Gut,1961,2,23Studiesonthesiteoffatabsorption1.Thesitesofabsorptionofin - PDF document

Gut,1961,2,23Studiesonthesiteoffatabsorption1.Thesitesofabsorptionofincreasingdosesof3'I-labelledtrioleinintheratC.C.BOOTH,A.E.READ,ANDE.JONESFromtheDepartmentofMedicine,PostgraduateMedicalSchoolofLondon,andtheDepartmentofPhysics,HammersmithHospital,DuCaneRoad,LondonSYNOPSISThesestudiesonfatabsorptioninratsshowthatasmalldosewasrelativelyrapidlyabsorbedinthejejunum.Withlargerdosesitwasalsoabsorbedintheileum.Whenlargerdosesweregiventhejejunumrespondedtotheincreasedloadwithanincreasedabsorption,andfatwasalsoabsorbedintheileum.Absorptionofdifferentsubstancesdoesnotoccuruniformlyinthesmallintestine.Glucose,forinstance,isabsorbedrapidlyfromthejejunumbutmoreslowlyinthedistalsmallintestine(CumminsandJussila,1955;VerzarandMcDougall,1936)andunderphysiologicalconditionsvitaminB12ispre-dominantlyabsorbedfromtheileum(Booth,1958;BoothandMollin,1959).Althoughthesiteoffatabsorptionhasbeenstudiedbyseveralworkers,therelativeimportanceoftheproximalanddistalsmallintestineintheabsorptionoffatisuncertain.Kremen,Linner,andNelson(1954),fromobservationsindogssubjectedtointestinalresection,haveconcludedthatfatisprimarilyabsorbedintheileum,andtheupperileumissaidtoplayamajorr6leintheabsorptionof'311-labelledoliveoilintherat(Benson,Chandler,Vansteenhuyse,andGagnon,1956).Ontheotherhand,Turner(1958)considersthatthejejunumisthemainsiteofabsorptionoffatinthedog,andBorgstrom,Dahlqvist,Lundh,andSj6vall(1957)havesuggestedthatinmanfatisabsorbedintheproximal100cm.ofthejejunum.Inviewoftheseconflictingreports,wehavemadeafurtherstudyofthesiteoffatabsorptionintherat,using'31L-labelledtriolein.Animalswerekilledatfrequentintervalsduringabsorptionofdifferentdosesoflabelledfatandtheamountofradioactivityinthevariouspartsofthesmallintestinewasmeasured.Theuseof1311-labelledtrioleinprovidesasimplemethodforthispurpose.Digestionofthelabelledmaterialdoesnotreleasesignificantamountsoffreeiodide(Balint,Pendower,andRamsey,1960)and1311thereforeremainsboundtotrioleinduringabsorptionatleastuntilithasreachedtheintestinalmucosalcell.Studiesofthoracicductlymphafteroraladministrationof1311-labelledtrioleinsuggestthatitmaybepartiallyde-iodinatedduringabsorp-tion(VanHandelandZilversmit1958),butthisprobablyoccurswithintheintestinalmucosalcellssincefreeiodidemaybereleasedfrom'311-labelledtrioleinbyincubationwithfreshlypreparedextractsofintestinalmucosa(Balintetal.,1960).Thispaperisconcernedwiththedetectionoflabelledtrioleinonlyduringitspassagealongtheintestinaltractandentryintothemucosalcellandthemeasurementofthetotalradioactivityinthevariouspartsofthegutisthereforeasatisfactorymethodoftracingtheabsorptionof1311-labelledtriolein.ThetechniquesusedweresimilartothosereportedbyParkins,Dimitriadou,andBooth(1960).MATERIALSANDMETHODSEXPERIMENTALANIMALSLaboratory-bredmaleorfemalewhiteratsweighingapproximately200g.wereusedthroughouttheexperiments.1311-LABELLEDTRIOLEINTrioleinwasiodinatedwith1311usingamodificationofthemethoddescribedbyLubranandPearson(1958).The1311-labelledtrioleinwaspurifiedfirstbydialysisandthenbypassagethroughanion-exchangeresintoremovefreeiodide.Theresultingmaterialusuallycontainedaconcentrationof30to40mg.oftrioleinperml.,ofspecificactivity0.8to0.91£c.permg.Itwastestedforpuritybyascensionchromatographyinabutanollaceticacid/watermedium,thedistributionofradioactivityinthechromatogramsbeingdeterminedbyautoradiography.Chromatogramswerealsomadeafterincubationofthe1311-labelledtrioleinwithpan-23 