Functional adaptation of thick ascending limb and internephron heterogeneity to urine concentration

Functional adaptation of thick ascending limb and internephron heterogeneity to urine concentration

Kidney International, Vol. 31(1987), pp. 549—555 Functional adaptation of thick ascending limb and internephron heterogeneity to urine concentration ...

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Kidney International, Vol. 31(1987), pp. 549—555

Functional adaptation of thick ascending limb and internephron heterogeneity to urine concentration MARIE—MARCELLE TRINH—TRANG—TAN, NADINE BOUBY, WILHELM KRIz,

and LIsE BANKIR INSERM Unite 90, Hôpital Necker, F-75743 Paris Cedex 15, France, and Anatomisches Istitut der Universität Heidelberg, Im Neuenheimerfeld 307, D-6900 Heidelberg, Federal Republic of Germany

The kidney is able to adapt to a variety of chronic changes in excretory needs, for example, to high or low sodium or potassium intake [1] and to acidosis and alkalosis [2]. One of the main functions the mammalian kidney assumes is the concentration

of solutes in the urine. The diversity of mammals affords examples of renal adaptation to water conservation. Comparison among animals living in fresh water, in temperate regions, or inhabiting desert areas reveals several differences in kidney morphology and function [3]. Recent experiments have shown, not only among different species but within a given species as well, remarkable functional adaptation of the kidney to chronic variations in urine concentration. Urine concentration can be altered for sustained periods of time by modifying either antidiuretic hormone (ADH) or fluid intake. The Brattleboro rat offers a unique model in which the

interest is the fact that the renal hypertrophy did not involve all zones of the kidney equally (Fig. 1). The inner stripe of the outer medulla increased in thickness and volume to a much greater than other zones, so that it represented 17.9 1.0% of total kidney volume in treated DI rats versus only 10.9 0.9% in DI

rats (P < 0.001). This disproportionate increase of the inner stripe reflects a lengthening of all nephron segments in this zone (Fig. 1). In addition, marked hypertrophy of the thick ascending limb (TAL) epithelium was also observed [5]. The diameter of

this segment increased in its most proximal part (in the deep inner stripe) by more than 30% (Fig. 2). Its volume of tissue per unit length increased 100% in both superficial and deep neph-

rons (Fig. 3). This was mostly due to an increase in size of pre-existing cells, that is, to true hypertrophy, not to hyperplasia. The TAL hypertrophy was less marked in the more

effects of the total absence of ADH can be studied [4]. By

superficial part of the inner stripe and was no longer detectable studying homozygous Brattleboro rats with diabetes insipidus in the outer stripe and cortex. The pars recta of proximal tubule (DI) and comparing their kidney function and morphology with and the collecting duct were unchanged in size (Fig. 3) [5]. Additional experiments were performed in normal Wistar rats that of heterozygous rats of the same strain, normal Long Evans rats (the strain from which the Brattleboro are derived), in which the endogenous ADH level was either depressed or and DI rats chronically receiving antidiuretic hormone, we were increased, by giving the rats 5% glucose in water as drinking able to demonstrate several features of the renal adaptation to fluid, or by restricting their water intake to a few mis per day, water conservation. Several of these features were also ob- respectively. Results of this study were very similar to those in served in normal rats in response either to an increase or to a Brattleboro rats, that is, the water—restricted group exhibited decrease in water intake for several weeks. These changes are marked increases in kidney weight relative to body weight, described in the first part of this paper. The second part will inner stripe thickness and volume, and thickness of TAL address the mechanisms by which these changes are induced, epithelium compared to the respective values in water—drinking group [6, and Bankir, Kriz et a!, unpublished observations]. and how they improve the urine concentrating process. Morphometric measurements of basolateral membrane surface area and mitochondrial volume in cells of the early TAL of Adaptive changes linked to urine concentration DI and treated DI rats showed that density of both structures Continuous infusion of antidiurectic hormone in the form of per unit cell volume was unchanged by dDAVP. However, due its nonpressor analog dDAVP (200 ng/day by means of Alzet to the increase in cell volume, the amount of basolateral cell minipumps) for several weeks, caused a high urine concentra- membrane and the number of mitochondria per unit tubular tion in DI rats (2700 mOsm/kg H20 versus 220 mOsm/kg H2O in

length were significantly greater in the treated DI rats [5].

