NaCl transport and ultrastructure of opercular epithelium from a freshwater‐adapted euryhaline teleost, Fundulus heteroclitus

Abstract

We adapted killifish to defined freshwater (FW: 1.0 mM Na+, 1.0 mM Cl−, + 0.1 mM Ca2+) and by fluorescence light microscopy and scanning and transmission electron microscopy found that the opercular epithelium retained mitochondria rich (MR) cells that were significantly larger but less numerous than in SW-acclimated tissues. Opercular epithelia mounted in vitro with FW bathing the mucosal surface take up Cl− against a large negative inside transepithelial potential (Vt, grand mean −64.1 mV) and concentration gradient; the observed flux ratio was significantly different (P < 0.001) from that predicted for passive ion distribution but the net flux was consistently negative. The Na+ flux ratio suggested that Na+ was passively distributed. Vt was largely a Na+ diffusion potential, based on unilateral manipulations of [Na+]. Cl− unidirectional uptake was unaffected by mucosally added SITS (0.1 mM) but was inhibited by SCN− (1.0 mM) and by anaerobiosis. Killifish transferred from SW to FW for 48 h had reduced Cl− secretion by the opercular epithelium compared to SW controls but had not yet developed Cl− uptake, indicating a slow adaptive process for development of Cl− absorptive transport. Opercular epithelia of FW adapted fish, if bathed with isotonic saline on both sides, has a modest net Na+ and Cl− uptake, unlike SW opercular epithelium that strongly secretes Cl− under similar conditions. FW killifish opercular epithelium may provide a model to study ion regulation by euryhaline fish. J. Exp. Zool. 277:23–37, 1997. © 1997 Wiley-Liss, Inc.