Although studies in mammalian cells and yeast suggest that Mg2+ plays an important role in cell growth and hormone response, intracellular roles of Mg2+ are poorly understood. Thus, we are developing methods to study Mg2+ regulation of growth and hormonal response. Preliminary data using cell-permeable Mg2+ indicators based on tropolone suggest the feasibility of the dynamic and selective determination of intracellular free Mg2+ concentration. "Mg2+-deficient" cell lines have also been developed. Murine S49 lymphoma cells in normal 0.8 mM Mg2+ medium double in 17 hours, but die when placed in 0.2 mM Mg2+ medium. Two classes of S49 clones have been isolated which grow in 30 microM Mg2+ with doubling times of 22 and 60 hours. Although total cell Mg2+ is decreased by 50%, the decrease is selective since cytoplasmic Mg2+ is decreased 75% while particulate Mg2+ is unchanged. Hormonal response in the Mg2+ -deficient cells is defective. Cyclic AMP accumulation in response to beta-adrenergic receptor activation is decreased more than 95%. In contrast, the Mg2+ -deficient cells lose only about 50% of their response to PGE1 receptor activation, retain 50% of their beta-receptors, and accumulate cyclic AMP in response to cholera toxin at the wild-type rate. Mg2+ transport also occurs at the wild-type rate, but with a slightly higher affinity and is no longer hormone-sensitive. Ca2+ content is normal or slightly high. T-lymphocytes isolated from rats made Mg2+ -deficient for 8 weeks give similar results, indicating that the Mg2+ -deficient S49 lymphoma cell clones are a good model for Mg2+ -deficiency. The data suggest that lack of Mg2+ causes growth abnormalities and leads to markedly altered receptor-G-protein coupling, but may have less effect on G-protein-adenylate cylase interaction.