The penetration of urea into the central nervous system at high blood levels

Untargeted serum metabolic profiling of diabetes mellitus among Parkinson's disease patients

Type 2 diabetes mellitus (T2DM) is a common comorbidity among Parkinson's disease (PD) patients. Yet, little is known about dysregulated pathways that are unique in PD patients with T2DM. We applied high-resolution metabolomic profiling in serum samples of 636 PD and 253 non-PD participants recruited from Central California. We conducted an initial discovery metabolome-wide association and pathway enrichment analysis. After adjusting for multiple testing, in positive (or negative) ion mode, 30 (25) metabolic features were associated with T2DM in both PD and non-PD participants, 162 (108) only in PD participants, and 32 (7) only in non-PD participants. Pathway enrichment analysis identified 17 enriched pathways associated with T2DM in both the PD and non-PD participants, 26 pathways only in PD participants, and 5 pathways only in non-PD participants. Several amino acid, nucleic acids, and fatty acid metabolisms were associated with T2DM only in the PD patient group suggesting a possible link between PD and T2DM.

Central mechanisms of osmosensation and systemic osmoregulation

Systemic osmoregulation is a vital process whereby changes in plasma osmolality, detected by osmoreceptors, modulate ingestive behaviour, sympathetic outflow and renal function to stabilize the tonicity and volume of the extracellular fluid. Furthermore, changes in the central processing of osmosensory signals are likely to affect the hydro-mineral balance and other related aspects of homeostasis, including thermoregulation and cardiovascular balance. Surprisingly little is known about how the brain orchestrates these responses. Here, recent advances in our understanding of the molecular, cellular and network mechanisms that mediate the central control of osmotic homeostasis in mammals are reviewed.

Does uremia protect against the demyelination associated with correction of hyponatremia during hemodialysis? A case report and literature review

Rapid correction of chronic hyponatremia is known to cause demyelination syndromes, which are attributed to the rapid shift of water out of the brain. In uremic patients with hyponatremia, depending on the dialysate sodium concentration and delivered Kt/V, serum sodium levels may be rapidly corrected inadvertently during the hemodialysis (HD) session. It is not known whether uremic patients are as susceptible to the development of demyelination as patients with normal renal function. Since urea diffuses slowly across the blood-brain barrier, it can act as an effective osmole between plasma and the brain if levels are changed abruptly. During HD, blood urea levels drop suddenly and significantly and cerebral edema may develop (dialysis disequilibrium syndrome). This effect may counteract the fluid shift out of the brain during correction of hyponatremia. Therefore, theoretically, uremic patients may be less prone to develop demyelination. We present a patient with renal failure whose hyponatremia was corrected rapidly during HD to illustrate the potential problem. The patient tolerated rapid correction of hyponatremia without sustaining any neurologic damage. We performed a literature search looking for similar case reports and reviewed the scientific evidence behind the above hypothesis.

Effects of urea on taurine efflux and cell volume in incubated rat cerebral cortical minislices

The effects of urea on the rate of efflux of preloaded taurine and volume regulation have been examined in incubated minislices from rat superficial cerebral cortex. As external urea was increased in the range 0-100 mmol/L, there was a concentration-dependent slowing of cellular taurine efflux. Cell volumes progressively increased over the range 0-50 mmol/L urea, but decreased slightly in 100 mmol/L. Urea had no effect on cell volume in the absence of taurine. Retardation of efflux, and cell swelling in the presence of 50 mmol/L urea were entirely abolished by trimethylamine (100 mumol/L). TMA had no effect on either variable in the absence of urea. It is suggested that impaired loss of taurine and accompanying cell swelling may be factors contributing to the neurological disturbances accompanying uremia.

Brain swelling after dialysis: old urea or new osmoles?

The pathogenesis of brain swelling and neurological deterioration after rapid hemodialysis (dialysis disequilibrium syndrome) is controversial. The "reverse urea hypothesis" suggests that hemodialysis removes urea more slowly from the brain than from the plasma, creating an osmotic gradient that results in cerebral edema. The "idiogenic osmole hypothesis" proposes that an osmotic gradient between brain and plasma develops during rapid dialysis because of newly formed brain osmoles. In this review, the experimental basis for the two hypotheses are critically examined. Based on what is known about the physiology of urea and water diffusion across the blood-brain barrier, and empiric observations of brain solute composition after experimental hemodialysis, we conclude that the "reverse urea hypothesis" remains a viable explanation for dialysis disequilibrium and that rapid reduction of a high urea level in and of itself predisposes to this condition.

