9-Phenanthrol and flufenamic acid inhibit calcium oscillations in HL-1 mouse cardiomyocytes

Department of Biomedical Sciences, James H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN 37614, USA.
Cell calcium (Impact Factor: 4.21). 07/2013; 54(3). DOI: 10.1016/j.ceca.2013.06.003
Source: PubMed

ABSTRACT It is well established that intracellular calcium ([Ca(2+)]i) controls the inotropic state of the myocardium, and evidence mounts that a "Ca(2+) clock" controls the chronotropic state of the heart. Recent findings describe a calcium-activated nonselective cation channel (NSCCa) in various cardiac preparations sharing hallmark characteristics of the transient receptor potential melastatin 4 (TRPM4). TRPM4 is functionally expressed throughout the heart and has been implicated as a NSCCa that mediates membrane depolarization. However, the functional significance of TRPM4 in regards to Ca(2+) signaling and its effects on cellular excitability and pacemaker function remains inconclusive. Here, we show by Fura2 Ca-imaging that pharmacological inhibition of TRPM4 in HL-1 mouse cardiac myocytes by 9-phenanthrol (10μM) and flufenamic acid (10 and 100μM) decreases Ca(2+) oscillations followed by an overall increase in [Ca(2+)]i. The latter occurs also in HL-1 cells in Ca(2+)-free solution and after depletion of sarcoplasmic reticulum Ca(2+) with thapsigargin (10μM). These pharmacologic agents also depolarize HL-1 cell mitochondrial membrane potential. Furthermore, by on-cell voltage clamp we show that 9-phenanthrol reversibly inhibits membrane current; by fluorescence immunohistochemistry we demonstrate that HL-1 cells display punctate surface labeling with TRPM4 antibody; and by immunoblotting using this antibody we show these cells express a 130-150kDa protein, as expected for TRPM4. We conclude that 9-phenanthrol inhibits TRPM4 ion channels in HL-1 cells, which in turn decreases Ca(2+) oscillations followed by a compensatory increase in [Ca(2+)]i from an intracellular store other than the sarcoplasmic reticulum. We speculate that the most likely source is the mitochondrion.

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