Synergistic interactions between Ca²⁺ entries through L‐type Ca²⁺ channels and Na⁺–Ca²⁺ exchanger in normal and failing rat heart

Abstract

We used confocal Ca 2+ imaging and the patch-clamp technique to investigate the interplay between Ca2+ entries through L-type Ca 2+ channels (LCCs) and reverse-mode Na + -Ca 2+ exchange (NCX) in activating Ca 2+ -induced Ca 2+ release (CICR) from the sarcoplasmic reticulum (SR) in cardiac myocytes from normal and failing rat hearts. In normal myocytes exposed to N (6),2 -O-dibutyryl adenosine-3 ,5 -cyclic monophosphate (db-cAMP, membrane-permeable form of cAMP), the bell-shaped voltage dependence of cytosolic Ca 2+transients was dramatically broadened due to activation of SR Ca 2+ release at high membrane potentials (30-120 mV). . This is consistent with the spatial localization of the LCCs just above the RyRs across the junctional cleft (Franzini-Armstrong et al. 1999 (Leblanc & Hume, 1990;Lipp & Niggli, 1994;Kohomoto et al. 1994;Sipido, 1998). Additionally, there may be interplay between different Ca 2+ -entry pathways in initiation of SR Ca 2+ release. For example, it has been proposed that Ca 2+ entry through LCCs and reverse-mode NCX interact synergistically to activate RyR channels in the SR (Haworth et al. 1991;Litwin et al. 1998; Viatchenko-Karpinski & Györke, 2001). Integration of Ca 2+ -input signals from different entry pathways could provide an important mechanism for regulation of cardiac EC coupling and contractility. However, whether and how such integration of triggers for CICR occurs in cardiac muscle remain to be determined.Stimulation through the β-adrenergic pathway is a classical regulatory mechanism for controlling the contractile state of the myocardium. β-adrenergic agonists increase the amount of Ca 2+ released from the SR, resulting in the development of increased contractile force in cardiac muscle. The biochemical signalling mechanisms underlying β-adrenergic stimulation involve cAMP-dependent phosphorylation by protein kinase A (PKA) of certain target proteins including the LCC, RyR and phospholamban (Kranias et al. 1985;Shannon et al. 2001). In heart failure (HF), the Ca 2+ -releasing function of the SR appears to be compromised, resulting in depressed intracellular Ca 2+ transients and impaired contraction Eisner et al. 2003). Additionally the Ca 2+ -release machinery loses its ability to respond to adrenergic stimulation in myocytes from failing hearts, contributing to reduced contractile reserve in HF (Gomez et al. 1997;Pogwizd et al. 2001). The precise reasons for these HF-related changes in Ca 2+ transients and responsiveness to adrenergic stimulation are not known, and are currently the subject of intense debate.In the present study, we investigated the interplay between Ca 2+ entries via L-type Ca 2+ channels and reverse-mode NCX in activation of SR Ca 2+ release, focusing on the influence of adrenergic stimulation and changes occurring after induction of HF. Significantly, our results show that LCCs and NCX interact in a synergistic manner resulting in a larger total Ca 2+ entry than the arithmetic sum of entries mediated by each of these transport systems alone. This syne...

High Basal Protein Kinase A–Dependent Phosphorylation Drives Rhythmic Internal Ca 2+ Store Oscillations and Spontaneous Beating of Cardiac Pacemaker Cells

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