Gut, 1981, 22, 558-565

Prolactin and the small intestine*Effect of hyperprolactinaemia on mucosal structure in the rat

E MULLERt AND R H DOWLINGt

From the Gastroenterology Unit, Guy's Hospital Medical School, London

SUMMARY To study the mechanism for the adaptive mucosal hyperplasia which occurs indepen-dent of luminal nutrition and pancreatico-biliary secretions in isolated Thiry-Vella segments ofintestine from lactating rats, and to examine the effects of prolactin on small bowel mucosalstructure in the rat, we used two models of experimental hyperprolactinaemia and comparedquantitative histology and several markers of mucosal mass in jejunum and ileum from control ratsand from test and lactating animals. Hyperprolactinaemia, induced by perphenazine injections (5mg/kg/day for two or seven weeks) or transplantation of four pituitary glands from donor animalsto beneath the renal capsule in the recipient, was confirmed by radioimmunoassay. Proof of itsbiological activity was obtained by weighing the mammary pads and by demonstrating true breasthyperplasia on histological section. Median serum prolactin levels increased from 50 ng/ml in thecontrols to 570 ng/ml in the perphenazine treated animals and to 600 ng/ml in the pituitarytransplanted rats-levels comparable with those seen in lactation (870 ng/ml). In the lactating rats,there was striking mucosal hyperplasia of both jejunum and ileum but, despite the hyperprolac-tinaemia, there were no such changes in villus height, crypt depth, or in mucosal wet weight,protein, or DNA/unit length intestine in the perphenazine-injected or pituitary-transplantedanimals. We conclude that prolactin is not trophic to the intestine in rats and that hyperprolac-tinaemia cannot explain the intestinal adaptive changes of lactation.

Recent studies of intestinal adaptation haveattempted to identify trophic factors responsible forstimulating small bowel mucosal growth. Such studiesare important because of the close relationshipbetween small bowel structure and absorptivefunction and because, in theory, such trophic factorscould play a therapeutic role in the management ofpatients with malabsorption, secondary to smallbowel resection or mucosal damage. Of the factors sofar identified, it is known that luminal nutrition,pancreatico-biliary secretions, and hormonal factorscan all stimulate mucosal hyperplasia. ' 2 Of thehormones, candidates for the role of enterotrophininclude enteroglucagon,34 gastrin,5 cholecystokinin,

*Presented in part at the annual meeting of the British Society of Gastroen-terology, September 1977, York, and published in abstract form: Muller E,Bates T, Sampson D, Dowling R H, Gut 1977; 18: A976.tPresent address: Dept. fur Innere Medizin, University Hospital, Ramistrasse100, CH-8091 Zurich, Switzerland.4Correspondence to RHD, 18th Floor, Guy's Tower, Guy's Hospital MedicalSchool, London SEI 9RT.

Received for publication 9 January 1981

and secretin,6 the anterior pituitary hornones' andprolactin.8-10

Further studies on the effect of prolactin on theintestine were of interest for several reasons. First, weshowed recently that the marked adaptive mucosalhyperplasia and hyperfunction seen in lactating ratsis independent of luminal nutrition and pancreatico-biliary secretions, as the degree of adaptive changewas the same in isolated, Thiry-Vella by-passed loopsas in intact intestine.9 This suggested that neuro-vascular or, more likely, hormonal factors must havereached the excluded intestine to stimulate mucosalgrowth. Secondly, Bates et al. ` showed that, whenrats are injected with large doses of prolactin, gutlength (from pylorus to anus) and empty gut weightincrease when compared with controls, but there wasno information about the effect of these injections onthe small bowel (independent of the colon) nor aboutthe changes induced in the intestinal mucosa asopposed to in its muscle coats." Thirdly, prolactin istrophic to organs other than the gut, including themammary gland'2 and the liver.'3

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To study further the effects of prolactin on smallbowel mucosal growth, we used two animal modelsof hyperprolactinaemia-perphenazine injections'4and transplantation of pituitary glands from donoranimals to beneath the renal capsule ofrecipients. ``16Having confirmed the presence of hyperprolac-tinaemia, we compared resultant changes in mucosalmass with those found in lactating animals. This paperreports our findings.