untreated DI rats) and induced a 40 to 50% increase in kidney weight, without a significant change in body weight [5]. Relative kidney weight was 1109 22 mg/i00 g body weight in DI rats and 1515 84 mg/l00 g body weight in treated DI rats. Of

Parallel changes were observed in the activity of two enzymes

Received July 9, 1986

unit tubular length, but not if expressed per unit tubular

© 1987 by the International Society of Nephrology

volume, demonstrating that the enzymatic changes are depen-

located on the basolateral cell membrane (Fig. 4). Na-KATPase and ADH- and glucagon—stimulated adenylate cyclase

were significantly increased in medullary TAL (MTAL) of treated DI compared to MTAL of DI rats. However, these changes were apparent only when results were expressed per

549

550

Trinh-Trang-Tan et a!

33

1

Os Is

0.98

n

1.75 H

+1 C,)

IM

3.67

neity (Table 1 lines 5 and 7) [10] as had been observed in

younger rats. In contrast to these results obtained after

1.08

060

increase in deep SNGFR, thus accentuating nephron heteroge-


VI

4.20

V fl DI

Fig. 1. Height of the renal zones along the cortico-papillary axis in lzomogous (DI.) and heterozygous (HZ) Brattleboro rats and in DI rats infused with the ADH analogue dDAVP for several weeks (TDI.

long—term exposure to ADH, acute changes in ADH status (acute infusion of AVP in DI rats, or discontinuation of dDAVP in treated DI rats) did not induce significant effects on SNGFR heterogeneity, that is, did not increase heterogeneity in DI rats or decrease it in treated DI rats (Table 1, lines 6 and 8),

suggesting that the hemodynamic changes seen in chronic

studies are not the result of an acute direct effect of the hormone on glomerular filtration rate but due to a chronic indirect effect. The volume of superficial and deep glomeruli was also estimated in kidney sections from water—restricted and hy-

drated Wistar rats. Heterogeneity of GV was significantly

greater in the former group [6, and Bankir, Kriz et al, unpublished observations]. Both the TAL and the GV measurements The mean thickness of each zone and the mean SEM total kidney performed in Wistar rats show that the changes described in height are given in mm. Abbreviations are: (C) cortex; (OS) and (IS) Brattleboro rats after dDAVP administration also occur in outer and inner stripe of outer medulla, respectively; (IM) inner normal rats when their endogenous ADH levels are altered for medulla. Student's t-test between TDI and DI:NS for C and for OS; P a sustained period. <0.05 for TM; P < 0.005 for IS. Reproduced from [5], with permission.

dent upon and proportional to the changes in cell size and

Mechanism and functional relevance of changes in the medullary thick ascending limb and nephron heterogeneity

corresponding changes in basolateral membrane area [7]. These

The morphologic and enzymatic changes occurring in the results and the finding of a greater short circuit current in medullary thick ascending limb suggest that active transport is isolated perfused medullary TAL taken from DI rats chronically treated with AVP as compared to untreated DI rats [8] suggests that active salt transport is chronically stimulated in the MTAL of TDI rats.

intensified in this segment when ADH is infused and/or urine

minipumps which induced a state of higher urine concentration

dependency of this reabsorption is well demonstrated [13]. It is

(mean 24 hr urine osmolality, 2700 mOsm/kg H20) than did daily injections [10]. Maintenance of dDAVP infusion for several weeks resulted in a marked increase in kidney weight with a proportional increase in whole kidney filtration (+34% and +33%, respectively). This was accounted for by an increase in

conceivable that the amount of salt entering the early TAL could be greater in the high concentrating than in the non- or

concentrating processes stimulated. The factors capable of influencing transport activity in TAL include certain hormones

