Physiological and Intracellular pH Response of Cells Subjected to Environmental Changes
 

Pinar Ozkan, Bhargavi Garapati, Raj Mutharasan
Drexel University
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Philadelphia, PA, 19104
pinar@drexel.edu
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Large-scale cultivation processes using animal cells as hosts for production of biotechnological products such as antibodies and hormones are generally based on results obtained from small-scale tests. Such systems subject cells to different environmental conditions, which result in physiologic response different from small systems such as T-flasks. Intracellular pH (pHi) is one of the indicators of energy status and metabolic state of the cells. Intracellular pH and NAD(P)H levels of Spodoptera frugiperda (Sf9) cells are monitored to study the metabolic response when cells are subjected to changes in shear rate, extracellular pH, substrate (sucrose) concentration and dissolved oxygen levels. Upon exposure to sublethal levels of shear, Sf9 cells were found to exhibit cytosolic acidification, the magnitude of which depended on the magnitude of imposed hydrodynamic shear (5-8 dynes/cm2). Under similar shear rates intracellular NAD(P)H levels increased and oxygen uptake rates decreased, indicating a possible increase in anaerobic metabolism. Studies in pH shift (6.0-6.6) showed that intracellular pH followed extracellular pH very closely. The change in pHi was very similar to the level of change in extracellular pH. In a like manner, increase in sucrose level was followed by an overall acidification of cells up to 0.15 pH units for 54 g/l sucrose. Dissolved oxygen (DO) level had no significant immediate effect on pHi of the cells under the test conditions. Although NAD(P)H levels were affected clearly by DO changes, no change in pHi was observed.