The interaction of ionophore A23187 with the uridine uptake system of quiescent and serum-activated hamster fibroblasts

We have studied the effect of the divalent metal ion ionophore A23187 on uridine uptake by quiescent Nil 8 cells in the presence and absence of dialysed serum and MgCl₂. In the absence of serum at low (50 μM) Mg²⁺ concentration, treatment of the cells with the ionophore caused a dramatic inhibition of uridine uptake. This inhibition is due to an effect on phosphorylation of the nucleoside rather than its transport across the plasma membrane. The inhibition did not occur in the presence of dialysed serum or at high concentrations of MgCl₂. The protecting effect of Mg²⁺ is saturable and the concentration at half-maximum uptake rate is of the same order as the K_m for Mg²⁺ of the uridine-phosphorylating system as assayed in cell-free extracts. The inhibition is completely reversed by the addition of 10% serum to cultures which were treated with ionophore at low Mg²⁺ concentration. This activation occurs with no apparent lag even after 60 min exposure to the ionophore.