for which this segment is a target site, and alterations in the Another consequence of the long term operation of urine composition of luminal and/or pentubular (interstitial) fluid. Let concentrating processes in treated DI rats versus DI rats is the us consider each of these factors. The medullary thick ascending limb is a target site for restoration of normal nephron heterogeneity with regard to single nephron filtration rate (SNGFR), glomerular volume antidiuretic hormone [12]. Both in vitro [review in 131 and in (GV), and proximal tubule length (PTL), a heterogeneity which vivo [14, 15], ADH has been shown to stimulate active salt is absent or greatly attenuated in DI rats [9, 10]. These pa- transport in this segment. Maintenance of high circulating levels rameters were assessed by the ferrocyanide and nephron micro- of ADH, induced either by infusing dDAVP in DI rats or by dissection technique. The first three lines in Table 1 show that restricting water intake in Wistar rats, chronically stimulates homozygous Brattleboro rats with DI display abnormally low the concentration ability of the kidney. Both the collecting duct SNGFR, GV, and PTL in deep nephrons compared to normal and the TAL are target segments of ADH, whereas the Wistar rats [11] or to heterozygous Brattleboro rats (HZ) able to hypertrophy was confined to the TAL. This may be due to the synthetize ADH [91. Superficial nephrons show no anomaly. fact that, in the TAL, ADH induces an active energy—demandDaily injections of dDAVP in oil for several weeks, begun when ing transport. This requires a sustained increase in cell metaDI rats were two weeks old, restored normal urine concentra- bolic activity, contrary to the change in water permeability of tion (mean 24 hr urine osmolality, 1700 mOsmlkg H20) and the collecting duct which, once intramembranous particules are increased SNGFR, GV, and PTL in deep nephrons with no inserted in their appropriate location, apparently requires no change of these parameters, respectively, in superficial neph- significant energy. In addition to this direct effect of ADH on the TAL, other rons (Table 1, line 4) and no significant change in whole kidney filtration or in kidney weight [9]. In a second group, chronic factors, linked secondarily to the effect of ADH on collecting dDAVP treatment was delayed in DI rats until after completion duct water permeability, could also increase salt transport in of kidney maturation (2 months of age). In this experiment the TAL indirectly. First, an increase in salt delivery to the dDAVP was administered continuously by implanted Aizet early TAL may increase salt reabsorption since the flow—

poorly—concentrating kidney. Salt addition (salt recycling) in the thin descending limb has not been observed in the rat by micropuncture. However, in the experimental setting for the superficial and in deep SNGFR but with a much greater latter, urine osmolality and the osmotic pressure gradient in the

551

'4

1 a

- aM

At

TAL functional adaptability

Fig. 2. Cross section in the deep inner stripe in a DI (A) and a i'D! rat (B). A few thick ascending limbs are marked by asterisks. C = collecting ducts. Note the difference in diameter and in epithelium thickness of the TALs. (Magnification x400.) Adapted with modification from [37].

papilla are greatly reduced, thus perhaps impeding salt recy-

cling. It is conceivable that salt may be added in the thin

diffusion through the basolateral membrane will be greater, thus increasing the workload for the TALs.

That an increased workload is able to cause a transporting descending limbs of long—looped nephrons in the highly concentrating rat kidney, as is the case in the Psammomys [161. epithelium to hypertrophy, in the absence of hormonal manipThe high permeability to sodium recently demonstrated by Imai ulations, was demonstrated by Ecknauer, Feyerabend and [17] in the initial part of the thin descending limb of long loops Raffler [19] and Kaissling, Bachmann and Kriz [20]. In the latter of the rat gives some credibility to this hypothesis. If so, this study, marked hypertrophy of the convoluted distal tubule would lead to increased delivery of salt only to the TAL of epithelium was observed after salt reabsorption in the preceding long—looped nephrons and could not play a role in the hyper- segment had been inhibited for several days by furosemide. Our experimental results do not permit further speculation on trophy of short loop TAL. An alternative hypothesis is that the factors which induce the TAL and the inner stripe hyperactive transport of salt by the TAL of all nephrons in the highly trophy seen in the concentrating kidney, but it is reasonable to concentrating kidney is increased by factors independent of salt think that the factors listed above are not mutually exclusive delivery from the preceding segment. The salt concentration in and that each of them may contribute to the effects observed. the inner stripe interstitium is greater in the concentrating than How can the effect of ADH to increase nephron heterogenein the poorly concentrating kidney, as part of the increased ity be explained? After their initial work showing an increase in corticopapillary osmotic pressure gradient [181. Consequently, deep nephron filtration rate after acute AVP infusion in DI rats, back diffusion of salt via the paracellular pathway, and passive Davis and Schnermann suggested that this increase was the

552

Trinh-Trang-Tan et a! Long loops

Short loops

Collecting ducts E

I

1200

ci)

Level 5

E

[\

900

(-I

0

Level 4

E

600

Q

Level 3 a)

0. (0

Level 2

a)

C C

T

.

