Neutral amino acid transport and in vitro aging.

Human fibroblasts possess five distinguishable systems for amino acid transport. Initial rates for three of these decline with in vitro age. The decline most noticeably affects the L system, whose rate for L-phenylalanine transport at 10 microM declines by 44% between the 27th and 39th population doubling levels. Accumulation of L-phenylalanine is similarly affected. The insensitivity of older cells to reduction of the amino acid concentration in culture medium casts doubt on possible interpretation of the transport decline as a causal factor in limiting growth.