Using traction force microscopy to study the role of fibroblast contractility in hypertensive cardiac dysfunction

Hypertensive cardiac dysfunction (HCD) is a consequence of chronic hypertension and is characterized by progressive impairment of diastolic filling and systolic contraction that eventually lead to heart failure. A leading hypothesis to explain the pathogenesis of HCD focuses on the poor regulation of the renin-angiotensin system, with consequent increase in the hormone angiotensin II (Ang II). This idea is supported by the fact that Ang II stimulates the cardiac fibroblast to produce extracellular matrix (ECM) and proliferate, thereby contributing to the massive remodeling, hypertrophy, and eventual stiffening of the myocardium that occur in HCD.; This study uses traction force microscopy (TFM) to investigate the involvement of fibroblast contractility in HCD. The basic idea behind TFM is to culture cells onto a flexible substratum of known mechanical properties and then to utilize the deformations of the substratum as a basis for making contractility measurements. Inherent to TFM are two computational challenges. The first deals with measuring the deformations given images of the substratum and is overcome using an optical flow algorithm. The second focuses on computing cell contractility given the deformation data and is overcome by numerically solving an inverse problem based upon linear elasticity theory.; Contractility measurements are obtained using TFM for cardiac fibroblasts isolated from the hearts of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. This allows insights into understanding the role of fibroblast contractility in HCD since the SHR develops chronic hypertension and HCD with symptoms and onset similar to humans. The involvement of Ang II in HCD is also investigated by measuring the contractility of both WKY and SHR fibroblasts while they are stimulated with Ang II alone and while blocking the functional Ang II receptor with the anti-hypertensive drug irbesartan.; The data indicate that the average traction exerted by SHR fibroblasts is ∼1.8 times higher than in WKY cells. Stimulating WKY fibroblasts with Ang II increases their tractions ∼2 fold, thereby eliminating the differences in contractility. In contrast, Ang II has no effect on the tractions exerted by SHR fibroblasts. Surprisingly, the tractions exerted by both WKY and SHR fibroblasts decrease by ∼50% when they are exposed to irbesartan both in the absence and presence of Ang II. Thus, irbesartan not only blocks the binding of Ang II, but it also has some adverse effect on contractility. These facts suggest that the renin-angiotensin system within the SHR fibroblast is in a constitutively activated state and locally stimulates this cell type with endogenous Ang II leading to a maximal increase in traction. On the other hand, the renin-angiotensin system within the WKY fibroblast is not as active and thus, this cell type is not as contractile as the SHR cell unless it is stimulated with Ang II.