Effects of phenylalkylamines and benzothiazepines on Cav1.3-mediated Ca2+ currents in neonatal mouse inner hair cells

Calcium currents (ICa) in inner hair cells (IHCs) are carried by the Cav1.3 subtype of L-type calcium channels. They play an important role in synaptic transmission of sound-evoked mechanical stimuli. L-type calcium channels are targets of the organic blocker classes dihydropyridines, phenylalkylamines and benzothiazepines. Previously a low sensitivity of the Cav1.3 subtype towards dihydropyridines has been demonstrated. Therefore, this study evaluates the effect of two phenylalkylamines (verapamil and gallopamil) and the benzothiazepine diltiazem on ICa through Cav1.3 channels in mouse IHCs. Whole-cell ICa was measured using the patch-clamp technique in mouse IHCs aged postnatal day 3–7 with 5 mM calcium as a charge carrier. The phenylalkylamines verapamil and gallopamil and the benzothiazepine diltiazem inhibited ICa in IHCs in a concentration-dependent manner. This block was largely reversible. Dose–response curves revealed IC50 values of 199 ± 19 μM for verapamil, 466 ± 151 μM for gallopamil and 326 ± 67 μM for diltiazem. The inhibition of peak ICa by phenylalkylamines and benzothiazepines was voltage-independent. Verapamil (300 μM) enhanced current inactivation from − 20 to + 20 mV while diltiazem (300 μM) did so only at very depolarised potentials (+ 20 mV). In conclusion, the concentrations of phenylalkylamines and benzothiazepine necessary to inhibit 50% of ICa in IHCs were one order larger compared to concentrations which inhibited ICa through Cav1.2 channels in native cells or expression systems. However, inhibitory concentrations were in the same range as those required for block of ICa in turtle hair cells.