Alternatively, the assay can be modified to be much more sensitive by using the proton channel Gramicidin D (Gra), a compound capable of highly efficient proton shuttling. The Gale group have predominantly used Gra instead of FCCP and found Gra to give a greater rate enhancement than FCCP for anion transporters with rate-limiting H + /OH -transport. However, Gramicidin D is also capable of sodium cation transport, so the NaX (100 mM) intra-and extravesicular salts must be switched to N-methyl-Dglucamine (NMDG) salts. The highly hydrophilic nature of the NMDG + cation has led to it being used as a common alternative in the HPTS transport assay, as it neither aids nor hinders anion transport due to the inability of Gra to transport the bulky counterion.