Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC267
Characterization of a TRPM7-like cation current in human atrial cardiomyocytes
I. Martisiene1, V. Gendviliene1, D. Zablockaite1, R. Macianskiene1
1. Institute of Cardiology, Lithuanian University of Health Sciences, Kaunas, Lithuania.
Transient receptor potential melastatin 7 (TRPM7) channels, which is known as Mg2+-inhibitable cation current (MIC) or Mg nucleotide-regulated metal ion current (MagNuM) have recently been discovered to play important roles in Mg2+ and Ca2+ homeostasis, which is critical to both human health and cell viability. However, most of these data were obtained using the heterologous expression systems, very few data in the heart cells of different animals, while in native human cardiomyocytes has not yet been described. We aimed to identify the presence of TRPM7-like current in human atrial cardiomyocytes and evaluate the origin of the permeability of the channel and its regulation using the whole-cell voltage clamp technique. Atrial myocytes were isolated from right atrial appendages from 52 adult patients in sinus rhythm (SR) with and without ischaemic heart disease, with coronary artery disease or valve diseases. We demonstrate that under voltage clamp conditions using voltage ramps between -120 mV and +80 mV, an outward-rectifying current at positive potentials and small inward current at negative potentials, was present in cells dialyzed with zero [Mg2+]i, i.e. current increased from 1.67±0.07 to 3.73±0.20 pA/pF at +80 mV, and from -0.26±0.02 to -0.50±0.03 pA/pF at -120 mV, (n=81, p<0.001). The current was suppressed by raising [Mg2+]o and was absent in cells dialyzed with physiological [Mg2+]i, indicating that it was due to the Mg2+i-sensitive, TRPM7-like current. Extracellular application of Gd3+, 2-APB and spermine (at 100 µM), known to block TRP channels, decreased the peak density of TRPM7-like current outward component almost completely. It is interesting that additional large current was induced in divalent free solutions, when cells were dialysed with physiological or high intracellular free Mg2+ (up to 10 mM) or MgATP (5 mM) concentration, which did not protected the spontaneous run-up in a concentration dependent manner. This was due not to the movement of monovalent ions via L-type Ca2+-channels because that current was resistant to 100 µM nifedipine. Moreover, the difference current obtained on the ascending limb of the 4 sec ramp was the same or nearly the same as on the descending limb, that implies that the current we measure is TRPM7-like. In addition, we established that the density of activated TRPM7-like current was related to the clinical history of the donor, i.e. in cells from patients with the ischaemic heart disease the outward current was higher (3.92±0.24 pA/pF, n=57, p<0.001) as compare with those without ischaemia (3.19±0.32 pA/pF, n=20, p<0.001). In conclusion: to our knowledge, this is the first study, which describes TRPM7-like current in native human atrial cardiomyocytes that operates under physiological/pathophysiological conditions.
Where applicable, experiments conform with Society ethical requirements