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Journal of Physiology (2002), 541.3, pp. 929-936
© Copyright 2002 The Physiological Society
DOI: 10.1113/jphysiol.2002.019430
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ABSTRACT |
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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.
(Resubmitted 21 February 2002; accepted after revision 27 March 2002)
Corresponding author M. L. Halperin: Division of Nephrology, St Michael's Hospital Annex, 38 Shuter Street, Toronto, Ontario, Canada M5B 1A6. Email: mitchell.halperin{at}utoronto.ca
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INTRODUCTION |
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Two factors are needed to excrete a concentrated urine (Sands & Layton, 2000). First, a hyperosmolar medullary interstitial compartment provides the osmotic driving force to reabsorb osmole-free water from the medullary collecting duct (MCD). Second, the MCD must be permeable to water - this occurs when vasopressin causes the insertion of aquaporin-2 (AQP-2) water channels into the luminal membrane of the distal nephron (Nielsen et al. 2002).
The inner and outer areas of the renal medulla carry out unique roles to concentrate the urine. The main function of the inner medulla is to permit the excretion of a minimum volume of urine when there is a water deficit. To achieve this aim, urea must become permeable so that it is an 'ineffective' urine osmole, i.e. have near-equal concentrations in the lumen of the inner MCD and in the papillary interstitial compartment (Gowrishankar et al. 1998). This requires actions of vasopressin to insert urea transporters into the luminal membrane of the inner MCD (Smith et al. 1995; Sands, 1999).
The main function of the outer medulla is to permit the excretion of hypertonic urine when there is a large intake of salt with little water - the urine often has both a very high electrolyte concentration (tonicity) and volume (eqn (1)). The excretion of hypertonic urine is probably the most important medullary function in rats because their daily intake of electrolytes is both large and hypertonic. In quantitative terms, rats weighing 350 g consuming their usual chow excreted close to 8 mmoles of sodium (Na+) plus potassium (K+) in 24 h (Lin et al. 1998); this electrolyte excretion rate is almost an order of magnitude higher than in humans on a typical Western diet when the comparison is made on a 'per kg' basis. This electrolyte load is due to a much higher intake of calories needed to maintain body temperature in a small animal rather than a difference in composition of the diet (electrolytes/unit calorie) (Schmidt-Nielsen, 1991). If this electrolyte load were not excreted promptly in a very hypertonic form, body tonicity would rise and thirst would be stimulated (Fitzsimons, 2000). To make this intake into an isotonic solution, an imaginary 70 kg rat would need to drink 10-15 l of water per day. To do so and drink by licking would seriously compromise other essential activities. Hence the renal responses to excrete a large hypertonic salt load promptly in the rat represents its usual physiology (see Appendix for a more detailed consideration of the need to have a high urine tonicity):
(1) Our purpose was to evaluate two factors that might be important to achieve both a large urine volume and a very high urine electrolyte concentration (tonicity). Two hypotheses were tested. First, will the induction of hypernatraemia raise the urine tonicity? Second, will the rate of excretion of urea influence the urine tonicity? Therefore two protocols were employed. In the first, rats consuming a low-electrolyte diet were given vasopressin and an equal Na+ load in an isotonic or a hypertonic form. In the second protocol, urea excretion was varied over an 8-fold range in rats that drank the 600 mmol l-1 NaCl solution. The results reported here suggest that the urine tonicity was higher in rats with hypernatraemia. The urine tonicity was also somewhat higher in rats with a very low rate of excretion of urea.
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METHODS |
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Rats were cared for in accordance with the principles and guidelines of the Canadian Council on Animal Care. The Animal Care Committee of St Michael's Hospital approved this study protocol.
