DEVELOPMENTALLY REGULATED SECRETED FACTORS CONTROL EXPRESSION OF MUSCARINIC RECEPTOR SUBTYPES IN EMBRYONIC CHICK RETINA

Two molecular mass subtypes of muscarinic receptor are expressed by th e chick retina (72 and 86 kDa). During development, the ratio of subty pes changes, with the 72-kDa form becoming predominant. We have found that subtype switch can occur in retina cell culture, and have investi gated factors that influence this in vitro increase in the 72-kDa rece ptor. Increases similar to those in vivo occurred when cells were cult ured at 10(5) cells/cm2, but not at 10-fold lower density. High-densit y cultures, maintained on coverslips, showed no receptor development w hen transferred to large volumes of fresh medium, indicating that cell -cell contact alone was not responsible for induction. However, replac ement of fresh medium with conditioned medium (from high-density cultu res) resulted in normal induction. There were no morphological differe nces between cultures with high and low levels of the 72-kDa receptor. Conditioned medium also induced 72-kDa receptors in low-density cultu res, consistent with a minimal role for cell-cell contact. Efficacy of conditioned medium was markedly dependent on age. Media from cells cu ltured 1-4 days had no effect, but media from cells cultured 5-8 and 1 -8 days elicited 1.6-fold and fourfold increases in the 72-kDa subtype , respectively. The data indicate that maturing retina cells secrete d evelopmentally regulated factors that are necessary for abundant expre ssion of the 72-kDa muscarinic receptor subtype.