Action of 2,3-butanedione monoxime on capacitance and electromotility of guinea-pig cochlear outer hair cells

1. Whole-cell patch-clamp recordings were obtained from isolated cochlear outer hair cells (OHCs) while applying 2,3-butanedione monoxime (BDM) by pressure. BDM (5 mM) shifted the range of voltage sensitivity of membrane capacitance and cell length in the hyperpolarised direction by -49.6 ± 4.0 mV (n = 12; mean ± S.E.M.), without appreciable effects on membrane conductance. The shift was completely reversible and dose dependent, with a Hill coefficient of 1.8 ± 0.4 and a half-maximal dose of 3.0 ± 0.8 mM (values ± S.D). 2. The shift of the capacitance curve was also reproducible in cells whose natural turgor had been removed. BDM had no detectable effect on the capacitance of Deiters' cells, a non-sensory cell type of the organ of Corti. 3. The effect of BDM on membrane capacitance was faster than that of salicylate. At similar saturating concentrations (20 mM), the time constant of the capacitance changes was 1.8 ± 0.3 s (n = 3) for salicylate and 0.75 ± 0.06 s (n = 3) for BDM. The recovery periods were 13 ± 1 s and 1.7 ± 0.4 s, respectively (means ± S.E.M.). 4. The effect of BDM, a known inorganic phosphatase, was compared to the effects of okadaic acid, trifluoperazine and W-7, which are commonly used in studies of protein phosphorylation. Incubation of OHCs with okadaic acid (1 μM, 30-60 min) shifted the voltage sensitivity of the membrane capacitance in the hyperpolarised direction. Incubation with trifluoperazine (30 μM) and W-7 (150 μM) shifted it in the opposite, depolarised direction. BDM induced hyperpolarising shifts even in the presence of W-7. 5. Simultaneous measurement of membrane capacitance and intracellular free Ca²⁺ concentration ([Ca²⁺]_i) showed that BDM action on OHC voltage-dependent capacitance and electromotility is not mediated by changes of [Ca²⁺]_i. 6. Our results suggest that: (a) the effects of BDM are unrelated to its inorganic phosphatase properties, cell turgor conditions or Ca²⁺ release from intracellular stores; and (b) BDM may target directly the voltage sensor of the OHC membrane motor protein.