Administration of intravenous urea and normal saline for the treatment of hyponatremia in neurosurgical patients

Hyponatremia frequently complicates the care of neurosurgical patients and requires prompt effective therapy. These patients commonly fulfill the laboratory criteria of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) or cerebral salt wasting; the classification depends on the volume status of the patient. The authors have been dissatisfied with the standard therapy of fluid restriction for the critically ill neurosurgical patient because of 1) slow rates of sodium correction; 2) poor applicability in patients requiring multiple intravenous medications and/or nutritional support; and 3) possible dangers of inducing or enhancing cerebral ischemia in patients who already may be fluid-depleted. Reported successes in the treatment of hyponatremia due to SIADH by administration of urea and normal saline led to the authors' routine use of this therapy for hyponatremic neurosurgical patients. A retrospective review of an 18-month period revealed 48 patients (3% of all neurosurgical inpatients) with hyponatremia from various causes who received 62 treatments of urea and normal saline. Treatment consisted of 40 gm urea dissolved in 100 to 150 ml normal saline as an intravenous drip every 8 hours and an intravenous infusion of normal saline at 60 to 100 ml/hr for 1 to 2 days. The mean pretreatment serum sodium level (+/- standard deviation) was 130 +/- 3 mmol/liter (range from 119 to 134 mmol/liter). There was a significant mean posttreatment elevation to 138 +/- 4 mmol/liter (range 129 to 148 mmol/liter) (p less than 0.001, Student's t-test). Average daily fluid intake and output on treatment days were 2719 +/- 912 and 2892 +/- 1357 ml, respectively. There were no treatment complications in this group. It is concluded that urea and saline administration results in a rapid, safe, and effective correction of hyponatremia, making this method superior to fluid restriction in many neurosurgical patients.

Effects of acute hyperosmolar NaCl or urea on brain H2O, Na+, K+, carbohydrate, and amino acid metabolism in weanling mice: NaCl induces insulin secretion and hypoglycemia

This study compares early and late effects of the injection of hyperosmolar NaCl and urea of equal osmolarity on selected aspects of brain water, electrolyte, carbohydrate, amino acid, urea, and energy metabolism in normal suckling-weanling mice. One hour after treatment, salt-treated mice were critically ill, while the behavior of urea-treated animals could not be distinguished from that of controls. This clinical difference could not be explained on the basis of differences in plasma osmolality, the brain water content, or the degree of hemorrhagic encephalopathy. The injection of NaCl induced a 14-fold increase in plasma insulin and a progressive fall in the plasma glucose concentration (a reduction of 66% at 1 hr). In contrast, plasma glucose levels in urea-injected mice were unchanged. Prior to the fall in plasma glucose levels, metabolite changes in the brains of NaCl-injected mice were compatible with facilitation of transfer of glucose from the blood to the brain, increased metabolic flux in the Embden-Meyerhof and Krebs citric acid cycle pathways, and increased energy production. With the exception of the glucose content (unchanged), similar metabolite changes were seen in brain soon after urea injection. In the brains of the hypoglycemic NaCl-treated mice, glucose levels were reduced 80%, and glycogen 41%. Other metabolite changes were compatible with decreased glycolysis and metabolic flux through the Krebs citric acid cycle. In contrast, with few exceptions, at a similar time after injection, metabolite levels had returned to normal in the urea-treated mice. Permeability of the brain to urea was also examined. Brain urea reached high levels at 2 hr but returned to near baseline at 6 hr. Both hyperosmolar solutions increased the brain content of aspartic and glutamic acids 1 hr after injection. The failure of hypoglycemic mice with hypernatremia and elevated plasma osmolality (range, 416-434 mOsm/kg H2O) to respond to 1 M glucose (30 ml/kg) may have been due to the ill effects of the additional hyperosmolar load. The possibility remains that the encephalopathy induced by hyperosmolar NaCl, but not by hyperosmolar urea, is in some way related to the sudden elevation of brain Na+ and/or Cl- ions.

