Testosterone Modulates Renal Ammonia Metabolism

Abstract

Ammonia metabolism is the primary component of net acid excretion and thus has a critical role in acid‐base homeostasis. Our recent studies show there are substantial sex‐dependent differences in ammonia metabolism that correlate with sex‐dependent differences in renal structure and in expression of proteins involved in ammonia generation and transport. This study's objective was to determine the role of testis‐derived testosterone in these differences. We used 4 month‐old C57BL/6 male mice divided into 3 groups: orchiectomy plus placebo (ORCH+P), ORCH plus testosterone (ORCH+T), and sham operated control (SOC). We replaced testosterone using time‐delayed pellets implanted subcutaneously; ORCH+P received a similar pellet that did not release testosterone. Plasma testosterone levels did not differ significantly between ORCH+T and SOC mice (ORCH+T, 159±36; SOC, 120±140; P = NS). Mice were studied four weeks after surgery. Food, and thus protein, intake, which is a major determinant of endogenous acid production, was similar in all three groups (SOC, 12.3±1.7; OCRH+P, 12.6±1.2; ORCH+T, 13.3±1.2 g/d; P=NS). Despite similar food intake, ORCH+P mice excreted more ammonia than did SOC mice; this was reversed by testosterone replacement (ORCH+P, 135±32; SOC, 55±13; ORCH+T, 48±28 μmol/day; P<0.001). Similar findings were observed in a separate set of ORCH+P and SOC mice studied at 8 weeks. ORCH decreased kidney size, and this was completely reversed by testosterone replacement (ORCH+P, 187±18; SOC, 220±16; ORCH+T, 236±19 mg; P<0.001). In parallel, proximal tubule volume density in the ORCH+P kidney was less than in the SOC kidney, and testosterone replacement completely reversed this difference (ORCH+P, 47±2; SOC, 63±1; ORCH+T, 62±1%; P<0.001). Phosphoenolpyruvate (PEPCK), a major proximal tubule (PT) ammonia generating protein, was significantly greater in ORCH+P mice than SOC mice, and this effect was completely reversed by testosterone replacement. Glutamine synthetase, which recycles ammonia, and NBCe1, a basolateral PT transporter that regulates PT ammonia metabolism, did not differ significantly between the groups. Expression of NKCC2, which mediates thick ascending limb ammonia reabsorption, was significantly greater in ORCH+P mice than SOC mice, and this effect was completely reversed by testosterone replacement. Finally, renal androgen receptor (AR) mRNA and protein expression were detected by real‐time RT‐PCR and immunoblot analysis, and immunohistochemistry showed AR expression throughout the proximal tubule (PT), with no detectable expression in non‐PT epithelial cells. These findings demonstrate that the loss of testis‐derived sex steroid hormones dramatically alters ammonia excretion, proximal tubule structure, and expression of key proteins involved in ammonia metabolism and transport. Furthermore, these dramatic changes derived from the loss of testis‐derived sex steroid hormones were reversed by physiologic testosterone replacement. We conclude that testosterone, likely acting through AR, decreases ammonia excretion through its effects on proximal tubule structure and on several key proteins involved in ammonia metabolism and transport.Support or Funding InformationNIH R01‐DK045788, R01‐DK107798 and 5T32‐DK‐104721.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.