300

4000

200:

N

0

0

6000

A

•0

a)

.E

C)

t0

ATPase activity

Adenylate—cyclase activity

0

AVP

GLU

-

Na-K Mg ATPase ATPase

Fig. 4. Adenylate cyclase and A TPase activities in medullary TAL

segments microdissected from DI (white bars) and dDA VP-treated DI rats (hatched bars). Stars indicate a significant difference by analysis of variance. Adenylate cyclase activity was measured in presence of l06 M arginine—vasopressin (AVP) or 10-6 M glucagon (GLU). Na-K ATPase was calculated as total ATPase minus ouabain—insensitive ATPase (= Mg ATPase) [7].

Level 1

a)

C

Ca,

Fig. 3. Ratios of volume of epithelium per unit tubular length in TDI over DI rats. Volume of epithelium was calculated from the mean area of epithelium per TAL cross section measured by morphometry at different levels within the kidney. Ratios exceeding that of the whole kidney enlargement (1.35, in this case) are indicated by thick boxes. Reproduced from [5], with permission.

available experimental results, may explain the effect of ADH

on nephron heterogeneity. The hypothesis is that increase in nephron heterogeneity is the consequence of increased transport activity in the medullary TAL, influencing differently the tubuloglomerular feedback control of the nephron filtration rate in superficial and deep nephrons because of the different lengths of their TAL. Due to the relative atrophy of the medullary TAL

in DI rats, salt reabsorption may be abnormally low in this segment, in all nephrons. In deep nephrons, which lack a cortical thick ascending limb, the signal at the macula densa would be higher than in normal rats and would depress deep nephron glomerular filtration. In superficial nephrons additional

consequence of increased vascular resistance in the vasa recta, due to the higher viscosity of the blood in the concentrating medulla [21]. However, this hypothesis is not compatible with

the recent observation of persistent high SNGFR in deep nephrons two days after dDAVP discontinuation, when diabetes insipidus has reappeared [10] (Table 1). A direct vasoconstrictor effect of ADH on vasa recta can also be ruled out as the explanation because of the following observations: 1) dDAVP infused in our rats is virtually devoid of pressor effect. 2) Mice with nephrogenic diabetes insipidus also show reduced nephron heterogeneity [22], although they have endogenous ADH and normal pressor response to the hormone (change in arterial blood pressure after acute infusion of 60 pg AVP was + 27 mm

Hg, N = 5, P < 0.01 [M.C. Veale and H. Valtin, personal communication]). 3) Acute AVP infusion induces a much smaller increase in deep SNGFR than does chronic treatment (Table 1). Were the effects due to a direct action, in the long run decreased sensitivity and response would more likely be ob-

served than an increase. 4) High SNGFR persists in deep

reabsorption would take place in the cortical TAL, thus compensating for the insufficient reabsorption in the medullary TAL. The signal at the macula densa, and hence the nephron filtration rate, would be close to normal [10]. According to this hypothesis, MTAL atrophy results in a

reduction in filtration rate only in the deep nephrons, as a consequence of the unequal length of the cortical TAL among nephrons throughout the kidney. Long—term ADH administration corrects the TAL epithelium defect and salt reabsorption in the MTAL becomes normal, enabling filtration to proceed at a normal level in deep nephrons. This situation is maintained, even after an abrupt cessation of ADH infusion, because the hypertrophy of the TAL is not immediately reversible. The persistence of increased transport in the hypertrophied diluting segment after discontinuation of the treatment is indeed suggested by the aggravation of diabetes insipidus observed in this situation. Urine osmolality becomes significantly lower and urine volume significantly higher than in never treated DI rats [10, 23]. In experiments in which dDAVP was infused with minipumps and urine concentration maintained at high levels for several weeks, filtration rate was increased in deep but also (to a lesser extent) in superficial nephrons, resulting in a clear increase in whole kidney GFR. It may be assumed that, in this case, the intense stimulation of active salt transport in the TAL was able to lower the "static head" reached in the TAL [24] and thus to

nephrons two days after discontinuation of ADH infusion, as already mentioned (Table 1). These observations suggest that ADH-induced changes in nephron heterogeneity are the result of chronic, long—lasting modifications in the kidney caused by long—term exposure to the hormone. Given the changes observed in the TAL epithehum, the following hypothesis, which seems compatible with decrease the signal at the macula densa, even in superficial

