Laboratory experiments on the retention of 235Np by plaice (Pleuronectes platessa L.)

J. Environ. Radioactivity 21 11993} 189--201

Laboratory Experiments on the Retention of 235Np by Plaice (Pieuronectes platessa L.)

D. J. Swift Ministry of Agriculture Fisheries and Food, Directorate of Fisheries Research, Pakeficld Road. Lowcstoft. Suffolk NR33 0HT. UK (Received 5 June 1992: revised version received 7 August 1992: accepted 3 September 1992~

A BSTR,.I ( ' T

Gastrointestinal ahsorpti, m re" "S~Nl, from hd,clh'd ./hod in phticc was e.~timatcd at about 0.2";; with no .~'(k,ni/i'cant transli°r ofactivit.v / m m the gut lining to other or ffans or ti.~sucs. Thc hiolo.k, ical half ti,te [br "¢~.Vp in the gut lining was ctth'uhtted as 24 &tv.v. 'SS Np inh'ctcd intrwmtscuhtr(v ht plaice wtts translocatcd shnt'ly./?om the ina'ction site, and this was imhTcnth'nt o f the rate o f growth re" the .fish. ,-|t~out 40'.";; o f thc infi'ctcd : ~ Np was tran.~/i'rrcd over 148 tht vs and about I0",, was excretcd. Al~out 35%; o f the di.~pt,r.vt'd 2 ¢SNp ( 14"4, re" the ttntouJtt inlcctcd ) was Jotmd in scab" and bonc tissues in slow-growing fish and ahmtt 60",, ( 24% o f the ttmottnl injected) in/ilst-grmcing fish.

INTRODUCTION Neptunium (Np, atomic number 93) is an artificial transuranium element produced by neutron activation in nuclear weapon detonations and in irradiated uranium fuel in nuclear reactors. Fourteen isotopes are known and all are radioactive, although only six have half-lives longer than a few minutes. The alpha-emitter 237Np has the longest half-life, 2.16 x l0 t' years, and is the most important radiologically when considcring potential longterm effects of radioactive waste disposal (Thompson, 1982a). About 3 I,~9 ~. Crown Copyright 1993

tonnes of :-~Np are estimated to have been released following atmospheric testing of thermonuclear weapons (Efurd et al.. 1984): this activity has been detected in human tissues I Efurd et aL. 19861. Neptunium has entered the marine environment from nuclear weapons fallout and from authorized low-level nuclear waste discharges (Pentreath & Harvey. 1981 ). The decay of"41Am and 2-~1Pu(via -"~lAm)also present in the waste from nuclear fuel reprocessing, produces additional 237Np in the sea (Harvey. 1981: Holm. 1981). There are few data on neptunium accumulation in the marine ecosystem. The short-lived -'~'~Np (half-life 2.35 days) was detected in seawater and plankton immediately after underwater nuclear weapons tests in the Pacific ILowman, 1960: Freiling & Ballot,. 1962: Ballou. 1963). Noshkin ~'t al. (1974) detected 237Np in seawater and sediments at Enewetak atoll. Holm (1981) measured 2~"Np in sediments from Thule. Bombay harbour. Bikini atoll and from Windscalc (Scllafield). and for seawater and seaweed collected near Windscalc. Pcntrcath & |larvcy (1981) five more detailed 237N p nlcasuremen ts in scawater, a seaweed (F.c..s .serratus) and for several species of benthic invertebrates from the Scllaficld area. Germain and Pinto (1990) have measured -'37Np in littoral plant and animal taxa from a site within 5 km of the ot, tfall from the Cap l.a I laguc fuel rcprocessing plant. Valt,cs lbr the 237Np content of the edible fraction of several molluscan and crustacean species havc bccn publishcd as part of the routine radiological monitoring of the Scllaficld coastal area (Hunt, 1985. 1986. 1987. 19b;8: MAFF. 19,v,9. 1990). (;uary and Fowler (1977. 1978, 1990) have studied 237Np accumuhltion under laboratory conditions in shrimp (Lysmata selicauthtta), mussel {Alt'til..~" gulh~provincialis), the edible crab {Cancer pagurus) and the shore crab (('~lrcintts i~t~t~,n¢l,v). However. 2~TNp mr, st bc chemically separated from biological tissues bclbrc radiomctric analysis, thus precluding the t, sc of this isotope for acct, mulation time-series sit, dies on individual animals. Fowlcr and Aston (1982) suggested the use of the X-ray emitting isotope -'3SNp (half-life 396.1 d:lys) for such expcrimcnts. Fishcr et ul. (1983) studied e3"~Np accumulation by phytoplankton and Fowler and Aston (1982) similarly studicd zooplankton. However. no data on "~~SNp accumulation by fish seem to have been published: this promptcd the laboratory study of -'35Np assimilation from lbod and the retention and tisst, c distribution following injection in the plaice, Ph'uronecle,s' phttessa. described in this paper. MATERIALS AND METHODS Plaice taken from a stock acclimated to the laboratory conditions (water temperature 1 0 + I C: photopcriod 12h) for 8 months were used.

