The novel Na+/Ca2+ exchange inhibitor KB-R7943 also blocks native and expressed neuronal nicotinic receptors

1 We studied the effects of the novel Na+/Ca2+ exchange inhibitor KB-R7943, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate, on the native nicotinic receptors present at the bovine adrenal chromaffin cells, as well as on rat brain alpha(3)beta(4) and alpha(7) nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. 2 As expected, KB-R7943 blocked the Na+-gradient dependent Ca-45(2+) uptake into chromaffin cells (IC50 of 5.5 mu M); but in addition, the compound also inhibited the Ca-45(2+) entry and the increase of cytosolic Ca2+ concentration, [Ca2+](c), stimulated by 5 s pulses of ACh (IC50 of 6.5 and 1.7 mu M, respectively). 3 In oocytes expressing alpha(3)beta(4) and alpha(7) nicotinic AChRs, voltage-clamped at -60 mV, inward currents elicited by 1 s pulses of 100 mu M ACh (I-ACh) were blocked by KB-R7943 with an IC50 of 0.4 mu M and a Hill coefficient of 0.9. 4 Blockade of alpha(3)beta(4) currents by KB-R7943 was noncompetitive; moreover, the blocker (0.3 mu M) became more active as the ACh concentration increased (34 versus 66% blockade at 30 mu M and 1 mM ACh, respectively). 5 Inhibition of alpha(3)beta(4) currents by 0.3 mu M KB-R7943 was more pronounced at hyperpolarized potentials. If given within the ACh pulse (10 mu M), the inhibition amounted to 33, 64 and 80% in oocytes voltage-clamped at -40, -60 and -100 mV, respectively. The onset of blockade was faster and the recovery slower at -100 mV; the reverse was true at -40 mV. 6 In conclusion, KB-R7943 is a potent blocker of nicotinic AChRs; moreover, it displays many features of an open-channel blocker at the rat brain alpha(3)beta(4) AChR. These results should be considered when KB-R7943 is to be used to study Ca2+ homeostasis in cells expressing nicotinic AChRs and the Na+/Ca2+ exchanger.