Glucose and urea kinetics in cows in early lactation

1. Four mature Friesian cows were used. For 2 weeks after calving they were fed 3 kg hay (147 g crude protein (nitrogen X 6.25; CP)/kg dry matter (DM) plus 9 kg concentrates (177 g CP/kg DM) per d and thereafter 4 kg hay plus 12 kg concentrates/d. 2. At approximately the 2nd, 4th and 9th weeks post-partum each cow was given a single intravenous dose of [14C]urea (1 mCi) and [6-3H]glucose (1.5 mCi). Following this, jugular venous blood samples were withdrawn up to 26 h post-injection. 3. The log (specific activity) v. time curves were not linear for either metabolite. Glucose irrversible loss rates (IRL) were calculated by a stochastic procedure. The linear part of the urea log (specific activity) v. time curve gave the same estimate for urea IRL rate as a stochastic method of calculation, but urea pool sizes were overestimated so that stochastic analysis of results was preferred. 4. Mean milk yields at the time of the three measurements were 27.1, 30.8 and 27.9 kg milk/d. 5. Urea IRL was significantly lower (P < 0.05) in the first 4 weeks of lactation than in the 9th, and blood urea concentration was lower (P < 0.05) in the 4th than in the 2nd and 9th week, but there was no close correlation between urea IRL and concentrations. 6. Glucose IRL rose (P < 0.05) between the 2nd and 9th weeks post-partum, perhaps due to the increase in feed intake. The results were generally consistent with the relationship between milk yield and glucose IRL established by Paterson & Linzell (1974). 7. Urea IRL (y) and milk protein-N (x) production values were used to calculate the efficiency of protein utilization (EPU) as x divided by (x+0.35 y). This assumes that 0.35 of urea IRL was due to protein catabolism. EPU was found to be 0.59-0.80 (mean 0.69). 8. The possible contribution of catabolized amino acids to glucose IRL was calculated by assuming that 0.35 of urea IRL was due to protein catabolism and 0.2 of catabolized protein-C passed through the glucose pool. On this basis the contribution of protein-C top glucose-C was < 25 g/kg glucose IRL.

Uptake of [14C]urea by the in vivo choroid plexus--cerebrospinal fluid--brain system: identification of sites of molecular sieving

1. The time course of the uptake of [(14)C]urea by the lateral ventricular choroid plexus of the adult rat in vivo was analysed to delineate further the permeability characteristics of the epithelial membrane of this secretory tissue.2. Eight hours after I.P. injection, [(14)C]urea attained a steady-state distribution in 78% of the tissue water of lateral ventricular choroid plexus; similarly, approximately 8 hr was required for radiourea to reach a steady-state concentration in both the cerebral cortex and cerebrospinal fluid (c.s.f.).3. Results obtained for compartment analysis were used to calculate the concentration of [(14)C]urea in the epithelium of the lateral ventricular plexus during the approach to and at steady-state distribution. Even after 1 hr of distribution, the [(14)C]urea concentration in choroid cell water was less than 15% of that in plasma water.4. Although the concentration of radiourea in choroid cell water continually increased after 3 hr, it remained in equilibrium with the concentration of [(14)C]urea in c.s.f. water. At the steady state (i.e., 8 hr), the distribution of [(14)C]urea between the water of plasma and that of the choroidal epithelium was considerably away from equilibrium (i.e., by 25-30%).5. An analysis of the concentration gradients for [(14)C]urea across both the apical (c.s.f.-facing) and basolateral (plasma-facing) membranes of the epithelium of the lateral ventricular plexus suggests that the movement of urea is hindered to a greater extent by the basolateral membrane than by the apical membrane.6. Only a single half-time component (1.3 hr) can be resolved from analysis of the curve describing the time course of uptake of radiourea by the choroid epithelial cell compartment.7. The concentration gradient data suggest that urea penetrates from blood to c.s.f. via the choroid plexus by a transcellular pathway; however, it is not possible to rule out a paracellular pathway for urea movement.8. At the steady state, radiourea distributes into 88% of the water of cerebral cortex. This observation, together with the finding of a steady-state concentration gradient for [(14)C]urea from cortical tissue to c.s.f., constitutes evidence that urea movement is hindered at the blood-brain barrier as well as at the blood-c.s.f. barrier.

Compartments and barriers in the sciatic nerve of the rabbit

Near constant concentration of various radioactively labelled polar nonelectrolytes were maintained in the blood plasma of renal ligated rabbits. The uptake of [14C]urea, [51Cr]EDTA and [14C]inulin into sciatic nerve was interpreted in terms of a two-compartment model. There was a rapidly equilibrating space representing about 20% of the tissue weight which is almost certainly the interstitial fluid of the epineurium and outer layers of the perineurium. Urea penetrated the blood-nerve barrier at a similar rate to the entry of this compound into brain. Sucrose, chromium-EDTA and sulphate passed into the true interstitial fluid approaching a total volume of distribution of about 36% of the tissue weight at 6-8 h. Since movement of these 3 solutes is severely restricted by the blood-barrier, the blood-nerve barrier in the rabbit must contain channels of greater than 1 nm which are not present in the developed blood-brain barrier. Uptake of [3H]3-O-methylglucose into nerve was examined but the results did not give a conclusive answer to the question of whether there is specific transport of sugars at the blood-nerve barrier. The wet nerve contained (in mmole/kg) sodium 74.3; potassium 42.6 and chloride 48.8. Water was 64.6 ml/100 g. Intracellular electrolyte concentrations were estimated. Approximately 30% of the total sodium content cannot be accounted for as being free in the interstitial fluid at a Gibbs-Donnan distribution with plasma. Analysis of metabolically poisoned nerves which have equilibrated with salt solution in vitro suggests that negatively charged polyelectrolyte molecules retain about 14 mmole of sodium/kg wet normal nerve.