C.C.Booth,A.E.Read,andE.Jonescreaticextractsorwithratintestinaljuicefor24hours.AsreportedbyBalintandhiscolleagues(1960),digestionfailedtoreleasesignificantamountsof1311fromthematerial.Testdosesof10,50,100,and500mg.of1311-labelledtrioleinin1-ml.volumewerepreparedbydilutionandtheadditionofsuitableamountsofunlabelledtriolein,theradioactivityofthetestdosesbeingadjustedtoapproximately2fuc.of131J.EXPERIMENTALPROCEDUREABSORPTIONOF131I-LABELLEDTRIOLEINBYINTACTANIMALSSuccessivedosesof10,50,100,and500mg.of13I-labelledtrioleinweregiventoagroupofsixrats.Theanimalswerestarvedfor12hoursbeforehandandthelabelledfatwasfedfromagraduatedsyringethroughafinepolythenetubepassedintothestomach.Eachanimalwasthenplacedinanindividualmetabolismcageandthefaeceswerecollectedfor48hours,urinebeingcollectedseparately.Theexcretionofradio-activityinthefaecesoftheseanimalswasmeasuredandabsorptionwascalculatedbyassumingthattheradio-activematerialnotrecoveredinthefaeceshadbeenabsorbed.MeasurementofradioactivityThefaecalspecimenswerecollectedinwaxedcartonsandcountedbyposition-ingeachcartonbetweentwoscintillationcounters,accordingtothemethoddescribedbyLewisandPorter(1960).Thiscountingarrangementmadeitpossibletocounttheradioactivitywithouthomogenization.Thesensitivityofthecounterwasapproximately60,000countsperminuteper,ecof131I.Backgroundcountingratesrangedfrom400to600perminute.DISTRIBUTIONOFRADIOACTIVITYINGASTROINTESTINALTRACTDURINGABSORPTIONFourgroupsof21animalswereused.Theanimalswerestarvedfor12hoursandeachgroupofratsreceivedintragastrictestdosesofeither10,50,100,or500mg.of"3'l-labelledtriolein.Threeanimalsfromeachgroupwerethenkilledateachofthefollowingtimesafterthetestdosesweregiven:quarter,half,one,oneandahalf,two,three,andfourhours.Thegastro-oesophagealjunction,pylorus,andrectumwereclampedandtheentiregastrointestinaltractwasremoved.Thesmallintestinewasdividedintofoursegmentsofequallengthasfollows:(1)duodenumandupperjejunum,(2)lowerjejunum,(3)upperileum,(4)lowerileum.Thestomachanddifferentsegmentsofthesmallintestineandcolonwerethenplacedinseparatecontainersanddissolvedin20ml.ofasolutioncontainingequalquantitiesof40%sodiumhydroxideandabsolutealcohol,themixturebeingincubatedovernightat56°C.MeasurementofradioactivityTheradioactivityofthestomachandintestinalspecimenswasmeasuredinacoronet-typecountersimilartothatdescribedbyVeallandBaptista(1954).Withthiscounter,1jacof1311recordedapproximately9,000countsperminute.Back-groundcountingratesonthiscounterrangedfrom25to45perminute.RESULTSABSORPTIONOFTESTDOSESOF1311-LABELLEDTRIO-LEINBYINTACTRATSTheamountsofradioactivityrecoveredfromthefaecesofthesixanimalswhichreceivedtestdosesof10,50,100,and500mg.of1311-labelledtrioleinaregiveninTableI;themeanexcretionofradioactivityaftereachdoseisshowninFig.1.Theactualamountsof'311-labelledfatTABLEIFAECALEXCRETIONOF131I-LABELLEDTRIOLEIN(%OFDOSE)AFTERINTRAGASTRICDOSESOF10TO500MG.RatNo.AmountFed(mg.)1050100500111-0211-03156410-5513-569-2Mean±S.D.