Evaluation of Mg2+ as an intracellular regulator of uridine uptake

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Abstract
The rate of uptake of the nucleoside uridine increases within minutes after adding a growth stimulus to quiescent 3T3 cells. We have previously shown this uptake rate to be highly sensitive to changes in the intracellular concentration of Mg2+. In the present paper, the alteration of uptake by Mg2+ is shown to occur at the phosphorylation step — the same point at which serum acts to modulate uridine uptake. The serum stimulation of uridine uptake can be mimicked by Mg2+ alone or blocked by partially depleting cells of their Mg2+. Work with cell-free extracts shows that the uridine kinase enzyme responds to Mg2+ in a manner similar to that exhibited by whole cells whose concentrations of Mg2+ have been raised. In addition, the enzyme's inhibition by ATP is relieved by raising the Mg2+ concentration. Thymidine uptake, a reaction which does not respond quickly to mitogenic stimulation, is unaffected by alterations in Mg2+ concentration. These results are discussed in terms of a possible role for Mg2+ as an intracellular regulator of uridine uptake and other reactions of the coordinate response of cells to external effectors.