Procedure
Adult male Wistar rats (weight 300-400 g) were housed in individual metabolic cages so that complete collections of urine could be obtained for balance studies. Sucrose (5 %) was added to the saline drinking solution to ensure its complete consumption. Rats consuming isotonic saline drank 4-fold more drinking solution so that the Na+ load would be similar in the two salt-loaded groups. Urine was collected in two portions, one over the 7 h period between 10.00 and 17.00 h and the second over the remaining 17 h period which represented an overnight collection. Thymol was added as a urine preservative. Results are reported for the night-time period of the second day for the timed collections to minimize risk of admixture of urine before rats consumed the saline-containing drinking water. Blood was drawn under light anesthesia at the end of the balance period.
Experimental protocols
Two protocols were followed.
Protocol 1: Effect of the concentration of sodium chloride in the drinking solution on the maximum urine tonicity. After 2 days on the low-electrolyte diet (Na+ 0.2 mmol kg-1, K+ 0 mmol kg-1, Cl- 0 mmol kg-1), sodium chloride (NaCl) was added to the drinking water as isotonic saline (150 mmol l-1) (n = 6) or hypertonic saline (600 mmol l-1) (n = 7) on days 3 and 4 on this diet. The quantity of Na+ and Cl- ingested was virtually identical in these two groups of rats. To ensure that V2 actions of vasopressin were not a variable, all rats were given 2 µg of desmopressin acetate (dDAVP, Ferring Co., Ontario, Canada) by the intraperitoneal route at 10.00 and 17.00 h on each of the two experimental days. Another subgroup of rats (n = 6) that ingested isotonic saline was given vasopressin (1.2 USP pressor units (kg body weight)-1) to ensure that the absence of V1 receptor agonists was not the reason for the differences observed. Since there were no observable effects of vasopressin, the data from these rats are not shown.
Protocol 2: Effect of the rate of excretion of urea on the urine tonicity in rats with hypernatraemia. The purpose of this series of experiments was to examine the effect of altering the rate of excretion of urea on the urine tonicity in hypernatraemic rats. The rate of excretion of urea was varied by 8-fold by having two extremes of protein intake and by adding urea to the drinking water. Varying the diet minimized the risk of having a large and sudden oral urea load that might cause a transient urea-induced osmotic diuresis. To induce hypernatraemia, rats drank 600 mmol l-1 NaCl in their 5 % sucrose drinking solution - urine samples were collected as described above. Four groups of rats were studied. Group 1 rats (n = 8) were fed a protein-free diet that consisted of hydrogenated vegetable oil supplemented with carbohydrate. Groups 2 (n = 8), 3 (n = 5) and 4 (n = 6) consumed regular rodent laboratory chow. The drinking solution of group 2 did not contain urea whereas the final urea concentration in the drinking solution of groups 3 and 4 was 500 and 2000 mmol l-1, respectively.
Analytical techniques
Na+ and K+ in plasma and urine were determined by flame photometry, Cl- was determined by electromimetic titration, osmolality was measured by freezing point depression (Advanced Instruments Inc, Needham Heights, MA, USA), and blood gas analysis was performed at 37 °C with a digital pH/blood gas analyser (Corning 178 blood pH analyser). Ammonium (NH4+) in urine, and urea and creatinine in plasma and urine were measured as previously described (Halperin et al. 1985; Cheema-Dhadli & Halperin, 1993).
Calculations
The balance calculations for all ions and water plus the rate of excretion of urea, NH4+ and creatinine were determined daily as previously described (Lin et al. 1998).
The quantity of electrolytes excreted in hypertonic form was calculated in two steps. First, the concentration of Na+ + K+ in plasma was subtracted from the total cation concentration in the urine to yield the concentration of cations in hypertonic form. Second, the resultant cation concentration in hypertonic form was multiplied by the urine flow rate to obtain an excretion rate. To convert this excretion rate to a negative free-water excretion rate (TcH2O), the rate of excretion of cations in hypertonic form was divided by the sum of the Na+ + K+ concentrations in plasma.
Statistical analysis
Results are reported as mean ± S.E.M. Statistical analysis was performed on the group mean values using an unpaired Student's t test. A P value of less than 0.05 was considered to be statistically significant.
