Living with water stress: evolution of osmolyte systems.

Striking convergent evolution is found in the properties of the organic osmotic solute (osmolyte) systems observed in bacteria, plants, and animals. Polyhydric alcohols, free amino acids and their derivatives, and combinations of urea and methylamines are the three types of osmolyte systems found in all water-stressed organisms except the halobacteria. The selective advantages of the organic osmolyte systems are, first, a compatibility with macromolecular structure and function at high or variable (or both) osmolyte concentrations, and, second, greatly reduced needs for modifying proteins to function in concentrated intracellular solutions. Osmolyte compatibility is proposed to result from the absence of osmolyte interactions with substrates and cofactors, and the nonperturbing or favorable effects of osmolytes on macromolecular-solvent interactions.

Effects of sodium concentration and osmolality on water and electrolyte absorption form the intact human colon.

The influence of sodium concentration and osmolality on net water and monovalent electrolyte absorption from or secretion into the intact human colon was studied in healthy volunteers. WHEN ISOTONIC SOLUTIONS CONTAINING NACL AND/OR MANNITOL WERE INFUSED INTO THE COLON: (a) a direct linear relationship between luminal sodium concentration (in the range of 23-150 mEq/liter) and rate of net water, sodium, and chloride absorption was found. No water absorption was found when sodium concentration in the luminal fluid was below 20 mEq/liter; (b) water and sodium absorption from the isotonic test solutions was not enhanced by addition of 80-250 mg/100 ml of glucose; and (c) the rate of water and sodium absorption was decreased markedly when chloride was replaced by bicarbonate in the test solution. WHEN THE COLON WAS PERFUSED WITH HYPERTONIC TEST SOLUTIONS CONTAINING NACL AND MANNITOL OR UREA: (a) water was absorbed from hypertonic NaCl solutions against a lumen-to-blood osmotic gradient of 50 mOsm/kg; (b) when the osmolality of the mannitol solution was increased, water entered the colonic lumen at a more rapid rate. The relationship between the rate of water entering the colon and the osmolality of the test solution was a parabolic one; (c) sodium and chloride entered the colonic lumen at a rate that was lineraly related to that of water entrance when the lumen-to-blood osmotic gradient exceeded 150 mOsm/kg; (d) water flow into the colonic lumen was identical when equimolar urea or mannitol solutions were infused; (e) neither urea nor mannitol was absorbed in significant amounts from the hypertonic solutions; and (f) our results suggest that the equivalent pore radius of the human colon is smaller than the molecular radius of urea (2.3 A).

Urea and ethylene glycol-facilitated transport systems in the human red cell membrane. Saturation, competition, and asymmetry.

The equilibrium exchange of [14C]urea and ethylene glycol was measured using a new type of fast flow system. Approximately equal volumes of saline and air were mixed to form a segmented fluid stream into which 14C-loaded red cells are injected. The stream flows through three filter chambers which allow sampling of the 14C in the extracellular fluid at three time points. The chambers are designed so that they do not disrupt the segmented bubble pattern. The alternating air and saline segments prevent laminar dispersion in the flowing stream and ensure good mixing at the injection and sampling sites. The equilibrium exchange of both urea and ethylene glycol showed saturation kinetics. The maximum permeability (Po) measured in the limit of zero solute concentration is 1.6 X 10(-3) cm/s for urea and 4.8 X 10(-4) cm/s for ethylene glycol (T = 23 degrees C). The apparent dissociation constant (Km) was 218 mM for urea and 175 mM for ethylene glycol. The Po for thiourea is 2.3 X 10(-6) cm/s and the Km is 19 mM. Urea and thiourea inhibit the transport of each other and the inhibition constant (KI) is approximately equal to the Km for both compounds. 53 other analogues of urea were screened for their inhibition of urea or thiourea transport. Several analogues [e.g., 1-(3,4-dichloro-phenyl)-2-thiourea] had a KI in the range of 0.03 mM. The affinity of the inhibitor increased as it was made more hydrophobic. The urea analogues did not significantly inhibit the ethylene glycol or osmotic permeability. Glycerol inhibited ethylene glycol permeability with a KI of 1,200 mM.

Blood urea nitrogen and serum creatinine. Physiology and interpretations.

