Human fibroblasts shrink and are unable to recover their initial volume when incubated in hypertonic saline solutions, whereas an efficient volume restoration takes place in hypertonic media containing substrates of the highly concentrative transport system A (amino acids and methylamines). Amino acid substrates of barely concentrative transport systems are ineffective in sustaining the volume recovery. The activity of system A increases following incubation of fibroblasts under conditions promoting cell shrinkage and decreases upon cell swelling. These results stress the role of system A in the regulation of cell volume. Adaptive changes in the activity of system A are also induced by cell starvation and refeeding (adaptive regulation). Cell starvation is associated with cell shrinkage, while cell refeeding is accompanied by cell swelling. It is suggested that nutritional regulation and cell volume control by system A may be part of a common regulatory network.

The cell-to-medium distribution ratios at steady state of L-arginine (RArg) and of the lipid-soluble cation tetraphenylphosphonium (RTPP) were studied as a function of the membrane potential (Em) in adult human fibroblasts. The relationship between RArg and Em was qualitatively similar to that of RTPP and Em. Quantitatively, RArg and RTPP differed in that 1) RTPP was much greater than RArg when Em was near zero, indicating a significant binding component in the uptake of TPP+ but not of L-arginine, and 2) after a correction for binding when Em is near zero, RTPP was still greater than RArg so that RT/F . ln RTPP exceeded RT/F . ln RArg by 10-25 mV. The pattern of the redistribution of accumulated TPP+ and arginine after an alteration of Em was identical. In null-point experiments, the external [K+] for which there were no changes in cellular TPP+ or L-arginine in the presence of high valinomycin (the null points) were very similar for the two probes. Em calculated from the null-point measurements (-70(-)-80 mV) was also very similar to RT/F . ln RArg and thus smaller than RT/F.ln RTPP. It was concluded that 1) there was an additional TPP+ binding as cellular [TPP] rose in response to more negative membrane potentials, 2) the transport system for L-arginine in these cells (system y+) operates as a facilitated diffusion system driven by the membrane potential, and 3) in some circumstances, L-arginine could be employed as a probe of Em.

The net influx of L-arginine (JARG) was employed as an indicator of the membrane potential in human fibroblasts. Cell depolarization, obtained by increasing [K+]out, decreased both JARG and the net influx of the lipid soluble cation tetraphenylphosphonium (JTPP), a probe of membrane potential. JTPP, but not JARG, was influenced by the mitochondrial potential and exhibited a component dependent on intracellular and/or extracellular binding. JARG was sensitive to changes in the membrane potential induced by Na+-dependent transport of L-proline or by the activity of Na+-K+-ATPase. In the presence of 50 microM valinomycin, JARG was markedly influenced by the distribution ratio of K+ in a range of [K+]out from 1.5 to 100 mM. In this range of [K+]out, membrane potential (Em) varied from -90 to -23 mV, and calibration of JARG vs. the membrane potential yielded a linear relationship. These results indicate the following: 1) that the net influx of TPP+ is not a reliable indicator of membrane potential in cultured human fibroblasts; 2) that in the same cells the net influx of L-arginine can be employed as an index of membrane potential; 3) that in a range of Em from -23 to -90 mV the activity of system y+ (the membrane agency devoted to L-arginine transport in cultured human fibroblasts) exhibits no saturation of potential-dependent activation of transport.

No abstract

An enhancement of sugar transport is among the most characteristic biochemical markers of cellular transformation.' In the case of amino acid transport, an increased uptake has been reported for virus-and chemically transformed cell lines, but not for cells derived from spontaneously occurring tumors.2 In any case, it was not possible to correlate variations in amino acid transport to the expression of a specific oncogene.Here we report amino acid and sugar transport in mouse 3T3 cells transformed by ras or neu oncogenes.Oncogenes were integrated in the genomic DNA by transforming 3T3 mouse cells with Harvey sarcoma virus (line XHT), or with DNA fragments from human bladder carcinoma3 (line EJ) and from rat neuroblastoma cells4 (line 8104). The activity of