Activation of Ca2+-Dependent Cation Current by Fluid Shear Force in Atrial Myocytes

Abstract

Atrial myocytes are subjected to fluid shear force (FSF) during each contraction and relaxation. Ionic currents regulated by shear force and their molecular integrity in cardiac myocytes have not been well-understood. We examined whether FSF activates specific current in atrial myocytes and underlying mechanisms for FSF-sensitive ionic current using whole-cell patch-clamp technique. A FSF of ∼16 dyne/cm2 was applied to entire single atrial myocyte using automated micro-puffing apparatus. A FSF-sensitive current (IFSF) was detected in lowly Ca2+-buffered (0.5 mM EGTA) atrial myocytes, but not in highly Ca2+-buffered (≥4 mM EGTA or 10 mM BAPTA) myocytes. The IFSF showed an outward rectification with a reversal potential of about -6 mV. The IFSF was inhibited by high concentrations (20-50 μM) of ryanodine and by replacement of external and internal cation with impermeant NMDG+, suggesting that IFSF is a Ca2+ release-dependent cation current. Application of either transient receptor potential melastatin subfamily 4 (TRPM4) inhibitor 9-phenanthrol or TRPM4-specific antibodies removed most of inward IFSF and ∼80% of outward IFSF. However, stretch-activated cation channel blocker GsMTx-4 did not affect IFSF. Interestingly, IFSF was strongly inhibited by inositol 1,4,5-trisphosphate receptor (IP3R) blockers, 2-APB (2 μM) or xestospongin C. In addition, in atrial myocytes isolated from type 2 IP3R (IP3R2) knock-out mouse, IFSF was not detected, although 9-phenanthrol-sensitive IFSF was recorded in wild-type myocytes. Co-immunostaining of TRPM4 and IP3R2 in rat atrial myocytes revealed peripheral localization of these proteins with some co-localizations. These results suggest that fluid shear stimuli may activate TRPM4 channels in atrial myocytes via Ca2+ releases triggered by the activation of nearby IP3R2.