=11-8±22334030qoso20uL18-311-69-614-612-59.411-0±2.3414-813-914-314-820-618-316-0±2.730.322-823-023-822-423-124-2±3:01..10so100500Dosegiven(mg.)FiG.1.Meanpercentageofadministereddose(±t2S.D.)excretedinthefaecesofsixratsgiven10,50,100,or500mg.of"31I-labelledtriolein.absorbedfromthedifferentdoses,calculatedbyassumingthattheradioactivematerialnotrecoveredinthefaeceswasabsorbed,aregiveninTableII;themeanabsorptionateachdoselevelisillustratedinFig.2.When10mg.of1311-labelledtrioleinwasgiven,thepercentageofthisdoseexcretedinthefaecesrangedfrom9.2to15.6(mean11.8%).Similaramountswereexcretedafterdosesof50mg.,butthepercentageexcretionofthedoseincreasedwhendosesof100or500mg.werefed,thefaecalexcretionrangingfrom13.9to20.6(mean16.0%)when100mg.wasgiven,andfrom22.4to30.3(mean24.2%)afterthe500mg.dose.Althoughthepercentageofthedoseexcretedinthefaeceswasgreaterwhenthelargerdosesweregiven,therewasaprogressiveandlinearincreaseintheactualamountof1311_labelledfatwhichwasabsorbedwhentheconcen-trationofthedosewasincreased(TableII,Fig.2).24 StudiesonthesiteoffatabsorptionTABLEIICALCULATEDABSORPTION1OF'35I-LABELLEDTRIOLEIN(MG.)FROMINTRAGASTRICDOSESOF10TO500MG.AmountFed(mg.)RatNo.105010050085286-185-785S279.481-783-98-945-88-944-28-445-28-942-78-643.79-145.3Mean=8-844.5348-0386-03850381.0388-0384-03790'Theabsorptionof"'1I-labelledtrioleinhasbeencalculatedbyassumingthattheradioactivitynotrecoveredinthefaeces(TableI)wasabsorbed.400X300o20011001050100Doltgiven(,.g.)500FIG.2.Meanabsorptionoflabelledfatbysixratsgivendosesof10,50,100,or500mg.of1311-labelledtriolein.DISTRIBUTIONOFRADIOACTIVITYINSTOMACHANDGastricemptyingThepercentagesofgastricINTESTINESDURINGABSORPTIONThepercentagesofemptyinginthefourgroupsofanimalsaredetailedthedifferentdosesfoundinthestomachanddif-inTablesIIItoVI.ThemeangastricemptyinginferentsegmentsofthesmallintestineandthecolonthethreeratskilledateachtimeafterthedifferentofthefourgroupsofratskilledatvaryingtimedosesisshowninFig.3.intervalsafterreceiving10to500mg.of1311-labelledAtquarterofanhourafterfeeding,therewasnotrioleinaregiveninTablesIII(10mg.),IV(50mg.),significantdifferenceinthepercentagesoftheV(100mg.),andVI(500mg.).Fromtheseresultsdifferentdoseswhichhadleftthestomach,themeanthepercentagegastricemptyingandpercentagepercentagegastricemptyingrangingfrom41.2toabsorptionofthetestdoseshavebeencalculated503%(Fig.3).Byhalfandonehour,thepercentagesforeachanimal.ofgastricemptyinghadincreasedafterallfourTABLEIIIPERCENTAGEOF10MG.DOSEOF1311-LABELLEDTRIOLEININSTOMACHANDINTESTINESOFRATSKILLEDATINTERVALSOFQUARTERRatTimeNo.Killed(hr.)789101112131415161718192021222324252627I39.545.0645Mean=49.722-317-231-2Mean=23-6136-8129-61295Mean=31.91428-51410-414255Mean=21*5220-02167217-4Mean=18-0310-635.4319-4Mean=11-8417-6415-849.3Mean=14-3StomachSmallIntestine123414-025020-219-76-826-611-414-93-15.54-64.45-86-85.96-24-23.74-14-01-61-01-11-20-8091-10933.413-812-019-79.721-410613-94.53.34-2403.97.44.75.32-32.57.74-22-2121-01-51-61-82-31-9523-10.63013.90210682658111.7881069860887.55.94.7601.3120608081.80o91.2OFANHOURTOFOURHOURSColonTotal%'Gastric'AbsorbedEmptying0o00-1030-10o00.2020-10.53.4021.40o503461.81-9291-120485.74.44.97.986467-00o00203020o01.30707040.51.5080.61-01.2090848072.13.97.75s25s63.15.75.94.992.187397.992.452.766964767451850451.751.349.935.747.944.534.737.535.736.324022231725.93183462413027.91272.17647.333135.332648249648348750-164352.155.565362564363776077.868.374.168265475.969.861-555s035.550.377.782868876463270470568.171.589674.578.580.083.382682089494680688.282484290785s7'Percentageabsorptionwascalculatedbyassumingthatthematerialnotrecoveredhadbeenabsorbed.2Percentagegastricemptyingistheamountofmaterialnotrecoveredfromthestomach.2523456lrl)l C.C.Booth,A.E.Read,andE.Jones60/v40.20!