MECHANISMS OF FLUID SECRETION BY POLYCYSTIC EPITHELIA

We have sought to determine the mechanisms driving fluid secretion by the cystic epithelium in autosomal dominant polycystic kidney disease (ADPKD). We have performed in vitro experiments on intact cysts dissec ted from discarded ADPKD kidneys, on monolayers of cells cultured from the cystic epithelium and on microcysts clonally derived from single cultured cells. These preparations absorb fluid in the control state b ut secrete fluid in response to native cyst fluid, to adenylate cyclas e agonists and to permeant analogues of cAMP. Measurements of short-ci rcuit current and transepithelial voltage in the monolayers indicate t hat anion secretion must drive the fluid secretion. Fluid secretion by the intact cysts was inhibited by basolateral application of ouabain but not by apical application. The effect of ouabain on fluid secretio n and short-circuit current in the monolayers followed the same patter n. Thus the functional Na,K-ATPase enzyme complex is located only in t he basolateral membrane of the cystic cells and serves to maintain the transmembrane chemical and electrical gradients that drive anion secr etion by other transport mechanisms. Fluid secretion and short-circuit current in the cultured monolayers was inhibited by the basolateral a pplication of the Na-K-2Cl cotransporter inhibitors, bumetanide and fu rosemide, and by apical application of the chloride channel blocker, d iphenylamine-2-carboxylate (DPC). These data suggest that chloride is the anion that is actively secreted. Preliminary experiments utilizing the monolayers and the microcysts and measuring cell chloride concent ration and chloride efflux across the apical membrane support this con clusion. Other preliminary data indicate that the cystic fibrosis tran smembrane conductance regulator is present in the apical membrane. Thu s active chloride transport generates fluid secretion by the cystic ep ithelium.