Cl^- secretory effects of EBIO in the rabbit conjunctival epithelium

Experiments were conducted to determine whether the Cl^- secretagogue, 1-ethyl-2-benzimidazolinone (EBIO), stimulates CP transport in the rabbit conjunctival epithelium. For this study, epithelia were isolated in an Ussing-type chamber under short-circuit conditions. The effects of EBIO on the short-circuit current (I_sc) and transepithelial resistance (R _t) were measured under physiological conditions, as well as in experiments with altered electrolyte concentrations. Addition of 0.5 mM EBIO to the apical bath stimulated the control I_sc by 64% and reduced R _t by 21% (P < 0.05; paired data). Under Cl^--free conditions, I_sc stimulation using EBIO was markedly attenuated. In the presence of an apical-to-basolateral K⁺ gradient and permeabilization of the apical membrane, the majority of the I_sc reflected the transcellular movement of K⁺ via basolateral K ⁺ channels. Under these conditions, EBIO in combination with A23187 elicited nearly instantaneous 60-90% increases in I_sc that were sensitive to the calmodulin antagonist calmidazolium and the K⁺ channel blocker tetraethyl ammonium. In the presence of an apical-to-basolateral CP gradient and nystatin permeabilization of the basolateral aspect, EBIO increased the CP-dependent I_sc, an effect prevented by the channel blocker glibenclamide (0.3 mM). The latter compound also was used to determine the proportion of EBIO-evoked unidirectional ³⁶Cl^- fluxes in the presence of the Cl^- gradient that traversed the epithelium transcellularly. Overall, EBIO activated apical CP channels and basolateral K⁺ channels (presumably those that are Ca²⁺ dependent), thereby suggesting that this compound, or related derivatives, may be suitable as topical agents to stimulate fluid transport across the tissue in individuals with lacrimal gland deficiencies.