Methods

ANIMALS STUDIEDAdult female rats were used throughout. For the 2-and 7-week pherphenazine injection rats and theirappropriate controls (see below) we used COBD-Wistar virgin rats (Charles River UK Ltd, Margate)with a mean initial body weight of 235 (±SEM) 4 g.For the pituitary transplantation experiments,however, to ensure histocompatibility we had to usean inbred Wistar strain of rats (f. AGUS f/Lac; Bantinand Kingman Ltd, Hull, UK), with an initial bodyweight of 191 (+2-0) g.The lactating rats were also from the COBD Wistar

strain, and, to avoid the possibility of changes inintestinal structure persisting from a previouspregnancy, we mated nulliparous female rats whenthey weighed 185+±2-0 g. However, because of themarked changes in food intake and body weightknown to occur during pregnancy and lactation, themean final body weight of the lactating rats (295 ±6 g)was greater than that in the perphenazine injectionand pituitary transplantation groups.

EXPERIMENTAL DESIGNThe study design is summarised in Table 1.

PERPHENAZINE INJECTION STUDIESThe principal study was carried out in six rats treatedwith perphenazine for 14 days. This time was chosen

Table 1 Summary ofanimal groups studied andexperimental design

Group a Experiment Lactation(Wistar rats)

Perphenazine injected Pituitary transplantation(Female Wistar rats) (Female in-bred Wistar

rats)

Test Perphenazine injected Pituitary isografts pair-fed Lactating rats(PI) with adipose tissue suckling 5-9

grafted controls offspringControls 1. Solvent injected 1. Adipose tissue grafted,

and pair-fed with fed ad libitumP1 group

2. Non-injected, 2. Non-operated,weight-matched fed weight-matched fedad libitum ad libitum

because it is known that the mucosal hyperplasia oflactation has fully developed by two weeks. 7 To see iflong-term hyperprolactinaemia had differing effectson the small bowel mucosa from those seen after twoweeks' treatment, a less complete study was made inanother group of six rats treated with perphenazinefor 50 days.Apart from the stress of the daily injections,

treatment with this phenothiazine drug induceddrowsiness with a resultant reduction in food intake.As this, of itself, could have affected gut structure andfunction, we included two control groups: (1) a pair-fed group of six rats in which food intake wasrestricted to match, 24 hours later, the reduced foodintake of the perphenazine injection group. Theseanimals also had daily injections but with the solventalone. (2) Another group of six weight-matched, non-injected rats fed ad libitum.

PITUITARY TRANSPLANTATION STUDIESThe seven pituitary transplantation rats were alsostudied 14 days after transplantation-again with theaim of simulating the serum prolactin response tolactation. With this experimental model, hyperpro-lactinaemia becomes fully established one week aftergrafting.' Pituitary transplantation rats grow quickerthan normal'5 and to exclude the possibility that this isdue to hyperphagia (again with secondary effects onthe gut), we limited the food intake of these rats tomatch that of the seven adipose tissue-graftedcontrols. Again, there were seven nornally-fed,weight-matched, non-operated controls.

LACTATIONEach of the six lactating rats had delivered andsuckled five to nine offspring until the time of death,14 days post-partum. This group was included toprovide a definitive model of hyperprolactinaemia'8and intestinal mucosal hyperplasia`7 (for comparisonwith the two major test groups) and to establish thatthe radioimmunoassay used in the present studiescould, indeed, measure raised serum prolactin levels.

TECHNIQUESPerphenazine studiesPerphenazine BP (Fentazin, Allen and HanburysLtd, London) was supplied in sterile ampoules con-taining 5 mg perphenazine/ml solvent (see below).Five milligrams/kg/day was given at 9.15 am by sub-cutaneous injection near the root of the tail. Theperphenazine solvent was made up from a formulasupplied by the manufacturer (citric acid 1-75 g,sodium hydroxide 0-67 g, and sodium metabisulphite016 g in 80 ml distilled water) and, in the pair-fed,solvent-injected controls, the volume (05 ml) and

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timing of the solvent injections were the same asthose used in the perphenazine injection rats.

