Water balance and food consumption in dehydrated growing jirds (Meriones shawi shawi)

MALIKA SAHNI,' JACQUELINE PEIGNOUX-DEVILLE, AND EVELYNE LOPEZ Laboratoire de Physiologie Geizeiale et ComparPe du Museum National dJHistoire Naturelle et Laboratoire d'Endocrinologie ComparPe, Centre National de la Recherche scienti$que, 75231, Paris CEDEX 05, France

Received May 14, 1992 Accepted October 27, 1992

SAHNI, M., PEIGNOUX-DEVILLE, J., and LOPEZ, E. 1993. Water balance and food consumption in dehydrated growing jirds (Meriones shawi shawi). Can. J. Zool. 71: 651 -656.

This study was an investigation of the degree to which water conservation is adapted in jirds (Meriones shawi shawi) when dehydration occurs during growth. During total water deprivation, three different stages became evident. For the first 7 days of total dehydration food intake decreased, then it stabilized over the next 4 weeks, until it approached the initial amounts, and finally increased to twice control levels. Analysis of changes water flux in growing M. s. shawi, with and without a supply of water, revealed that dehydrated animals are able to develop physiological tolerance and performance with respect to their water balance. The excretion of hyperosmotic urine in response to prolonged dehydration is intimately linked to the adaptive morphological kidney changes, i.e., selective hypertrophy, that we observed.

SAHNI, M., PEIGNOUX-DEVILLE, J., et LOPEZ, E. 1993. Water balance and food consumption in dehydrated growing jirds (Meriones shawi shawi). Can. J. Zool. 71 : 651 -656.

Nous avons examine l'adaptation des mecanismes de conservation de l'eau chez la Merione (Meriones shawi shawi) lorsque la deshydratation se produit durant la croissance. Au cours d'une privation totale d'eau, on observe trois etapes. Durant les premiers sept jours la consommation de nourriture diminue. Ensuite, pendant quatre semaines, l'ingestion de nourriture se stabilise autour des niveaux initiaux. Finalement, la consommation augmente, atteignant deux fois le niveau temoin. Une analyse du flux hydrique chez des meriones en croissance, avec ou sans acces a d~ l'eau, montre que les animaux deshydrates peuvent developper une tolerance physiologique et savent economiser l'eau. A la suite d'une deshydratation prolongee, l'excretion d'une urine hyperosmotique est liee aux changements morphologiques adaptatatifs qui se produisent dans le rein, soit une hypertrophie selective.

[Traduit par la redaction]

Introduction

The survival of animals in the desert depends o n physiologi- cal and behavioural adaptation that permits conservation of water. Indeed, adult rodents adapted to arid areas can survive in the absence of free water (Schmidt-Nielsen and Schmidt- Nielsen 1952; MacMillen and Lee 1967).

Several studies carried out in both the field (Petter and Lachiver 1983; Bradshaw e t al. 1976) and the laboratory (de Rouffignac and Morel 1965) have shown that adult jirds (Meriones shawi shawi) a re able to maintain water homeo- stasis in the absence of drinking water. The relationship between water conservation and renal anatomical - functional adaptation has been investigated in adult M. s. shawi and other species of desert rodents (Bankir and d e Rouffignac 1985), but little is known about the effect of water deprivation o n renal development in juveniles. Adult jirds subjected to dehydra- tion showed a significant decrease in food consumption and, consequently, a reduction in body weight (F. Lachiver and K. Taktoye, unpublished data; Ben Chaoucha-Chekir 1989). In previous work (Sahni e t al. 1987) w e have shown that pro- longed dehydration during growth alters body weight. O n the other hand, w e have also demonstrated that adult jirds deprived of water since weaning a re able to maintain their water balance in equilibrium. Based on these data, w e were interested to know whether prolonged dehydration would also modify food intake during growth and, if so, whether food consumption would generate enough metabolic water to cover

'Author to whom all correspondence should be sent at the follow- ing address: Departments of Orthopedics and Cell Biology, School of Medicine, Yale University, 333 Cedar Street, New Haven, CT 06510, U.S.A.

growth requirements. The ability of the kidney of adult desert rodents to adapt to alterations in water intake by changing urine concentration status is a well-documented phenomenon. However, very little is known about the impact of dehydration o n the kidney performance of growing jirds.

