Rate-dependent blockade of a potassium current in human atrium by the antihistamine loratadine

1 The antihistamine loratadine is widely prescribed for the treatment of symptoms associated with allergies. Although generally believed to be free of adverse cardiac effects, there are a number of recent reports suggesting that loratadine use may be associated with arrhythmias, in particular atrial arrhythmias. 2 Nothing is known regarding the potassium channel blocking properties of loratadine in human cardiac cells. Using the whole-cell patch clamp technique, the effects of loratadine on the transient outward K current (I-to), sustained current (I-sus), and current measured at -100 mV (I-Kl and I-ns), the major inward and outward potassium currents present in human atrial myocytes, were examined in order to provide a possible molecular mechanism for the observed atrial arrhythmias reported with loratadine use. 3 Loratadine rate-dependently inhibited I-to at therapeutic concentrations with 10 nM loratadine reducing I-to amplitude at a pacing rate of 2 Hz by 34.9 +/- 6.0%. In contrast, loratadine had no effect on either I-sus or current measured at -100 mV. 4 These results may provide a possible mechanism for the incidences of supraventricular arrhythmias reported with the use of loratadine.