Retention of :-~~Np b.v plaice

191

23SNp was prepared by AERE, Harweli and supplied as Np(V) in 4M hydrochloric acid. 235Np-labelled food pellets were prepared by evaporating 4 ml of the isotope stock solution (about 163 kBq ml -E ) to dryness and redissolving the residue in 5 ml of a 0-9% (w/v) sodium chloride solution. This procedure has been shown experimentally not to change the oxidation state of the tracer (McCubbin, D., pers. comm.). Gelatine powder (1 g) was added and the mixture warmed and stirred until the gelatine dissolved. Then 0-8 g of homogenized freeze-dried shrimp powder was mixed in. Six plastic l-ml disposable syringes were modified by cutting off the needle holder. These were filled with the warm gelatine mixture. When cold, the gelatine was extruded, cut in half and the Z35Np activity of each pellet measured, it was considered that the conditions used in the preparation of the pellets did not change the oxidation state of the tracer. One pellet was fed to each of 12 plaice chosen at random from the stock fish. These fish had been starved for 2 days to encourage rapid feeding. All 12 fish were held in a tank supplied with running seawater for 2 h. During this time, one fish regurgitated its pellet, which was immediately eaten by another fish. After the 2 h, the fish were removed individually, measured and weighed. The '35Np activity of each fish was measured in a whole-body gamnla counter; K-capture X-ruys at 0.094 and 0.098 MeV were counted (Nichols, 1981). After the initial measurcmcnt, the Ii remaining fish wcrc placed in a large tank supplied with running seawater and fitted with a plastic mesh l]oor to reduce the possibility of faeces being reingcsted. The 23SNp content was measured at 1 and 5 days aftcr the initial labelling. Five fish selected at random on day 5 were killcd and dissected and the 23SNp activity of the dissectcd organs and tissues measurcd immediately. The remaining fish were then fed for the first time. Subsequently, the fish were ted normally with approximately !% of the fish's wet weight per day of live ragworm (Nereis sp.). On day 35, all of the fish were killcd, measurcd and weighed. The whole-body activity was measured. Each fish was dissected and the 235Np activities of the organs and tissues were measured. In a second experiment, the difference in 235Np tissue distribution pattern in growing fish was compared with non-growing fish. Twelve plaice were selected at random from the stock fish and each was injected with 0.1 ml of a saline solution of the tracer (about 107 kBq ml j). The tracer was injected into the muscle above the lateral line and immediately behind the head via the dorsal surface. The injected fish were placed in batches of three into each of four large tanks supplied with running seawater. Two hours post-injection the -"35Np activity of each fish was measured, as in the first experiment. Further measurements were made I and 4 days postinjection and then at 7-day intervals until day 148. Two tanks of the fish were fed a maintenance diet of about 1% wet body weight per day. The fish

192

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in the other tanks were fed as much as they would take at a single |~:edmg each day; ragworm were used as food for both groups. Alter the experiment, all of the fish were killed and dissected and the ~'35Np activity of the organs and tissues were analysed.

