Mammary protein synthesis is acutely regulated by the cellular hydration state.

The effect of cell-volume pertubations on mammary tissue protein synthesis has been examined. Cell-swelling, induced by a hyposmotic shock, increased the rate of incorporation of radiolabelled leucine and methionine into trichloroacetic acid precipitable material. The incorporation of radiolabel under both isosmotic and hyposmotic conditions was inhibited by cycloheximide. The increases in mammary protein synthesis as a result of cell-swelling may be attributable to an increase in casein synthesis. Conversely, cell-shrinking, as a consequence of a hyperosmotic challenge, almost abolished mammary protein (casein) synthesis. The finding that cell-volume pertubations had no significant effect on steady-state casein mRNA levels suggests that the regulation, within the time course of the experiments, is at the level of translation. The results strongly suggest that mammary cell volume may be an important cellular signal in the control of mammary protein synthesis in general and casein synthesis in particular.