Control of membrane permeability by external and internal ATP in 3T6 cells grown in serum-free medium.

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Abstract
Cultures of mouse embryofibroblast 3T6 cells were plated in serum-free medium and grown in the presence of insulin (1 .mu.g/ml) and epidermal growth factor (0.5 ng/ml). External ATP (250 .mu.M) applied to such cultures caused a rapid efflux of acid-soluble pools labeled with [3H]uridine, 2-deoxy[3H]glucose, or 86Rb+ and allowed the entry of p-nitrophenylphosphate. This increase in passive membrane permeability depended on ATP concentration, pH and time of ATP contact with the cells, and it was not produced by GTP, UTP or Pi. In the presence of compounds that decrease intracellular ATP, low concentrations of external ATP (40 .mu.M) caused a massive synergistic stimulation of efflux. The efflux of acid-soluble pools was stopped (sealing) by bringing the cultures of 3T6 cells to neutral pH in the presence of Ca2+ and Mg2+. Exposure of 3T6 cells grown in serum-free medium to [.gamma.-32P]ATP under the conditions of permeabilization led to the selective labeling of a membrane protein with a MW of 44,000 as revealed by NaDodSO4 polyacrylamide gel electrophoresis and autoradiography. The control of membrane permeability by ATP apparently is completely independent of serum-derived proteins. The protein band (MW 44 .times. 103) that shows selective labeling by [32P]ATP during permeabilization is not an adsorbed serum component.