RESULTS

Feeding experiment The Ii fish used had a mean weight of 78-8±2-53g (+SE) and a mean overall length of 20.2 + 0-28 cm. The mean whole body '3~Np content on day 0 was 293 ± 8 Bq (n = 10). The fish that ingested two pellets had an initial content of 686 Bq. During the experiment, there was no significant increase in weight or length of any of the fish. "~3SNp retention was low (Fig. I), the whole-body percentage retention (A,) at time t days after ingestion being approximated by the equation: A, = 99-8e i~q,,,+0.2e i~0_,,,

(1)

The bh~logical hall:tmle for the transit of 2~SNp through the gut is thus estimated as 0-5 days and that lbr assimilated 2JSNp as 24 days. After 5 days, about 65% of the retained tracer, representing about 0.2% of the initial body burden, was present in the alimentary tract (Table I). 235Np was found on the gill filaments of two fish. This may have been due to labelled food particles becoming trapped on the gills. None of the other organs (liver, kidney, heart, spleen and gonad) or tissue samples (blood, skin, muscle and bone) analysed contained a detectable amount of -"~SNp However, the remainder (consisting of the fins, head and the undissected skin, muscle and skeletal tissue) had a mean rcsidual activity of about 7 Bq kg t (Table i ). This activity was possibly present in the bone and scale tissues within the remainder sample or it could have been present as general contamination of the skin by egested tracer. Aftcr 35 days, -'~SNp was detected in the stomach and the intestine but not in the rectum (Table 1). 235Np was not detected in any of the other organ or tissue samples analysed. However, as with the samples analysed after 5 days. the undissected remainder had a mean residual activity of about 8 Bq kg ~. The similar activity value for the remainder sample after 35 days to that after 5 days suggests that this activity is probably due to external contamination rather than to actual assimilation into the body from the gut.

Retention of "-~,'qP by plaice

193

10(

cO

i

¢0

i

E ¢J t'l

0.1

0.011 0

I 10

I 20 Time (days)

I 30

Fig. I. Mean percentage retention of '~SNp by plaice fotlowmg a single labelled meal.

Injection experiment After 148 days, high-ration plaice had significantly increased body weights and lengths compared with the low-ration fish (Table 2). Very little 235Np had been excreted. The mean whole-body retention was above 90% for both groups (Table 2). There was no significant difference between the two groups in retention of tracer for the injected muscle blocks (Table 3). The liver, kidney, skin, bone and non-injected muscle (Table 3) contained the major fraction of the 2JSNp translocated from the injected muscle. The liver was the only organ or tissue of those examined which showed a statistically significant difference in 23SNp concentration between the two test groups,

194

D J S**t/t

TABLE I Distribution of 235-Np (mean ± SE) in Plaice 5 Days and 35 Da~s after a Single Meal of Labelled Pellet. Neptunium v, as Undetectable m the other organs Analysed (see text) A l t e r 5 thtl ~ Bqkg

Stomach Upperintestme

Lower intestine Rectum Gut contents

Remainder"

~

.

230±40 I 170±160 530 ± 93

5 "~ 5

65(1~1(/2 It)() ± II 7 ~ 13

,~t/ter 35 ~htt

'% BodL bur~h'n n

8 . 1 : 1 75 39 z 3 6 3 12.3 ~ 2 0 l

5 5 5

3

52:t0-81

5

2 5

S S ~ 1.65 2t~.8 ~ 3-06

5 5

Bqkg

i

180±13 530:~ 1(t7 450 ± 93 ND ND S ~ 0.3

n

5 6 6

5

% B o d v hur~h'n n

12.9±091 488±3.20 19.7 ~ 2 25 ND ND 25 ± 2 3 0

~, 6 6

5

"Remainder fraction consisted o f undissectcd head, lins, skin. b o d ) muscle and skeletal bone. TABLE 2 Changes in the Mean Weight and Length of Injected Plaice (mean k: SE) and the Mean Percentage Retention of 235-Np at the End of the Experiment Day

