No abstract

Aldosterone-independent mechanisms may contribute to K + homeostasis. We studied aldosterone synthase knockout (AS 2/2 ) mice to define renal control mechanisms of K + homeostasis in complete aldosterone deficiency. AS 2/2 mice were normokalemic and tolerated a physiologic dietary K + load (2% K + , 2 days) without signs of illness, except some degree of polyuria. With supraphysiologic K + intake (5% K + ), AS 2/2 mice decompensated and became hyperkalemic. High-K + diets induced upregulation of the renal outer medullary K + channel in AS 2/2 mice, whereas upregulation of the epithelial sodium channel (ENaC) sufficient to increase the electrochemical driving force for K + excretion was detected only with a 2% K + diet. Phosphorylation of the thiazide-sensitive NaCl cotransporter was consistently lower in AS 2/2 mice than in AS +/+ mice and was downregulated in mice of both genotypes in response to increased K + intake.Inhibition of the angiotensin II type 1 receptor reduced renal creatinine clearance and apical ENaC localization, and caused severe hyperkalemia in AS 2/2 mice. In contrast with the kidney, the distal colon of AS 2/2 mice did not respond to dietary K + loading, as indicated by Ussing-type chamber experiments. Thus, renal adaptation to a physiologic, but not supraphysiologic, K + load can be achieved in aldosterone deficiency by aldosteroneindependent activation of the renal outer medullary K + channel and ENaC, to which angiotensin II may contribute. Enhanced urinary flow and reduced activity of the thiazide-sensitive NaCl cotransporter may support renal adaptation by activation of flow-dependent K + secretion and increased intratubular availability of Na + that can be reabsorbed in exchange for K + secreted.

Ammonium chloride addition to drinking water is often used to induce metabolic acidosis (MA) in rodents but may also cause mild dehydration. Previous microarray screening of acidotic mouse kidneys showed upregulation of genes involved in renal water handling. Thus, we compared two protocols to induce metabolic acidosis in mice and rats: standard 0.28M NH 4 Cl in drinking water or an equivalent amount of NH 4 Cl in food. Both treatments induced MA in mice and rats. In rats, NH 4 Cl in water caused signs of dehydration, reduced mRNA abundance of the vasopression receptor 2 (V2R), increased protein abundance of the aquaporin water channels AQP2 and AQP3 and stimulated phosphorylation of AQP2 at residues Ser256 and Ser261. In contrast, NH 4 Cl in food induced massive diuresis, decreased mRNA levels of V2R, AQP2, and AQP3, did not affect protein abundance of AQP2 and AQP3, and stimulated phosphorylation at Ser261 but not pSer256 of AQP2. In mice, NH 4 Cl in drinking water stimulated urine concentration, increased AQP2 and V2R mRNA levels, and enhanced AQP2 and AQP3 protein expression with higher levels of AQP2 pSer256 and pSer261. Addition of NH 4 Cl to food, stimulated diuresis, had no effect on mRNA levels of AQP2, AQP3, and V2R, and enhanced only AQP3 protein abundance whereas pSer256-AQP2 and pSer261-AQP2 remained unaltered. Similarly, AQP2 staining was more intense and luminal in kidney from mice with NH 4 Cl in water but not in food. Pendrin, SNAT3 and PEPCK mRNA expression in mouse kidney were not affected by the route of NH 4 Cl application. Thus, addition of NH 4 Cl to water or food causes MA but has differential effects on diuresis and expression of mRNAs and proteins related to renal water handling. Moreover, mice and rats respond differently to NH 4 Cl loading, and increased water intake and diuresis may be a compensatory mechanism during MA. It may be necessary to consider these effects in planning and interpreting experiments of NH 4 Cl supplementation to animals. 1060