The Involvement of NAADP and Two-Pore Ca2+ Channels in the Cardiac Beta-Adrenergic Response

Abstract

NAADP is a potent endogenous Ca2+-mobilising agent with actions in many cell types, including cardiac myocytes1,2. Our previous work suggests that NAADP causes Ca2+ release from an acidic endolysosomal store, leading to additional Ca2+ uptake by the SR1,2. Endolysosomal Ca2+ release is thought to occur via a two-pore Ca2+ channel (TPC), which exists in several subtypes including TPC1 and TPC23,4. The aim of this study was to investigate whether Ca2+ mobilised by NAADP is important in cardiac beta-adrenergic signalling.In guinea pig ventricular myocytes, photorelease of NAADP elicited an increase in the amplitude of the Ca2+ transient (42±11% from control; n=6, P<0.05). Isoprenaline increased the amplitude of the Ca2+ transient by 148±6% (n=10, P < 0.05); this response was reduced to 102±12% in the presence of bafilomycin (to disrupt the endolysosomal Ca2+ store; n=6, P<0.05) and to 106±6% in the presence of NED-19, an inhibitor of the NAADP pathway (n=6, P<0.05).In ventricular myocytes from transgenic mice lacking TPC1 and 2 proteins (TPCDKO), the increase in amplitude of contraction in response to isoprenaline was reduced from 243±31% in wild type (WT) to 140±24% in TPCDKO (both n=8, P<0.05). In Langendorff-perfused hearts lacking TPC2 proteins (TPC2KO), the isoprenaline-induced increase in contractile force was reduced from 92±4% in WT (n=5) to 63±9% in TPC2KO hearts (n=7, P<0.05).These data support the hypothesis that NAADP-induced Ca2+ mobilisation contributes to the cardiac beta-adrenergic response. It appears that TPC proteins are involved in this mechanism, presumably as a consequence of increased synthesis of NAADP.1. Collins et al., (2011) Cell Calcium 50: 449-4582. Macgregor et al., (2007) J Biol Chem 282: 15302-153113. Calcraft et al., (2009) Nature 459: 596-604. Elson et al., (2010) Biophys J 98: 100a