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RESULTS |
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The effect of hypertonic saline intake on the urine tonicity
Rats drinking both isotonic and hypertonic NaCl had similar Na+ intakes (3.5 ± 0.31 and 3.6 ± 0.23 mmol (24 h)-1) and outputs (1.5 ± 0.20 and 1.7 ± 0.21 mmol (24 h)-1) over 24 h (Table 1). Thus they excreted almost half of the ingested Na+. Rats drinking hypertonic saline (600 mmol l-1) developed a significant degree of hypernatraemia (148 ± 1.0 mmol l-1, Table 1). In contrast, the plasma Na+ concentration in rats that had isotonic saline added to their drinking water was in the normal range (142 ± 0.6 mmol l-1). The plasma Cl- concentrations changed in a parallel fashion (107 ± 1.1 mmol l-1 and 101 ± 0.6 mmol l-1, respectively, P < 0.05). There were no significant differences in the concentrations of K+, creatinine and urea in plasma between these two groups (Table 1). Although sucrose was added to the drinking water, hyperglycaemia and/or glucosuria were not observed.
Rats consuming hypertonic saline had a significantly lower urine flow rate (2.9 ± 0.3 vs. 4.7 ± 0.7 µl min-1) and a higher concentration of the major cations (Na+ + K+ + NH4+) in their urine (731 ± 31 vs. 412 ± 37 mmol l-1, P < 0.05, Table 1). The concentration of urea in the urine was similar in both groups of rats (1215 ± 125 and 1144 ± 157 mmol l-1) so the urine osmolality was higher in the rats given the hypertonic salt load (2602 ± 203 vs. 2079 ± 235 mosmol (kg H2O)-1, Table 1). The quantity of electrolytes excreted in hypertonic form and the TcH2O excretion rates were significantly higher in the hypernatraemic rats (1.64 ± 0.13 vs. 1.16 ± 0.13 µmol min-1, and 10.9 ± 0.87 vs. 7.7 ± 0.87 µl min-1, respectively, Table 1).
The effect of the urea excretion rate on the urine tonicity
In the second protocol, there was an 8-fold range of urea excretion (1.3 ± 0.1 to 12.8 ± 1.1 mmol day-1) and all the rats had hypernatraemia (see legend to Fig. 1). The maximum urine Na+ + K+ + NH4+ concentration was close to 800 mmol l-1 in all but the rats with the lowest urea excretion rate - in these rats, the urine tonicity was 1099 ± 118 mmol l-1.
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Figure 1. Effect of the rate of excretion of urea on the maximum urine tonicity The rate of urea excretion is shown on the x-axis. The urine Na+ + K+ + NH4+ concentrations (mmol l-1) are depicted by the open symbols and urine urea concentration (mmol l-1) by the filled symbols. Rats on the protein-free diet are shown in the square symbols while those on regular chow with a 0, 500 and 2000 mmol l-1 urea supplements are shown by the circles, diamonds, and triangles, respectively. The plasma Na+ concentration in each set of rats, beginning with those with the lowest urea excretion rate, were 148 ± 0.5, 153 ± 1.4, 148 ± 1.1 and 147 ± 0.6 mmol l-1 (mean ± S.E.M.). | ||
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DISCUSSION |
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The principal finding using the first protocol was that hypernatraemic rats had a higher urine tonicity than did the normonatraemic rats with a similar intake and excretion of NaCl. The degree of extracellular fluid (ECF) volume expansion was unlikely to be very different in either of these groups because of their similar positive Na+ balances and because their plasma Na+ concentrations differed by only 6 mmol l-1 (Table 1). Therefore other factors are needed to explain why a higher urine tonicity was present in the hypernatraemic rats. There were two differences between the hypernatraemic and normonatraemic rats. First, hypernatraemia should contract the intracellular fluid volume (Robertson & Berl, 1996). Second, the filtered load of Na+ was greater in the hypernatraemic rats because their glomerular filtration rate (GFR) was not different as reflected by the identical plasma creatinine values (32 µmol l-1 in both the isotonic and hypertonic saline groups, Table 1).
