Long-term effects of intracellular calcium and growth factors on excitation and contraction in smooth muscle

Modulation of vascular smooth muscle cells from a contractile to a syntheti c phenotype is thought to be important in the development of the atheroscle rotic lesion. Such modulation depends on growth factors and is influenced b y cell-cell and cell-matrix interactions. Whereas smooth muscle cells in th e vessel wall are contractile, dispersed cells in culture rapidly modulate to synthetic phenotype, which complicates long-term in vitro studies. In co ntrast, vascular segments or smooth muscle strips in organ culture can main tain contractility for at least a week, sufficient for studies involving al tered metabolism or protein expression. Examples are effects of endogenous polyamines on membrane ion channels and excitation-contraction coupling. Wh ile smooth muscle tissue is well preserved In serum-free culture, growth st imulation with fetal calf serum (FCS) causes multiple effects, including de creased contractility, ultrastructural changes, decreased expression of L-t ype Ca2+ channels, and increased SR release of Ca2+ via ryanodine receptors . These are all consequences of increased basal [Ca2+](i) caused by FCS, as they are reversed by culture with verapamil in a concentration (1 mu M) th at does not inhibit stimulation of DNA and protein synthesis by FCS. The ef fects of FCS on contractility and Ca2+ channel expression are mimicked in s erum-free culture with increased [Ca2+](i). Contractile protein patterns, i ncluding myosin isoform composition, are unaffected by FCS, suggesting that reversal to synthetic phenotype is limited and not the immediate cause of decreased contractility.