Dear Ian
As with the UV-probe for sodium there is preliminary indication that
the spectral characterisitcs of the probe differe markedly within the
cell environment from those expected from in vitro information.
With SBFI there is a marked shift in the wavelength dependencies, so
much so that  in some cell types (including cardiac cells and renal tubules)
at the lower UV-range where you would expect fluorescence  increases with
Na increases you see no fluorescence change, while the upper UV range
shows significant fluorescence decreases at the same time, hence allowing
ratiometric analyses. You would not expect this from in vitro spectral
data.  Something equally weird seems to occur with Sodium green such that
given the way you would look at it on the MRC-600 you may not see anything.
Sounds like its "Back to the cuvette and fluorimeter" to work out what is
happening in your loaded cells. The dye certainly loads into cardiac cells, but
resting images of renal tubules look much "brighter" and I'm sure that a lot of
this is due to a much higher resting [Na+].
As a start scan your loaded cells and then lyse them and look at what comes out in the spec. My guesss is the 488nm/530nm combination is less than ideal for
intracellular Sodium green.
 
Cheers,  David.
 
PS. Hope its going well in the US otherwise.