Interactions between benzylamiloride and Fura-2:: Studies in vitro and in cardiac myocytes

Amiloride derivatives are commonly used inhibitors of Na+/H+- and Na+/Ca2+-exchange. Because they are fluorescent molecules the use of benzylamiloride (BZA), an inhibitor of Na+/Ga2+ exchange, in conjunction with Fura-2, a commonly used fluorescent Ca2+ indicator, might complicate interpretation of fluorescence data obtained. In vitro data show that BZA decreases the Fura-2 fluorescence at all useful wavelengths in a concentration-dependent manner. The Fura-2 ratio 340/380 (used to estimate intracellular Ca2+ ([Ca2+](in))) also decreased with increasing BZA concentrations. The Stern-Volmer relation suggests that this phenomenon is due to either static or dynamic quenching. Varying temperatures from 4 to 37 degrees C did not alter Stern-Volmer constants, consistent instead with fluorescence resonance energy transfer (FRET), The in situ relevance of these interactions was evaluated in adult rat cardiac myocytes which exhibit Na+/Ca2+ exchange reflected by rapid [Ca2+](in) increase following Nat removal. Pretreatment with BZA greater than or equal to 25 mu M decreased the magnitude of Fura-2 changes induced by Na+ removal. Analysis of the individual Fura-2 useful wavelengths indicated that greater than or equal to 25 mu M BZA altered the Fura-2 signal in a manner consistent with the quenching effects noted in vitro, Together, these data show that BZA interacts with Fura-2 in vitro and in situ and suggest caution when interpreting Fura-2 fluorescence data derived in conjunction with BZA, (C) 1998 Academic Press.