sodium-22 retention as a function of water intake by citellus lateralis

7/29/2019 Sodium-22 Retention as a Function of Water Intake by Citellus lateralis

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L969.In Hrysiological $ystems in Semiarid Environments.C. Clayton Hoff and. i{arvin L. Ried.esel, Eid.itors.University of t{ew Mexico Press. pp. 15-52.

Sodium-Z2 Retention as a Functionof Water Intake by Citellus lateralis'Gnnv L. BrNrz

Department of Biology, The University of New Mexico,Albuquerque, New MexicoNncenvn wATER balance is one aspect of the stress involved inhibernation. Hibernation has been observed to continue uninter-rupted as long as 3 weeks by animals maintained in a laboratory coldroom. Such observations imply that in the field (natural conditions)animals may hibernate, without waking, for periods uP to at least 3weeks in duration. During a period of hibernation an animal has nowater intake, but water loss still occurs. Although water loss is prob-ably slight during hibernation, 3 weeks is a long period to be withoutwater intake.

Because of the ability of hibernators to tolerate negative water bal-ance for extended periods, perhaps hibernators demonstrate quanti-tative differences in water metabolism, especially tolerance to negativewater balance. If such is the case, the capacity of hibernators to toler-ate negative water balance may not be restricted to periods of hiber-nation, but would be demonstrable during periods of activity of theanimals. This hypothesis has been tested and found true by studyingresponses to negative water balance in nonhibernating animals (lab-oratory rats) as controls versus animals capable of hibernation (Citel'/zzs sp.) (Riedesel, Klinestiver, and Benaliy, 1964). In subsequentstudies (Bintz, 1965; Bintz and Riedesel, 1967) laboratory rats andground squirrels were deprived of drinking water unti\ 30/o of origi-nal body weight had been lost, after which the water content of vari-ous tissues was determined. All rat tissues analyzed were dehydrated.In ground squirrels, blood only n'as dehydrated, liver had an in-creased water content, and kidney, heart, lung, and muscle had nochange in rvater content. Also, employing weight loss as a criterionof tolerance, we found that under various conditions (temperature,

'Srpport"a in part by U.S. Atomic Energy Commission Contract AT-(29'2)-1629..'vt, L . K.'eciegE I l*-ln*i-a-tt' l.f


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46 PHYSIOLOGICALSYSTEMSseason, food allotment) ground squirrels were more tolerant to drink-ing-water deprivation than were laboratory rats.Following experimenrs in studying rolerance of Citellus lateralisto drinking-water deprivation, we became interested in studyingwater requirements in this species. Water requirements cannot bedetermined simply by measuring rhe animal's ad-lib. consumprionof water in the laboratory. Many animals have a tendency to over-hydrate under laboratory conditions (Hudson, 1962; Chew, lg65).In this study a water-soluble isotope, 22Na, was employed to studythe water requirements of C. lateralis. During the study the follow-ing assumptions were made: (i) excretion of 22Na is directly propor-tional to water intake and turnover, and (ii) when water requirementsof the animal are satisfied, rate of excretion of 22Na will be maximal.It is becoming well known that laboratory-acclimated animalsoften differ physiologically from rhe same individuals under naturalconditions. For example, Young and Riedesel (I967) demonstratedthat laboratory-acclimated C. lateralis tend to be more homeother-mic than the same animals living under natural conditions. Becauseof the tendency of animals to overhydrate in a laboratory environ-ment, perhaps the water requirements of laboratory-acclimated ani-mals are difierent from the water requirements of animals livingunder natural conditions where water is not always readily available.This hypothesis was tested by comparing rhe rates of excretion of22Na in recently caught and in laboratory-acclimated C. lateraliswhen all animals were allowed equivalent amounts of water daily.Marrnrarsexo MBrHoosAnimals. Specimens of C. lateralis were captured in Sherman trapsin the mountains of northern New Mexico. Animals were dustedwith l/o rotenone powder shortly after capture. During the timeinterval between capture and return to. the laboratory, animals werefed raw carrot and apple.IJpon return to the laboratory and until the time of experimenta-tion, animals to be studied as laboratory-acclimated animals wereindividually caged and allowed free access to drinking water andWayne Lab Blox. Acclimation to laboratory conditions (temperature22-+2 C; relative humidity 30-+5Tot photoperiod, light from 7 errto 7 rlc) was for 6 to 9 months.Animals to be studied as field-adapted animals were individually