The principal finding using the second protocol was that the urine tonicity was similar over a wide range of urea excretion rates in hypernatraemic rats. There was a minor exception, however - rats with the lowest urea excretion rate had a somewhat higher urine tonicity.
As background for the remainder of the discussion, the process of raising the tonicity in the renal medullary interstitial compartment will be evaluated in quantitative terms. For simplicity, values in humans will be utilized.
Events in the vasa recta
All the water and solutes reabsorbed in nephron segments that traverse the renal medulla must exit the medulla with a similar composition to the plasma that entered via the descending vasa recta because both limbs of the vasa rectae are very permeable to water, electrolytes, and urea (Sands & Layton, 2000). Therefore a larger volume will exit via the ascending vessels. The source of the NaCl that makes the reabsorbed osmole-free water into an isosmotic solution will be considered in the next section.
Events in the descending thin limb (DtL) of the loop of Henle (LOH)
The volume delivered to the LOH is close to one-third of the GFR, or 60 l day-1 (Lassiter et al. 1961; Windhager & Giebisch, 1961). The presence of water channels in the DtL ensures the luminal and medullary interstitial osmolalities will be equal at a given horizontal plane (Agre et al. 1993). Because the osmolality of the interstitial compartment rises from 300 to 900 mosmol (kg H2O)-1 as one proceeds from the cortex to the junction of the outer and inner medulla (Steinmetz & Smith, 1963; Oh & Halperin, 1997), almost two-thirds of the 60 l (i.e. 40 l) will be absorbed in the DtL. Because of a higher urea concentration in the interstitial compartment and the presence of transporters for urea in the DtL (Sands & Layton, 2000), urea enters the lumen of this nephron segment (called recycling of urea). The DtL is also permeable to Na+ and the direction of Na+ diffusion must be down its concentration gradient at any horizontal plane. With identical luminal and interstitial osmolalities at each horizontal plane, the sum of the osmotic contributions of Na+ + Cl- and urea must be equal in these two compartments. Comparing the DtL lumen and the interstitial compartment, if the luminal urea concentration is lower in the DtL, its luminal Na+ concentration must be higher because of identical osmolalities. Therefore when urea diffuses from the interstitial compartment to the lumen of the DtL, Na+ can only diffuse in the opposite direction.
Events in the medullary thick ascending limb of the LOH (mTAL)
In this water-impermeable segment, Na+ and Cl- , but not water, are reabsorbed. For each litre of osmole-free water reabsorbed from the water-permeable DtL and MCD, enough Na+ and Cl- must be reabsorbed from the mTAL to make this osmole-free water ultimately become isosmotic to the fluid that enters the medulla via the descending vasa recta. Because 40 l were reabsorbed from the DtL and approximately 4 l from the MCD (Oh & Halperin, 1997), 90 % of the NaCl reabsorbed in the mTAL provides the osmoles needed to make this osmole-free water from the DtL become isosmolar to plasma. Therefore only 10 % of the Na+ and Cl- reabsorbed from the mTAL contributes directly to creating the driving force to reabsorb osmole-free water from the MCD (concentrating the urine).
Transport systems for Na+ and Cl- reabsorption in the mTAL
The 'single effect' that raises the Na+ concentration in the medullary interstitial compartment is the result of two separate Na+ reabsorbing pathways in the mTAL that each reabsorb an equal amount of Na+. Active reabsorption of Na+ occurs by the combined flux via the luminal Na+-K+-2Cl--cotransporter (NKCC) and the Na+-K+-ATPase on the basolateral membrane of mTAL cells. K+ enters the lumen via ROM-K channels to provide K+ for the NKCC - this K+ entry also causes a lumen-positive voltage in the mTAL. The second pathway is passive and is the result of diffusion of Na+ through the paracellular pathway driven by this lumen-positive voltage (Fig. 2). To absorb equivalent amounts of Na+ and Cl- in the mTAL, there must be a 1 : 1 link between these two transport systems. The magnitude of this lumen-positive voltage dictates the maximum ratio of the concentrations of Na+ in the luminal fluid of the mTAL relative to that in the interstitial compartment at any given horizontal plane (Hebert & Andreoli, 1986). In quantitative terms, if this lumen-positive voltage were 6 mV, the concentration of Na+ in the lumen of the mTAL would be 80 % of the interstitial Na+ concentration at that horizontal plane at equilibrium (Nernst equation). Because flux through the NKCC and the paracellular Na+ channel both depend on the net entry of K+ through the ROM-K channel, the latter is a likely control site. As we shall hypothesize later, control of the open-probability of ROM-K and/or its conductance for K+ could regulate the reabsorption of Na+ and Cl- in the mTAL and thereby the maximum urine tonicity.
