Expression of acid-base-related proteins in mesonephric kidney of the rabbit

The mesonephric kidney, precursor to the metanephric kidney, comprises 30- 50 nephrons, each with a glomerulus and proximal, distal, and collecting tubules. Although two different cell types have been identified in the mesonephric collecting tubule, no relationship to cells of the metanephric collecting duct has been established. To characterize expression of some of the acid-base-related proteins, we assayed for carbonic anhydrase (CA) activity and performed immunocytochemistry in mesonephroi from 15- to 20- day-old fetal rabbits. From total RNA, we detected expression of CA II and CA IV mRNA. Microdissected proximal and collecting tubules abundantly expressed both CA II and CA IV, at least to the extent observed in mature metanephric proximal tubules and collecting ducts. Histochemistry confirmed the expression of CA activity in these segments; in the collecting tubule, 28% of the collecting tubule cells were CA rich. Most CA-rich cells showed apical H⁺-ATPase and basolateral band 3 anion exchanger staining consistent with the findings in mature H⁺-secreting (α) intercalated cells of the metanephric collecting duct. CA-negative cells could be labeled with an antibody that identifies mature metanephric principal cells. Thus the mesonephric collecting tubule has many cells resembling mature α- intercalated cells and a majority of cells resembling principal cells. The similarity to the metanephric collecting duct suggests that the lineages of metanephric α-intercalated and principal cells may be closely related to those of the mesonephros.