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SODIUM.22 RETENTION 47caged. Food and water consisted of Wayne Lab Blox and raw potato.Fach animal was given daily l0 g of porato per 100 g of body weightdyliig the period prior to ."p"rirrr.rrtution. Experimentation beganrvithin I week of the date that animals were brought into the labora-tory, thus acclimation ro laborarory conditions (especialry drinkinglvater ad lib.) was minimized.Pre.exP-erimental procedure. one week prior to experimentation a]ranimals were weighecr and transferred to metabolic cages located inan environmental chamber (temperature 2r -+0.5 c; relaiive humidityand photoperiod as above). Bohy weights of animals ranged from170 to 390 g. Feces and urine producecl-were weighed daily through-out the experiment for ail animals. Ad_lib. drinking water consump-lio"-bI laboratory-acclimatecl animars was derermi.,Ja auity by weigh-ing drinking-warer bottles.Experimental procedure..Animals were given a single ip injectionof 22fa in Ringer's solution. Specific acivity of the" r2Na was 0.30,.:/"tJ. Each injectior y^: O.Z miTtOO g body weight. As soon as possi_ble after injection whole-body radioaciivity was counred with a pack-ard Auto-Gamma model 410 A single-channel spectrometer and anArmac deep-well, exrernal liquid scintillator model 440. For each1ni1al radioactivity was counted at least once daily untit b\/o ofthe isotope was excreted. Experimental animals were dividea'intothe following groups:

Laboratory-acclimated animalsgroup a ("-6), drinking water ad lib.group b (n-8), no drinking water or potarogroup c ("-8), b ml rvater/ 100 g body weight per daygroup d (n-8), l0 ml water/100 g body weighi per dayField-adapted animals

group e (n-8), no drinking water or poratogroup f (n-8), 5 ml warer/100 g bocly weight per daygroup S (n-8),warer from poraro ad lib.In all groups where b or l0.ml water/100 g body weight per daywere allowed, the water was given in raw potato. some animars didnot eat all the potaro provided. Only animals that readily ate rhepotato were included in the experiment.


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48 PH''SIOLOGICALSYSTEMSRnsurrsLaboratory-acclimated, animals. Mean biological half-life values(To) and standard errors for four groups (a, b, c, d) of Citellus lateralisare presented in the upper part of Table l. In all four experimentalgroups excretion of the isotope was rapid during the first 24 hr follow-ing injection. Mean values ranged from l6 to 22/o of total isotopeinjected Fig. t). Animals of the three groups that had nrater available

C, LATERALIS22 No , sinqle i.p. lnjeclionTemp.= 2lt o.5"c, Rel Humid = 30l 57'Photoperiod, Light 7A.M.-7P.M.o= Lob Acclimoted, Drinking Woter od lib , n=5O: , lOml HaOllOOg BodyWeight/Dov, n=8A: ,5mlHaO/lOOqBodyWeiqht/Dov'n=8a: , No Drinking Woler, n = 8

24 48 72 96 t20 t44HOURS , POST INJECTION

Ftcunr l Nlean percentage retention of injected "Na by laboratory-accli-mated Citellus lateralis allowed different amounts of water'continued to lose isotope rapidly after the first 24 hr (Fig. l)' In fact,rate of excretion of the isotope was nearly linear with respect totime, at least until biological half-life was reached' On the otherhand, rate of 22Na excretion in animals with no drinking water avail-able was considerably less afrer the first 24 hr than during the first24 hr following injection.Statistically the biological half-life for those animals allowed nodrinking water difiered from the biological half-life values for thoseanimals allowed 5 ml or more of water/100 g body weight per day(p 4 0.01). None of the biological half-life values for animals of thethree groups given water differed from each other.

90

80


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SODIUAI.22 RETENTION 49L'ield-adapted animals. Rate of excretion of the isotope in the twogroups of animals allowed 5 ml water/100 g body weight per day(group f) and potaro ad lib. (group g) was rapid during the first 24hr following injection. Mean values ranged from 17 to 20/o of thetotal isotope injected Fig. 2). Rate of isotope excretion continued

C LATERALIS?2No , Single i.p. lnj.ction , Tcmp.. 21 : 0.5 . C .Rol.Homid..30: 5%, Pholoperiod, Lighl 7A.lr.-7PM.oeFiald Anlmols, NoDrinkihg Wotcr, n. 4

, 5ml H2O /lOO9 Body W.ighl/Doy, n.8

^____-'r_

,)---1 ,)240 336HOURS, POST INJECTIONFrcunr 2. Mean percentage rerention of injecred ",Na by field-adapted Cilei-lus lateralis allowed different amounts of water.

essentially in a linear fashion with respect to time after the first 24hr following injection and ar least until biological half-life valueswere reached (Fig. 2). Biological half-life values are given in thelower part of Table lField-adapted animals of group e were allowed no drinking waterand biological half-life was reached by only one animal. Three of theeight animals died before reaching the biological half-life. Isoropeexcretion by animals in this group was slow during the first 24 hrfollowing injection and conrinued to be very slow until 836 hr post-injection, at which time four of the animals were sacrificed becauseit appeared that they would die before reaching the biological half-life.Statistically the amount of isotope excreted by animals that wereallowed no water was different borh at 24 hr (P40.01) and at 48 hr

