reversed changes in surface membrane conformation and polysome profile induced by the transfer of actively growing Balb/c 3T3 fibroblasts from a serum-containing to a serum-free medium. Morphometric analysis of polysome profiles revealed a 94% aggrega-

By scanning and transmission electron microscopy we have shown that insulin rapidly reversed changes in surface membrane conformation and polysome profile induced by the transfer of actively growing Balb/c 3T3 fibroblasts from a serum-containing to a serum-free medium. Morphometric analysis of polysome profiles revealed a 94% aggregation of total f ribosomes during logarithmic growth. This figure fell to 78% after 18 h of serum starvation. The number of f ribosomes per unit area of cytoplasm also fell. 1 h of insulin treatment restored aggregation to 92 % and increased the number of f ribosomes per unit area of cytoplasm by 22 %. Scanning electron microscopy of logarithmically growing cells revealed an abundance of surface microviUi, whereas serum starvation promoted a smooth surface with few microvilli. After 1 h of insulin treatment, microvilli reappeared with a distribution and subcellular organization characteristic of exponential growth. This study shows the combined and rapid effect of insulin on the regulation of polysome formation and the promotion of a specific surface membrane conformation in cultured cells. The observations are consistent with the knowledge that insulin, acting on the surface membrane, can influence such parameters as membrane transport, and the rates of protein and R N A synthesis. I N T R O D U C T I O N Transferring logarithmically growing Balb/c 3T3 fibroblasts to a serum-free medium modifies rates of various cellular processes including decreases in the rates of protein and RNA synthesis, reduction in the rate of uridine, leucine, and glucose uptake, and an increase in the rate of protein degradation (1, 2). After 18-20 h of serum starvation the addition of insulin or fresh serum to the medium rapidly restores the rates of the above reactions to values associated with the exponential cell growth (1). The integrated manner in which these reactions are regulated by extraceUular factors led to the hypothesis that they are coordinated by a common intracellular regulatory mechanism (1). Furthermore, since sepharosebound insulin elicits responses similar to those brought about by the unconjugated hormone, it was assumed that the growth-promoting reTHE JOURNAL OF CELL BIOLOGY • VOLUME 61, 1974 " pages 95-106 95 on Jne 6, 2017 D ow nladed fom Published April 1, 1974