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Figure 3. Model for the concentrating process in the renal medulla When more water is reabsorbed from the MCD (see control panel on the right), the concentration of ionized Ca2+ falls in the interstitial compartment. As a result, less Ca2+ binds to the Ca2+ receptor (Ca2+-Rec) on the basolateral aspect of cells of the mTAL. This in turn leads to a greater open-permeability of ROM-K channels in the luminal membrane of the mTAL. Hence more K+ enters the lumen of the mTAL which augments the reabsorption of Na+ by the NKCC and the paracellular pathway until the lumen voltage rises sufficiently to drive the reabsorption of sufficient Ca2+ to again inhibit the process of Na+ reabsorption in the mTAL, completing the feedback loop. The net result could be a higher concentration of electrolytes in the medullary interstitial compartment. | ||
In the paragraphs to follow, we shall consider how the two major differences attributed to the hypertonic saline load (higher PNa, larger filtered load of Na+) might contribute to a higher urine tonicity.
A higher Na+ concentration in fluid entering the LOH might increase Na+ reabsorption in the mTAL
Hypernatraemia leads to a higher filtered load of Na+ if the GFR does not decline. If the same proportion of filtrate were reabsorbed in the proximal convoluted tubule when the PNa was higher, the luminal Na+ concentration at the junction of the inner and outer renal medulla will increase by the same multiple (e.g. one would now triple a higher number because of the hypernatraemia). Therefore, there should be a higher concentration of Na+ in each litre of filtrate delivered to the mTAL. Hence with no change in lumen-positive voltage, more Na+ and Cl- would be reabsorbed in the mTAL (some percentage decline in its luminal Na+ concentration, but starting from a higher luminal value). Thus this could be a partial explanation for the higher urine tonicity in hypernatraemic rats.
Possible stimulus for the reabsorption of Na+ and Cl- in the mTAL
A larger delivery of Na+ and Cl- to the MCD would be expected when there is a higher filtered load of Na+ and Cl- in these hypernatraemic, ECF-volume expanded rats. When vasopressin acts, the osmolalities of fluid in plasma and the lumen of the terminal cortical collecting duct should be equal (Steele et al. 1994). Therefore there will be a higher osmole delivery to the MCD which implies that volume delivery to the MCD will also be larger. Hence the same percentage but a larger volume of osmole-free fluid will be added to the medullary interstitial compartment when more Na+ is delivered from upstream nephron sites. Moreover, because the urine tonicity and osmolality were actually higher in the hypernatraemic rats (Table 1), even more osmole-free water was reabsorbed from the MCD. If this larger addition of osmole-free water to the medullary interstitial compartment were an isolated event, the interstitial osmolality should fall, but it actually rose in vivo. Therefore the question is, how might this addition of water to the medullary interstitial compartment stimulate Na+ reabsorption from the mTAL? Our speculation is outlined in the next paragraph.