In the present work, w e describe a detailed analysis of kid- ney morphology that w e carried out in an attempt to determine whether prolonged dehydration influences the ontogeny of the kidney, increasing renal efficiency to conserve water in grow- ing jirds.

Materials and methods

Animals The Meriones shawi shawi used in this study were raised in the

laboratory from a colony originally captured in the field in Tatouine (southern Tunisia). Prior to the experiments the jirds were held in individual cages. All animals were maintained at 25 f 2"C, at a rela- tive humidity of 65 k 4 % and a 12 h L : 12 h D photoperiod.

The experiments were carried out on twenty 30-day-old (weaning age) jirds. We divided the animals into two groups. Group 1 sonsisted of 10 control animals which were fed rat chow with 10% humidity ad libitum and had free access to apples, which represented their water supply until the end of the experiment. Animals in group 2 were deprived of water progressively from weaning. From days 30 to 46, growing jirds received 4 g of apples per day, which was reduced at day 47, 2 g of apples being given to animals until day 57. During these two periods, jirds were considered to be partially dehydrated. From day 57, animals in group 2 were maintained with- out an apple supply and were considered totally water deprived.

Methods Water and sodium flux Each animal was weighed and injected intraperitoneally with 1 pCi

(1 Ci = 37 Bq) of tritiated water (HTO) and 0.2 pCi '2Na/g body

Pr~nted In Canada I lrnprlrne au Canada

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652 CAN. J . ZOOL. VOL. 7 1 , 1993

No. of days

FIG. 1. Change in food intake as a function of time. 0 , controls; 0 , water-deprived animals. Values are shown as the mean f SE (n = 10 animals in each group).

> partial

0 total dehydration

dehydration , , I I I 1

40 60 80 100

No, of days

FIG. 2. Effect of water deprivation on body weight. 0 , controls; 0 , dehydrated animals. Values are shown as the mean f SE (n = 10 animals in each group).

weight. After the 2 h needed for equilibration of the isotopes with the exchangeable body fluids, a 70-pL blood sample was collected by retro-orbital puncture with a heparinized haematocrit tube (Clay Adams). After 3 days the jirds were weighed and another blood sample was taken. They were again injected with the same amounts of HTO and 22Na. Following a second 2-h equilibration period, blood samples were again collected. All blood samples were centrifuged immediately at 20 000 x g. The HTO and 22Na radio- activity levels in plasma were measured by liquid scintillation count- ing using 20-pL samples mixed with 4 mL of Instagel (Packard). All samples were counted with an accuracy of less than 1 % error, and quenching was corrected by automatic external standardization. Total body water and exchangeable sodium pools were determined by dilu- tion of the injected isotopes. Water and sodium influx and efflux rates were calculated according to Lifson and MacClintock (1966).

The sodium concentration in the plasma and hydrolyzed chow was determined by atomic absorption spectrometry, using 2- and 5-pL aliquots, respectively. The osmotic pressure of the plasma and urine was measured with a micro-osmometer, using 10- and 5-pL aliquots, respectively.

Food intake Food intake was calculated from water and sodium influx and from

total dietary water (0.798 mL - g- ' chow; 0.68 mL . g- ' apple) and sodium content (0.107 mmol . g- show; 0.00065 mmol . g - ' apple). Total water intake was calculated as the sum of dietary water and metabolic water, and was calculated according to Schmidt-Nielsen (1964). If nc and n, represent the sodium concentrations in chow and apple, respectively, then kc and k, are the hydration coefficients for chow and apple, and cc and c, represent the quantities of chow and apple ingested per day, respectively.

