STIMULATION OF CHLORIDE TRANSPORT BY CAMP IN RAT PROXIMAL TUBULES

We have previously demonstrated that formate and oxalate stimulate tra nscellular Cl- absorption (J(Cl)) in the rat proximal tubule by a mech anism involving DIDS-sensitive anion exchange across the luminal membr ane and diphenylamine-2-carboxylate (DPC)-sensitive Cl- channels in th e basolateral membrane. Recent evidence indicates cAMP activation of C l- channels in apical and basolateral membranes of proximal tubule cel ls. We therefore tested the effect of cAMP on Cl- and fluid transport in rat proximal tubule studied by luminal and capillary microperfusion in situ. The luminal perfusate contained 5 mM HCO3- and 145 mM Cl-, a nd the capillary perfusate contained 25 mM HCO3- and 110 mM Cl-, simul ating conditions in the late proximal tubule. Addition of 0.5 mM dibut yryl cAMP markedly stimulated fluid absorption (J(v)) and J(Cl). Simil ar effects resulted from addition of forskolin (10 mu M) to stimulate cAMP production. The increments in J(v) and J(Cl) due to dibutyryl cAM P were abolished when the Cl- channel blocker DPC (200 mu M) was added to the capillary perfusate but not when it was added to the lumen. Th e increments in J(v) and J(Cl) due to dibutyryl cAMP were unaffected b y luminal DIDS (100 mu M), which abolishes the increments in J(v) and J(Cl) induced by addition of oxalate. In contrast, the increments in J (v) and J(Cl) due to dibutyryl cAMP were abolished by luminal 5-nitro- 2-(3-phenylpropylamino)benzoate (NPPB; 10 mu M), another Cl- channel b locker. Luminal NPPB had no effect on baseline J(v) and J(Cl) nor on t he increments in J(v) and J(Cl) induced by addition of oxalate. In the absence of a Cl gradient (symmetrical NaCl concentration = 135 mM), c AMP was ineffective. Addition of 5 mu M oxalate to both intratubular a nd peritubular perfusates led to a doubling of J(Cl) and J(v) compared with control conditions. We conclude that 1) cAMP markedly stimulates J(v) and J(Cl) in the rat proximal tubule; 2) in the presence of a tr anstubular Cl- gradient cAMP activates passive transcellular J(Cl) via NPPB-sensitive Cl- channels in the apical membrane and via DPC-sensit ive Cl- channels in the basolateral membrane; and 3) cAMP and oxalate stimulate proximal tubule Cl- transport via different mechanisms.