Plasma membrane associated metabolic parameters and the aging of human diploid fibroblasts.

Substrate uptake, portions of the cyclic AMP system, membrane fluidity and cellular phospholipid content are some of the parameters which are structurally associated with the plasma membrane and which have been linked to the control of cell proliferation. These parameters were studied with respect to cellular aging of human embryo lung fibroblasts (HELF) in culture. We observed in late passage an increase in the rate of uridine transport and in cellular cyclic AMP levels. These results were examined in relation to the increase in cell volume which occurs in senescing HELF. We also observed an increase in Vmax of uridine transport, and a decrease in the Km of cyclic AMP phosphodiesterase (PDE) as quiescent, passage 18-25, HELF were stimulated to divide with fresh serum. A similar effect of serum occurred in late passage (p. 43) cells despite the inability of these late passage cultures to undergo further proliferation. There was no change in cAMP-PDE activity with increasing passage number suggesting that the observed alterations of the cAMP levels, basal and in response to extracellular effectors, were due to alterations in the adenyl cyclase system. We observed no change in senescent HELF in membrane fluidity or phospholipid and neutral fat content.