Pituitary transplantation studiesPituitary isotransplantation was carried out asdescribed by Welsch et al.'6 The pituitaries wereharvested from mature female donor rats and therewere four transplants per recipient-two beneatheach kidney capsule.The sham operated controls underwent an identical

operation except that a slice of subcutaneous adiposetissue from the abdominal wall of donor rats wasplaced beneath the capsules of both kidneys.Although the presence of hyperprolactinaemia per

se (see below) strongly suggested that the pituitarytransplants were viable, in two animals we alsoobtained histological confirmation that the graftedglands had 'taken' satisfactorily. By light microscopy,the transplants were all well vascularised, theendocrine cells appeared healthy, and there was nomorphological evidence of either inflammation orrejection.

EVIDENCE OF HYPERPROLACTINAEMIAI Timing ofblood samplesAlthough the effect of suckling on rat serum prolactinlevels throughout the day is well established'9 theduration of hyperprolactinaemia after perphenazineinjections was not known. Therefore, in pilot studieson four groups of animals given perphenazine at 09.00hours, we measured serum prolactin levels by radio-immunoassay at 10.30 (n=5), 14.30 (n=5), 18.30 (n=6), and at 08.30 the following morning (n=4). Themedian serum prolactin values at these times were1320, 640, 530, and 90 ng/ml respectively, thusestablishing that hyperprolactinaemia persists for atleast nine hours after perphenazine injection.

During lactation, serum prolactin levels fluctuatemarkedly throughout the day'9 mainly because ofintermittent suckling which provides a potent stimulusto prolactin release. Because of this, we standardisedthe timing of blood samples in the lactating rats asfollows. First, to ensure that they were hungry, thepups were separated from their mothers for a four-hour period at 0900 hours. Then they were allowed tosuckle for one hour after which the mothers werepromptly killed. In the case of the pituitary trans-plantation rats, it was assumed that the model wouldprovide a constant release of prolactin with no diurnalvariations in serum levels. This too was tested inseven rats by measuring serum prolactin levels inblood samples taken at varying times between 0900and 1700 hours. The results of these studies showedthat there were no major changes in serum prolactinlevels throughout the day; they ranged from 420-850

ng/ml. There was no significant difference betweenthe mean serum prolactin levels in the various controlgroups (the normally-fed, weight-matched, and thesolvent-injected rats from the perphenazine injectionstudy and the weight-matched and adipose tissuegrafted animals from the pituitary transplantationstudies). Therefore, the results from all these animals(n=38) were pooled to provide control data for com-parison with the two major experimental groups andwith the lactating rats.

2 RadioimmunoassayTo avoid the transient, although modest, increases inserum prolactin levels known to occur after stress orafter ether anaesthesia,'9 the rats were stunned by ablow on the head, decapitated, and the blood samplescollected from the neck vessels. The samples weremaintained at 240C over ice and allowed to clot forone hour before centrifugation to obtain serumsamples which were then stored at -20°C untilanalysed for their prolactin content within one monthof the animal being killed.The prolactin concentrations were measured with a

rat prolactin radioimmunoassay (RIA) provided byA F Parlow, the Pituitary Hormone DistributionProgram, The National Institutes of Arthritis,Metabolism and Digestive Diseases, the NationalInstitutes of Health, Bethesda, Md, USA. Thereference preparation was rat prolactin (NIAMD-rat-prolactin-RP-1) and for iodination with 131 I (Radio-chemical Centre, Amersham, UK), purified rat pro-lactin (NIAMD-rat-prolactin 1-2) was used. In ourexperience, the standard curve obtained by plottingpercentage of radioactivity bound against prolactinconcentration gave reliable results in the 1-0-50-0 ngprolactin range. To ensure that the unknown sampleswere within this range, duplicate sera were diluted1:2, 1:4, 1:8, 1:16, and 1:32 before assay.

3 Evidence that immunoreactive serum prolactin wasbiologically activeIt is well known that hyperprolactinaemia induced byperphenazine injection'4 and by pituitary trans-plantation'6 causes gynaecomastia in rats. To confirmthat the immunoreactive hyperprolactinaemia foundin the present study was indeed biologically active, weremoved and weighed the right inguinal mammarypad (which contains the three posterior mammaryglands with associated adipose tissue), and processedthe tissue for light microscopy.