553

TAL functional adaptability

Table 1. Single nephron glomerular filtration rate (SNGFR), glomerular volume (GV), and proximal tubular length (PTL) in Wistar rats and in Brattleboro rats in various conditions

GV

SNGFR S

Reference N

Line# 1

WISTAR

2 3

HZ

4

TDP

5 6

DIC

7 8

TDII,c

DI

DI +

acute ADH

TDI 2 days after ADH discontinuation

[11]

5

S/JM ratio

JM

nI/mm

nI/mm

32

49

[9] [9]

4 36.4 6 33.6

3.5 51.7 3.8 32.2

6.7

0.71

3.4

[91

6

33.0

1.8

42.0

1.5

1.04 1.30 0.78 1.28

4.8 31.2 3.4 39.2 5.6 58.6

4.7

37.4 42.2 43.1

2.1

[10] [10] [10] [10]

4 34.4 7 7 4

53.7

3.7

7.0 2.4

JM nl

S

ni

0.65 1.93 1.26

1.12 1.68 0.96 0.71 1.93 0.81

PTL

3.56

S/JM ratio

JM mm

S

mm

0.56

9.3

0.50 0.77 0.59

9.2 9.2 9.4

0.3 0.5 0.4

12.7 10.3

0.08 2.16

0.28 0.17 0.14

0.19 2.08

0.29

0.81

10.2

0.14 3.12

0.24

0.62

10.5

0.07 2.53 0.13 1.70

S/JM ratio 0.76

12.2

12.1

0.7 0.5 0.4

0.73 0.90 0.78

0.9

11.3

1.1

0.91

0.4

12.6

0.3

0.84

Abbreviations are: S and JM, superficial and juxtamedullary nephrons, respectively; DI, Homozygous Brattleboro rats with diabetes insipidus; HZ, Heterozygous Brattleboro rats; TDI, DI rats receiving dDAVP for several weeks. Began to receive dDAVP before kidney maturation was complete. The drug was given by daily injections. b Received dDAVP only in adulthood but had a continuous delivery of the drug through Alzet minipumps. For homogeneity between the different studies, glomerular volumes, reported in reference 10 after correction for shrinkage, are given here without this correction. This correction, however, does not affect the direction of the changes between the groups. [9, 10]

nephrons. The concomitant increase in kidney weight may be an adaptation of the whole tubule to the enhanced reabsorptive need resulting from the increased GFR. However, as described in detail by Bouby et al [5], in these hypertrophied kidneys the inner stripe and the MTAL increased more than expected from the overall kidney increase, suggesting the primary involvement of this zone and this nephron segment in the whole process. How do the hypertrophy of the inner stripe and of TAL, and the increased nephron heterogeneity affect urine concentration? The creation and maintenance of the corticopapillary osmotic gradient depend on the balance between forces which generate it and those which dissipate it. The TAL is responsible for the "single effect" which contributes actively to creating the gradient. The TAL hypertrophy provides more tissue in the deep outer medulla, close to the inner medullary border, to perform the active salt transport that contributes to building the gradient.

vary with the state of urine concentration. Increasing deep nephron filtration rate, and probably also deep glomerular blood flow, may serve to deliver more salt to the inner medulla via the long loops, and on the other hand, to provide increased blood

supply to the outer medulla to sustain the energy demanding work of the TALs. Because oxygen consumption in this zone represents a very large fraction of the oxygen supply [33],

increased active transport in the MTALs may in fact be

dependent upon increased blood flow. The accentuation of internephron heterogeneity with regard to SNGFR, GV, and PTL in the rat, in relation to the chronic status of urine concentration, parallels the relation observed among different mammalian species. As shown in Figure 5, animals living in fresh water show a reduced nephron heterogeneity, whereas the largest differences between superficial and deep nephrons are observed in desert—adapted species. This shows that the renal adaptations seen among mammals with

