Mibefradil alters intracellular calcium concentration by activation of phospholipase C and IP receptor function.

Mibefradil is a tetralol derivative originally developed as an antagonist of T-type voltage-gated calcium (Ca) channels to treat hypertension when used at nanomolar dosage. More recently, its therapeutic application in hypertension has declined and has been instead repurposed as a treatment of cancer cell proliferation and solid tumor growth. Beyond its function as a Ca blocker, the micromolar concentration of mibefradil can stimulate a rise in [Ca] although the mechanism is poorly known. The chanzyme TRPM7 (transient receptor potential melastanin 7), the release of intracellular Ca pools, and Ca influx by ORAI channels have been associated with the increase in [Ca] triggered by mibefradil. This study aims to investigate the cellular targets and pathways associated with mibefradil's effect on [Ca]. To address these questions, we monitored changes in [Ca] in the specialized mouse epithelial cells (LS8 and ALC) and the widely used HEK-293 cells by stimulating these cells with mibefradil (0.1 μM to 100 μM). We show that mibefradil elicits an increase in [Ca] at concentrations above 10 μM (IC around 50 μM) and a fast Ca increase capacity at 100 μM. We found that inhibiting IP receptors, depleting the ER-Ca stores, or blocking phospholipase C (PLC), significantly decreased the capacity of mibefradil to elevate [Ca] Moreover, the transient application of 100 μM mibefradil triggered Ca influx by store-operated Ca entry (SOCE) mediated by the ORAI channels. Our findings reveal that IPR and PLC are potential new targets of mibefradil offering novel insights into the effects of this drug.