Any elevations in levels of blood urea nitrogen and/or serum creatinine do not necessarily indicate structural renal disease. Conversely, blood urea nitrogen or serum creatinine values, which appear to be within the range of normal, do not by themselves rule out significant reduction in glomerular filtration rate. Any interpretation of the blood levels of these two substances must be done with the awareness that a variety of extrarenal factors can affect them. The blood urea nitrogen to serum creatinine ratio can be a valuable tool in the determination or renal functional and structural integrity.

Pre-postdilution haemofiltration.

Postdilution haemofiltration is a treatment indicated for patients who are unable to tolerate standard dialysis. However, its duration is uncomfortably prolonged in cases of low blood flow from the vascular access. In a predilution mode greater clearance rates can be reached even if a greater amount of sterile and pyrogen-free solutions are required. Although an in-line solution-producing system lowers the cost, theoretical calculations indicate that a predilution mode is not advantageous, in terms of the amount of infused solutions, if urea clearances greater than 150 ml/min at 300 ml/min blood flow are required. Theoretically, the combination of predilution and postdilution is the best system to utilise relatively small amounts of sterile solution in order to enhance treatment efficiency at low blood flow. However, the predilution mode may ultimately be preferable because of easier utilisation of the hardware. The results of clinical application of pre-postdilution haemofiltration indicate that at a 400 ml/min blood flow, a urea clearance of more than 220 ml/min can be obtained, and the duration of treatment can be reduced to only 3 h in the majority of patients.

Questions and replies: renal mechanisms for urinary concentrating and diluting processes.

Mechanisms for urinary concentration or dilution depend on counterflow processes, both tubular and vascular, within the renal medulla. Recently, there have emerged differing hypotheses about the renal tubular processes responsible for maintaining a hypertonic medullary interstitium. In this Editorial Review, R.W. Berliner frames three questions germane to this issue, and J.P. Kokko and D.J. Marsh provide their responses to these queries. The major issues addressed are: 1) What are the major unresolved question(s) concerning the mechanism by which concentrated urine is formed? 2) Current evidence suggests that the urea concentration in thin ascending limbs is slightly lower in the lumen than in interstitial fluid. Is the transepithelial concentration gradient between thin ascending limb and renal medullary interstitium sufficient to permit an entirely passive mechanism for diluting tubular fluid in the thin ascending limb? 3) A simple three-compartment model for the renal medullary concentrating process would include the tubular lumen, peritubular capillary, and the interstitium. Is it possible to generate a model that, by juxtaposing medullary structures, might explain renal medullary counterflow processes more adequately than the simple three-compartment model?

Renal concentrating ability in the uninephrectomized rat.

To investigate the effects of uninephrectomy on renal concentrating ability, studies were performed on unanesthetized rats 5-11 days after uninephrectomy (UN) or a sham operation (SO). Female rats were deprived of water for 27 h prior to the infusion of inulin and para-aminohippurate and urine collection. They were also preconditioned to being handled and to the experimental locale. During a nondiuretic state urine osmolality was the same for all UN and SO groups (mean about 1,700 micro osmol/g H2O), whereas the mean solute excretion rate (micro osmol/min per kg body wt per kidney) was 74 in the UN and 35 in the SO rats. When SO rats were infused with mannitol or isotonic saline to increase their solute excretion rate per kidney to the level of the UN rats, urine osmolality dropped 200-1,000 micro osmol/g H2O; when urea was infused, urine osmolality did not drop. Thus, after uninephrectomy and a consequent doubling of the solute excretion rate per kidney, renal concentrating ability was higher than predicted on the basis of a comparable but acute elevation of the solute excretion rate. The glomerular filtration rate was about 17 ml/min per kg body wt in the SO rats and was 1.2 times greater (on a per kidney basis) in the UN rats. These exceptionally high glomerular filtration rats are attributed to preexperimental conditioning of the rats and the absence of stress during urine collection.

Urea elimination using a cold activated-carbon artificial tubulus for hemofiltration.

