Calcium and magnesium ions and the regulation of multiplication in normal and transformed cells

Abstract

THE relative importance of Ca²⁺ and Mg²⁺ in the regulation of cellular multiplication and alteration of that regulation by viral transformation has recently become a subject of considerable interest^1–5. Extracellular Ca²⁺ and Mg²⁺ both have effects on metabolic parameters and macromolecular synthesis in cultured cells. Mg²⁺, because of its substrate level intracellular concentration and its role in macromolecular synthesis and intracellular transphosphorylation reactions, has been suggested as a possible coordinate regulator of the positive pleiotropic response, a coordinated array of reactions associated with initiation of DNA synthesis and multiplication in cultured cells^6–9. Ca²⁺ on the other hand, has been suggested as a short term regulator of cell growth and function, largely on the basis of its tightly regulated low cytoplasmic concentration^10,11. In this report, we describe a kinetic analysis of the effects of extracellular concentrations of Ca²⁺ and Mg²⁺ on survival and multiplication of normal and transformed human cells in culture. We have found that cellular multiplication rate (number of population doublings per unit time) and also cellular ‘survival rate’ (percentage survival divided by time) can be substituted in place of reaction rate in classical enzyme kinetic formulae (Fig. 1). Both responses exhibit saturation kinetics, with a transition from first order response to zero order, as the concentration of calcium, magnesium, or any other essential component of the culture medium is increased. Rate plotted against concentration yields the expected right rectangular hyperbola described by the Henri–Michaelis–Menten formulation, and the expected linear relation when plotted by the Lineweaver–Burke or van Hofstee methods. Linear regression of the Lineweaver–Burke double reciprocal plot yields from its intercepts a maximal rate for multiplication or survival (equivalent to V_max in classical enzyme kinetics) and a Michaelis–Menten constant (K_m), equal to the substrate (or ligand) concentration yielding a half maximal rate of multiplication or survival.