Ration

~Veight (g)

n

L e n g t h (cm)

n

0 148 148

l.ow l'ligh

56.8 t:- 3.111 81.5 ~ 6.13 135.5 t-9-14

12 6 6

18.9 r. 0-31 202 L0-51 23.3 t':0 56

12 6 6

ll7toh'-t~odv Yt'lt'lllloH ( '~o )

n

9 4 2 ~ I 4q 93.5 L2.63

6 6

with the high-ration lish showing a lower content per kilogram (Table 4~ There wcrc no significant differences between the radionuclidc concentration of bone and skin from the two test groups (Table 4), although the highration fish had statistically higher percentage dispersion to bone and skin (Table 3). Calculation of the -'-~SNppercentage content in bone, muscle and skin tissues assumed that plaice of the length range used consisted of 58-3% muscle, and 13.2% bone by weight. This assumption was based on an earlier series of dissections of similar-sized plaice. The liver and kidney values (Table 3) were calculated using the measured organ weights.

DISCUSSION The physiology and biochemistry of neptunium in marine animals have not been widely studied. Fowler and Aston (1982) and Fisher et al. (1983) measured accumulation from seawater by zooplankton and Guary and Fowler (1977, 1978, 1990) similarly measured accumulation in mussel,

Retention of :-)"Np b.v plaice

195

TABLE 3

Dispersion of 235-Np (mean 4- SE) in the Tissues of Two Groups of Plaice Following Intramuscular Injection of the Tracer and Then Feeding Each G r o u p with a Different Food Ration for 148 Days

Low ration

Bone* Skin** Liver Kidney Muscle Spleen Gill Heart Gonad Stomach Gut Injected muscle Whole body

High ration

Bq

% Whole Body

Bq

96 Whole body

108 4- 8.2 34 4- 2.6 48 4- 6.6 51 4- 8.2 12 4- 0.9 2.3 4- 0-7 0-5 4- 0.04 0-9 4- 0.2 0.2±0-02(5) 0.4 ± 0.1)6 0-6 4- 0-I I 274 ± 30 700 + 80

17 4- 0-5 5-2 4- 0.13 6-8 ± 0.04 7. I 4- 0.02 1.8 4- 0-05 0.3 4- 0-05 < 0. I 0-I ± 0.05 <0-1 < O-I < 0. I 38.2 • 2.1 -

197 4- 13 52 5:3-5 52 4- 8.7 38 4- 8.2 13 4- 0-9 1.8 4- 0.47 I-0 ± 0-19 0.9 ± 0.19 1-1 4-0-49(5) 0.7 ± 0-09 1.0 ± 0.33 330 4- 48 730 ± 93

29.2 ± 0.66 7-7 4- 0.13 7. I 4- 0.13 5.0 4- 0.05 1.9 4- I).03 0-4 + 0-03(5) 0-2 ± 0-003(4) 0-2 4- 0-00113) 0.3±0-006(2) 0-1 4- 0.003(2) 0.2 4- 0006(3) 45.8 4- 5.38 --

n = 6 except where number given in parentheses.

*Statistically significant different (P < 0.001) between groups. **Statistically significant difference (P < 0OI) between groups.

TABI.E 4 235-Np ('onccntrations in the Tissues of hucctcd Plaice (Bq kg ~ wet; mean ± SI-) Alter 148 Days Dcpuration; the Values arc for Two Groups of Plaice Fed on I)iffcrent Food Rations

Low ration Bk)od Heart Spleen Liver Kidney Stornach Gut (}ill filaments Skin Uninjected muscle Injected muscle Bone Bone/uninjected muscle quotient

3118 ± 108(2) 14 781) ± 4 471) 85 560 ± 22 650 39 61)1) ± 7 41)0 181 200 5:62 400 800 ± 80(5)

550 ± 131) I 150 ± 320 5 770 ± 650(5) 250 ± 50 27 200 ± 5 400 10000 4- 1 150 40 ± 8

n = 6 except where number given in parentheses. *Statistically significant difference between the two groups (P < 002).

tligh rathm 7 700 ± I 800 64 000 5:20 000 20 000 + 3 500* 67 000 + 15 000 620 ± 85 520 ± 230 I 400 + 4011 5 4O0 ± I 100 170 ± 35 18 220 + 3 71.10 I 1 000 4- 1 900 65 5: 8*