Urea adsorption on active carbon is reversible and temperature-dependent. Urea adsorption isotherms of different carbons were determined at 0 degrees C and 65 degrees C within the equilibrium concentration range of 1.0-3.4 gm/L. At low urea concentrations considerable differences (3.4-13.0 gm/kg carbon at concentrations of 1.0 gm/L) were found between different types of activated carbon. The overall internal surface area was of minor importance compared to the pore size distribution. Adsorbing at low temperature, desorbing at high temperature, and flushing the carbon adsorber with a limited volume of the liquid to be purified yielded an "artificial urine." Compared to the original urea concentration of the filtrate, this "artificial urine" had an increased urea concentration. From a 36-liter volume containing 90 grams urea dissolved in saline, 18 liters were recirculated at a flow rate of 100 ml/min. The influence of adsorption and desorption time intervals was evaluated. After one to one and a half hours the carbon was saturated with urea. After saturation, about 1.4 grams urea were eliminated per cycle. In the "artificial urine" urea concentrations of up to 4.5 gm/L were found when the original solution contained only 2.5 gm/L. In the "patient" volume the urea concentration decreased from 2.5 gm/L to 1.9-2.1 gm/L. Within three hours a total of 22 grams of urea was removed by 3 x 120 grams activated carbon corresponding to removal of 50% of the urea passing the "artificial tubulus." The advantage of this system is that after priming, no additional physiological solution would be necessary. The necessity of excessive safety controls, additional electrolyte adjustment, energy demand in the form of direct current, and great amounts of waste in solid form lead to the conclusion that for intermittent hemofiltration treatment, commercially produced and controlled infusion solution is preferable.

Alveolar epithelium permeability to small solutes: developmental changes.

To determine whether alveolar epithelium permeability to small lipid-insoluble solutes changes during development we measured transport across the blood-gas barrier in isolated Ringer-perfused lungs from prenatal, 1-day-old, 4-wk-old, and adult rabbits. Radioactive test molecules, one of which was always sucrose, were dissolved in Ringer solution and instilled into the trachea of degassed lungs. Samples taken from recirculating perfusate were used to calculate permeability-surface area (PS) products. Results were expressed as the ratio (PS)/(PS)sucrose, and as absolute permeability. Lungs from 4-wk-old rabbits were studied most thoroughly; the (PS)/(PS) sucrose ratios obtained are urea 4.0, erythritol 1.3, mannitol 0.98, L-glucose 1.4, and D-glucose 5.6. These and other data imply that the most lipid-insoluble molecules (erythritol, mannitol, L-glucose, and sucrose) are transported by a nonselective bulk process. Urea transport is primarily through lipid membranes; D-glucose seems to involve a special process. Sucrose and L-glucose permeability decreased during development, but their relative permeabilities did not change. Small lipid-insoluble solutes apparently do not cross the alveolar epithelium through small water-filled pores, and their permeability decreases as the animal matures.

Effects of urea on electrolyte transport in the dog kidney.

Three-phase re-collection micropuncture experiments were undertaken to study the effect of 2.5% and 5% urea infusion on tubular handling of sodium, potassium, calcium, magnesium, phosphate, and chloride in 10 acutely parathyroidectomized dogs. The fractional excretions of water and electrolytes were increased in response to graded urea infusion. The late proximal TF/P inulin fell from 1.59 to 1.25, but there was no change in TF/UF Osm, TF/UF calcium, or TF/P sodium, so that the proximal fractional reabsorption of water decreased by 12% and that of sodium and calcium by 11% and 13%, respectively. Proximal TF/UF magnesium fell from 1.33 to 1.16, and fractional magnesium reabsorption decreased by 9%. Distal TF/UF Osm increased from 0.31 in control to 0.67 with 2.5% urea to 0.80 with 5% urea infusion. Distal TF/P inulin ratios fell strikingly (3.89 to 2.05 to 1.52), accompanied by similar increases in TF/P sodium (0.24 to 0.46 to 0.57) and TF/UF calcium (0.31 to 0.51 to 0.62), whereas TF/UF magnesium did not change (0.90 to 0.79 to 0.94). The fraction of potassium remaining at the distal tubule exceeded that measured at the late proximal tubule, indicating potassium secretion between the proximal and distal puncture sites during urea administration. Urea-induced phosphaturia was mainly a result of decreased proximal phosphate reabsorption, with additional inhibition occurring beyond the proximal sampling site. Thus urea infusion (1) inhibits reabsorption of sodium, potassium, and calcium proportionately more than of magnesium in the proximal tubule, (2) inhibits sodium, calcium, and magnesium reabsorption in the loop, (3) promotes potassium secretion into the pars recta of descending limb in the loop of Henle, (4) has little effect on ion transport beyond the distal sampling site, and (5) causes a slight phosphaturia at both levels of urea infusion by inhibiting proximal phosphate reabsorption.

