PubMed · 15219308
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Elizabeth N Jacobson, Latha G Stead. 2004. Syncope due to hyperkalemia.. https://doi.org/10.1016/j.jemermed.2003.12.028
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The armoured catfish, Liposarcus pardalis, tolerates severe hypoxia at high temperatures. Although this species can breathe air, it also has a strong anaerobic metabolism. We assessed tissue to plasma glucose ratios and glycogen and lactate in a number of tissues under "natural" pond hypoxia, and severe aquarium hypoxia without aerial respiration. Armour lactate content and adenosine in brain and heart were also investigated. During normoxia, tissue to plasma glucose ratios in gill, brain, and heart were close to one. Hypoxia increased plasma glucose and decreased tissue to plasma ratios to less than one, suggesting glucose phosphorylation is activated more than uptake. High normoxic white muscle glucose relative to plasma suggests gluconeogenesis or active glucose uptake. Excess muscle glucose may serve as a metabolic reserve since hypoxia decreased muscle to plasma glucose ratios. Mild pond hypoxia changed glucose management in the absence of lactate accumulation. Lactate was elevated in all tissues except armour following aquarium hypoxia; however, confinement in aquaria increased armour lactate, even under normoxia. A stress-associated acidosis may contribute to armour lactate sequestration. High plasma lactate levels were associated with brain adenosine accumulation. An increase in heart adenosine was triggered by confinement in aquaria, although not by hypoxia alone.
A novel family of proteins, the Mep/AMT/Rh glycoprotein family may mediate important roles in transmembrane ammonia transport in a wide variety of single-celled and multicellular organisms. Results from our laboratory have examined the expression of the non-erythroid proteins, Rh B Glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), in a wide variety of mammalian tissues. In the kidney, Rhbg and Rhcg are present in distal nephron sites responsible for ammonia secretion. In the mouse kidney, Rhbg immunoreactivity is exclusively basolateral and Rhcg immunoreactivity is exclusively apical, whereas in the rat kidney Rhcg exhibits both apical and basolateral expression. Chronic metabolic acidosis increases Rhcg expression in the outer and inner medulla of the rat kidney; these changes, at least in the outer medullary collecting duct, involve changes in total cellular protein expression in both principal and intercalated cell and changes in its subcellular localization. In the liver, Rhbg is present in the basolateral plasma membrane of the perivenous hepatocyte and Rhcg is present in bile duct epithelia. In the gastrointestinal tract, Rhbg and Rhcg exhibit cell-specific, axially heterogeneous, and polarized expression. These patterns of expression are consistent with Rhbg and Rhcg mediating important roles in mammalian ammonia biology. The lack of the effect of chronic metabolic acidosis on Rhbg expression raises the possibility that Rhbg may function either as ammonia sensing-protein or that it may mediate roles other than ammonia transport.
Ammonium excretion into urine is a major process essential to the regulation of acid-base homeostasis. We have shown that Rh-type proteins, including renal RhCG, belong to the Mep/Amt family of ammonium transporters and promote bi-directional ammonium transport upon heterologous expression in yeast. To study the physiological role of RhCG and to test a potential function in ammonium excretion, we have generated mice bearing an invalidation of the corresponding gene.