Ondansetron blocks wild-type and p.F503L variant small-conductance Ca²⁺-activated K⁺ channels

Apamin-sensitive small-conductance Ca²⁺-activated K⁺ (SK) current ( I_KAS) is encoded by Ca²⁺-activated K⁺ channel subfamily N ( KCNN) genes. I_KAS importantly contributes to cardiac repolarization in conditions associated with reduced repolarization reserve. To test the hypothesis that I_KAS inhibition contributes to drug-induced long QT syndrome (diLQTS), we screened for KCNN variants among patients with diLQTS, determined the properties of heterologously expressed wild-type (WT) and variant KCNN channels, and determined if the 5-HT₃ receptor antagonist ondansetron blocks I_KAS. We searched 2,306,335 records in the Indiana Network for Patient Care and found 11 patients with diLQTS who had DNA available in the Indiana Biobank. DNA sequencing discovered a heterozygous KCNN2 variant (p.F503L) in a 52-yr-old woman presenting with corrected QT interval prolongation at baseline (473 ms) and further corrected QT interval lengthening (601 ms) after oral administration of ondansetron. That patient was also heterozygous for the p.S38G and p.P2835S variants of the QT-controlling genes KCNE1 and ankyrin 2, respectively. Patch-clamp experiments revealed that the p.F503L KCNN2 variant heterologously expressed in human embryonic kidney (HEK)-293 cells augmented Ca²⁺ sensitivity, increasing I_KAS density. The fraction of total F503L-KCNN2 protein retained in the membrane was higher than that of WT KCNN2 protein. Ondansetron at nanomolar concentrations inhibited WT and p.F503L SK2 channels expressed in HEK-293 cells as well as native SK channels in ventricular cardiomyocytes. Ondansetron-induced I_KAS inhibition was also demonstrated in Langendorff-perfused murine hearts. In conclusion, the heterozygous p.F503L KCNN2 variant increases Ca²⁺ sensitivity and I_KAS density in transfected HEK-293 cells. Ondansetron at therapeutic (i.e., nanomolar) concentrations is a potent I_KAS blocker. NEW & NOTEWORTHY We showed that ondansetron, a 5-HT₃ receptor antagonist, blocks small-conductance Ca²⁺-activated K⁺ (SK) current. Ondansetron may be useful in controlling arrhythmias in which increased SK current is a likely contributor. However, its SK-blocking effects may also facilitate the development of drug-induced long QT syndrome.