Development aspects of peritoneal dialysis kinetics in dogs.

To determine whether solute transfer during peritoneal dialysis is age related and to identify those factors which might explain age-related differences in dialysis kinetics, the peritoneal dialysance of [14C]urea (DU), [3H]inulin (DI), and the permeability index (DR = DI/DU) were examined in six puppies and five adult dogs. Exchange volume of lactated Ringer's (40 ml/kg) and exchange times (30 min) were identical in all studies. Theoretical calculations for urea dialysance for animals of differing body size were made. Assuming the existence of a similar functional peritoneal surface area per kg and the use of similar exchange volumes per kg and dwell times, theoretical values for the urea dialysance per kg for different sized animals were identical. The experimental studies demonstrated that DI per kg and DU per kg were higher in the puppies (0.146 +/- 0.023 and 0.765 +/- 0.054 ml/min kg; X +/- S.E.) than in the adult (0.052 +/- 0.01 and 0.462 +/- 0.05 ml/min/kg) (P less than 0.01). Also, DR was higher in the puppies (0.187 +/- 0.026), than in the adults (0.11 +/- 0.015) (P less than 0.05). The greater values for DI and DU per kg and DR in the young are best explained by the young having an increased peritoneal membrane permeability as well as an increase in functional peritoneal surface area relative to body weight. This increase in solute movement is independent of the dialysis mechanics used in an exchange and reflects age-related differences in the intrinsic characteristics of the peritoneal membrane.

Abundance of mRNAs encoding urea cycle enzymes in fetal and neonatal mouse liver.

The relative abundances of mRNAs encoding the five urea cycle enzymes during development of mouse liver have been determined and compared with those of mRNAs encoding four other liver-specific proteins (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase, alpha-fetoprotein, and albumin). Urea cycle enzyme mRNAs in fetal liver are expressed at 2-14% of the abundance in adult liver as early as 6 days before birth. Expression of the urea cycle enzyme mRNAs is not coordinate during the fetal and neonatal period. However, profiles of three urea cycle enzyme mRNAs are quite similar to that of alpha-fetoprotein mRNA, suggesting the possibility of a common response to regulatory signals during fetal development. With the exception of ornithine transcarbamylase mRNA, the urea cycle enzyme mRNAs have been shown previously to be inducible by cAMP and glucocorticoids. However, only argininosuccinate lyase mRNA exhibits any significant change in abundance at birth, resembling postnatal expression of tyrosine aminotransferase mRNA. The results indicate that the urea cycle enzyme mRNAs are potentially useful markers for elucidating various features of hepatocyte differentiation in mammals.

Na-K-Cl cotransport in apical membrane of rabbit renal papillary surface epithelium.

The renal papillary surface epithelium is exposed to pelvic urine on its apical surface and to inner medullary interstitium on its basolateral surface. To investigate transport in this epithelium, we dissected it free from the renal papilla of rabbits and mounted it in a chamber that allowed both sides to be bathed independently. Cell volume was measured at 25 degrees C utilizing computerized quantitative microscopy. Addition of ouabain (10(-4) M) to the basolateral solution induced a 20% volume increase. This volume increase was completely inhibited by the removal of apical bath NaCl, Na+, K+, or Cl- but not by the removal of urea. Bumetanide, down to 10(-9) M in the apical bath, completely inhibited the ouabain-induced swelling. Changes in apical bath osmolality, resulting from addition or removal of NaCl, caused cell volume changes that were greater than could be accounted for by osmotic water flow alone. This hyperresponse was blocked by bumetanide and was stimulated by vasopressin (10(-8) M). These observations are consistent with the presence of Na-K-ATPase in the basolateral membrane and a bumetanide-sensitive, vasopressin-responsive Na-K-Cl co-transporter in the apical membrane.