cAMP-dependent fluid secretion in rat inner medullary collecting ducts

We used an unambiguous in vitro method to determine if inner medullary coll ecting ducts (IMCD) have intrinsic capacities to absorb and secrete solutes and fluid in an isotonic medium. IMCD1, IMCD2, and IMCD3 were dissected fr om kidneys of young Sprague-Dawley rats. 8-Bromo-3',5'-cyclic monophosphate (8-BrcAMP) stimulated lumen formation and progressive dilation in all IMCD subsegments; lumen formation was greatest in IMCD1. Benzamil potentiated t he rate of lumen expansion in response to 8-BrcAMP. Fluid entered tubule lu mens by transcellular secretion rather than simple translocation of intrace llular fluid. Secreted lumen solutes were osmometrically active. Inhibition of protein kinase A with H-89 and Rp diastereomer of adenosine 3',5'-cycli c monophosphorothioate blocked fluid secretion. The rate of lumen expansion was reduced by the selective addition of ouabain, barium, diphenyl-2-carbo xylate, bumetanide, glybenclamide, or DIDS, or reduction of extracellular C l-. We conclude that IMCD absorb and secrete electrolytes and fluid in vitr o and that secretion is accelerated by cAMP. We suggest that salt and fluid secretion by the terminal portions of the renal collecting system may have a role in modulating the composition and volume of the final urine.