Adaptive physiological water conservation explains hypertension and muscle catabolism in experimental chronic renal failure

Abstract

Aim: We have reported earlier that a high salt intake triggered an aestivation-like natriuretic-ureotelic body water conservation response that lowered muscle mass and increased blood pressure. Here, we tested the hypothesis that a similar adaptive water conservation response occurs in experimental chronic renal failure.

Methods: In four subsequent experiments in Sprague Dawley rats, we used surgical 5/6 renal mass reduction (5/6 Nx) to induce chronic renal failure. We studied solute and water excretion in 24-hour metabolic cage experiments, chronic blood pressure by radiotelemetry, chronic metabolic adjustment in liver and skeletal muscle by metabolomics and selected enzyme activity measurements, body Na⁺ , K⁺ and water by dry ashing, and acute transepidermal water loss in conjunction with skin blood flow and intra-arterial blood pressure.

Results: 5/6 Nx rats were polyuric, because their kidneys could not sufficiently concentrate the urine. Physiological adaptation to this renal water loss included mobilization of nitrogen and energy from muscle for organic osmolyte production, elevated norepinephrine and copeptin levels with reduced skin blood flow, which by means of compensation reduced their transepidermal water loss. This complex physiologic-metabolic adjustment across multiple organs allowed the rats to stabilize their body water content despite persisting renal water loss, albeit at the expense of hypertension and catabolic mobilization of muscle protein.

Conclusion: Physiological adaptation to body water loss, termed aestivation, is an evolutionary conserved survival strategy and an under-studied research area in medical physiology, which besides hypertension and muscle mass loss in chronic renal failure may explain many otherwise unexplainable phenomena in medicine.

Keywords: aestivation; body sodium; body water; dehydration; double-barrier concept; glucose-alanine-shuttle; glycine methylation; hepato-renal; hypertension; kidney; liver; muscle mass loss; organic osmolytes; purine metabolism; skin; transamination; transepidermal water loss; urea cycle; urine concentration.

FIGURE 1

Chronic renal failure leads to renal water loss. A, Contribution of 24‐h urine Na⁺, K⁺ and urea solute concentration to urine osmolality (UOsm) in control rats (n = 6) and in rats with 5/6 renal mass reduction (5/6 Nx; n = 12). Note that Na⁺ and K⁺ excretion are calculated twofold to account for the excretion of unmeasured accompanying anions. B, Relative contribution of Na⁺, K⁺ and urea solutes to urine solute formation in the same rats. Data expressed as average ± SD. C, Relationship between urine osmolality and urine volume in the same rats. D, Relationship between water intake and urine osmolality in the same rats

FIGURE 2

Relationship between renal water loss, tissue water content, and blood pressure. A, Relationship between urine osmolality and mean arterial blood pressure (MAP). B, Relationship between urine volume and blood pressure. C, Relationship between absolute Na⁺+ K⁺ and absolute water content in the skin. ( ): we excluded the data points with lowest and with highest Na⁺+ K⁺ content from regression analysis. D, Relationship between skin Na⁺+ K⁺ content per gram dry skin and mean arterial blood pressure. Data are from the same rats shown in Figure 1 and Tables 1 and 2

FIGURE 3

Chronic renal failure leads to reprioritization of liver and muscle nitrogen metabolism for water conservation. A, Metabolomic analysis of differences in nitrogen utilization for urea osmolyte production, urate production, creatine and NO production and nitrogen transfer from the muscle branched‐chain amino acids (BCAA) and muscle dipeptides, carnosine and anserine, between control (n = 6) and 5/6 Nx (n = 10) rats. Key enzymes for urea generation are arginase (#1), cytoplasmatic malate dehydrogenase 1 (MDH1; #2), and aspartate aminotransferase (ASAT; #3). B, Arginase activity in control (n = 6) and 5/6 Nx rats (n = 10). C, Malate dehydrogenase 1 (MDH1) activity in the same rats. D, Aspartate aminotransferase (ASAT) activity in the same rats. *P < .05; †P < .01. For abbreviations of the metabolites, see Table S1

FIGURE 4

Reprioritization of purine and nitrogen metabolism in favour of methylamine production in rats with chronic renal failure. A, Metabolomic analysis of purine nitrogen and glycine utilization for betaine osmolyte production in the same rats as reported in Figure 3. The biosynthesis of S‐adenosyl‐methionine (SAM) in 5/6 Nx rats, which is responsible for the methylation of glycine during betaine synthesis, is promoted by the cataplerotic enzymes MDH1 (#2) and aspartate aminotransferase (#3) in the purine nucleotide cycle. Parallel increases in arginase activity (#1), in conjunction with reduced activity of guanidino‐acetate methyl transferase (GAMT; #4), reduces nitrogen transfer from arginine to creatine, and increases the availability of glycine as a substrate for organic osmolyte production. B, Densitometric quantification of guanidino‐acetate methyl transferase (GAMT) protein levels in liver and muscle of six control and six rats with 5/6 Nx. C, Densitometric quantification of the ratio of the phosphorylated and unphosphorylated for AMP‐activated kinase (AMPK) in the same rats. D, Densitometric quantification of phosphorylated acetyl‐CoA carboxylase (p‐ACC) protein levels in the same rats. E, Western Blots of AMPK, pAMPK, pACC and GAMT protein in liver and muscle of the control and the 5/6 Nx rats. *P < .05

FIGURE 5

Rats with chronic renal failure reduce cutaneous blood flow to limit transepidermal water loss, albeit at the expense of arterial hypertension. A, Plasma copeptin and skin angiotensin II (Ang II) levels in the same rats as described in Figures 1 and 2. B, 24‐h urine norepinephrine (NE) excretion and plasma Ang 2 levels in control (n = 10) and 5/6 Nx rats (n = 14), in which we in parallel measured their plasma Ang 2 levels (controls: n = 7; 5/6 Nx: n = 12). C, Time‐dependent increase in body core temperature in anaesthetized control (n = 3) and 5/6 Nx rats (n = 3) exposed to 33°C ambient temperature; and D, Changes in intra‐arterial mean arterial blood pressure (MAP) in response to increasing body temperature. E, Cutaneous blood volume in response to increasing body core temperature in control (n = 5) and 5/6 Nx (n = 6) rats. F, Transepidermal water loss (TEWL) in response to increasing body core temperature in the same rats as in (E). G, Relationship between skin blood volume and TEWL in the same rats as in (E and F). Data are expressed as average ± SD and analysed by multivariate or univariate General Linear Model (A and B), univariate General Linear Model for Repetitive Measurements (C‐F), and simple linear regression (G). AU, arbitrary units. *P < .05