The peptide ”FRCRCFa”, dialysed intracellularly, inhibits the Na/Ca exchange in rabbit ventricular myocytes with high affinity

 We investigated the effect in rabbit ventricular myocytes of ”FRCRCFa”, a newly developed peptide inhibitor of the Na/Ca exchange. Myocytes were whole-cell patch clamped and experiments were carried out at 36°C. The Na/Ca exchange was measured selectively, by blocking interfering ion channel currents and the Na/K pump, as the membrane current which could be inhibited by 5 mM nickel (Ni; a known blocker of the Na/Ca exchange). Increasing concentrations of FRCRCFa dialysed into the cell from the patch-pipette inhibited the Na/Ca exchange current. The dose/response curve could be fitted by a function for co-operative ligand binding, which predicted a K_D for FRCRCFa-mediated inhibition of 22.7 ± 3.7 nM, with a Hill coefficient of 0.61 ± 0.06. Pipette FRCRCFa concentrations of 1 μM and above were sufficient to cause complete inhibition of Na/Ca exchange current. The inhibitory effect of FRCRCFa was independent of membrane potential and relatively selective: 10 μM FRCRCFa dialysed into the cell had no effect on the L-type Ca current and delayed rectifier and inward rectifier K currents. Thus FRCRCFa appears to be a potent and relatively selective inhibitor of the Na/Ca exchange in intact cardiac myocytes, and may be of value for studies of the Na/Ca exchange. Received: 9 July 1996 / Received after revision: 18 September 1996 / Accepted: 20 September 1996