Ionic mechanisms for bumetanide enhancement on aminoglycoside ototoxicity: a study with MDCK cells

Abstract

It is well known that loop diuretics such as bumetanide dramatically enhances ototoxicity of aminoglycosides (AG) when they are administered together to mammalians including humans. The mechanism(s) to cause such enhancement remain poorly understood. MDCK cells share many membrane properties with cochlear marginal cells and have been used to investigate the AG‐permissive channel mechanisms. Using gramicidin‐perforated patch‐clamp techniques and fluorescent gentamicin (GTTR) imaging analysis, we found: 1) MDCK uptake of GTTR was concentration‐dependently enhanced by bumetanide and furosemide; the enhancement was suppressed by La3+, the Cl−‐channel blocker N‐phenylanthranilic acid (PAA) or extracellular low Cl− (60 mM) but not by TEA, 4‐AP or glipizide. 2) La3+ (5 mM) reduced input conductance in all cells tested with a net current reversal potential of ~−10 mV. 3) Bumetanide hyperpolarized MDCK cells for 10.5 mV from a mean of −32.1 (p<0.01). 4) Bumetanide caused an increase in slope conductance of the whole‐cell I/V curve (from ~0.2 to ~1.5 nS at −60 mV). Bumetanide‐induced net current I/V curve reversed its polarity at ~ −45 mV. 5) The membrane action of bumetanide was suppressed by PAA and low Cl but not by FAA, NFA, 4AP, TEA, glipizide, La3+ or Gd3. We conclude: (1) Bumetanide enhances the La3+‐sensitive AG‐uptake by MDCK cells; (2) The enhancement is likely due to bumetanide‐induced hyperpolarization thus an increased driving force for cationic AG influx; (3) Bumetanide causes the hyperpolarization via its activation of a PAA‐sensitive Cl−‐channel; (4) The Cl− equilibrium potential is a main source of membrane potential of the MDCK cells we used. Funded by NIDCD R01 04716 (ZGJ) R01 04555 (PSS) and P30 05983.