Pharmacological characterisation of the human small conductance calcium-activated potassium channel hSK3 reveals sensitivity to tricyclic antidepressants and antipsychotic phenothiazines

A stable CHO-K1 cell line was developed which expresses the human small con ductance calcium-activated potassium channel hSK3. Immunofluorescence micro scopy using an anti-SK3 antibody and radioligand binding using [I-125]-apam in demonstrated the presence of hSK3 channel in the recombinant cell line. This cell line was utilised in a fluorescence assay using the membrane pote ntial-sensitive dye DiBAC(4)(3) to functionally analyse and pharmacological ly characterise this potassium channel. The analysis of known blockers of c alcium-activated potassium channels revealed the highest potency for apamin (IC50=13.2 nM). This result was confirmed by direct recordings of SK3 curr ents using the whole-cell patch-clamp technique. Tricyclic antidepressants such as desipramine. imipramine and nortriptyline as well as phenothiazines such as fluphenazine, promethazine, chlorpromazine and trifluoperazine blo cked the hSK3 channel with micromolar potencies. These compounds also displ aced [I-125]-apamin binding to the hSK3 channel thus suggesting direct and competitive channel blocking activity. Since these compounds: share a commo n three-ring molecular core structure, this feature seems to be important f or channel blocking activity. The serine/threonine protein phosphatase inhi bitors okadaic acid and calyculin A were able to abolish channel activation with nanomolar potencies, but did not displace [I-125]-apamin binding. Thu s, phosphorylation of hSK3 or an accessory channel subunit seems to be invo lved in its modulation. (C) 2001 Elsevier Science Ltd. All rights reserved.