REGULATION OF HCO3- ABSORPTION BY PROSTAGLANDIN E(2) AND G-PROTEINS IN RAT MEDULLARY THICK ASCENDING LIMB

Arginine vasopressin (AVP) inhibits HCO3- absorption (J(HCO3)) in the medullary thick ascending limb (MTAL) of the rat by increasing adenosi ne 3',5'-cyclic monophosphate. Hyperosmolality also inhibits J(HCO3) v ia a pathway additive to inhibition by AVP. To determine whether these regulatory effects are modulated by prostaglandin E(2) (PGE(2)), MTAL were isolated and perfused in vitro with 25 mM HCO3- solutions (pH 7. 4; 290 mosmol/kgH(2)O). PGE(2) (10(-6) M in the bath) had no effect on J(HCO3) in the absence of AVP. In contrast, with 10(-10) M AVP in the bath solution, addition of 10(-8) or 10(-6) M PGE(2) to the bath incr eased J(HCO3) from 9.7 +/- 0.8 to 14.3 +/- 1.1 pmol . min(-1). mm(-1) (P < 0.001). In the presence of AVP and hyperosmolality (75 mM NaCl ad ded to perfusate and bath), PGE(2) increased J(HCO3) from 1.4 +/- 0.1 to 7.5 +/- 0.5 pmol . min(-1). mm(-1) (P < 0.005). PGE(2) also stimula ted J(HCO3) in the presence of AVP and hypertonic urea. Cholera toxin (CTX, 10(-12)-10(-9) M in the bath) inhibited JHCO(3) by 40%, and this inhibition was reversed by PGE(2). PGE(2) dia not reverse inhibition of J(HCO3) by forskolin. The stimulation of J(HCO3) by PGE(2) in the p resence of AVP was blocked by pretreatment with pertussis toxin (PTX, 2 x 10(-11) or 10(-8) M). Neither CTX nor PTX affected inhibition of J (HCO3) by hyperosmolality. These results demonstrate that PGE(2) rever ses inhibition of J(HCO3) by AVP by acting via a PTX-sensitive G prote in (presumably G(i)) to inhibit AVP-stimulated adenosine 3',5'-cyclic monophosphate production. PGE(2) may act as a counterregulatory factor to maintain a stable rate of HCO3- absorption in the MTAL during anti diuresis when circulating AVP levels and medullary osmolality are elev ated.