To raise the medullary interstitial osmolality, the added water might have lowered the interstitial concentration of an inhibitor of mTAL Na+ and Cl- reabsorption (Fig. 3). A possible candidate for this function is ionized calcium (Ca2+) because its interstitial activity is linked to ROM-K flux in the mTAL. In more detail, when Ca2+ binds to its receptor on the basolateral aspect of mTAL cells, ROM-K channels in their luminal membrane are inhibited (Hebert, 1996), thereby depressing the reabsorption of Na+ and Cl- by the NKCC cotransporter. In addition, with less entry of K+ into mTAL luminal fluid, its voltage would become less positive, thereby diminishing the electrogenic reabsorption of Na+ by the paracellular pathway. On the other hand, with a lower Ca2+ activity in the medullary interstitial compartment, ROM-K channel flux would increase. As a result, the reabsorption of Na+ + Cl- in the mTAL would be accelerated. Its lumen voltage would become more positive which would also accelerate the paracellular reabsorption of Na+ + Ca2+ until a new steady state was achieved. The net result would be a more hypertonic medullary compartment.
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Figure 2. The reabsorption of Na+ in the LOH There are two major issues with respect to permeabilities in the mTAL. This nephron segment is impermeable to water (||, site 1) and its paracellular area is permeable to the cations Na+, Ca2+ and Mg2+ (site 2). There are two major luminal ion transport systems, the electroneutral NKCC cotransporter (site 3) and an electrogenic ROM-K ion channel (site 4). Reabsorption of Na+ via the NKCC and via passive diffusion occurs with a 1 : 1 stoichiometry. | ||
Urea and the maximum urine non-urea osmolality
When urea recycles, the medullary interstitial osmolality declines because osmoles (urea) leave the interstitial compartment and enter the DtL (Fig. 4). To prevent this fall in interstitial osmolality, there must be an equivalent addition of osmoles (Na+ + Cl-) without water from the mTAL. Because there is no rise in interstitial osmolality by this component of mTAL Na+ + Cl- reabsorption, a driving force to reabsorb water from the MCD was not created. Therefore if less urea were to recycle and there were no changes in Na+ and Cl- reabsorption in the mTAL, more water could be reabsorbed from the MCD. To test the potential impact of reduced urea recycling on the maximum urine tonicity, rats in the second series of experiments were given hypertonic saline. Variations in dietary protein ± the use of urea supplements ensured that there was an 8-fold range in the rate of excretion of urea. Only at the extremely low rate of urea excretion was the urine tonicity somewhat higher (Fig. 1). Therefore recycling of urea did not seem to have an important effect on the excretion of a maximally hypertonic urine.
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Figure 4. Urea recycling and mass balance in the outer medulla The LOH in the outer medulla with its DtL and mTAL is represented by the loop structure on the left; the MCD is represented by the cylindrical structure on the right. There is a rise in osmolality in the interstitial compartment from 300 mosmol (kg H2O)-1 at the cortical-medullary junction to 900 mosmol (kg H2O)-1 deeper in the outer medulla. Close to 50 % of the urea delivered to the MCD is said to recycle (urea is reabsorbed in the inner MCD and enters the DtL). For osmotic balance in the interstitial compartment in this example, 450 mosmol of Na+ + Cl- will be reabsorbed from the mTAL (shown on the same horizontal plane). There must be a negative balance of 300 mosmol over the LOH to reabsorb 1 l of osmole-free water from water-permeable structures (shown as the MCD for convenience). Hence there must be a total of 750 mosmol of Na+ + Cl- reabsorbed in the LOH, of which, 450 mosmol were used to match the recycling of 450 mmol of urea. | ||
Limitations and contingencies
Whenever experiments are conducted in vivo, there are variables that cannot be controlled. For example, water intake was greater in the rats given isotonic as compared to hypertonic saline. This difference is required, however, if the goal of the experiment is to provide an equal Na+ load to both groups of rats. Another design element was to make as few manipulations as possible to minimize stress and its resultant hormonal changes. Therefore we did not use gavage feeding or anaesthesia. Despite these limitations, whole animal experiments are the best way to provide insights into a possible relationship between hypernatraemia, the rate of excretion of urea, and the concentrating mechanism. We acknowledge that many of the conclusions must be indirect because certain data could not be obtained in unanaesthetized rats. For example, the activity of ionized Ca2+ in the interstitial compartment at a specific horizontal plane in the renal medulla could not be measured. Therefore, some speculation has been required and this has been clearly identified in this report.
