Verbost P M, Schoenmakers T J, Flik G, Wendelaar Bonga S E
Department of Animal Physiology, Faculty of Science, University of Nijmegen, The Netherlands.
J Exp Biol. 1994 Jan;186:95-108. doi: 10.1242/jeb.186.1.95.
Plasma membranes of the gills of freshwater- and seawater-adapted tilapia were analyzed for Ca(2+)-ATPase and Na+/Ca2+ exchange activity. The relative importance of ATP-driven and Na(+)-gradient-driven Ca2+ transport in Ca2+ extrusion was evaluated on the basis of kinetic analyses in vitro. The Na+/Ca2+ exchangers in branchial membranes from freshwater or seawater fish displayed similar kinetics. The ATP-driven Ca2+ pump, however, showed a somewhat lower affinity for Ca2+ in membranes isolated from seawater gills than in membranes from freshwater gills; no difference in Vmax was found. The activity of the exchanger was estimated to be 50% of that of the ATP-driven pump at prevailing cytosolic Ca2+ concentrations (10(-7) mol l-1). Opercular ionocyte densities and branchial Na+/K(+)-ATPase content were not significantly different in fish residing in fresh water or sea water. We conclude that the gills of tilapia living for prolonged periods in fresh water or sea water do not differ in the make-up of their basolateral membrane with regard to Ca(2+)-ATPase, Na+/Ca2+ exchange and Na+/K(+)-ATPase activity. Apparently, the densities of these carriers suffice for calcium and sodium homeostasis under these vastly different ambient conditions.
对适应淡水和海水环境的罗非鱼鳃的质膜进行了钙（2 +）-ATP酶和钠/钙交换活性分析。基于体外动力学分析评估了ATP驱动和钠（+）梯度驱动的钙转运在钙排出中的相对重要性。淡水或海水鱼鳃膜中的钠/钙交换器表现出相似的动力学。然而，ATP驱动的钙泵对从海水鳃分离的膜中的钙的亲和力略低于从淡水鳃分离的膜中的钙；未发现最大反应速度（Vmax）有差异。在主要的胞质钙浓度（10（-7）mol l-1）下，交换器的活性估计为ATP驱动泵活性的50%。生活在淡水或海水中的鱼的鳃盖离子细胞密度和鳃钠/钾（+）-ATP酶含量没有显著差异。我们得出结论，长期生活在淡水或海水中的罗非鱼鳃在基底外侧膜的组成方面，在钙（2 +）-ATP酶、钠/钙交换和钠/钾（+）-ATP酶活性方面没有差异。显然，在这些截然不同的环境条件下，这些载体的密度足以维持钙和钠的体内平衡。
