Mechanical forces regulate focal adhesion and costamere assembly in cardiac myocytes

-To determine whether the formation and maintenance of focal adhesions and costameres in cardiac myocytes are influenced by the mechanical forces that they transmit, we mechanically unloaded these cells by inhibiting their spontaneous contractile activity with the calcium-channel blocker nifedipine (12 μM). Interferencereflection and fluorescence microscopy revealed that within 24 h of arrest, β₁-integrin- and vinculin-positive focal adhesions and costameres were disrupted. Loss of mature β₁-integrin from the cell surface was observed in cell surfacelabeling experiments and in Western blots. Subjecting nonbeating cells to a 5% static stretch for 24 h resulted in an increase of 21% for β₁-integrin and 39% for vinculin. Stretching beating cells resulted in 71 and 9% increases, respectively. Intracellular concentrations of pre-β₁ were not affected by contractile activity or by stretch. Our results indicate that mechanical forces stabilize the cellular levels of β₁-integrin and vinculin by possibly regulating their association with the formation and maintenance of focal adhesions and costameres. β₁-integrin; vinculin