Validation of high throughput screening assays against three subtypes of Ca(v)3 T-type channels using molecular and pharmacologic approaches.

T-type Ca(2+) channels encoded by voltage-gated Ca(2+) channel (Ca(v)) 3.1, 3.2, and 3.3 genes play important physiological roles and serve as therapeutic targets for neurological and cardiovascular disorders. Currently there is no selective T-channel blocker. To screen for such a blocker, we developed three stable cell lines expressing human recombinant Ca(v)3.1, 3.2, or 3.3 channels and then examined their usefulness in high throughput screens. All three cell lines displayed an increase in intracellular Ca(2+) in response to changes in extracellular Ca(2+) as detected with Ca(2+)-sensitive dyes using a fluorometric imaging plate reader (FLIPR [Molecular Devices, Sunnyvale, CA] or FlexStation [Molecular Devices]). The signal-to-noise ratio was 2-4. Co-expression of Ca(v)3.2 with a mouse leak K(+) channel, which by virtue of being open at rest hyperpolarizes the cell membrane, blocked the fluorescent signal. Co-addition of KCl to these cells induced a Ca(2+) signal that was similar to that observed in the cell line expressing Ca(v)3.2 alone. These results confirm that the detection of intracellular Ca(2+) increase in cells expressing Ca(v)3.2 alone results from Ca(2+) entry through channels that are open at the resting membrane potential of each cell line (i.e., window currents). Testing known drugs on Ca(v)3 channels showed that block could be reliably detected using the FlexStation assay, FLIPR assay, or voltage clamp recordings using the IonWorks HT system (Molecular Devices). These results support the use of the FLIPR window current assay for primary drug screening and high throughput patch recordings for secondary screening of novel T-channel blockers.