COMPOSITION IN-SITU AND IN-VITRO OF VASCULAR SMOOTH-MUSCLE LAMININ INTHE RAT

Vascular smooth muscle cells are surrounded by a basal lamina containi ng an array of macromolecules: included among these are the laminins, a family of oligomeric glycoproteins composed of subunits encoded by d ifferent genes. In this study, we have used monoclonal antibodies to s everal of these subunits, including S-laminin, laminin B2, and laminin B1, to study these proteins in tail artery, superior mesenteric arter y, and aorta of rats. In situ, immunostaining for the B2 and S chains was present in the basal lamina, between the smooth muscle cells, thro ughout the tunica media. In contrast, B1 chain immunostaining was conc entrated around cells in the inner media. To investigate whether smoot h muscle cells can produce S-laminin, laminin B2, and laminin B1, smoo th muscle cells from the superior mesenteric artery were grown in cult ure and laminin subunit expression determined. In early culture (4 day s), immunostaining showed abundant laminin B2 and less B1 synthesis an d incorporation into the matrix. Staining for S-laminin was even less intense than for B1 and was localized to areas where cells were densel y packed. The same pattern of S-laminin immunostaining was seen during early culture in cells grown on fibronectin, type IV collagen, or gel atin. Immunoblotting detected S-laminin in the conditioned medium from early cultured cells. In later culture (12 days), S-laminin incorpora tion into the matrix increased markedly compared to incorporation at 4 days. At this time, cells are much more densely packed and multilayer ed with extensive matrix accumulation. Cyclical stretching of cells in vitro did not increase immunostaining for S-laminin. Together these d ata show that S-laminin is a component of the arterial media in situ a nd that in vitro S-laminin is synthesized by smooth muscle cells. Incr eased incorporation of S-laminin into the matrix in later culture corr elates with the presence of a more extensive matrix, suggesting that m atrix organization may be critical to S-laminin incorporation.