Escape of solutes via the ascending vasa recta tends to regard to water conservation may also occur, within certain dissipate the gradient. Thickening of the inner stripe lengthens

limits, in a given species, under the influence of environmental

the distance along which countercurrent exchange between or hormonal factors. vasa recta ascending from the inner medulla and descending Intriguingly, internephron heterogeneity is observed among thin limbs of superficial nephrons can occur within the vascular thick ascending limbs in the rat. In the mid-inner stripe, where bundles [3, 25—27]. This reduces solute escape from the me- the distinction between TALs of short loops and TALs of long dulla. Urea is the major solute thought to be recycled via this loops is possible on transverse sections (Fig. 3 [5]), morphoroute [3, 25, 281. Thus, the increase in inner stripe thickness metric measurements show that TALs of short loops have a creates conditions for more efficient recycling of urea. greater diameter and a thicker wall than do long loops. StimuInternephron heterogeneity is now recognized to be an important factor in the process of urine concentration. The existence of short—and long—looped nephrons, their various location within the kidney, and the different permeability properties of

lation of urine concentration by dDAVP infusion in DI rats [5] or by water restriction in Wistar rats (unpublished data) results in greater hypertrophy in TALs of short loops than in those of

plasma osmolality, by mechanisms discussed in several books and reviews [3, 25, 28—32]. Although they do not concentrate their urine, Brattleboro DI rats do have an apparently normal proportion of short and long loops. However, filtration in the deep nephrons and related morphologic factors (GV and PTL)

about the possible influence of the luminal urea. Recycling of urea in superficial nephrons is well demonstrated by the finding

long loops in the mid-inner stripe (Fig. 3). The mechanism their thin descending limbs to sodium and urea enable the responsible for this TAL heterogeneity and its accentuation in mammalian kidney to concentrate its urine several fold above the concentrating kidney is not known. One may speculate

of a greater flow of urea in the early distal than in the late proximal tubule by micropuncture techniques [3, 28]). Most likely, the recycled urea enters the nephron via the thin

Trinh-Trang-Tan et a!

554 1.2

A

interest because the intensity of urea recycling differs among these species [3, 28], according to the urea permeability [17] and

A

1.0

the location of descending thin limbs with respect to vascular bundles in the inner stripe [25, 31, 34]. The accentuation of internephron TAL heterogeneity in the inner stripe when urine

U



0.8 U,

•A

0.6

I

S

. A

.

0.4

S

0.2

I

I

No ADH

Dl

Normal AD

Exogenous ADH Subnormal ADH administration Treated Dl HZ

Normal rat

Wistar

Brattleboro B

1.2

U

1.0

.

. A

S

(1)

0.6

A

.

A

.

.

S

0.4

S

A 0.2

loops increase their salt transport (or their metabolic activity) even more than do long loops in this situation. That some adaptive changes of the kidney to the chronic state of hydration may also occur in man is suggested by studies performed in the late fifties. Water deprivation [35] or forced drinking [35, 36] for a few days in healthy volunteers increased or decreased, respectively, maximum urinary concentrating ability, as tested after Pitressin administration. The authors concluded that "the renal concentrating process is conditioned to an important degree by the state of hydration" [35] and that

"the kidney's ability to concentrate urine is related to the

U

0.8

concentrating processes are stimulated suggests that short

I

's., Around 1,000

I

I

2,000—3,500

I

<.

e

I.

o, I

Above 4,000

individual's average intake of water" [361. In both studies the changes required days to appear and were slow to regress when conditions were reversed. This is compatible with a progressive adaptation of TAL epithelium and nephron heterogeneity similar to that we observed in rats. In summary, the rat kidney (and probably the mammalian kidney in general) shows several functional and anatomical adaptations to long—term operation of urine concentrating

mechanism. These effects are observable only if ADH is present, but they do not seem to depend strictly on direct

actions of the hormone. Rather, they seem to result from the intensity of work imposed on the medullary thick ascending limbs in the concentrating kidney. Reprint requests to Lise Bankir, INSERM Unite 90, Hopital Necker, F-75 743 Paris Cedex 15, France.

Urine osmolality, mOsm/kg H20

Fig. 5. lnternephron heterogeneity and urine concentration. Ratios of superficial over juxtamedullary nephrons (SliM) are shown for SNGFR (triangles), glomerular volumes (circles), and proximal tubular length (squares). Measurements were made by ferrocyanide technique and nephron microdissection. A shows results obtained in rats in various

conditions of ADH status, in our (Brattleboro) or de Rouffignac's laboratory (Wistar). B shows results from the literature concerning various mammals, including 3 desert—adapted rodents. A similar relationship exists between internephron heterogeneity and urine concentration within a given species and among different mammalian species. Reproduced with permission from [30].

