Measurement of cellular stimulation through monitoring pH changes by bead injection fluorescence microscopy

This work presents a method for extracellular and intracellular pH measurements in live cells based on a combination of the bead injection (BI) technique and fluorescence microscopy. For extracellular pH measurement, cells are grown on fluorescent beads, packed into a small column by a sequential injection instrument, and fluorescence intensity from the beads stained by the indicator is recorded by a fluorescence microscope. The method is applied to quantifying carbachol stimulation of Chinese hamster ovary (CHO) cells transfected with the m1 muscarinic receptor and is verified by a glucose depletion experiment. The results yield an EC50 value of 1 μM for carbachol, which is in reasonable agreement with the literature value 3 μM determined by an existing potentiometric technique for measuring acid release. The intracellular measurement utilizes CHO M1 cells growing on non-fluorescent beads. For this method the cells rather than the beads are stained by incubating them in a solution of the fluorescent pH indicator BCECF. The cells are also stimulated with carbachol and the intracellular pH dependent fluorescence from the cells is recorded. The results show dependence between intracellular pH changes and carbachol concentration and yield an EC50 value of 4 μM.