No. of days

FIG. 3. Change in water turnover rate under dehydration, expressed as a percentage of total body water. 0 , controls; 0 , water-deprived animals. Values are shown as the mean f SE (n = 10).

No. of days

FIG. 4 . Changes in water influx (-) and efflux (---) during growth. 0 , controls; 0 , dehydrated animals. Values are shown as the mean f SE (n = 10).

The following equations established by M. Znari (unpublished data) were used to determine food consumption:

[ I ] influx (Na) = nccc + n,c,

[2] influx (H20) = kccc + k,c,

K,(influx Na) - n,(influx H20) [31 cc =

ncKa - naKc

For water-deprived animals, eq. 1 was simplified as follows:

[4] influx,, = ccnc

influx,, [5] cc = - nc

Renal morphometry Animals were sacrificed at days 64, 86, and 1 10, and the kidneys

were weighed and placed in Bouin's solution. Following fixation, the kidneys were dehydrated in alcohol and embedded in paraffin. Sec- tions (7 pm) were cut sagitally and stained with haematoxylin-eosin.

Morphometric measurements of the kidney zones were performed using a semi-automatic digitizer (ASM, Leitz) equipped with an electromagnetic ball pen pointer. The thickness of each renal zone (cortex and outer stripe of the outer medulla, C + 0 s ; inner stripe

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No. of days

FIG. 5. Effect of water deprivation on water balance. a, controls; 0 , water-deprived animals. Values for the ratio of influx to efflux are shown as the mean f SE (n = 10).

of outer medulla, IS; and inner medulla, IM) was determined from the sagittal sections.

Results Food consumption

Figure 1 shows that at days 30 and 46 there was no signifi- cant difference in food intake between partially water-deprived animals and controls. From days 64 to 98 the food intake of totally deprived jirds remained constant, and was significantly lower than in controls between days 64 and 86 ( p < 0.01). Beyond day 98, food intake decreased in controls and increased in water-deprived animals; it was twice the control level at the end of the experiment (day 110).

-4 Body weight

The body weight curves (Fig. 2) indicate that animals having 1 1 0

free access to apples gradually increased their body weight between days 30 and 80. After this growth period, stabiliza- No. of days tion of body weight was observed in control animals, whose FIG. 6. Effect of total dehydration on ~ l a s m a (---) and urine (-)

ages ranged from 86 to 110 days. osmotic pressure. a , controls; 0 , totally dehydrated animals. Values

The body weight curve for animals subjected to partial are shown as the mean f SE (n = 10).

dehydration was similar to that of controls. During total water deprivation three different stages became evident.%or the first 7 days of total water deprivation, body weight fell to 13 % of Water

the initial value and was significantly less than in controls The ratio of water influx to efflux was calculated from the

(p < 0.01). Over the following 4 weeks body weight remained data given in Fig. to determine water (Fig. 5). The

unchanged. From day 98 body weight increased, and at day ratio was (P < Oeo5) greater than in growing

110 it was close to that of controls. control animals, whose ages ranged from 30 to 64 days. Dur- ing the same period, partially water-deprived animals showed

Water turnover rate During both partial and total dehydration the water turnover

rate, expressed as percentage of total body water (Fig. 3), was significantly lower than in controls. From day 57, the begin- ning of total water deprivation, a decline in WTR was observed. At day 64, water turnover rate was 15% per day in deprived animals versus 30% per day in controls ( p < 0.01).

During the last part of the total dehydration phase (days 86 - 1 lo), jirds stabilized their water turnover rate at 7% per day, which was one-third that of controls ( p < 0.01).

Water flux Water influx and efflux increased between days 30 and 46

(Fig. 4) in growing control animals; these values were signifi- cantly higher than those obtained in partially water-deprived animals ( p < 0.01). From days 46 to 86, water influx and efflux declined progressively in controls, and at the end of the experiment there was no significant difference between values calculated at days 110 and 86 ( p < 0.01). In dehydrated animals, water flux was significantly ( p < 0.01) lower than in controls at all ages.

a progressive decrease in this ratio, and at day 64 they had a negative water balance. At days 86 and 110, controls and water-deprived animals maintained their water flux at equilibrium.

