NACL TRANSPORT AND ULTRASTRUCTURE OF OPERCULAR EPITHELIUM FROM A FRESH-WATER-ADAPTED EURYHALINE TELEOST, FUNDULUS-HETEROCLITUS

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 b ut less numerous than in SW-acclimated tissues. Opercular epithelia mo unted in vitro with FW bathing the mucosal surface take up Cl- against a large negative inside transepithelial potential (V-t, grand mean -6 4.1 mV) and concentration gradient; the observed flux ratio was signif icantly different (P < 0.001) from that predicted for passive ion dist ribution but the net flux was consistently negative. The Na+ flux rati o suggested that Na+ was passively distributed. V-t was largely a Nadiffusion potential, based on unilateral manipulations of [Na+]. Cl- u nidirectional uptake was unaffected by mucosally added SITS (0.1 mM) b ut was inhibited by SCN- (1.0 mM) and by anaerobiosis. Killifish trans ferred from SW to FW for 48 h had reduced Cl- secretion by the opercul ar epithelium compared to SW controls but had not yet developed Cl- up take, indicating a slow adaptive process for development of Cl- absorp tive transport. Opercular epithelia of FW adapted fish, if bathed with isotonic saline on both sides, has a modest net Na+ and Cl- uptake, u nlike 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. (C) 1997 Wiley-Liss, Inc.