INTESTINAL STRUCTUREImmediately after the animal was killed, the abdomenwas opened and the macroscopic appearance of thesmall bowel noted. The small intestine was then

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removed, stripped of its mesentery and its lengthmeasured against a vertical scale using a 5 g stretch.

1 Quantitative histologyTwo-centimetre lengths of jejunum, starting 3 cmdistal to the ligament of Treitz and of ileum, ending 3cm proximal to the ileocaecal valve, were removed,split open, pinned flat on cork, fixed in formol saline,and sectioned (5 jam) parallel to the long axis of theintestine for histological measurements of villusheight and crypt depth as previously described.9 Carewas taken to measure only well-orientated parts ofthe tissue sections.

2 Mucosal massMucosal wet weight and the protein andDNA contentof the mucosa were measured in mucosal scrapesfrom 10 cm lengths of jejunum and ileum takenimmediately distal and proximal to the segmentsremoved for quantitative histology (see above) aspreviously described.9DNA results were not obtainedin the lactating rats or in 7-week perphenazine in-jection animals.

For the protein and DNA assays, the tared mucosalscrapes were suspended in 0 15M saline and stored at-20°C for a maximum period of one month. At thetime of assay, the sampleswere thawed, homogenised,and sonicated (3 x 15 s at an amplitude of 18 microns)in an MSE Mark II ultrasonic disintegrator. Proteinwas estimated by the Lowry method20 using bovineserum albumin (Sigma Chemical Company, St. Louis,Mo., USA) as standard. DNA was measured inaliquots of the same mucosal broth by the ethidium-bromide method2` after inactivation of RNA bybovine ribonuclease (Sigma). In this assay, calfthymus DNA (Koch-Light Laboratories, Colnbrook.Bucks, UK) was used for the standard curve.

STATISTICAL METHODSTo assess the significance of differences between theresults for the indices of mucosal mass and for themammary gland wet weight, the null hypothesis, thatany differences between the three subgroups in theperphenazine injection and in the pituitary trans-plantation experiments reflected only variations in acommon parent population, was advanced and testedby a Gaussian one-way analysis of variance for threeindependent samples.The non-parametric Wilcoxon rank sum test for

two independent samples was used to test for signifi-cance of difference in mean serum prolactin levels.

Results

FOOD INTAKE AND BODY WEIGHT

In all major experimental groups, the animals

remained well throughout the study. However, in thefirst 24 to 48 hours after operation, the adipose tissue-grafted and pituitary transplantation rats ate less thantheir weight-matched controls. Thereafter, the fat-grafted animals fed ad libitum ate the same amount offood as the weight-matched controls while the trans-plantation animals, which were pair-fed with fat-grafted group, invariably consumed their dailyallowance (mean 17 g/day) of food completely.Although both sets of operated rats lost a little weightin the immediate postoperative period, there was nosignificant difference in the final body weights in thethree subgroups from the pituitary transplantationstudy (207+3 g in the pituitary grafted group; 204±3 gand 205 +3 g in the corresponding controls).As expected, perphenazine injection induced

marked drowsiness with an associated reduction infood intake over the first three days of the study.Thereafter, even though the lethargy persisted, themean food intake increased to 95% of the controlvalue (20 g/day) by two weeks and, by virtue ofchoosing heavier rats for the perphenazine treatmentthan for the solvent injections, there was no significantdifference between the final body weights in the threesubgroups (245±7 g in the treated group; 248±7 g and247±7 g in the corresponding controls). The final bodyweights in the seven weeks' perphenazine experimentwere 265 +7 g in the treated group and 262±7 g in bothcontrols groups. The lactating rats showed markedhyperphagia with a mean food intake of 58 g/day.

EVIDENCE OF HYPERPROLACTINAEMIA1 Immunoreactive serum prolactin levelsThe serum concentrations of immunoreactive pro-lactin in the controls, the two major experimentalgroups, and the lactating rats are shown in Fig. 1.

