TRPM4, a Ca2+-activated nonselective cation channel in mouse sino-atrial node cells.
OBJECTIVE
A calcium-activated nonselective cation channel (NSC(Ca)) 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 I(f) current activity, arguments are also in favor of participation of an NSC(Ca). Similarly, the NSC(Ca) may be expressed in specialized pacemaker cells, i.e. sino-atrial 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 NSC(Ca) 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-bisphosphate (10 microM). 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.


CONCLUSIONS
TRPM4 is functionally expressed in SAN cells and may be a key player in the generation and/or perturbation of heart rhythm.
Demion
Marie
M
Institut de Physiologie et Biologie Cellulaires, CNRS UMR 6187, Université de Poitiers, F-86022 Poitiers Cedex, France.
Bois
Patrick
P
Launay
Pierre
P
Guinamard
Romain
R
D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology; D015153:Blotting, Western; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002122:Calcium Chloride / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D005439:Flufenamic Acid / Q000494:pharmacology; D005905:Glyburide / Q000494:pharmacology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D051379:Mice; D032383:Myocytes, Cardiac / Q000737:chemistry / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000494:pharmacology; D012333:RNA, Messenger / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012849:Sinoatrial Node / Q000737:chemistry / Q000378:metabolism*; D050053:TRPM Cation Channels / Q000032:analysis* / Q000235:genetics
D000894:Anti-Inflammatory Agents, Non-Steroidal; D002121:Calcium Channel Blockers; D019269:Phosphatidylinositol 4,5-Diphosphate; D012333:RNA, Messenger; D050053:TRPM Cation Channels; C437888:TRPM4 protein, human; D005439:Flufenamic Acid; D002122:Calcium Chloride; D005905:Glyburide