References 1. KAI55LING B: Structural aspects of adaptative changes in renal electrolyte excretion. Am J Physiol 243 (Renal Fluid Electrolyte Physiol 12):F211—F226, 1982

2. AL—AWQATI Q: The cellular renal response to respiratory acid—base disorders. Kidney mt 28:845—855, 1985

3. BANKIR L, DE R0UFFIGNAc C: Urinary concentrating ability: Insights from comparative anatomy. Am J Physiol 249 (Regulatory Integrative Comp Physiol l8):R643—R666, 1985

4. VALTIN H, SOKOL HW: The Brattleboro rat. Ann NY Acad Sci 394: 1—828, 1982

5. BOUBY N, BANKIR L, TRINH—TRANG-TAN MM, MINUTH WW,

descending limb. Both the location and the permeability prop-

KRIz W: Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats. Kidney mt 28:456—466,

erties of their respective thin limbs suggest that this urea recycling occurs preferentially in short— rather than in

6. BANKIR L, JUKKALA K, FISCHER 5, FISCHER C, SPECHT HC, KRIz

long—looped nephrons [28]. In the rat kidney, thin limbs of short

loops are more permeable to urea than are those of long loops [17]. In addition, and in contrast to those of long loops, the short descending limbs run near to the vascular bundles, in

1985

W: Adaptation of the rat kidney to high or low urine concentration. Influence of ADH and water availability. (abstract) Renal Physiol 9:117—1 18, 1986 7. TRINH—TRANG—TAN MM, BANKIR L, DOUCET A, EL MERNISSI G, IMBERT—TEBOUL M, MONTEGUT M, SIAUME S, MOREL F: Influ-

close apposition to the vasa recta ascending from the inner

ence of chronic ADH treatment on adenylate cyclase and ATPase

medulla and carrying urea—rich blood [3, 25, 34]. The presence of variable amounts of urea in the TAL lumen might influence

tleboro rats. Pfluigers Arch 405:216—222, 1985 8. BESSEGHIR K, STONER L: Effect of chronic treatment with arginine

either the intensity of or the energy requirement for salt reabsorption by this segment. This suggestion could be tested

by adding urea to the luminal perfusate during in vitro microperfusion experiments. Comparison of results obtained in

the rabbit, rat, mouse, and hamster would be of particular

activity in distal nephron segments of diabetes insipidus Bratvasopressin (AVP) on the function of in vitro medullary thick ascending limb of Henle (mTALH) of the Brattleboro (diabetes insipidus, DI) rat. IXth mt Cong Nephrol, 1984, p. 405 9. TRINH—TRANG—TAN MM, DIAz M, GRUNFELD JP, BANKIR L:

ADH-dependent nephron heterogeneity in rats with hereditary hypothalamic diabetes insipidus. Am J Physiol 240 (Renal Fluid

TAL functional adaptability Electrolyte Physiol 9):F372—F380, 1981 10. TRINH—TRANG—TAN MM, BOUBY N, DOUTE M, BANKIR L: Effects

of long—term and short—term antidiuretic hormone availability on

internephron heterogeneity in the adult rat. Am J Physiol 246 (Renal Fluid Electrol Physiol 15):F879—F888, 1984

11. BAINES AD, DE ROUFFIGNAC C: Functional heterogeneity of

nephrons. II. Filtration rates, intraluminal flow velocities and fractional water reabsorption. Pfiflgers Arch 308:260—276, 1969 12. IMBERT—TEB0UL M, CHABARDES D, MONTEGUT M, CLIQUE A,

MOREL F: Vasopressin—dependent adenylate cyclase activities in the rat kidney medulla: Evidence for two separate sites of action. Endocrinology 102:1254—1261, 1978 13. HEBERT SC, ANDREOLI TE: Control of NaCl transport in the thick

ascending limb, Am J Physiol 246 (Renal Fluid Electrol Physiol 15):F745—F756, 1984 14. DE ROUFFIGNAC C, CORMAN B, ROINEL N: Stimulation by antidi-

uretic hormone of electrolyte tubular reabsorption in rat kidney. Am J Physiol 244 (Renal Fluid Electrol Physiol 13):F156—F164, 1983

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