Electrolyte and fluid secretion by cultured human inner medullary collecting duct cells

Inner medullary collecting ducts (IMCD) are the final nephron segments through which urine flows. To investigate epithelial ion transport in human IMCD, we established primary cell cultures from initial (hIMCD(i)) and terminal (hIMCD(t)) inner medullary regions of human kidneys. AVP, PGE(2), and forskolin increased cAMP in both hIMCD(i) and hIMCD(t) cells. The effects of AVP and PGE2 were greatest in hIMCD(i); however, forskolin increased cAMP to the same extent in hIMCD(i) and hIMCD(t). Basal short-circuit current (I(SC)) of hIMCD(i) monolayers was 1.4 +/- 0.5 microA/cm2 and was inhibited by benzamil, a Na+ channel blocker. 8-Bromo-cAMP, AVP, PGE(2), and forskolin increased I(SC); the current was reduced by blocking PKA, apical Cl- channels, basolateral NKCC1 (a Na+ - K+ - 2Cl- cotransporter), and basolateral Cl-/HCO(3)(-) exchangers. In fluid transport studies, hIMCD(i) monolayers absorbed fluid in the basal state and forskolin reversed net fluid transport to secretion. In hIMCD(t) monolayers, basal current was not different from zero and cAMP had no effect on I(SC). We conclude that AVP and PGE2 stimulate cAMP-dependent Cl- secretion by hIMCD(i) cells, but not hIMCD(t) cells, in vitro. We suggest that salt secretion at specialized sites along human collecting ducts may be important in the formation of the final urine.