Kleine, Teri J.

Eosinophil peroxidase (EPO), a cationic protein found in eosinophils, has been reported to be cytotoxic independent of its peroxidase activity. This study investigated with electrophysiological methods whether EPO is toxic to mammalian urinary bladder epithelium. Results indicate that EPO, when added to the mucosal solution, increases apical membrane conductance of urinary bladder epithelium only when the apical membrane potential is cell interior negative. The EPO-induced conductance was concentration dependent, with a maximum conductance of 411 μS/cm2 and a Michaelis-Menten constant of 113 nM. The EPO-induced conductance was nonselective for K+ and Cl−. The conductance was partially reversed using voltage but not by removal of EPO from the bulk solution. Mucosal Ca2+reversed the EPO-induced conductance by a mechanism involving reversible block of the conductance. Prolonged exposure (up to 1 h) to EPO was toxic to the urinary bladder epithelium, as indicated by an irreversible increase in transepithelial conductance. These results suggest that EPO is indeed toxic to urinary bladder epithelium via a mechanism that involves an increase in membrane permeability.

The eosinophil granule protein major basic protein (MBP) is toxic to a wide variety of cell types, by a poorly understood mechanism. To determine whether the action of MBP involves an alteration in membrane permeability, we tested purified MBP on rabbit urinary bladder epithelium using transepithelial voltage-clamp techniques. Addition of nanomolar concentrations of MBP to the mucosal solution caused an increase in apical membrane conductance only when the voltage across the apical membrane was cell interior negative. The magnitude of the MBP-induced conductance was a function of MBP concentration, and the rate of the initial increase in conductance was a function of the transepithelial voltage. The MBP-induced conductance was nonselective for K+ and Cl−. Mucosal Ca2+ reversed the induced conductance, whereas mucosal Mg2+partially blocked the induced conductance and slowed the rate of the increase in conductance. The induced conductance was partially reversed by changing the voltage gradient across the apical membrane to cell interior positive. Prolonged exposure resulted in an irreversible loss of the barrier function of the urinary bladder epithelium. These results suggest that an increase in cell membrane ion permeability is an initial step in MBP-induced loss of barrier function.