Angiotensin II inhibits HCO₃/^- absorption via a cytochrome P-450- dependent pathway in MTAL

The role of ANG II in the regulation of ion reabsorption by the renal thick ascending limb is poorly understood. Here, we demonstrate that ANG II (10^-8 M in the bath) inhibits HCO₃/^- absorption by 40% in the isolated, perfused medullary thick ascending limb (MTAL) of the rat. The inhibition by ANG II was abolished by pretreatment with eicosatetraynoic acid (10 μM), a general inhibitor of arachidonic acid metabolism, or 17-octadecynoic acid (10 μM), a highly selective inhibitor of cytochrome P-450 pathways. Bath addition of 20-hydroxyeicosatetraenoic acid (20-HETE; 10^-8 M), the major P- 450 metabolite in the MTAL, inhibited HCO₃/^- absorption, whereas pretreatment with 20-HETE prevented the inhibition by ANG II. The addition of 15-HETE (10^-8 M) to the bath had no effect on HCO₃/^- absorption. The inhibition of HCO₃/^- absorption by ANG II was reduced by >50% in the presence of the tyrosine kinase inhibitors genistein (7 μM) or herbimycin A (1 μM). We found no role for cAMP, protein kinase C, or NO in the inhibition by ANG II. However, addition of the exogenous NO donor S-nitroso-N- acetylpenicillamine (SNAP; 10 μM) or the NO synthase (NOS) substrate L- arginine (1 mM) to the bath stimulated HCO₃/^- absorption by 35%, suggesting that NO directly regulates MTAL HCO₃/^- absorption. Addition of 10^-11 to 10^-10 M ANG II to the bath did not affect HCO₃/^- absorption. We conclude that ANG II inhibits HCO₃/^- absorption in the MTAL via a cytochrome P-450- dependent signaling pathway, most likely involving the production of 20- HETE. Tyrosine kinase pathways also appear to play a role in the ANG II- induced transport inhibition. The inhibition of HCO₃/^- absorption by ANG II in the MTAL may play a key role in the ability of the kidney to regulate sodium balance and extracellular fluid volume independently of acid-base balance.