CALCIFICATION OF CARDIAC-VALVE BIOPROSTHESES - BIOCHEMICAL, HISTOLOGIC, AND ULTRASTRUCTURAL OBSERVATIONS IN A SUBCUTANEOUS IMPLANTATION MODEL SYSTEM

Abstract

To study the process of calcification in bioprostheses, 108 glutaraldehyde-treated porcine aortic valve leaflets were implanted s.c. in rabbits and removed 1 day to 6 mo. later; morphologic findings were correlated with biochemically determined levels of calcium (Ca2+) and gamma-carboxyglutamic acid (Gla), a vitamin K-dependent Ca2+-binding amino acid known to be present in a variety of tissues with pathological calcification. Gla and Ca2+ levels began to increase about 2 mo. after implantation and increased progressively with time. Ca/Gla molar ratios were comparable to those in leaflets of bioprostheses explanted from patients, 22-64 mo. after implantation. Morphologically evident calcification began at the same time that Gla and Ca2+ increases were detected biochemically and also increased in severity with time. EM showed that calcification primarily involved the surface of collagen fibrils and the interfibrillar spaces. The biochemical and morphologic findings in this experimental system are similar to those described in calcified porcine bioprosthetic valve leaflets removed from patients, but occurred much more rapidly. As with pathological calcification of other tissues, progressive calcification was accompanied by increased Gla levels, suggesting that Gla plays a role in the calcification of the leaflets. This model allows rapid comparative evaluation of large numbers of valve leaflets maintained under similar host conditions.