In the 38 controls, the median serum prolactin levelwas 50 ng/ml (range 8-360) but the results were notnormally distributed, as high values in five ratsskewed the data upwards (Fig. 1). Despite this, withthe exception of one rat in the perphenazine injectiongroup, there was no overlap between the serum pro-lactin levels in the perphenazine injection (median670 ng/ml), pituitary transplantation (500 ng/ml), andlactating rats (870 ng/ml), the results in all threegroups being significantly greater than in the controls(P<0-001). However, there was no significantdifference in serum prolactin levels between the per-phenazine injection and the pituitary transplantationrats or between either of these groups and. thelactating rats.

2 Evidence for bioactivity of immunoreactive serumprolactinThe results of mammary pad wet weight are shown inFig. 2.

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Fig. 1 Immunoreactiveserum prolactin levels incontrol rats, in theperphenazine-injectedand in the pituitary-transplanted models ofhyperprolactinaemiaand in lactating rats(14th day post-partum).

1000-

Serumprolactin(ng, ml)

500 -

0

S00

0

0

0

0@0

.0

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A a5 ^on J a IX

Controls Perphenazine Pituitaryinjected transplantation

The immunoreactive hyperprolactinaemia wasassociated with marked and highly significantincreases in the weight of the mammary pad, themean values in the perphenazine injection (3.53±0.17) and pituitary transplantation rats (3-36±0-13)being 110 and 100% greater respectively than that inthe controls, both these differences being significantat the 0-1% level.

Histological sections showed that these increases inmammary pad wet weight were almost exclusivelydue to hyperplasia of the glandular tissue withdevelopment of alveoli and proliferation of ducts.Conversely, most of the pad weight in the virgincontrols was accounted for by adipose tissue.

Despite the enlargement of the mammary glands inthe two major test groups (apparent even on nakedeye examination), the changes were much moremarked during lactation where the pad weight of6-3+0-47 g was again significantly greater (p<0001)than that in both experimental groups.

INTESTINAL STRUCTURE1 MacroscopicWhen the abdomen was opened at the end of thestudy, there was marked enlargement of the entiresmall bowel in the lactating rats but there was noobvious change in the appearance of either jejunumor ileum in the two test groups when compared withtheir corresponding controls. There was no differencein the length of the small bowel from the two experi-mental hyperprolactinaemic groups and for theirappropriate controls, the mean values ranging from113-119 (+4-5) cm. However, there was an obviousand significant increase in small bowel length in thelactating animals (145+±7 cm; P<0-001).

2 Quantitative histologyThe results for the histological measurements of villusheight and crypt depth in both jejunum and ileum aregiven in Fig. 3.

8

6Fig. 2 Mammary padwet weight in thedifferent experimentalgroups (see legendFig. 1). The barsrepresent mean valuesand closed circles theindividual data points.

Mammary padwet weight(grams)

4

2

0Controls Perphenazine Pituitary

injected transplantation

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600

400

200

Fig. 3 Quantitative0 histological

measurements of villusheight and crypt depth inthe jejunum (upper

200 panel) and ileum (lowerpanel) ofrats in theperphenazine-injected (2weeks) and pituitary-transplanted groups,together with their

200 corresponding controls,compared with lactatingrats (14th day post-partum). The bars

0 represent mean valuesand the closed circles theindividual data points.

200

Pituitary transplantation Lactationexperiiments

Despite the immunoreactive and bioreactivehyperprolactinaemia, there were no significantdifferences for the results of either villus height or

crypt depth between the perphenazine injection ratsand their two control groups, the pituitary trans-plantation rats and their controls or between the twoexperimental test groups.A similar pattern of results was seen with 'long-

term' hyperprolactinaemia induced by seven weeks'perphenazine injection: there was no significantdifference in jejunal or ileal villus height or cryptdepth between the test and control groups at thistime.

In keeping with the results of many previousstudies, there was marked villus hyperplasia in the

lactating rats, the mean villus height in this group

being significantly greater than in all the other sub-groups (P<O001). This was not true for crypt depth,as the mean value in the lactating rats was not signifi-cantly different from the other groups with the resultthat the ratio of mean villus height:'crypt depth was3-3 in the lactating animals compared with valuesranging from 2.12 to 2.43 in the other sets of rats.

