Regulation of HCO₃^- absorption by prostaglandin E2 and G proteins in rat medullary thick ascending limb

-Areinine vasopressin (AVP) inhibits HCO₃^- absorption (J_HCO3 ) in the medullary thick ascending limb (MTAL) of the rat by increasing adenosine 3′,5′-cyclic monophosphate. Hyperosmolality also inhibits e/H_CO3 via a pathway additive to inhibition by AVP. To determine whether these regulatory effects are modulated by prostaglandin E₂ (PGE₂), MTAL were isolated and perfused in vitro with 25 mM HCO₃^- solutions (pH 7.4; 290 mosmol/kgH₂O). PGE₂ 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 PGE2 to the bath increased 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 added to perfusate and bath), PGE₂ increased J_HCO3 from 1.4 ± 0.1 to 7.5 ± 0.5 pmol-min^-1. mm^-1 (P < 0.005). PGE₂ also stimulated JHCO. in the presence of AVP and hypertonic urea. Cholera toxin (CTX, 10^-12-10^-9 M in the bath) inhibited J_HCo3 by 40%, and this inhibition was reversed by PGE₂. PGE₂ did not reverse inhibition of J_HCO3 by forskolin. The stimulation of J_HCO3 by PGE₂ in the presence of AVP was blocked by pretreatment with pertussis toxin (PTX, 2 × 10^-11 or 10^-8 M). Neither CTX nor PTX affected inhibition of J_HCO3 by hyperosmolality. These results demonstrate that PGE₂ reverses inhibition of JHCO, by AVP by acting via a PTX-sensitive G protein (presumably G;) to inhibit AVP-stimulated adenosine 3′,5′-cyclic monophosphate production. PGE₂ may act as a counterregulatory factor to maintain a stable rate of HCO₃^- absorption in the MTAL during antidiuresis when circulating AVP levels and medullary osmolality are elevated. vasopressin; adenosine 3′,5′-cyclic monophosphate; hyperosmolality; acid-base regulation; pertussis toxin