LONG-TERM ACCLIMATION OF THE TELEOST OREOCHROMIS-MOSSAMBICUS TO VARIOUS SALINITIES - 2 DIFFERENT STRATEGIES IN MASTERING HYPERTONIC STRESS

Laboratory-reared tilapia (Oreochromis mossambicus) were long-term acc limated to freshwater (FW), brackish water (BW 10 parts per thousand s alinity), seawater (SW, 35 parts per thousand salinity) and two hypers aline media (45 and 60 parts per thousand salinity). We examined the i nfluence of these ambient salinities on the density (D-cc) and diamete r (d(cc)) of DASP-MI-stained chloride cells and on the capacity for el ectrogenic Cl- secretion of the in vitro opercular epithelium. To prov ide a characterisation of Cl- secretion, transepithelial potential dif ference (PDte), conductance (G(te)) and short-circuit current (I-sc) w ere measured after mounting the respective epithelium in an Ussing-cha mber. The cellular electromotive forces (E(c)) and conductances (G(c)) as well as the leak conductances (G(1)) were obtained from G(te):I-sc plots. In the salinity range between FW and SW both D-cc and d(cc) in creased. All electrophysiological parameters recorded increased in par allel, indicating a strong enhancement of the capacity for Cl- secreti on on the cellular and epithelial level. In the salinity range above S W a further increase of D-cc was observed. However, despite a higher c oncentration gradient across the body surface of the tilapia during ac climation to hypersaline media, the short-circuit current (I-sc) was n ot significantly different compared to SW preparations. This reflects proportional decreases of G(c) and increases of E(c), respectively. Of particular interest, we found a strong decrease of the leak conductan ce (G(1)) in preparations from tilapia acclimated to hypersaline media compared to those from SW fish, indicating that the tight junctions b ecome less permeable.