Starting with 0.1 mg of lyophilized exendin-4:

0.1 mg =1 x 10-4 g

1 x 10-4 g /(4186.7 g/mol) = 2.39 x 10-8 moles

10 nM = 1 x 10-8 mol/L

2.39 x 10-8 mol/(1 x 10-8 mol/L) = 2.39 L

i.e. dissolve 0.1 mg of protein in 2.39 L of buffer or medium.

If you do not need such a large volume of such a dilute solution, dissolve it in a smaller volume to make a concentrated stock solution, and use that to prepare the volume of the more dilute solution that you need. For example, make a 1000X stock solution by dissolving the protein in 2.39 ml.

Thanks for your help Dr Adam

But which buffer can I use to reconstitute the vial?

If you bought the protein, there should be instructions on how to prepare it e.g add 1 mL water to powder to get 1 mg/mL concentration. This is what we usually do when we buy recombinant BSA or GroEL. There are usually salts in the powder, so there is usually no need to reconstitute in buffer. You should check with the manufacturer on how to prepare the sample.