Pat- If by valvular interstitial cells you mean fibroblasts, I have found
that valvular interstitial fibroblasts express CD34, the human progenitor
antigen just as interstitial fibroblasts do in most collagenous mesenchyme.
 (Berrutti L, Silverman JS.   Cardiac myxoma is rich in FXIIIa+
dendrophages. Immunohistochemical study of four cases.  Histopathology
1996;28:529-35.)  Try Biogenex CD34 (QBend-10 clone) and let me know what
you find. 
Jeff Silverman