Mutual potentiation by magnesium and calcium of growth in animal cells

Abstract

The effects on DNA synthesis of various combinations of Mg2+ and Ca2+ in cultures of chick embryo cells have been studied. When [Mg2+] larger than or equal to 0.24 mM, reduction of Ca2+ from the standard concentration of 1.72 mM to 0.01 mM had no effect on the incorporation of [3H]thymidine ([3H]dThd) into DNA over a 16-hr period. When Mg2+ was reduced to 0.04 mM, [3H]dThd incorporation into DNA decreased directly with [Ca2+] below 1.72 mM and increased slightly up to [Ca2+] = 5.02 mM, where cell damage began to occur. The change in [Ca2+] necessary to maintain a half-maximal rate of [3H]dThd incorporation was found to depend inversely on the fourth power of the change in [Mg2+]. Chelation of Ca2+ with approximately equimolar ethylene glycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) in the presence of [Mg2+] larger than or equal to 0.24 mM reduced [3H]dThd incorporation about 10-fold, and large excesses of EGTA did not further reduce it. The amount of EGTA required to produce a detectable inhibition of [3H]dThd incorporation was independent of [Mg2+] larger than or equal to 0.24 mM, as was the level of residual incorporation in excess EGTA. When [Mg2+] was reduced to 0.04 mM, however, [3H]dThd incorporation declined even when [EGTA] less than [Ca2+], and vanished when EGTA was in large excess. The results are discussed within the framework of a model for the regulation of cell metabolism and growth in which the availability of free Mg2+ is the central coordinating factor. The metabolic effects of Mg2+ depend on its distribution between elements such as ATP and binding sites on membranes. We propose that the major metabolic effects of varying [Ca2+] are produced indirectly through its competition with Mg2+ for membrane sites, thereby making more or less Mg2+ available for rate-limiting transphosphorylation reactions.