Osmotic pressure (Fig. 6) The mean plasma osmotic pressure did not differ between

controls (300 + 2.5 mosmol . L-l) and totally water-deprived animals (302 + 4.6 mosmol L-I) ( p > 0.05). The urine osmotic pressure of controls increased significantly between days 64 and 1 10 ( p < 0.01). However, the values obtained in controls were significantly lower than in water-deprived jirds ( p < 0.01 at each age).

Kidney weight The data shown in Table 1 indicate that in both controls and

deprived animals, absolute kidney weight generally increased as a function of age. It was significantly higher in totally dehydrated animals than in controls at all ages ( p < 0.01). Likewise, kidney weight represented a significantly higher percentage of body weight in deprived jirds than in controls at all ages ( p < 0.01).

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CAN. J . ZOOL. VOL. 71. 1993

Controls Dehydrated animals

.........

86 days

FIG. 7 . Changes in thickness of the inner stripe zone in controls and dehydrated animals. Absolute values are given (in millimetres) as the mean f SE (n = 10). C +OS, cortex and outer stripe of the outer medulla; IS, inner stripe of the outer medulla; IM, inner medulla; *, p < 0.01.

TABLE 1. Changes in absolute and relative kidney weight as a function of age in hydrated animals (H)

and water-deprived animals (WD)

Age Body wt. Kidney wt. (days) (8) (mg) % body wt.

NOTE: Values are given as the mean + SE (n = 10 animals).

Thickness of different renal zones Total water deprivation does not seem to affect the thickness

of cortex + outer stripe (1.12 f 0.03 mm in controls; 1.14 f 0.05 mm in dehydrated animals) or the thickness of the inner medulla (2.90 f 0.37 mm in controls; 3.17 f 0.23 mm in

No. of days

FIG. 8 . Relative development of each renal zone as function of age and dehydration, expressed as a percentage of total kidney thickness. Values are given as the mean f SE (n = 10). Open bars show the data for controls and patterned bars show the data for dehydrated animals.

dehydrated animals), as these parameters were not signifi- cantly different between water-deprived and control animals at all ages. However, at days 86 and 110 the inner stripe (Fig. 7) was thicker in water-deprived animals than in controls ( p < 0.01).

Figure 8 shows the thickness of each kidney zone expressed as a percentage of total kidney thickness to facilitate compari- son of their relative development. The thickness of C + OS remains a constant proportion of total thickness relative to age for both controls and water-deprived animals (Fig. 8a). How- ever, in controls there was a tendency for IS to decrease (Fig. 8b) and IM to increase significantly with age ( p < 0.01)

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SAHNI ET AL. 655

(Fig. 8c). In contrast, both IS and IM remained a constant When the values for IS and IM were expressed as a propor- proportion of total thickness in water-deprived jirds (p > tion of total kidney thickness, two different patterns became 0.01). evident in controls and dehydrated animals. In view of changes

in the inner stripe in rats receiving a high water intake, as a Discussion result of the decrease in cell size (Bankir et al. 1988) and the

The changes in body weight under prolonged dehydration previously studied in growing Meriones shawi shawi (Sahni et al. 1987) are related to the changes in food intake demon- strated in the present study. Despite a reduction in food intake, the animals subjected to partial water deprivation showed an increase in their body weight in the same proportion as hydrated animals. These data clearly indicate that partial water deprivation had no effect on body weight, probably because free water and water in food were sufficient to cover water needs during growth. However, during the first 7 days of total water deprivation, a loss of body weight (about 13 % of initial value) was observed, after which body weight remained stable for several weeks. A similar pattern has been reported for adult Meriones and other rodents deprived of drinking water (Ben Chaoucha-C hekir 1989; Schmidt-Nielsen 1964). The gain in body weight noted during the last part of the total dehydration period is probably linked to the increase in food intake that we observed.

