Positively charged cyclic hexapeptide Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) represents a group of conformationally constrained peptides that block the cardiac sarcolemma Na+-Ca2+ exchanger [Khananshvili et al. (1995b) J. Biol. Chem. 270, 16182-16188]. Here, we study the kinetic mechanisms of FRCRCFa-induced inhibition of Na+-Ca2+ exchange and its partial reaction, the Ca2+Ca2+ exchange. The Na(i)-dependent 45Ca uptake and Ca(i)-dependent 45Ca uptake were measured by adding the EGTA quench in the semirapid mixer. The reverse mode of Na+-Ca2+ exchange (Na(o)-dependent Ca efflux) was monitored (t = 10-5000 ms) in the stopped- flow machine by measuring extravesicular free calcium with a fluorescence probe fluo-3. Saturating concentrations of FRCRCFa inhibit completely the forward and reverse modes of exchange, suggesting that the inside-out vesicles contribute to most (if not all) of the exchange activity. A short time exposure (t = 10-20 ms) of FRCRCFa with the vesicles is enough to reach a rapid equilibrium between FRCRCFa and a putative inhibitory site at the extravesicular (cytosolic) side of the membrane. A lower limit for the second-order rate constant of FRCRCFa binding can be estimated as a k(on) of > 106 M-1 s-1. A possible competition between FRCRCFa and either Na+ or Ca2+ has been tested at the extravesicular (cytosolic) side of the membrane. At different extravesicular Ca(o) concentrations of 13-250 μM, FRCRCFa inhibits the Na+-Ca2+ and Ca2+-Ca2+ exchanges with an IC50 of 11-16 μM, suggesting no competition between FRCRCFa and Ca2+. At different extravesicular Na(o) concentrations of 40-160 mM, FRCRCFa inhibits Na(o)-dependent Ca efflux with an IC50 of 12-18 μM, suggesting that FRCRCFa and Na+ do not compete for binding at the extravesicular side. A mild proteolytic treatment of vesicles activates the Na(i)-dependent 45Ca uptake, but has a little effect on the FRCRCFa-induced inhibition. Thus, the 'inhibitory site' is still functional after the proteolytic treatment of the inside-out vesicles. In conclusion, a rapid (<20 ms) interaction of extravesicular (cytosolic) FRCRCFa with the exchanger prevents the ion translocation through the exchanger, while the inhibitory peptide does not interact with the ion transport sites of the exchanger at the cytosolic side of the membrane.