The effects of acute gentamicin application on hair cells isolated from the frog semicircular canals have been tested by using the patch-clamp technique in the whole-cell configuration. Extracellular gentamicin (1 mM) mostly affected the Ca2+ macrocurrent, ICa, and the Ca-dependent K+ current, IKCa. The drug, applied to the hair cell basolateral membrane through a fast perfusion system, produced a rapid and relevant decrease (∼34%) of ICa amplitude, without apparently affecting its activation–deactivation kinetics. The IKCa component of the delayed IKD was similarly affected: peak and steady-state mean amplitudes were significantly reduced, by about 47 and 54%, respectively, whereas the time constant of the mono-exponential current rising phase did not change. The Ca2+ independent fraction of IKD, IKV, and the fast IA current were unaffected. Transduction channels (permeable to and blocked by gentamicin) are not available in the isolated hair cell, so the effect of intracellular gentamicin was tested by applying the drug through the patch pipette (1 mM in the pipette): again, it significantly reduced both ICa and IKD amplitude, without affecting currents kinetics. IA properties were also unaffected. The drug did not affect the onset and removal of IKD inactivation, although the changes were scaled to the reduced IKD amplitude.
From these observations, it is expected that hair cells exposed to gentamicin ‘in vivo’ become unresponsive to physiological stimulation (block of the transduction channels) and transmitter release at the cytoneural junction be drastically depressed due to reduced Ca2+ inflow. In particular, functional impairment ensues much earlier than biochemical events that lead to hair cell apoptosis.