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

Abstract

Renal ammonia excretion is a critical mechanism through which the kidneys maintain acid‐base homeostasis. We have previously identified that ammonia excretion is less in male mice than in female mice under basal conditions, that this correlates with differences in expression of key proteins involved in ammonia metabolism, and involves both testosterone‐dependent and renal androgen receptor (AR)‐dependent signaling pathways. In addition, there are sex differences in the response to acid‐loading. This study’s objective was to determine renal AR’s role in the renal ammonia response to an exogenous acid load. To avoid the known systemic effects of AR blockade/deletion, we studied mice with kidney‐specific AR deletion (KS‐AR‐KO) using Cre/loxP techniques and the Pax8‐Cre transgene; control (C) mice were Pax8‐Cre‐negative littermates. Mice were acid‐loaded by adding HCl to powdered chow for 7 days; control mice had equal amounts of deionized water added. Urine ammonia under basal conditions was ~2 fold greater in male KO (M‐KO) than in male control (M‐C) mice. After acid‐loading, ammonia excretion was significantly greater in M‐KO than in M‐C mice on day 7 (M‐KO, 852±203; M‐C, 588±93 µmol/day; P < 0.05; N= 6 in each group). Basal urine pH was significantly lower in M‐KO than in M‐C mice, but the response to acid‐loading significantly between genotypes. There was no significant difference in serum Na+, K+, and HCO3‐ after acid loading. Expression of phosphoenolpyruvate carboxykinase (PEPCK), a major proximal tubule (PT) ammonia generating protein, was greater under control conditions in M‐KO, but the relative increase with acid‐loading was greater in M‐C mice. Expression of glutamine synthetase, which recycles ammonia, significantly decreased in response to an acid load, with no significant genotype difference. NBCe1‐A expression was significantly less under control conditions in M‐KO, but the relative increase with acid‐loading was greater in M‐KO mice. Expression of NHE3, believed to be the primary mechanism of PT ammonia secretion, increased in response to acid‐loading in M‐KO mice but not in M‐C mice. After acid‐loading, cortical NKCC2, which mediates thick ascending limb ammonia reabsorption, was significantly greater in M‐KO than M‐C mice. The collecting duct secretes the majority of urinary ammonia, and the Rhesus glycoproteins, Rhbg and Rhcg, are the primary collecting duct ammonia transporting proteins. Expression of Rhbg increased in response to an acid load with no significant genotype difference. Renal structural analysis showed AR deletion in male mice significantly decreased kidney size (M‐C, 275±33 mg; M‐KO, 225±32; P<0.05; N= 6 in each group) after acid loading. In summary, KS‐AR‐KO increases the urinary ammonia response to acid‐loading, through coordinated effects on PEPCK, GS, NBCe1‐A, NHE3 and NKCC2. Thus, in male mice renal AR activation inhibits the response to acid‐loading of multiple components of ammonia metabolism and transport, which leads to less ability to increase ammonia excretion.