Article first published online: 22 APR 2013
Copyright © 2013 American Association for the Study of Liver Diseases
Volume 57, Issue 5, pages 2061–2071, May 2013
How to Cite
Soria, L. R., Marrone, J., Calamita, G. and Marinelli, R. A. (2013), Ammonia detoxification via ureagenesis in rat hepatocytes involves mitochondrial aquaporin-8 channels. Hepatology, 57: 2061–2071. doi: 10.1002/hep.26236
Potential conflict of interest: Nothing to report.
Supported by Grant PICT 05-0946 from Agencia Nacional de Promoción Científica y Tecnológica and Grant PIP 01563 from Consejo Nacional de Investigaciones Científicas y Técnicas (to R. A. M.).
- Issue published online: 22 APR 2013
- Article first published online: 22 APR 2013
- Accepted manuscript online: 8 JAN 2013 05:30PM EST
- Manuscript Accepted: 27 NOV 2012
- Manuscript Received: 3 AUG 2012
Hepatocyte mitochondrial ammonia detoxification via ureagenesis is critical for the prevention of hyperammonemia and hepatic encephalopathy. Aquaporin-8 (AQP8) channels facilitate the membrane transport of ammonia. Because AQP8 is expressed in hepatocyte inner mitochondrial membranes (IMMs), we studied whether mitochondrial AQP8 (mtAQP8) plays a role in ureagenesis from ammonia. Primary cultured rat hepatocytes were transfected with small interfering RNAs (siRNAs) targeting two different regions of the rat AQP8 molecule or with scrambled control siRNA. After 48 hours, the levels of mtAQP8 protein decreased by approximately 80% (P < 0.05) without affecting cell viability. mtAQP8 knockdown cells in the presence of ammonium chloride showed a decrease in ureagenesis of approximately 30% (P < 0.05). Glucagon strongly stimulated ureagenesis in control hepatocytes (+120%, P < 0.05) but induced no significant stimulation in mtAQP8 knockdown cells. Contrarily, mtAQP8 silencing induced no significant change in basal and glucagon-induced ureagenesis when glutamine or alanine was used as a source of nitrogen. Nuclear magnetic resonance studies using 15N-labeled ammonia confirmed that glucagon-induced 15N-labeled urea synthesis was markedly reduced in mtAQP8 knockdown hepatocytes (−90%, P < 0.05). In vivo studies in rats showed that under glucagon-induced ureagenesis, hepatic mtAQP8 protein expression was markedly up-regulated (+160%, P < 0.05). Moreover, transport studies in liver IMM vesicles showed that glucagon increased the diffusional permeability to the ammonia analog [14C]methylamine (+80%, P < 0.05). Conclusion: Hepatocyte mtAQP8 channels facilitate the mitochondrial uptake of ammonia and its metabolism into urea, mainly under glucagon stimulation. This mechanism may be relevant to hepatic ammonia detoxification and in turn, avoid the deleterious effects of hyperammonemia. (HEPATOLOGY 2013)