PROSTAGLANDIN-E2 INHIBITS SODIUM-TRANSPORT IN RABBIT CORTICAL COLLECTING DUCT BY INCREASING INTRACELLULAR CALCIUM

The mechanism by which prostaglandin E2 (PGE2) inhibits sodium absorption (J(Na)) in the rabbit cortical collecting duct (CCD) was explored. PGE2 activates at least three signaling mechanisms in the CCD: (a) by itself PGE2 increases cAMP generation (b) PGE2 also inhibits vasopressin-stimulated cAMP accumulation, and (c) PGE2 raises intracellular calcium ([Ca++]i). We tested the contribution of these signaling pathways to PGE2's effect on Na+ absorption, measuring Na-22 flux (J(Na)) and [Ca++]i (using fura-2) in microperfused rabbit CCDs. In control studies PGE2 reduced J(Na) from 28.2 +/- 3.4 to 15.6 +/- 2.6 pmol.mm-1.min-1. Lowering bath calcium from 2.4 to 45 nM did not by itself alter J(Na) but in this setting PGE2 failed to inhibit J(Na) (28.6 +/- 5.4 to 38.5 +/- 4.0). In separate tubules, PGE2 raised [Ca++]i in a spike-like fashion followed by a sustained elevation. However, in 45 nM bath Ca++, PGE2 failed to produce a sustained [Ca++]i elevation. While pretreatment of CCDs with pertussis toxin blocked PGE2 inhibition of vasopressin-stimulated water permeability, it did not block the effect of PGE2 on J(Na). To see if cAMP generation contributes to the effect of PGE2 on J(Na), we tested the effect of exogenous cAMP, (8-chlorophenylthio(CPT)cAMP) on J(Na). 0.1 mM 8-CPTcAMP reduced J(Na) from 35.75 +/- 2.3 to 21.6 +/- 2.2. However, the addition of PGE2 further blunted J(Na) to 15.9 +/- 1.3. In CCDs pretreated with indomethacin, 8-CPTcAMP did not significantly decrease J(Na) 33.6 +/- 2.8 vs. 28.4 +/- 2. However, superimposed PGE2 reduced J(Na) to 19.0 +/- 3.0. We conclude that PGE2 inhibits sodium transport predominantly by increasing intracellular calcium. This action is not mediated by a pertussis toxin-sensitive G protein. Finally, cAMP, through a cyclooxygenase-dependent mechanism, also inhibits CCD J(Na) and may contribute to the effects of PGE2 on J(Na) in the rabbit CCD.