Mechanotransduction through fibronectin-integrin focal adhesion in microvascular smooth muscle cells: is calcium essential?

Sun Z, Li Z, Meininger GA. Mechanotransduction through fibronectin-integrin focal adhesion in microvascular smooth muscle cells: is calcium essential?. Am J Physiol Heart Circ Physiol 302: H1965-H1973, 2012. First published March 16, 2012; doi: 10.1152/ajpheart.00598.2011.-It is believed that increased transmural pressure exerts force on vascular smooth muscle cells (VSMCs) and triggers Ca2+ signaling as an initiating event responsible for the arteriolar myogenic response. However, the mechanisms linking the pressure increase to Ca2+ signaling are unclear. We have shown previously using atomic force microscopy (AFM) that mechanical force induces a VSMC contractile response when applied to single fibronectin (FN; Sun Z, Martinez-Lemus LA, Hill MA, Meininger GA. Am J Physiol Cell Physiol 295; C268-C278, 2008) focal adhesion sites. This current study seeks to determine whether application of force to single focal adhesions can cause a change in VSMC Ca2+. Experiments were performed in low passage (p3 similar to 10) as well as in freshly isolated skeletal muscle arteriole VSMCs. AFM-attached microbeads (5 mu m) were coated with FN or collagen type I (CN-I) or type IV (CN-IV) and placed on a VSMC for 20 min, resulting in formation of a focal adhesion between the cell and the microbead. In low passage VSMCs, mechanically pulling on the FN-coated beads (800 similar to 3000 pN) did not induce a Ca2+ increase but did cause a contractile response. In freshly isolated VSMCs, application of an FN or CN-I-coated bead onto the cell surface induced global Ca2+ increases. However, these Ca2+ increases were not correlated with the application of AFM pulling force to the bead or with the VSMC contractile responses to FN-coupled pulling. Chelating cytosolic Ca2+ using BAPTA loading had no negative effect on the focal adhesion-related contractile response in both freshly isolated and low passage VSMCs, while the Rho-kinase inhibitor Y27632 abolished the micromyogenic response in both cases. These observations suggest that, in freshly isolated and cultured VSMCs, application of mechanical force to a focal adhesion does not invoke an acute global Ca2+ increase. On the other hand, our data support a role for Rho-linked signaling mechanism involved in mechanotransduction leading to focal contraction that is independent of the need for a global increase in VSMC Ca2+.