Measurement of Polyamine Efflux from Cells in Culture

The polyamines are cellular growth factors in most living organisms. Cells that are rapidly growing require high polyamine concentrations, whereas quiescent cells maintain comparatively low polyamine concentrations (1 ,2 ). This was shown to be the case in normal baby hamster kidney (BHK) fibroblasts grown to confluence (3 ). Here cells that were undergoing density-dependent inhibition of growth had significantly lower total polyamine content than cells in exponential growth. The question was, then, how did the cell decrease its polyamine content in response to the decreased growth rate? Analysis of the fate of radiolabeled polyamines showed that polyamines were released from quiescent cells into the extracellular medium (4 –6 ). The efflux was found to be specific for N ¹ -acetylspermidine and to be regulated in concert with the growth rate of the cell (3 ,7 ). Efflux is an integral part of the regulatory process responsible for controlling the intracellular polyamine content of cells. In addition, such factors as drugs (8 ,9 ), virus infection (10 ,11 ), and cell transformation (12 ) have all been found to alter the efflux of polyamines from cells in culture.