The role of K+ and Cl- conductances in chloride secretion by the opercular epithelium

The opercular epithelium of the teleost Fundulus heteroclitus, when mounted in a lucite chamber under short-circuited conditions, secretes Cl- at a rate equivalent to the short-circuit current (Isc). The transepithelial Na+ movements are passive and proceed by a paracellular pathway. The addition of 2 X 10(-3) M Ba2+ to the serosal bathing solution inhibited the Isc 76.8% with no effect on the transepithelial conductance (Gt). The addition of 5 X 10(-4) M Cu2+ to the mucosal bathing solution inhibited the Isc 79.6% and reduced the Gt 35.6%. These inhibitory effects of Ba2+ and Cu2+ on the Isc were initiated within 1 minute after exposure with maximum effects occurring within 20 and 30 minutes, respectively. Simultaneous 36Cl- and 22Na+ unidirectional fluxes were performed on paired epithelia from the same fish. Serosal Ba2+ and mucosal Cu2+ inhibited the Cl- secretory flux 30.2 and 58.9%, respectively. The resulting net Cl- flux after inhibition was not significantly different from the mean measured Isc. Neither ion had significant effects on the Cl- influx (mucosa to serosa) or the unidirectional Na fluxes. These results indicated that the effects of both Ba2+ and Cu2+ were most likely exclusive to the transcellular Cl- pathway. Ba2+ is proposed to inhibit Cl- secretion by blocking the basolateral K+ channels, depolarizing the cell, and reducing the electrochemical driving force for Cl- across the apical membrane. Cu2+ is proposed to inhibit Cl- secretion by blocking the apical membrane Cl- channels.(ABSTRACT TRUNCATED AT 250 WORDS)