TRPM4, a Ca2+-activated nonselective cation channel in mouse sino-atrial node cells

Objective: A calcium-activated nonselective cation channel (NSCCa) has been recently described in several cardiac preparations. This channel is over-expressed in models of ventricular hypertrophy showing electrophysiological perturbations of heart activity, including occurrence of spontaneous activity. While these perturbations are Currently attributed to a modification of the pacemaker If current activity, arguments are also in favor of participation of an NSCCa. Similarly, the NSCCa may be expressed in specialized pacemaker cells, i.e. sinoatrial node (SAN) cells. The aim of the present study was to detect such current in mouse pacemaker cells. Methods: The inside-out configuration of the patch-clamp technique was used in freshly isolated SAN cells from adult mice. Also, RT-PCR and Western-blotting studies were used to probe for TRPM4 mRNA and protein expression. Results: In these cells, an NSCCa activity was detected. The channel is voltage dependant with a conductance of 20.9 +/- 0.5 pS (n=11). It is equally permeable for Na and K+ but does not conduct Ca2+. It is activated by rise in intracellular calcium concentrations and blocked by intracellular ATP (0.5 mmol/L). Also, as a new property in cardiac cells, the channel is activated by internal application of phosphatidylinositol 4,5-bisphospliate (10 mu M). It is reversibly inhibited by flufenamic acid and glibenclamide. This channel shows; the hallmarks of the TRPM4 molecule, a member of the TRP melastatin subfamily. We confirm the expression of this TRP channel on SAN cells by Western blotting and RT-PCR and validate that TRPM4 is glibenclamide sensitive. Conclusion: TRPM4 is functionally expressed in SAN cells and may be a key player in the generation and/or perturbation of heart rhythm. (c) 2006 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.