Concluding remarks
The maximum tonicity of the urine achieved with the use of vasopressin was significantly higher when a large hypertonic salt load was ingested. A series of factors that could permit the mTAL to increase its reabsorption of Na+ and Cl- to draw more water out of the MCD were evaluated in this context. Of these, hypernatraemia appeared to be important whereas the rate of excretion of urea was not critical for this function. The importance of the urine tonicity, which is not considered when describing events in TcH2O terms, is evaluated in the Appendix.
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APPENDIX |
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The importance of having a very high tonicity
The traditional way to evaluate the excretion of a hypertonic electrolyte load is to measure the quantity of electrolytes excreted in hypertonic form - TcH2O excretion (Schuster & Seldin, 1993). The TcH2O analysis does not include an assessment of the urine tonicity.
The following example illustrates the importance of a higher urine tonicity. A human subject has a daily urine volume of 1 l that contains 300 mosmol of electrolytes. Three conditions are set in an imaginary experiment. First, he will ingest and excrete an extra 300 mosmol of electrolytes without additional water in 1 day (Fig. 5). Second, his body tonicity must not change. Therefore he must excrete the extra 300 mosmol of electrolytes in a hypertonic form in his urine. Third, his maximum urine tonicity will be 600 mosmol l-1 on one day and 400 mosmol l-1 on another day.
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Figure 5. Urine volume and compositions in the two examples where the intake is hypertonic saline Urine (1 l) is depicted as a rectangle; the osmolality due to Na+ salts in each litre is shown in an oval inside the rectangle with its osmolality due to hypertonic Na+ salts added below. The subject usually excretes 300 mosmol of Na+ salts and 1 l of water. On an experimental day, an extra 300 mmol of hypertonic Na+ is ingested (total daily osmoles due to Na+ salts is now 600 mosmol). The plasma Na+ concentration remains constant in each setting. In the first condition (top panel of the figure), the urine osmolality due to Na+ salts may rise to 600 mosmol l-1 whereas in the second condition (bottom panel of the figure), it may rise to only 400 mmol l-1. Therefore the final urine volume will be 1 l in the first setting and 3 l in the second setting in order to excrete 300 mosmol of Na+ salts in a hypertonic form (same TcH2O). | ||
In the first condition, his 300 mosmol hypertonic electrolyte supplement could be eliminated in his usual 1 l urine volume because its tonicity can rise from 300 to 600 mosmol l-1 (top portion of Fig. 5).
In the second condition, because his maximum urine tonicity is 400 mosmol l-1, he can only add 100 mosmol of electrolytes in hypertonic form to the original 1 l of urine (it contained 300 mosmol l-1 of electrolytes before the electrolyte supplement). Therefore, to excrete the remaining 200 mosmol of electrolytes in hypertonic form without causing a change in body tonicity, he must add the remaining 200 mosmol to an extra 2 l of isotonic urine because his maximal urine tonicity is now 400 mosmol l-1. This would create a negative balance of 2 l of water and 600 mosmol of electrolytes because no change in water intake was permitted (bottom portion of Fig. 5). In both settings, the urine TcH2O is identical.
We conclude that excreting the same volume of TcH2O at two different urine tonicities can lead to a major difference in body composition when water intake is limited. Thus it is less than ideal to focus solely on the rate of TcH2O excretion to evaluate the function of the outer medulla - the maximum urine tonicity should also be examined for this purpose.
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Acknowledgements
We are extremely grateful to Dr Kamel S. Kamel and Man S. Oh for very helpful discussions and suggestions during the preparation of this manuscript. We are also indebted to Stella Tang and Chee Kiong Chong for expert technical assistance and Jolly Mangat for secretarial assistance. This work was supported by grant MT-15485 from the Canadian Institutes for Health Research.
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