The Role of the Androgen Receptor in Sexual Dimorphisms of Renal Ammonia Metabolism

Abstract

Female mice excrete ~2x more ammonia than do male mice, and this appears to result from sex-differences in ammoniagenic enzyme expression in the proximal tubule (PT) and differences in ammonia transporter expression in the PT, TAL and collecting duct. Orchiectomy studies, with and without testosterone replacement, showed that many, but not all, of these sex-differences were mediated by testosterone. We showed recently that the androgen receptor (AR) is present in the PT in both male and female kidney, and not detectably in other renal epithelial cells. This study's objective was to determine AR's role in sexual dimorphisms in ammonia handling. To avoid known systemic effects of AR blockade/deletion, we generated mice with kidney-specific AR deletion (KS-AR-KO) using Cre/loxP techniques (AR floxed mice and Pax8-Cre mice); control (C) mice were Pax8-Cre-negative littermates. Plasma testosterone did not differ significantly between KO and C mice in either sex (M-C, 67±74 ng/dl; M-KO, 239±471; F-C, 27±10; F-KO, 29±11; P=NS; N= 8-11 in each group). In male mice, KS-AR-KO increased ammonia excretion significantly (M-C, 44±14 µmol/day; M-KO, 92±74; P < 0.05; N= 8-11 in each group); this occurred despite no significant difference in food intake, a major determinant of endogenous acid production; there was no significant difference in female mice. KS-AR-KO did not alter serum Na+, K+, or HCO3- significantly in either sex. Expression of phosphoenolpyruvate carboxykinase (PEPCK), a major proximal tubule (PT) ammonia generating protein, and NKCC2, the major mechanism of TAL ammonia reabsorption, were increased significantly by KS-AR-KO in male mice, but not in female mice. KS-AR-KO decreased expression of NHE-3, the major mechanism of PT ammonia secretion, and NBCe1-A, a basolateral PT transporter that regulates PT ammonia metabolism, in male mice, but not in female mice, and did not alter the sex-specific difference in collecting duct Rhbg and Rhcg expression in either sex. Renal structural analysis showed AR deletion decreased kidney size (M-C, 222±27 mg; M-KO, 175±23; P<0.05; N= 5-6 in each group) and cortical proximal tubule volume density (C, 61±2%; KO, 42±2; P<0.05; N= 5-6 in each group) in male mice, but did not alter either parameter in females. KS-AR-KO did not alter the sexual dimorphism of collecting duct volume density in either sex. We conclude that sex-differences in ammonia generation and excretion involve AR-dependent signaling pathways in male, but not female, kidney that lead to increased PT PEPCK and TAL NKCC2 expression. Adaptive responses in NHE-3 and NBCe1-A limit the magnitude of the effect on ammonia excretion, and the sex differences in the collecting duct are not mediated by AR. Since AR is not present in the TAL, the effect of KS-AR-KO on male NKCC2 expression likely involves PT-dependent paracrine signaling. Finally, the greater kidney size and PT volume density in male mice is the result of PT AR signaling.