I am wondering whether it makes a difference to use Na-ATP, K-ATP or Mg-ATP, if the composition of all ions in the solution (on paper) is the same. Some people seem to use combination of several types of ATP salt in their internal solution. Why?
I appreciate any insight.

The main question that you need to ask yourself is what do I want to do with the ATP?
I used ATP-Mg in order to stimulate the association of Heat Shock Chaperone complexes. All the others would not work for me.
Maybe one of our members would be able to tell us why?
Guy

It depends on if the channel that you are recording inhibited by internal divalent Cations. If for example you are studying MIC (Mg2+-inhibited cation) channels (part of the TRP family of channels), you may not want to have Mg in the pipette or have precise control over the final Mg conc in the pipette and so in the latter case you would have a mix of MgATP and NaATP or K-ATP in the pipette solution.

Otherwise, the accompanying cation with ATP that you choose is entirely up to you depending on how you want to balance your ionic compositions, osmolarity etc.

Thank you for your replies.
I am studying KATP channels which are sensitive to Mg-ATP. I started off using internal solution containing MgCl2 and Na-ATP thinking that the ions will dissociate and would have the same effect. However, I got some unexpected results and started wondering whether my assumption had been wrong.
I guess my specific question is whether there is any possibility of these ATP salts remaining as salts in solution so that they would have differential effects.

Have you read this paper?

MgATP activates the ß cell KATP channel by interaction with its SUR1 subunit

1. Fiona M. Gribble,
2. Stephen J. Tucker,
3. Trude Haug*, and
4. Frances M. Ashcroft†

http://www.pnas.org/content/95/12/7185.full