Fluoxetine inhibits K+ transport pathways (K+ efflux, Na+-K+-2Cl- cotransport, and Na+ pump) underlying volume regulation in corneal endothelial cells

We have studied regulatory volume responses of cultured bovine corneal endothelial cells (CBCEC) using light scattering. We assessed the contributions of fluoxetine (Prozac) and bumetanide-sensitive membrane ion transport pathways to such responses by determining KC efflux and influx. Cells swollen by a 20% hypoosmotic solution underwent a regulatory volume de crease (RVD) response, which after 6 min restored relative cell volume by 98%. Fluoxetine inhibited RVD recovery; 20 mu M by 26%, and 50 mu M totally. Fluoxetine had a triphasic effect on KC efflux; from 20 to 100 mu M it inhibited efflux 2-fold, whereas at higher concentrations the efflux first increased to 1.5-fold above the control value, and then decreased again. Cells shrunk by a 20% hyperosmotic solution underwent a regulatory volume increase (RVI) which also after 6 min restored the cell volume by 99%. Fluoxetine inhibited RVI; 20 mu M by 25%, and 50 mu M completely. Bumetanide(1 mu M) inhibited RVI by 43%. In a Cl--free medium, fluoxetine (50-500 mu M) progressively inhibited bumetanide-insensitive K+ influx. The inhibitions of RVI and K+ influx induced by fluoxetine 20 to 50 mu M Were similar to those induced by 1 mu M bumetanide and by CT-free medium. A computer simulation suggests that fluoxetine can interact with the selectivity filter of K+ channels. The data suggest that CBCEC can mediate RVD and RVI in part through increases in Kf efflux and Na-K-2Cl cotransport (NKCC) activity. Interestingly, the data also suggest that fluoxetine at 20 to 50 IJ mu M inhibits NKCC, and at 100-1000 mu M inhibits the Na+ pump. One possible explanation for these findings is that fluoxetine could interact with K+-selective sites in K+ channels, the NKC cotransporter and the Na+ pump.