3 Mucosal mass

The results for the indices of mucosal mass are givenin Table 2.As with the macroscopic findings and the quanti-

tative histology, there was no increase in any of theindices of jejunal or ileal mucosal mass in the rats with

Villushieight(mill)

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Table 2 Indices ofmucosal mass (mucosal wet weight, protein and DNA/cm intestine) in jejunum (upper panel)and ileum (lower panel)

Perphenazine experiments Perphenazine experiments Pituitary transplantation Lactation(2 weeks) (7 weeks) experiments

Weight- Pair-fed, Perphen- Weight- Pair-fed, Perphen- Weight- Adipose Pituitarymatched, solvent- azine matched, solvent- azine matched, tissue graftednon- injected injected non- injected injected non- grafted (PT)injected controls (PI) injected controls (P1) operated controlscontrols controls controls(n=6) (n=6) (n=6) (n=6) (n=6) (n=6) (n=7) (n=7) (n=7) (n=6)

JejunumWetweight(mg/cm)26-5+2-1 32-6±2-1 30-9+2-t 23-7±1-5 30-1±1-5t 25-4±1-5 23-9±0-6 22-4+0-6 23-3+0-6 53.9+3.8Protein (mg/cm) 2-61±0-16 3-77±0-16* 3-16±0-16 2-88±0-18 3-29±0-18 2-81+0-18 2-69±0-07 2-47±0-07 2-47*0-07 6-91+0-21DNA 0-098± 0-145+ 0-098± 0-127+ 0-107± 0-115±(mg/cm) 0-012 0-012t 0-012 - - - 0-0058 0-005 0-005

IleumWet weight (mglcm)23-5+1-7 27-3±1-7 25-1+1-7 20-8+1-3 24-9±1-3 24-1+1-3 17-2+0-8 16-6+0-8 19-1 0-8 41-7±3-4Protein (mg/cm) 2-10±0-19 2-94±0-19t 2-53±0-19 2-38+0-17 2-64±0-17 2-72*0-17 2-04*0-01 1-78±0-01 1-88*0-10 5-29±0-32DNA 0-155± 0-207* 0-151± 0-134± 0-121± 0-131±(mg/cm) 0-017 0-017 0-017 - - - 0-01 0-01 0-01

n: number of animals studied; results are mean values±SEMs.Significantly greater than weight-matched, non-injected controls (P<0-01).tSignificantly greater than weight-matched, non-injected controls and the PI group (r<0-05).*Significantly greater than weight-matched, non-injected controls (P<0-05).iSignificantly greater than adipose tissue grafted controls (P<0-05).

perphenazine injection or pituitary transplantation-induced hyperprolactinaemia when compared withtheir corresponding controls. There were inconsistentand minor, although significant, differences in someindices of mucosal mass-mainly in the pair-fed,solvent-injected group (Table 2)-but these dif-ferences are probably biologically unimportant.There was a similar trend in the control rats injectedwith solvent for seven weeks, although none of thesedifferences reached statistical significance.

Despite the negative results in the two major testgroups, there were again striking increases in themucosal mass during lactation. The mean jejunal wetweight/cm intestine increased by 203% and proteinby 265% when compared with the normally-fedcontrols used in the two-week perphenazine injectionexperiments; the corresponding data for the ileum ofthe lactating rats were 177 and 241%.

Discussion

The results of this paper clearly show that experi-mental hyperprolactinaemia is not associated withchanges in jejunal or ileal mucosal structure in therat. It seems, therefore, that prolactin is not trophicto the small intestine and that hyperprolactinaemiacannot explain the striking adaptive mucosal hyper-plasia and hyperfunction seen in isolated segments ofjejunum, by-passed from normal continuity as Thiry-Vella fistulae, in lactating rats.9 Our studies haveconfirmed, however, that perphenazine injection andpituitary transplantation are appropriate models forinducing hyperprolactinaemia.

EXPERIMENTAL DESIGNThe conclusion that prolactin is not the enterotrophichormone of lactation depends on the validity of themodels used. For several reasons, we believe that themodels were valid.

