The osteoglossid kidney: correlations of structure, function, and metabolism with transition to air breathing

Abstract

The cell composition, ultrastructure, ion-activated ATPase content, and metabolic enzyme activities of the kidneys in water- and air-breathing osteoglossids were compared. In both, the nephron consisted of three main segments. The first segment contained a well-developed brush border, numerous lysosomes, numerous mitochondria localized primarily in the basal two-thirds of the cell, and many apical tubules. The most striking feature of the second segment was a highly infolded basilar membrane, laying in close laminar association with large, elongate mitochondria, localized to the basal two-thirds of the cell. Particularly in Arapaima, cells of the third segment formed the longest homogeneous region of the nephron. In both species, the ultrastructure of the third segment was remarkably similar to that of the teleost chloride cell. The kidney in Osteoglossum, a water breather, was divided into head and trunk regions with nephrons only in the latter; in the air breather, the kidney was three- to four-fold larger than in aruana, and it was not divided into head and trunk regions, nephrons being found throughout its length. Total Ca2+-activated ATPase and Na+/K+-ATPase activities were, respectively, some 3- and 15-fold higher in Arapaima kidney than in aruana kidney, implying a much larger fraction of the ion transport work has been taken over by the kidney in the air breather. Glutamate dehydrogenase was found to be the primary ammonia-releasing reaction in both species. It occurred at about sevenfold higher activities in Arapaima kidney than in aruana. The increased potential for ammoniagenesis in Arapaima kidney appeared to correlate with an increased gluconeogenic capacity without which, it is postulated, the acid–base balancing function of ammonia production would be lost.