Different subcellular populations of L-type Ca2+ channels exhibit unique regulation and functional roles in cardiomyocytes

Abstract

Influx of Ca(2+) through L-type Ca(2+) channels (LTCCs) contributes to numerous cellular processes in cardiomyocytes including excitation-contraction (EC) coupling, membrane excitability, and transcriptional regulation. Distinct subpopulations of LTCCs have been identified in cardiac myocytes, including those at dyadic junctions and within different plasma membrane microdomains such as lipid rafts and caveolae. These subpopulations of LTCCs exhibit regionally distinct functional properties and regulation, affording precise spatiotemporal modulation of L-type Ca(2+) current (I(Ca,L)). Different subcellular LTCC populations demonstrate variable rates of Ca(2+)-dependent inactivation and sometimes coupled gating of neighboring channels, which can lead to focal, persistent I(Ca,L). In addition, the assembly of spatially defined macromolecular signaling complexes permits compartmentalized regulation of I(Ca,L) by a variety of neurohormonal pathways. For example, β-adrenergic receptor subtypes signal to different LTCC subpopulations, with β(2)-adrenergic activation leading to enhanced I(Ca,L) through caveolar LTCCs and β(1)-adrenergic stimulation modulating LTCCs outside of caveolae. Disruptions in the normal subcellular targeting of LTCCs and associated signaling proteins may contribute to the pathophysiology of a variety of cardiac diseases including heart failure and certain arrhythmias. Further identifying the characteristic functional properties and array of regulatory molecules associated with specific LTCC subpopulations will provide a mechanistic framework to understand how LTCCs contribute to diverse cellular processes in normal and diseased myocardium. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Figure 1. Cardiomyocyte L-type Ca²⁺ channels localized to caveolae are regulated by β₂-adrenergic receptor activation

(A,B) Immunogold electron micrographs of mouse neonatal ventricular myocytes demonstrate co-localization of the Ca_v1.2 subunit of L-type Ca²⁺ channel (large particle, arrows) with Cav-3 (small particle, arrowheads) within caveolae. Scale bars represent 200 nm. (C) β₂-adrenergic receptor activation with 10 μM salbutemol (Sal) plus 10 μM atenolol (Aten) increases peak I_Ca,L in neonatal mouse ventricular myocytes under control conditions. (D) β₂-adrenergic stimulation of I_Ca,L is lost in cells in which caveolae are disrupted using Cav-3 siRNA. Copyright (2006) National Acadamy of Sciences, USA [5].

Figure 2. Schematic of L-type Ca²⁺ channel subpopulations and associated macromolecular signaling complexes that contribute to diverse processes within the cardiac myocyte

Within T-tubules, some L-type Ca²⁺ channels (LTCCs) form dyadic junctions with ryanodine receptors (RyR2) on the apposing sarcoplasmic reticulum (SR). A small influx of Ca²⁺ through LTCCs triggers release of SR Ca²⁺ stores through RyR2, leading to the intracellular [Ca²⁺]_i transient, which is essential for activation of myofilament proteins leading to contraction. β-adrenergic receptor (β-AR) stimulation of [Ca²⁺]_i transients involves a signaling complex consisting of LTCCs, β-AR, adenylyl cyclase (AC), protein kinase A (PKA), caveolin-3 (Cav-3), and A-kinase anchoring proteins (AKAPs) [85]. Multiple AKAPs are important for regulating LTCC function in the heart including AKAP5 and AKAP15 [85,131]. Additional LTCC subpopulations in the surface sarcolemma are implicated in signaling to the nucleus to regulate the transcription of genes involved in cell survival or cardiac hypertrophy and may also contribute to membrane excitability. Within caveolae, LTCCs associate in a macromolecular protein complex with β₂-AR, AC, PKA, protein phosphatase 2A (PP2A), and Cav-3. Caveolar LTCCs are locally stimulated by β₂-AR activation [5]. Coupled gating of closely neighboring LTCCs results in persistent Ca²⁺ sparklets, which are enhanced by AKAPs and protein kinase C alpha (PKCα) and may serve as a local source of Ca²⁺ to stimulate calcineurin (PP2B) signaling to the nucleus [83,97].