The mechanism of urine concentration in the inner medulla

Intrarenal urea recycling leads to a higher rate of renal excretion of potassium: an hypothesis with clinical implications

PURPOSE OF REVIEW

This review aims to illustrate why urea recycling may play an important role in potassium (K⁺) excretion and to emphasize its potential clinical implications.

RECENT FINDINGS

A quantitative analysis of the process of intrarenal urea recycling reveals that the amount of urea delivered to the distal convoluted tubule is about two-fold larger than the quantity of urea excreted in the urine. As the number of osmoles delivered to the late cortical distal nephron (CCD) determines its flow rate when aquaporin 2 water channels have been inserted in the luminal membrane of principal cells, urea recycling may play an important role in regulating the rate of excretion of K⁺ when the distal delivery of electrolytes is not very high.

SUMMARY

Urea recycling aids the excretion of K⁺; this is especially important in patients with disorders or those who are taking drugs that lead to a less lumen-negative voltage in the CCD. As a large quantity of urea is reabsorbed daily in the inner medullary collecting duct, the assumption made in the calculation of the transtubular K concentration gradient that there is no appreciable reabsorption of osmoles downstream CCD is not valid.

Mechanisms to concentrate the urine: an opinion

PURPOSE OF REVIEW

Our goal is to suggest how the renal concentrating mechanism is regulated in vivo.

RECENT FINDINGS

The majority of descending thin limbs of the loop of Henle lack aquaporin-1 water channels, and loops of Henle in the inner medulla lack urea transporters.

SUMMARY

Lack of water permeability in the descending thin limbs of the loop of Henle offers several advantages. First, since much less water is added to the outer medullary interstitial compartment, inhibitory control mechanisms on sodium and chloride reabsorption from the medullary thick ascending of loop of Henle initiated by water addition from the medullary collecting duct can be effective. Second, recycling of urea is efficient, as little urea will be washed out of the medulla. Third, delivery of a larger volume of filtrate to the medullary thick ascending limb of the loop of Henle permits both an appreciable reabsorption of sodium along with only a small fall in the luminal concentration of sodium in each of these liters. Hence there need be only a small lumen positive voltage in the medullary thick ascending limb of the loop of Henle. The absence of urea transporters in the loop of Henle in the inner medulla is required for a passive mechanism of sodium and chloride reabsorption in the inner medulla. Control of urea reabsorption from the medullary collecting duct is needed to prevent excessive oliguria in electrolyte-poor urine.

Diagnostic approach to a patient with hyponatraemia: traditional versus physiology-based options

The usual diagnostic approach to a patient with hyponatraemia is based on the clinical assessment of the extracellular fluid (ECF) volume, and laboratory parameters such as plasma osmolality, urine osmolality and/or urine sodium concentration. Several clinical diagnostic algorithms (CDA) applying these diagnostic parameters are available to the clinician. However, the accuracy and utility of these CDAs has never been tested. Therefore, we performed a survey in which 46 physicians were asked to apply all existing, unique CDAs for hyponatraemia to four selected cases of hyponatraemia. The results of this survey showed that, on average, the CDAs enabled only 10% of physicians to reach a correct diagnosis. Several weaknesses were identified in the CDAs, including a failure to consider acute hyponatraemia, the belief that a modest degree of ECF contraction can be detected by physical examination supported by routine laboratory data, and a tendency to diagnose the syndrome of inappropriate secretion of antidiuretic hormone prior to excluding other causes of hyponatraemia. We conclude that the typical architecture of CDAs for hyponatraemia represents a hierarchical order of isolated clinical and/or laboratory parameters, and that they do not take into account the pathophysiological context, the mechanism by which hyponatraemia developed and the clinical dangers of hyponatraemia. These restrictions are important for physicians confronted with hyponatraemic patients and may require them to choose different approaches. We therefore conclude this review with the presentation of a more physiology-based approach to hyponatraemia, which seeks to overcome some of the limitations of the existing CDAs.

How to select optimal maintenance intravenous fluid therapy

Hyponatraemia is the commonest electrolyte abnormality in hospitalized patients. If the plasma sodium concentration (P(Na)) declines to approximately 120 mM in <48 h, brain cell swelling might result in herniation, with devastating consequences. The volume and/or the composition of fluids used for intravenous therapy often contribute to the development of acute hyponatraemia. Our hypothesis is that the traditional calculation of the daily loss of insensible water overestimates this parameter, leading to an excessive daily recommended requirement for water. We offer suggestions to minimize the risk of iatrogenic hyponatraemia.

Influence of hypernatraemia and urea excretion on the ability to excrete a maximally hypertonic urine in the rat

Rats normally excrete 20-25 mmol of sodium (Na+) + potassium (K+) per kilogram per day. To minimize the need for a large water intake, they must excrete urine with a very high electrolyte concentration (tonicity). Our objective was to evaluate two potential factors that could influence the maximum urine tonicity, hypernatraemia and the rate of urea excretion. Balance studies were carried out in vasopressin-treated rats fed a low-electrolyte diet. In the first series, the drinking solution contained an equivalent sodium chloride (NaCl) load at 150 or 600 mmol l(-1). In the second series, the maximum urine tonicity was evaluated in rats consuming 600 mmol l(-1) NaCl with an 8-fold range of urea excretion. Hypernatraemia (148 +/- 1 mmol l(-1)) developed in all rats that drank 600 mmol l(-1) saline. Although the rate of Na+ + K+ excretion was similar in both saline groups, the maximum urine total cation concentration was significantly higher in the hypernatraemic group (731 +/- 31 vs. 412 +/- 37 mmol l(-1)). Only when the rate of excretion of urea was very low, was there a further increase in the maximum urine total cation concentration (1099 +/- 118 mmol l(-1)). Thus hypernatraemia was the most important factor associated with a higher urine tonicity.