196

D J Swi/t

shrimp and crab. These studied showed that neptunium accumulation t¥om seawater by shellfish was generally similar to that found for plutonium, with most ( > 90%) of the whole-body radioactivity content being present on the shell or exoskeleton. Fowler and Aston (1982) showed that neptunium assimilation from food by the euphausid Meganyctiphanes norvegica was poor, with more than 95% of the ingested 235Np egested in about 30 h. 237Np content in plaice and cod fillets from fish caught in the Irish Sea are the only neptunium values to have been published for commercial marine fish (Pentreath & Harvey, 1981: Hunt, 1985, 1986: MAFF, 1989, 1990). Values are uniformly low, ranging from 0-4 to 0.65mBqkg -* for plaice and 0.15 to 0.8mBqkg * for cod. Germain and Pinte (1990) give some figures for 237Np in the blenny (Bh,nnius pholas): the highest measured activity concentration was present in the viscera (1.2 mBq kg I) with values for other tissues reported as being effectively undetectable. The most extensive studies on the biological effects of neptunium have bccn those using mammals (Thompson, 1982a, b). These studies have included estimation of neptunium toxicity following both injection and absorption from food its well as tissue distribution alter the administration of non-toxic doses, in general, bone is the principal accumulating tissue in mammals, rctaining about 35-60% of injected neptunium, depending on the species used (Thompson, 1982a). About 20'Vo of the administered dose accumulates in the liver while the kidneys retain about 4%. Ballou (1964) reported specific accumulation m the rat adrenal gland, I\~llowing intravenous injection. Mammals predominantly eliminate neptunium via their urine. Morin et al. (1973) showed that up to 40% of an injected neptunium dose is excreted by rats within 5 days. Neptunium appears to bind to high molecular weight proteins in mammalian blood and muscle (Nenot, 1982). These neptunium-bloodprotein complexes do not appear to restrict neptunium clearance, in contrast to plutonium protein complexes which do reduce clearance rates (Thompson, 1982a). Gastrointestinal absorption of neptunium is low in the few mammals studied, with absorption values ranging from 0.01 to 0-25% (Thompson, 1982a). However, David and Harrison (1984) reported that newborn mammals absorb up to 5% and Thompson (1982a) reported similar values lbr neptunium doses which approached toxic levels in adult mammals. Harrison et al. (1984) showed that gastrointestinal absorption depends on the chemical form in which the nuclide is fed as well as on the mammalian species studied. Plaice in many respects metabolize neptunium in a similar fashion to

Retention of "~¢:Np by plaice

197

mammals. However, following intramuscular injection, plaice retain an average of about 40% of the 235Np within the injected muscle tissue after 148 days. This retention is greater than that found in similar experiments with mammals. Rats retain about 30% of an intramuscular injection of 237Np nitrate 30 days post-injection with an estimated biological half-time of 45 days (Morin et al., 1973). This implies a retention of about 10% after 148 days. In contrast, rats injected with 237Np citrate retained only 12% after 16 days (Thompson, 1982a). The difference in retention rates between teleost and mammalian muscle may relate to differences in physiological function, in teleosts, white muscle makes up the bulk of the total muscle tissue (89% in plaice, Greer-Waiker & Pull, 1975) and consists of so-called fast muscle fibres that are large, poorly vasculated, lack myoglobin and are rich in low molecular weight proteins (Bone, 1978). The main function of this type of muscle is to operate for short periods during bursts of rapid swimming followed by longer periods of recovery: for most of the time, this tissue functions simply as a store for glycogen. The lower vascularization will inhibit neptunium diffusion away from the injection site but complex formation, probably with proteins, may also be a retaining factor. The rctcntion mechanism in rats intramuscularly injected with neptunium is unknown. Thompson (1982a) showed that only about I% of the dose is retained at the injection site in mammals. Nenot (1982) observed neptunium-protein complexes in mammalian blood but there is no evidence for a muscle protein complex. The mechanism by which tcleost and mammalian muscle retain neptunium remains as conjccture. Neptunium excretion in plaice is poor, at about 10% of the injected radionuclide over 148 days, compared with 25% in about I day and 40% in 5 days in mammals (Thompson, 1982a). Plaice also excrete 237pu poorly following injection, at less than 10% over a period of 103 days (Pentreath, 1978). Plaice arc similar to mammals in that the major accumulating tissue for dispersed neptunium is bone. in plaice, the mean bone accumulation represents skeletal bone plus scale growth. In the present study, this ranged from about 35% of the total whole-body retention for non-growing fish on a maintenance ration, to about 60% for rapidly growing fish on a high food ration (Table 3). These values were calculated as a percentage of the wholebody activity after the activity sequestered at the injection site had been subtracted. Pentreath (1978) showed a similar response in growing plaice injected with 237pu. The liver to bone content quotient was about 30 in slow-growing fish and less than 10 in fast-growing fish. However, in the present study, neptunium was dispersed predominantly to bone tissue with a much smaller proportion accumulating in the liver. The mean liver to

