Effects of rupatadine, a new dual antagonist of histamine and platelet-activating factor receptors, on human cardiac Kv1.5 channels

1 The effects of rupatadine, a new dual antagonist of both histamine H-1 an d platelet-activating factor receptors, were studied on human clotted hKv1. 5 channels expressed in Ltk(-) cells using the whale-cell patch-clamp techn ique. 2 Rupatadine produced a use- and concentration-dependent block of hKv1.5 ch annels (K-D = 2.4 +/- 0.7 mu M) and slowed the deactivation of the tail cur rents, thus inducing the 'crossover' phenomenon. 3 Rupatadine-induced block was voltage-dependent increasing in the voltage range for channel opening suggesting an open channel interaction. At potent ials positive to + 10 mV the blockade decreased with a shallow voltage-depe ndence. Moreover, rupatadine also modified the voltage-dependence of hKv1.5 channel activation, which exhibited two components, the midpoint of the st eeper component averaging -25.2 +/- 2.7 mV. 4 When the intracellular K+ concentration ([K+](i)) was lowered to 25% the voltage-dependent unblock observed at positive potentials was suppressed an d the activation curve in the presence of rupatadine did not exhibit two co mponents even when the midpoint of the activation curve was shifted to more negative potentials (-30.3 +/- 1.3 mV). 5 On channels mutated on the residue R485 (R485Y) which is located on the e xternal entryway of the pure the rupatadine-induced block did not decrease at potentials positive to + 10 mV. In contrast, on V512M channels rupatadin e reproduced all the features of the blockade observed on wild type channel s. 6 All these results suggest that rupatadine blocks hKv1.5 channels binding to an external and to an internal binding site but only at concentrations m uch higher than therapeutic plasma levels in man. Efflux of K+ promotes the unbinding from the external site. Furthermore, rupatadine binds to an inte rnal site and dramatically modifies the voltage-dependence of channel openi ng.