The cellular properties of semilunar cardiac valve leaflets may be more complex than previously assumed. In particular, the cells of the leaflet matrix which are likely critical to proper cusp function are a poorly described population to date. We hypothesized that, similar to the matrix cells of atrioventricular valves, aortic valve leaflet interstitial cells (AoLIC's) possess characteristics of both fibroblasts (matrix secretion) and smooth muscle cells (contraction). Porcine AoLIC's were structurally examined for contractile and stress fiber protein assemblies using transmission electron microscopy and immunocytochemistry. Contractile function in response to vasoactive stimuli was directly assessed using AoLIC's cultured on flame- polymerized silicone, with cell contraction identified by the appearance of wrinkles in the substratum after challenge with each agent. The structural analyses showed cellular microfilaments were often organized into various contractile arrangements including polygonal networks, and that AoLIC's are rich in smooth muscle-specific α-actin. Incomplete basal laminae often associated with myofibroblasts were observed. Contraction experiments indicated a responsivity of similar latency, but variable peak and duration to 10-7 M L-epinephrine, 3.2 x 10-7 M angiotensin II, 110 μM carbachol, 50 mM KCl, 3.2 x 10-7 M bradykinin, 110 μM isoproterenol, and 5 x 10-7 M endothelin I. Soluble and insoluble matrix secretion was confirmed with FITC- conjugated monoclonal antibodies to chondroitin sulfate, fibronectin, and prolyl-4-hydroxylase. These data show that the AoLIC's are best designated as myofibroblasts. The unusual features of the myofibroblast may be central to lifelong aortic leaflet durability.
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