Mechanisms of Regulatory Volume Decrease in Collecting Duct Cells

The mechanisms of cell volume regulation upon osmotic cell swelling were examined in the inner stripe of the outer medullary collecting duct (OMCDi). Segments of OMCDi were dissected from rabbit kidney and perfused in vitro. Using an image processing system, the cross-sectional area of tubule cells was monitored as an index of the relative cell volume. In response to a decrease in extracellular osmolality (290 to 190 mOsm), the tubule cells swelled promptly and restored gradually their original cell volume by a mechanism termed regulatory volume decrease (RVD). The initial response of RVD (the rate of the decrease in cell volume during the first 10 min) was inhibited by 79% at a high concentration of basolateral K+ (50 mM). By contrast, the same concentration of luminal K+ did not affect the response. When basolateral Cl- was removed 30 min before the experiment, the initial response of RVD was decreased by 77%, whereas the response was not affected 30 min after removal of luminal Cl-. Addition of basolateral Ba2+ and basolateral anthracene-9-CO2H inhibited the response by 70 and 65%, respectively. RVD response was accompanied by a transient rise in intracellular Ca2+. The Ca2+ rise was abolished when intracellular Ca2+ was chelated by acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). In this condition, the initial RVD response was decreased by 68%. Our data suggest that the exit of basolateral K+ and Cl- via conductive pathways mainly mediates the RVD response in rabbit OMCDi cells, and that intracellular Ca2+ is involved in this response.