Alpha and beta types of carbonic anhydrase-rich cells in turtle bladder

Abstract

The carbonic anhydrase-rich (CA) cell population of the turtle urinary bladder, which is responsible for the secretion of H+ and probably of HCO-3, was studied by freeze-fracture and thin-section electron microscopy. The apical membrane of the major CA cell type (alpha type) was characterized by microplicae and by a coat of studs on its cytoplasmic side; on freeze-fracture, it contained a dense population of rod-shaped intra-membrane particles. When fixed at low CO2 tension, the apical membrane area of the alpha cell was reduced; its surface displayed microplicae as well as microvilli, and the apical cytoplasm contained many vesicles with rod-shaped particles and studs. The apical membrane of the other (beta type) CA cell was characterized by numerous individual microvilli without microplicae and by a relative absence of rod-shaped particles and studs. Instead, the beta cell contained studs and rod-shaped particles in its basolateral membrane. The ultrastructure and frequency of the beta CA cell were not affected by changes in CO2 tension. We suggest that the alpha cell is responsible for H+ secretion. The reversal of the polarity of the membrane elements in the beta cell and failure to respond to CO2 with amplification of its apical membrane are consistent with a role in HCO-3 secretion.

Collecting Duct Intercalated Cell Function and Regulation

“…Moreover, it became clear that intercalated cells lacked a central cilium, at least in the cortex, which also differentiated them from the adjacent principal cells (Figure 3) (10,(28)(29)(30). Their peculiar morphology was reminiscent of acid-secreting cells in the turtle bladder (31,32) and frog skin (33), and, like these cells, intercalated cells participate in urinary acid secretion, bicarbonate reabsorption, and bicarbonate secretion (5).…”

Cellular composition and ultrastructure of the gill epithelium of larval and adult lampreys

“…This mechanism is dependent on the activities of both cytosolic carbonic anhydrase in the H + secreting cell and Na + /K + -ATPase in the basolateral membrane of the cell whose apical membrane contains the ENac (Harvey and Ehrenfeld, 1986;Harvey et al, 1988;Nagel and Dörge, 1996;Ehrenfeld and Klein, 1997). Furthermore, the toad and turtle urinary bladders, in which the epithelium has a cellular composition comparable with that of amphibian skin and consists also of granular cells and mitochondria-rich (MR) cells (Wade et al, 1975;Wade, 1976;Rick et al, 1978;Durham and Nagel, 1986;Brown and Breton, 1996), employ essentially the same mechanisms for reabsorbing Na + from the urine and for secreting H + into that urine (Stetson and Steinmetz, 1985;Durham and Nagel, 1986;Lang, 1988). In addition, the immunocytochemical demonstration that H + -ATPase and ENac are present in the gill epithelium of teleosts (Sullivan et al, 1995;Wilson et al, 2000a;Marshall, 2002) is consistent with the hypothesis that this mechanism for taking up Na + from a hypotonic environment is universal amongst vertebrates and invertebrates such as crustaceans, annelids and molluscs (Kirschner, 1983).…”

Ontogeny of Cell Type-Specific Enzyme Reactivities in Kidney Collecting Ducts

“…Further structural-functional specialization of intercalated cells into type A (acidifying) and B (alkalinizing) has been proposed recently on the basis of discoveries in the intracellular polarization of proton secretion in respect to the apex or base of these cells (13,14). However, little is known of the ontogeny of principal and intercalated cells or of the emergence of cell typespecific functions of collecting ducts, although this knowledge would provide better understanding of the unique functional features of the maturing kidney.…”