/4J1I2ll234HoursFIG.3.Meangastricemptyingatintervalsofquarterhourtofourhoursafterfeedingdosesof10(++),50(o0o),100(@),or500(&A)mg.of1311-labelledtriolein.doses.Betweenoneandthreehours,however,gastricemptyingwasinvariablylesscompleteafterthe500mg.dosethanafterdosesof10,50,or100mg.oflabelledfat.Althoughmorethan80%ofdosesof10to100mg.offathadleftthestomachattwoandthreehours,forinstance,themeangastricemptyingafterdosesof500mg.wasonly54-2%attwohoursand69.6atthreehours(Fig.3).Byfourhours,thedegreeofgastricemptyingwassimilarinthefourgroupsofanimals.RatesofabsorptionThetotalamountsofradio-activityrecoveredfromtheintestinaltractsofthefourgroupsofratsarealsogiveninTablesliItoVI.Theabsorptionbythedifferentratskilledateachtimeinterval,calculatedbyassumingthattheradioactivitynotrecoveredhadbeenabsorbed,isgivenintheseTables.ThemeanamountsabsorbedbythethreeratskilledateachtimeafterthefourdifferentdosesareshowninFigs.4and5.Theratesofabsorptionofthedifferenttestdosesvariedaccordingtotheamountoftrioleinwhichwasgiven.Intermsofthepercentageofeachdoseabsorbed,absorptionwasmostrapidafterfeedingthe10mg.doseandbecameprogressivelyslowerastheamountoffatinthetestdosewasincreased(Fig.4).Iftheseresultsareexpressedastheabsoluteamountoffatabsorbed,ontheotherhand,itcanbeseenthatthemorefatwasgiven,thegreaterwastheactualamountoffatabsorbedinagiventime(Fig.5).DistributionofradioactivityinsmallintestineThepercentagesoftheadministeredradioactivityfoundinthefoursegmentsofthesmallintestineofeachrataregiveninTablesIIItoVI.ThemeanamountsinthedifferentsegmentsofthethreeratskilledateachtimeafterthevariousdosesareshowninFig.6.LEIVPERCENTAGEOF50MG.DOSEOFM3I1-LABELLEDTRIOLEININSTOMACHANDINTESTINESOFRATSKILLEDATINTERVALSOFQUARTEROFANHOURTOFOURHOURSStomachSmnallIntestine123426-87-210-214-716-06-33-08-48-75-97-17-27-12-34-34-67-71-42-94-02-62-82-12-74-51-42-22-739-611.925-225-615-29-432-218-95-17-32-14-84-32-916-47-913-33-412-29-61.92-01-72-94-31-42-22-55-30-00-01-80-919-21-77-36-412-213-410-74-521-822-316-28-38-212-99-81-64-41-02-30-61-41.1100-00-00-00-00-20-30-00-21500-0055212-20-30-34-30511-80-44-32-03-08-44-515152-52-8ColonTotal%Gastric2AbsorbedEmptying1-20-00-00-40-80-80-0050-50-20-20-31-20-60-50-80-90-60-50-78-811-02-67-58-67-98-98-598-692-898-196-585-473-183-680-771-156-284-870-764-347-458-056-646-339-747-542-541-635-029-035-226-327-234-830-11-47-21-93-514-626-916-419-328-943-815-229-335-752-642-043-453-766-352-557-558-465-071-064-873-772-863-269-974.326-337.346-047.762-953.354-664-669-438-557.565-080-585-877-184-491*781-495-876-588-286-883-593-286-472-183-9'Percentageabsorptionwascalculatedbyassumingthatthematerialnotrecoveredhadbeenabsorbed.'Percentagegastricemptyingistheamountofmaterialnotrecoveredfromthestomach.26RatTinmeNo.Killed(hr.)282930313233343536373839404142434445464748X25-74-1.73.762-7Mean54-0252-3437-1246-7Mean45-4135-4130-6161-5Mean42-51)35-01419-51414-2Mean22-9215-628-3218-6Mean14-2324-5311-8313-2M-an16-546-8413-6427-9Mean-16-1 