The human Ca2+-activated K+ channel, IK, can be blocked by the tricyclic antihistamine promethazine

Phenothiazines can be used as psychopharmaceutical agents and are known to cause many side effects during treatment since they interfere with many different cellular systems. Recently, phenothiazines were reported to block Ca2+-activated potassium channels of the SK type. Therefore we investigated their effect on the functionally related class of Ca2+-activated potassium channels of the IK type. The representative phenothiazine derivative promethazine (PTZ) blocked IK channels almost independently from the extracellular pHo with an IC50 of 49 ± 0.2 μM (pHo 7.4, n = 5) and 32 ± 0.2 μM (pHo 6.2, n = 5) in whole cell experiments. The extracellularly applied membrane impermeable PTZ analogue methyl-promethazine (M-PTZ) had a strongly reduced blocking potency compared to PTZ. In contrast, intracellularly applied PTZ and M-PTZ had the same blocking potency on IK channels in excised inside out patch clamp experiments (Kd = 9.3 ± 0.5 μM for PTZ, n = 7 and 6.7 ± 0.4 μM for M-PTZ, n = 5). The voltage dependency of the PTZ and M-PTZ block was investigated in excised inside out patch clamp experiments at a concentration of 100 μM. For both compounds the block was more pronounced at positive membrane potentials. The steepness of the voltage dependency was found to be 70 ± 10 mV (for PTZ) and 61 ± 6 mV (for M-PTZ) indicating that both compounds sensed approximately 40% of the entire membrane spanning electrical field from the inside. We conclude that PTZ and M-PTZ bind to a side in IK channels, which is located within the electrical field and is accessible from the intracellular side.