First, the immunoreactive prolactin levels found inthe two experimental models were comparable withthose seen during lactation and, secondly, the ex-perimental hyperprolactinaemia seems to have beenwell maintained throughout the day. We establishedthat the hyperprolactinaemia persisted for at leastnine hours after perphenazine injection, while in thepituitary transplantation animals, the circulating pro-lactin levels were constantly high. Thirdly, we usedtwo completely different models to induce experi-mental hyperprolactinaemia and neither wasassociated with changes in gut structure. Finally, weshowed that the immunoreactive hyperprolacti-naemia was also biologically active-as judged bychanges in the wet weight and histological appearanceof the mammary gland.

Furthermore, considerable care was taken to in-clude appropriate controls: fat-grafted animals whosefood intake provided a standard for the pair-fedpituitary transplantation group, solvent-injected pair-fed rats for the perphenazine injection experimentand weight-matched, non-operated or non-injected adlibitum-fed controls. Despite these precautions, thepattern of food intake and the associated changes inluminal nutrition in a sedated rat may well have had dif-ferent effects on the gut from those produced when ahungry control animal is given a comparable daily-quotaof food which is consumed over the first few hours.

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PREVIOUS STUDIES ON EFFECT OF PROLACTININ INTESTINEThere have been few previous reports on the influenceof prolactin on the intestinal tract. The only otherstudy on adult rats was by Bates and colleagues" whonoticed an increase in small bowel plus colonic weightin animals injected subcutaneously with ovine pro-lactin for 16 days. Although they found an increase inoverall intestinal weight, for the reasons alreadystated, it is impossible to draw further conclusionsfrom these results. Yeh and Moog22 also studied therat but, unlike the present study and that reported byBates et al., " they used suckling animals and lookedat the effect of intraperitoneal ovine prolactin inhypophysectomised animals. They found no effect onsmall intestinal weight, villus or crypt size, or in themitotic index in the small bowel mucosa. The onlyother study of prolactin's effect on the gut was byCampbell and Fell.8 They, too, injected ovine pro-lactin, this time in mice, but found no changes in smallbowel weight. However, there was no evidence fromtheir report that the exogenous hormone was indeedbiologically active.

If prolactin is not the enterotrophic hormone oflactation, what are the other possibilities? Ignoringthe more remote possibility that neurovascular factorscould have caused the hyperplasia and hyperfunctionin by-passed jejunum from lactating rats, the otherhormonal candidates include gastrin' and entero-glucagon.34 We have already studied intestinal tissueand plasma enteroglucagon levels in three animalmodels where adaptive mucosal hyperplasia occurs-small bowel resection, hypothermia, and lactation-and in all three situations there were markedlyincreased enteroglucagon levels. Whether or notenteroglucagon is responsible for the adaptivemucosal hyperplasia of lactation, remains unproven.

References

'Dowling RH. Intestinal adaptation. In: Peters DK, edAdvanced medicine. London: Pitman Medical, 1976:251-61.2Williamson RCN. Intestinal adaptation. N Engl J Med1978; 298: 1393-402, 1444-50.3Gleeson MH, Bloom SR, Polak JM, Henry K,Dowling RH. Endocrine tumour in kidney affecting smallbowel structure, motility and absorptive function. Gut1971: 12:773-82.

4Jacobs LR, Bloom SR, Polak JM, Dowling RH. Doesenteroglucagon play a trophic role in intestinal adaptation?(abstract). Clin Sci Mol Med 1976; 50: 14P-15P.'Johnson LR, Lichtenberger LM, Copeland EM, DudrickSJ, Castro GA. Action of gastrin on gastrointestinalstructure and function. Gastroenterology 1975; 68:1184-92.

6Hughes CA, Bates T, Dowling RH. Cholecystokinin andsecretin prevent the intestinal mucosal hypoplasia of totalparenteral nutrition in the dog. Gastroenterology 1978; 75:34-41.7Taylor B, Murphy GM, Dowling RH. Pituitary hormonesand the small bowel: effect of hypophysectomy onintestinal adaptation to small bowel resection in the rat.Eur J Clin Invest 1979; 9: 115-27.8Campbell RM, Fell BF. Gastro-intestinal hypertrophy inthe lactating rat and its relation to food intake. J Physiol(Lond) 1964; 171: 90-7.9Elias E, Dowling RH. The mechanism for small boweladaptation in lactating rats. Clin Sci Mol Med 1976; 51:427-33.

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