It is evident that food and water are intimately linked, so that restricting one limits the intake of the other. Our results show that both partial and total water restriction induced a reduction in food consumption. In the animals subjected to partial water deprivation the rates of water influx and efflux declined pro- gressively. On the 7th day of total dehydration, the water balance became negative. This suggests that the animals pro- duced water endogenously. In agreement with this hypothesis, we observed a loss in body weight during th,e first 7 days of total water deprivation. Following this period, water balance was restored in water-deprived jirds as a result of their ability to reduce the rate of hater turnover. Indeed, water turn- over represented only one-quarter of control values at days 86 and 110. The same result was obtained by de Rouffignac and Morel (1965), who showed that dehydrated adult jirds reduced their rate of water turnover from 11.5 to 4 % . This reduction is primarily mediated through a drastic reduction in urine volume due to a marked urine concentrating ability. In our experiment we showed that maximum urine concentration was obtained after 7 days of total dehydration, but after this period urine osmotic pressure became similar to the values given by Ben Chaoucha-Chekir ( 1989) for dehydrated adult jirds. These data reflect an adaptive ludney function during prolonged dehydration. Renal morphometric studies demonstrated a marked increase in kidney weight which did not result from a uniform increase in all kidney zones but only from an increase in thickness of the inner stripe of the outer medulla. This indi- cates that one is dealing with a case of selective renal hyper- trophy. A similar feature has been reported in a few other instances in response to hormonal functional changes. As shown by Trinh-Trang-Tan et al. (1987), homozygous Brattel- boro rats with hereditary diabetes insipidus (lack of antidiuretic hormone (ADH) synthesis) exhibited cellular hypertrophy of the medullary thick ascending limb in the inner stripe when they received dDAVP (nonpressor analog of ADH) chroni- cally. The same authors demonstrated that the subsequent morphological changes were accompanied by a significant increase in Na+,K+-ATPase activity, allowing active trans- port of solutes in this renal zone. A similar adaptive concen- trating mechanism was also found in jerboas (Jaculus orientalis) when dehydrated (Baddouri et al. 1984).

reduction in Na+,K+-ATPase activity in the medullary thick ascending limb of jerboas given an ample water supply (Doucet et al. 1987), it may be assumed that the reduced development of IS in hydrated jirds is related to reduced active reabsorption in the medullary part of the thick ascending limb. However, our results indicate that urinary osmotic pressure increased with age in controls, probably because of progressive increase in IM thickness with age. In contrast to the kidney of controls, all sections of the kidney of dehydrated animals increase in thickness equally as the kidney becomes larger through growth and adaptation. This pattern, which differs from that of control animals, indicates that dehydration during the period of plas- ticity in renal development induced qualitative and quantitative changes in renal performance, which might be different when animals are dehydrated at the adult stage. This hypothesis correlates with the pattern described by Hewitt (1981). This author showed that renal hypertrophy in young mice (Notomys alexis) caused by changes in water intake during the period of plasticity in renal development is not found in mature animals.

Despite the fact that we did not actually measure plasma ADH levels or Na+,K+-ATPase activity in the kidney, it seems logical to suppose that the high urinary osmolarity observed during dehydration is a direct result of increased circulating ADH levels. This has indeed been shown for a different spe- cies of desert mammal (Yagil and Etzion 1979; Baddouri et al. 1981). Finally, it is evident that the selective hypertrophy of the kidney observed in dehydrated jirds is a morphological and functional adaptation allowing survival during this extreme physiological challenge.

Acknowledgements

The authors thank Prof. Y. A. Fontaine (Museum National d'Histoire Naturelle) for critical reading of the manuscript, Drs. C. Lim-Taylor and R. Miihlbauer for correcting it, and E. Martelly and B. Vidal for assistance with the figures. This work was supported by a grant from the CNRS (ATP 960089 (Reponses biochimique et physiologique a des situations vitales critiques)).

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