STIMULATION OF CHLORIDE TRANSPORT BY CAMP IN RAT PROXIMAL TUBULES

We have previously demonstrated that formate and oxalate stimulate transcellular Cl- absorption (J(Cl)) in the rat proximal tubule by a mechanism involving DIDS-sensitive anion exchange across the luminal membrane and diphenylamine-2-carboxylate (DPC)-sensitive Cl- channels in the basolateral membrane. Recent evidence indicates cAMP activation of Cl- channels in apical and basolateral membranes of proximal tubule cells. 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-, and the capillary perfusate contained 25 mM HCO3- and 110 mM Cl-, simulating conditions in the late proximal tubule. Addition of 0.5 mM dibutyryl cAMP markedly stimulated fluid absorption (J(v)) and J(Cl). Similar effects resulted from addition of forskolin (10 mu M) to stimulate cAMP production. The increments in J(v) and J(Cl) due to dibutyryl cAMP 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. The increments in J(v) and J(Cl) due to dibutyryl cAMP were unaffected by 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 blocker. Luminal NPPB had no effect on baseline J(v) and J(Cl) nor on the increments in J(v) and J(Cl) induced by addition of oxalate. In the absence of a Cl gradient (symmetrical NaCl concentration = 135 mM), cAMP was ineffective. Addition of 5 mu M oxalate to both intratubular and 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 transtubular Cl- gradient cAMP activates passive transcellular J(Cl) via NPPB-sensitive Cl- channels in the apical membrane and via DPC-sensitive Cl- channels in the basolateral membrane; and 3) cAMP and oxalate stimulate proximal tubule Cl- transport via different mechanisms.