What controls smooth muscle phenotype?

Enzyme-dispersed smooth muscle cells from the adult pig aortic media in the first few days of primary culture are in the contractile phenotype and do not divide when challenged with 5% WBS. After 6--8 days the isolated cells spontaneously undergo a change in phenotype where contraction cannot be stimulated and the cells respond to mitogens in WBS by logarithmic growth. The change in phenotype is reversible if the cells are seeded sufficiently densely (5 x 10(4) to 1 x 10(5)/ml) that a confluent monolayer results after less than 1 week of proliferation, but is irreversible if the cells are seeded sparsely (1 x 10(3) to 5 x 10(3)/ml) and take more than 2 weeks of proliferation to reach confluence. When the cells are seeded so densely (10(6)/ml) that a confluent monolayer is present from day 1, the cells do not undergo a change in phenotype but remain indefinitely in the contractile state. The spontaneous modulation of phenotype of isolated smooth muscle cells can be inhibited by a confluent monolayer of contractile smooth muscle or endothelial cells in co-culture with the sparsely seeded smooth muscle such that the two cell layers are not in contact but bathed by the same nutrient medium. Smooth muscle modulation can also be inhibited by a factor extracted from pig and rabbit aortic tissue, and its effects mimicked by commercially available sodium heparin at 50 units/ml.