Influence of medium exchange schedules on metabolic, growth, and GM-CSF secretion rates of genetically engineered...

The metabolic and secretory characteristics of NIH-3T3 fibroblasts transfected with a cDNA encoding human granulocyte-macrophage colony stimulating factor (GM-CSF) were examined as a function of the culture medium exchange schedule. The rates of glucose and glutamine consumption and of lactate and ammonia production were measured over exchange schedules ranging from complete medium replacement weekly (1/week) to complete medium replacement daily (7/week). All measured metabolic rates increased with increased medium exchange rates and accelerated sharply between exchange rates of 3.5/week and 7/week. The lactate/glucose and ammonia/glutamine yield coefficients, however, remained invariant at about 1.9 and 1.0 mol/mol, respectively, under all medium perfusion conditions. A shift-up in medium perfusion rates from 3.5/week to 7/week resulted in increased metabolic rates that resembled those observed in the cultures that were exchanged at the 7/week rate throughout, showing that the metabolic rates could be directly controlled by the perfusion rate. Differential regulation of medium versus serum perfusion demonstrated that increased NIH-3T3 cell metabolism was directly proportional to the serum flux to which the cells were exposed. Thus a limiting serum component is responsible for the altered metabolic and growth rates. The GM-CSF production by the transfected 3T3 cells was stable but exhibited substantial transient increases during periods of cell proliferation, demonstrating that the secretion of transfected gene products can be highly modulated even when the cDNA is driven from a constitutive promoter. These studies show that the metabolic and secretory behavior of genetically engineered cells is influenced by the medium exchange schedule.