Ion balance,acid-base regulation,and chloride cell function in the common killifish,Fundulus heteroclitus—a euryhaline estuarine teleost |
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| Authors: | Chris M Wood William S Marshall |
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| Institution: | 1. Department of Biology, McMaster University, L8S 4K1, Hamilton, Ontario, Canada
2. Department of Biology, St. Francis Xavier University, B2G 2W5, Antigonish, Nova Scotia, Canada
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| Abstract: | The common killifish,Fundulus heteroclitus, is a euryhaline teleost common throughout estuaries of eastern North America. This symposium paper reviews the important contributions of the killifish to our present understanding of ionoregulation in seawater (SW) fish and their mechanisms of euryhalinity, and presents new data developing the killifish as a freshwater (FW) model system. Experiments on killifish have characterized (i) drinking in SW and its reduction in FW; (ii) the adaptive roles of the kidney to SW and FW conditions; (iii) the instantaneous (Phase I) and delayed (Phase II) reductions in Na+ outflux that occur upon transfer from SW to FW; (iv) the importance of prolactin secretion in the Phase II effect; (v) the cortisol-stimulated induction of branchial Na+, K+-ATPase that occurs upon transfer from FW to SW; (vi) the accompanying changes in morphology of the mitochondria-rich (MR) or “chloride cells” on the gills; (vii) the localization of this Na+, K+-ATPase activity to the basolateral membrane of chloride cells; and (viii) the NaCl-secretory function of these cells in SW. The opercular epithelium, which is rich in chloride cells, has been used as an in vitro model to characterize the mechanisms and control of NaCl secretion in SW fish. Much less is known about gill function in fresh water (inward NaCl transport), primarily due to the absence of a comparable freshwater model. Here we show that killifish acclimated to dilute FW (NaCl] = 1 mmol I?1) possess large numbers of MR cells on the opercular epithelium. When mounted in vitro with FW on the outside, the preparation develops a large inside negative transepithelial potential (TEP) that is a Na+ diffusion potential. By the Ussing flux ratio criterion, Na+ fluxes are passive, but a small active influx of Cl? occurs, an observation that supports the involvement of MR cells in active Cl? uptake. This FW opercular epithelium if bathed with isotonic saline on both sides does not secrete Cl?, indicating that the MR cells indeed are of the FW type. In vivo, the fish exhibits a high rate of Na+ influx and outflux; Cl? outflux is much lower, and there is no detectable Cl? influx. Experimental variation of FW NaCl] reveals a saturable, low affinity Na+ uptake mechanism, a Cl? influx mechanism that is activated only at much higher concentrations, and no evidence of exchange diffusion. Acid-base disturbance appears to be corrected by differential regulation of the outflux components only. Hence, the FW killifish ionoregulates somewhat differently from the few other FW teleosts that have been examined, and its opercular epithelium will serve as a very useful model system. |
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