Title

Mechanisms and regulation of urate secretion in the avian renal proximal tubule

Date of Completion

January 2011

Keywords

Biology, Animal Physiology|Biology, Physiology

Degree

Ph.D.

Abstract

Urate is the end product of purine metabolism in humans and birds, as well as the product of nitrogenous waste in birds. Since urate cannot be further broken down in these species, the homeostasis of plasma urate relies heavily on a balance of hepatic production and urinary and intestinal excretion. While it is well known that the majority of urate excretion takes place via the kidney, the precise mechanisms of this process are unknown. The goal of this thesis research was to further elucidate the mechanisms of renal proximal tubule apical membrane urate secretion. The avian renal proximal tubule was chosen as an ideal model system for studying these mechanisms of secretion due to its lack of active urate reabsorption. ^ Basolateral membrane urate entry in the proximal tubule takes place via an anion exchange mechanism involving organic anion transporters 1 and 3 (Oat1/3); however, the mechanisms for apical membrane irate efflux are poorly understood. The data presented in this thesis show that an ATP-binding cassette (ABC) family transporter, multidrug resistance protein 4 (Mrp4), plays a pivotal role in this process. Previous studies isolated membrane vesicles have implicated a voltage-driven apical membrane Irate transport mechanism; however, the present research revealed no evidence for this in the intact epithelium. In addition, although another apical membrane ABC transporter, breast cancer resistance protein (Bcrp), has been previously shown to transport urate, the results here show that it does not contribute significantly to transepithelial urate secretion in the avian system.^ In addition to identifying membrane transporters involved in apical membrane urate secretion, another goal of the present study was to examine factors that modulate these pathways. This thesis focuses on the role of cellular stress activated pathways in regulating proximal tubule urate secretion. The data presented here show that heavy metal-induced cellular stress activated AMP-activated protein kinase (AMPK) and subsequently reduced transepithelial urate secretion. These findings suggest that Mrp4 activity is decreased during periods of cellular stress. The consequence may be to conserve energy and increase intracellular urate, which, as a potent antioxidant, can be important for protection from oxidative damage. ^