COUPLING OF CELL-VOLUME AND MEMBRANE-POTENTIAL CHANGES TO FLUID SECRETION IN A MODEL OF RENAL CYSTS

Renal tubular epithelia ordinarily absorb NaCl and water, although rec ent evidence indicates that renal cysts secrete fluid. We have utilize d the experimental advantages offered by cultured cysts, formed in a c ollagen matrix by propagating Madin-Darby canine kidney cells, to inve stigate the mechanisms involved in fluid secretion by this renal epith elium. The rate of fluid transport (adduced from changes in cavity vol ume), cell volume and changes in membrane potential were measured simu ltaneously in isolated cysts. Under basal conditions, cysts absorbed f luid (-0.83 +/- 0.34 x 10(-6) ml/min/cm(2) cavity surface area, N = 23 ). AVP and IBMX changed the direction of net fluid transport to secret ion (4.24 +/- 0.49 x 10(-6) ml/min/cm(2)). Cell volume initially fell 7.4 +/- 0.5% and remained stable thereafter as secretion continued. Me mbrane electrical potential (bis-oxonol epifluorescence) hyperpolarize d in 13 cysts and depolarized in 6, the mean change was 1.9 +/- 3.1%. Fluid secretion was abolished by 0.1 mM ouabain. Secretion was not aff ected by 0.1 mM DIDS and cell pH (bis-carboxyethyl-carboxyfluorescein epifluorescence) was not altered by the induction of secretion, sugges ting that secretion is not dependent on Cl-HCO3 exchange. Barium, in t he presence of AVP and IBMX, depolarized the cell membrane potential ( bis-oxonol fluorescence increased 22.3 +/- 0.03%), reversed secretion to absorption (from 3.21 +/- 0.93 to -1.52 +/- 0.61 x 10(-6) ml/min/cm (2)), and increased cell volume 2.7 +/- 0.5%. Bumetanide (100 mu M) re duced fluid secretion from 4.49 +/- 1.23 to -0.75 +/- 0.55 x 10(-6) ml /min/cm(2), further reduced cell volume 4.4 +/- 1.2% and hyperpolarize d the membranes (bis-oxonol fluorescence fell 24.3 +/- 5.0%). In the a bsence of AVP and IBMX bumetanide had no effect on fluid transport, ce ll volume or membrane potentials. We conclude that AVP reversed the di rection of fluid transport in these cultured renal epithelial cysts fr om absorption to secretion by stimulating a coordinated interaction of basolateral and apical K, Cl and Na transport mechanisms.