Transport of potassium at the blood-brain barrier

1. The pial surface of different regions of the central nervous system of the rabbit have been bathed with artificial cerebrospinal fluid (c.s.f.), containing different concentrations of potassium. The object has been to change the composition of the interstitial fluid with respect to this ion, where it is adjacent to subarachnoid c.s.f.2. Two techniques, subarachnoid perfusion from the supracallosal space between the hemispheres to the cisterna magna and barbotage from the cisterna magna, have been used. If the artificial c.s.f. contains Evans Blue, the former procedure results in maximum staining of the pia and underlying nervous tissue of the pons-medulla and spinal cord. The latter procedure results in maximum staining of the medial and supero-lateral surfaces of the cerebral hemispheres, particularly anteriorly.3. During subarachnoid perfusion at 0.06 ml./min with the potassium-free fluid, most regions of the brain took up significantly greater amounts of (42)K than was the case during perfusion with the fluids containing 3 and 10 m-equiv/l. For blue cerebral cortex, the tissue subjected directly to the inflowing fluid and showing the biggest differences, the ratios, c.p.m. per g brain/c.p.m. per ml. plasma, were 1.71 +/- 0.12 (+44%), 1.19 +/- 0.05 and 1.07 +/- 0.08 (-10%) during perfusion with the fluids containing, 0, 3 and 10 m-equiv/l. respectively.4. During barbotage, the uptake of (42)K into pons-medulla and spinal cord from blood plasma, the concentration in the latter being effectively kept near constant, was, at the end of 2 hr, greater when the fluid contained potassium, 0 m=-equiv/l. rather than 10 m-equiv/l. Thus the ratio, c.p.m per g brain/c.p.m. per ml. plasma was (0.99 +/- 0.04 (+36%) as against 0.73 +/- 0.05 for pons-medulla where the difference was greatest.5. Simultaneous measurements of the entry of [(14)C]urea from blood to different regions of the central nervous system revealed no significant differences due to the differing concentrations of potassium imposed by either barbotage or subarachnoid perfusion. This appears to exclude a non-specific cause for the changes in (42)K uptake, an example of which might be a changing blood flow.6. Reasons are given for supposing that the big increase in (42)K uptake due to the potassium-free fluid must be due to events occurring at the blood-brain barrier. This might be some form of interaction, possibly the single file effect, such that a low potassium concentration in the interstitial fluid potentiates (42)K influx across the blood-brain barrier. Alternatively it might be due to a low potassium concentration in this fluid greatly reducing active potassium movement from interstitial fluid to blood. The former explanation would conform neatly with the present results; but the latter would additionally be compatible with other evidence concerning the homoeostasis of potassium concentration in c.s.f. and the interstitial fluid of brain.

Release of neurohypophysial hormones in vitro

1. The rate of release of neurohypophysial hormones in vitro, using isolated, halved neural lobes of the rat in an incubation medium containing excess K(+) and Ca(2+), was measured. The highest average rate of release was observed between 10 and 20 min after commencement of incubation.2. Incubation of isolated, halved rat neural lobes in the presence of acetylcholine, with or without eserine, did not stimulate hormone release. When complete isolated hypothalamo-neurohypophysial systems were incubated in a suspension medium containing 10(-7) mg/ml. acetylcholine a significant increase in the release of oxytocin occurred (P < 0.01); the increase in vasopressin release was less pronounced (P < 0.05).3. Uptake of O(2) by the isolated, halved neural lobes and the hypothalamo-neurohypophysial systems continued for 2-3 hr, i.e. in excess of the experimental incubation time.4. During the first 40 min of incubation the control halved neural lobes increased in weight; the neural lobes incubated in buffer containing high potassium and calcium showed no increase in weight.5. Neural lobes incubated in buffer containing excess K(+) and Ca(2+) contained about 3 times as much potassium as controls. The sodium content was not affected significantly.6. Factors involved in the process of neurohypophysial hormone release are discussed.