Functional characterization of a Ca2+-activated non-selective cation channel in human atrial cardiomyocytes

Abstract

Cardiac arrhythmias, which occur in a wide variety of conditions where intracellular calcium is increased, have been attributed to the activation of a transient inward current (Iti). Iti is the result of three different [Ca]i-sensitive currents: the Na+-Ca2+ exchange current, a Ca2+-activated chloride current and a Ca2+-activated non-selective cationic current. Using the cell-free configuration of the patch-clamp technique, we have characterized the properties of a Ca2+-activated non-selective cation channel (NSCCa) in freshly dissociated human atrial cardiomyocytes. In excised inside-out patches, the channel presented a linear I-V relationship with a conductance of 19 ± 0.4 pS. It discriminated poorly among monovalent cations (Na+ and K+) and was slightly permeable to Ca2+ ions. The channel's open probability was increased by depolarization and a rise in internal calcium, for which the Kd for [Ca2+]i was 20.8 μM. Channel activity was reduced in the presence of 0.5 mM ATP or 10 μM glibenclamide on the cytoplasmic side to 22.1 ± 16.8 and 28.5 ± 8.6%, respectively, of control. It was also inhibited by 0.1 mM flufenamic acid. The channel shares several properties with TRPM4b and TRPM5, two members of the 'TRP melastatin' subfamily. In conclusion, the NSCCa channel is a serious candidate to support the delayed after-depolarizations observed in [Ca2+] overload and thus may be implicated in the genesis of arrhythmias. © The Physiological Society 2004.