Thank you! THF was also mentioned by SIGMA and in some publications.
...by the way - it does dissolve in DMSO, if you heat it up carefully in a waterbath at 60-80°C for ~1 hour; 1.29 mM Nitrate Ionophore VI in 25 mM DMSO, which I used as a stock-solution subsequently. However, I am not sure how stable Ionophores are at higher temperatures or whether this treatment affects the activity or not. Does anyone know more about this?