Inositol 1, 4, 5-Trisphosphate Receptors and Human Left Ventricular Myocytes

Abstract

Background Little is known concerning the function of inositol 1,4,5-triphosphate receptors (IP3Rs) in the adult heart experimentally. Moreover, whether these Ca2+ release channels are present and play a critical role in human cardiomyocytes remains to be defined. IP3Rs may be activated following Gαq-protein-coupled receptors (GPCR) stimulation affecting Ca2+ cycling, enhancing myocyte performance and, potentially, favoring an increase in the incidence of arrhythmias. Methods and Results IP3R function was determined in human left ventricular (LV) myocytes and this analysis was integrated with assays in mouse myocytes to identify the mechanisms by which IP3Rs influence the electrical and mechanical properties of the myocardium. We report that IP3Rs are expressed and operative in human LV myocytes. Following GPCR activation, Ca2+ mobilized from the sarcoplasmic reticulum via IP3Rs contributes to the decrease in resting membrane potential, prolongation of the action-potential, and occurrence of early after-depolarizations. Ca2+ transient amplitude and cell shortening are enhanced, and extra-systolic and dysregulated Ca2+ elevations and contractions become apparent. These alterations in the electromechanical behavior of human cardiomyocytes are coupled with increased isometric twitch of the myocardium and arrhythmic events, suggesting that GPCR activation provide inotropic reserve, which is hampered by electrical instability and contractile abnormalities. Additionally, our findings support the notion that increases in Ca2+ load by IP3Rs promote Ca2+ extrusion by forward mode Na+/Ca2+ exchange, an important mechanism of arrhythmic events. Conclusions Thus, the GPCR/IP3R axis modulates the electromechanical properties of the human myocardium and its propensity to develop arrhythmias.

Reduction in Kv Current Enhances the Temporal Dispersion of the Action Potential in Diabetic Myocytes: Insights From a Novel Repolarization Algorithm

“…The standard analysis of AP characteristics19, 30 was complemented with a newly developed algorithm for assessment of the repolarization phase.…”

“…Isolated LV myocytes were placed in a bath on the stage of an IX51, IX53, or IX71 (Olympus, Tokyo, Japan) microscopes for patch‐clamp measurements 19, 25, 30, 31. Experiments were conducted at 37°C.…”

“…Electrical signals were digitized using 250‐kHz 16‐bit resolution A/D converters (Digidata 1322, 1440A, and 1550; Molecular Devices) and recorded using pCLAMP 9.0 and 10 software (Molecular Devices) with low‐pass filtering at 2 kHz. Membrane capacitance (C m ) was measured in voltage‐clamp mode using a 5‐mV voltage step and pCLAMP software algorithm; this parameter was employed to normalize transmembrane currents 19, 20, 25, 30. Pipettes were pulled by means of a vertical (PB‐7; Narishige International Inc., East Meadow, NY), or horizontal (P‐1000; Sutter Instrument Co, Novato, CA) glass microelectrode pullers.…”

“…For AP measurements, current‐clamp mode was employed 19, 20, 30, 31. Cells were stimulated with current pulses ≈1.5 times threshold.…”

The SR/ER-mitochondria calcium crosstalk is regulated by GSK3β during reperfusion injury

“…16 Although ryanodine receptors (RyRs) are the major cardiac SR/ER Ca 2+ -release channels involved in excitation-contraction coupling (ECC) 17 and ischemia-reperfusion (IR) injury, 18 recent studies reported an increasing role for inositol 1,4,5-trisphosphate receptors (IP 3 Rs) Ca 2+ -release channels in the modulation of ECC and cell death. 19,20 Ca 2+ -handling proteins of ER and mitochondria are highly concentrated at mitochondria-associated ER membranes (MAMs), providing a direct and proper mitochondrial Ca 2+ signaling, including VDAC, Grp75 and IP 3 R1. [20][21][22] Here, we provide evidence that, following IR, a fraction of cellular GSK3β is localized at the SR/ER and MAMs.…”