19~

D d. S . tlt

bone quotient for slow-grov,ing fish was 0-44 and for last-growing fish 0.2~ (Table 3). These results highlight the greater affinity of neptunium for bone tissue compared with that of plutonium. Plaice kidney retained about 10% of the neptunium dose compared with about 4% in mammals. However, in teleosts, the kidney tissue consists ot both renal and adrenocortical (interrenal) tissue (Chester Jones et at.. 1969). The higher retention may have represented selective absorption of 235Np by the interrenal tissue in a similar manner to the known selective retention of neptunium by mammalian adrenal glands (Ballou. 1964). About 1% of the whole-body 2~SNp content on day 148 was found in other tissues, such as the heart, spleen, gut and gonads but with no evidence of retention differences related to feeding regime. From the present study, it appears that the fraction of neptunium assimilated from the gastrointestinal tract in plaice is similar to that assimilated in mammals, although any possible difference due to the chemical species used has not been investigatcd. Neptunium in seawater occurs predominantly as the stable pentavalcnt neptunyl ion, Np(V)O~ (Gcrmain ct al., 1987). About 5()".,,) of the 237Np present in the prodischarge Sellafield effluent is in the pentavalcnt state with the remainder in the tetravalent tbrm (Pcntreath ('t al., 1988). tlowcvcr, on contact with seawater post-discharge, over 99% of 237Np is present in coastal waters in the pcntavalcnt I\)rm (Pcntrcath (,t al., 1985). Reported marine bioaccumulation studies with ncptuniun~ do not address the question of wdencc state of the tracer. In the present study, it was assunlcd that Np(V)();, once absorbcd by biota, would remain in the pcntavalcnt form. Although not tested, the 2~SNp in the food pellets and injected saline solution wa.~ assumed to bc in the pcntavalent form and that this form rcprcscnts the normal state of neptunium present in the food of marine fish and present within the body following assimilation. Very little 23SNp :tppearcd to cross the mucosal serosal ix'Jtertacc in plaice. In contrast, in mammals, neptunium was detectable in the systemic organs, although in very small amounts prest, mably duc to the higher renal excretion rate (Harrison et al., 1984). These authors reported that increased absorption occurs in rats if the animals are starved Ibr 8h before the neptunium is fed. In the present study, the fish were starved for 2 days to encourage rapid feeding which is our normal procedure for this type of experiment. The gastrointestinal absorption estimate of about 0-2% for plaice may therefore be biased on the high sidc. The few environmental data on neptunium in fish make an assessment of the radioiogical aspects of fish consumption by man diMcult. The data suggest that 2~TNp in fish fillets is not a significant problem in the short term since annual intake is so small. Using the consumption rate figures

Retention O/"::: Np hy plaice

199

given by Hunt (1986) and the annual limit of intake for '37Np by the public of 300 Bq (Anonymous, 1982), the annual mean intake would be about 0.003% of the limit for plaice and 0.004% for cod; the maximum consumers o f fish would have intakes of 0.028% o f the limit for plaice and 0-035% for cod. However, the long half-life for 237Np (2.14× 106years), and its in-growth from the decay of z'*~Pu (half-life 15 years), via 24tAm, and directly from discharged -'41Am (half-life 433 years), are important factors to consider in dose commitment calculations,

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Retention of : ~"Np b.v plaice

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