Segmental heterogeneity in the biochemical properties of the Na+-K+-ATPasealong the intestine of the gilthead seabream (Sparus aurata L.)

The activity of the Na+-K+-ATPase along the intestinal mucosa of the gilthe ad seabream has been examined. Under optimal assay conditions, found at 35 degreesC, pH 7.5, 2-5 mM MgCl2, 5 mM ATP, 10 mM K+ and 200 mM Na+, maximal Na+-K+-ATPase activities were found in the microsomal fraction of pyloric c aeca (PC) and anterior intestine (Al), which were more than two-fold the ac tivity measured in the microsomes from the posterior intestine (PI). Na+-K-ATPase activities from PC, AI and PI displayed similar pH dependence, opti mal Mg2+ /ATP and Na+/K+ ratios, affinities for Mg2+ and ATP, and inhibitio n by vanadate. However, considerable differences regarding sensitivity to o uabain, inhibition by calcium and responses to ionic strength were observed between segments. Thus, Na+-K+-ATPase activity from the Al was found to be ten-fold more sensitive to ouabain and calcium than the enzyme from the PC and PI and displayed distinct kinetic behaviours with respect to Na+ and K + compared to PC and PI. Analysis of the data from the AI revealed the pres ence of two Na+-K+-ATPase activities endowed with distinguishable biochemic al characteristics, suggesting the involvement of two different isozymes. R egional differences in Na+-K+-ATPase activities in the intestine of the gil thead seabream are compared with literature data on Na+-K+-ATPase isozymes and discussed on the basis of the physiological differences between intesti nal regions.