupling to Gq/PLC is sufficient to induce ANF expression and that a Ras-dependent pathway contributes additional signals required for maximal M1mAChR-mediated ANF gene expression.