9080

70

60


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50 PHYSIOLOGICAL SYSTEMSTABLE IWATER CONSUMPTION AND BIOLOGICAL HALF-LIFEVALUES (Tb)OF PNA IN LABORATORY-ACCLIMATED ANIMALS AND INFIELD-ADAPTED ANIMALS

--:=Water intake in mlGroup per 100 gbodYweight per day

Field-adapted animals81-+-3.02 68-96

To in hrMean-+SE Range Mean bodyweightlLaboratory-acclimated animals

abc

UnlimitedNone5 ml in rawpotatol0 ml in rawpotato5 ml in rawpotato

83+ 11.8169+-14.7I07 -F I2.084-+6.9

57-r44r08-27264-t4669-128

Potato ad lib. 80-F3.02 65-93as percentage of body weight at timeof injection.

(f a0.0l) from the amount of isotope excreted by the animals in bothgroups allowed water. Biological half-life values of isotoPe in thetwo groups of animals allowed water were not different (P>0.8)'Fietd-adapted us. Iaboratory-acclimated animals. With respect tothe rate of excretion of 22Na, laboratory-acclimated animals demon-strated a greater variability from animal to animal (within the samegroup) than did the field-adapted animals. In the two groups of ani-mals allowed no water, excretion of the isotope during the first 24 hrfollowing injection was rapid (22/" of total isotope) in the laboratory-acclimated animals and slow (7f" of total isotope) in the field-adaptedanimals. Statistically biological half-tife values of the isotope in field-adapted and laboratory-acclimated animals allowed water ad lib. didnot differ (P>0.7). Also, biological half-life values of 22Na in field-adapted and laboratory-acclimated animals allowed 5 ml of water/100 g body weight per day did not difier (P20.05). The lack of statis-tical difference in the latter comparison was probably due primarilyto variability of rate of isotope excretion within each group, expe-cially in laboratory-acclimated animals.DrscussroNIn studies concerned with water intake of animals, one must be

98769398

100


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SODIUM.22 RETENTION 51aware not only of the drinking water but also of water in the animal'sfood. In the present study, animars alrowed at reast some water atel5 to 20 g of Lab Blox per day. Lab Blox has a warer contenr of 7_g/..In other words, the animals were obtaining about 1.5 ml of freewater for every 20 g of food eaten. Metaboric water may be greaterin amount than free warer. Lab Blox has a composition of afproxi_mately 24/o pr-otein, 4/o fat, b/o crude fiber, antl up to l0fo ash.Assuming that the remaining b\fo is carbohydrate, metabolic'watergained from oxidation of 20 g of Lab Blox to carbon dioxide andwater would be approximatery 8 ml, with an intake of 20 g of foodper day, an animal would be obtaining approximately l0 miof waterfrom food alone. Those animars alrowed no water ieither drinkingor potato) ate very little, if any, food after the first 2 to 3 days follow_ing water deprivation.Groups of laboratory animals allowed drinking water ad lib. orallowed l0 ml warer/100 g body weight per day excrered 22Na arsimilar rates (To-83 and 84 hr, respectively), whereas animals al_Iowed 5 ml water/100 g body weight per clayexcrered isorope some_what more slowly (Tb-107 hr). It *o,rtd seem, therefore, that in thecase of laboratory-acclimared animals, l0 ml water/100 g body weightPfr-day is not limiting and may approximate the water"requirementsof these animals.Groups of recently trapped animals allowed water ad lib. andthose allowed 5 ml water/100 g body weight per day excreted theisotope at similar rares, as biological half-rie oil,r., were g0 and glhr,.respectively. In the case of field animals, b ml water/100 g bodyweight per day approximates the drinking_warer reqriremJnts. Itis interesting that the maximum rate of e*cietion of the isotope wasvery similar in laboratory-acclimated animals alrowed drinking waterad lib. or allowed l0 ml warer/ 100 g body weight per day und in n"ld_adapted animals allowed porato ad lib. o, utt*.d 5 mi water/I00 g

body weight per day, even though several laboratory_acclimated anirnals drank over 100 ml of warer per day. Apparently, above a mini_mal-required water intake, Na excretion ii-independent of waterexcretion. Although laboratory-acclimated animals have a high waterintake, the osmotic control mechanism keeps Na excretion similar tothe rate observed in field-adapted animals.under conditions of no water, field-adapted animars were betterable to conserve body sodium than were raboratory-acclimated ani-

sodium-22 retention as a function of water intake by citellus lateralis

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