It is well known that acid/base disturbances modulate proton/bicarbonate transport in the cortical collecting duct. mRNA and protein large quantity in this nephron segment. There was a concomitant increase of basolateral BEZ235 AE1 and α-cell number. Intercalated cell proliferation did not seem to play a role in the adaptation to acidosis. Alkali loading for 6-20 h after acidosis doubled the bicarbonate secretory flux and reduced proton secretion. Pendrin and AE1 expression patterns returned to control levels demonstrating that adaptive changes by intercalated cells are rapidly reversible. Thus regulation of intercalated cell anion exchanger expression and distribution plays a BEZ235 key role in adaptation of the cortical collecting duct to perturbations of acid/base. secretion in CCDs taken from acidotic rabbits 4 or after acid incubation.3 5 In addition there is a loss of apical anion exchange in β-intercalated cells and a reduction of the apical membrane that binds peanut agglutinin.3 Recent studies suggest that the extracellular matrix (ECM) protein hensin is deposited in the fact a third of such adapting cells not only lost apical anion exchange but established basolateral anion exchange suggesting a reversal in functional polarity. Such an acid-induced insertion/activation of basolateral anion exchangers has also been observed by Merot isomerase) activity that block hensin secretion 7 inhibit acid-induced changes in intercalated cell physiology.8 Hensin is expressed in many epithelial cells and serves to induce a differentiated phenotype 9 suggesting that differentiation might be difficult to reverse once the matrix is laid down. BEZ235 It would follow that reversal of the BEZ235 acidosis might be associated with a significant delay in adaptive changes of intercalated cells. Accordingly we attempted to reverse acidosis by abruptly changing the diet of the rabbits and examined changes in cell physiology transport and phenotype. Would such ‘terminally BEZ235 differentiated’ intercalated cells respond rapidly to the correction of acidosis? The purpose of this study was to characterize in rabbits the changes in pendrin and AE1 distribution expression and synthesis and in bicarbonate transport in response to acid loading and then to determine what occurs within 12-16 h of NaHCO3 administration when acidosis has been rapidly reversed. Results Transitioning from acid to alkali loading as a model for recovery from acidosis In the rabbit CCD the adaptation to acidosis entails compensatory changes in H+/HCO3 transport by α- and β-intercalated cells respectively that is regulated in part by ETB receptor signaling12 and changes in the composition of ECM hensin predominantly surrounding β-intercalated cells.3 Because it Slc3a2 has been suggested that ECM hensin and galectin-3 mediate signals that promote acquisition of a terminally differentiated epithelial cell phenotype 9 10 it would seem that once hensin is deposited in the ECM a cell could not rapidly de-differentiate. Accordingly we sought to develop an model in which we could identify the parameters associated with reversible adaptive changes in intercalated cell phenotypes that define the response of α- and β-intercalated cells to perturbations in acid/base status. In this study we have compared the intercalated cell phenotypes in the CCD of normal rabbits with those from rabbits administered BEZ235 NH4Cl for 3 days (acidosis) versus rabbits administered NH4Cl for 3 days and abruptly transitioned to NaHCO3 for 12-18 h (recovery). As shown in Physique 1 normal rabbits fed an alkaline ash diet showed serum bicarbonate levels between 25 and 30 mmol/l (lower panel) with an alkaline urine (pH 8.1±0.1 upper panel) whereas NH4Cl loading induced noticeable acidosis characterized by reduction of serum bicarbonate to 15-16 mmol/l (lower panel) and acidification of urine to pH below 5 (upper panel). In rabbits transitioned from NH4Cl to NaHCO3 (recovery) the serum bicarbonate returned to essentially ‘normal’ levels (normal vs recovery; transport elicited by acidosis and the presence of mitotic cells in the CCD.1 In this study we have examined sections from rabbit small intestine and kidney for.