StudiesonthesiteoJ'fatabsorptionfi60,-/IOOOmg.20X-/OOgS4�2i2234HoursFIG.4.Meanpercentageabsorptionofdosesof10(+-+),50(O-O),100(-),or500(&^A)mg.of,fatatintervalsofquarterofanhourtofourhoursafterfeeding.0SOOmg.DIOOmg.50mg.KOmrg.2HoursFIG.5.Meanamountsoffatabsorbed(mg.)atintervalsofquarterhourtofourhoursafter,feedingdosesof10(xx),50(O--O),100(e),or500(AA)mg.of1311-labelledtriolein.HOURSAFTERDOSE1/4'12I1/223430L230*Duodenum&jjunum101234C234C234C234C1234C1234C1234CINTESTINALSEGMENTNUMBERFIG.6.Distributionofradioactivematerialinthedifferentsegmentsofthesmallintestineandinthecolo,iofratskilledatintervalsofquarterofanhourtofourhoursafterfeedingdosesof10,50,100,or150mg.of'311-labelledtriolein.Theintestinalsegmentswerenumberedasfollows:(1)Duodenumandupperjejunum,(2)lowerjejunum,(3)upperileum,(4)lowerileum.Cindicatesthecolon.An0C0I,3.el-2730LO-OM9.2010030500mg.2010 C.C.Booth,A.E.Read,andE.JonesWhen10mg.of1311-labelledtrioleinwasgiven,themeanpercentageofthedosefoundineachoftheproximaltwosegmentsatquarterofanhourwas19-7%(Fig.6,TableHI).Theradioactivityinthesesegmentsfellgraduallyduringthenextonetothreehoursasabsorptiontookplace(Fig.4),sothatatthreeandfourhoursnomorethan1-9%oftheadministeredradioactivitywaspresent.Atthesametime,themeanradioactivityinthethirdsegmentrosefrom30%atquarterofanhourto8.8%atoneandoneandahalfhours.Itthenfellto0.8and1.2%atthreeandfourhours.Thefourthsegmentcontainedonlysmallamountsofradioactivityuntilthreeandfourhours,atwhichtimesthemeanradioactivityinthissegmentwas4.9and7.0%respectively(Fig.6,TableIII).Whenthedosewasincreasedto50mg.,themeanpercentageoftheradioactivitypresentinthefirstsegmentatquarterofanhourwaslessthanafter10mg.,being14.7%.Atthistime,moreradio-activity(25.6%)wasfoundinthesecondsegment.Theradioactivityinthesetwosegmentsfellmoreslowlythanwhen10mg.wasgiven,9.6%ofthedosebeingstillpresentinthesecondsegmentattwohours(Fig.6,TableIV).Bythreeandfourhours,asabsorptionproceeded,lessthan2.9%waspresentineitherofsegments1and2.Themeanradioactivityinthethirdsegmentwasonly1.8%atquarterofanhourbutrosetoapeakof16.2%atoneandahalfhours.Itthenfellgraduallyto2-3%atthreehoursand1%atfourhours(Fig.6,TableIV).Thefourthsegmentcontainedlittleornoradio-activityatquarterorhalfanhourbutbetween4and5%oftheadministeredradioactivitywasfoundinthissegmentbetweenoneandthreehours.Afterthedoseof100mg.themeanpercentageradioactivityinthedifferentsegmentswasessentiallysimilartothatafteradoseof50mg.However,agreaterproportionoftheadministeredradio-activitywaspresentinsegments3and4attwohoursthanwheneither10or50mg.wasgiven(Fig.6,TableV).Whenadoseof500mg.wasgiven,thepercentageoftheradioactivityrecoveredfromsegments1and2betweenaquarterofanhourandtwohourswassimilartothatafter50or100mg.However,theradioactivityinthesesegmentsfellmoreslowlythanafterthesmallerdoses,forsegment1stillcon-tainedasmuchas4.6%andsegment2,10.4%atthreehours,atwhichtimethemeanradioactivityinthesesegmentswasinvariablylessthan2to3%afterdosesof10to100mg.(Fig.6,TableVI).ThemeanradioactivityinthethirdsegmentwasTABLEVPERCENTAGEOF100MG.DOSEOF1'11-LABELLEDTRIOLEININSTOMACHANDINTESTINESOFRATSKILLED1411111222333444ATINTERVALSOFStomachSmallIntestineQUARTEROFANHOURTOFOURHOURSColonTotal%1Gastric'AbsorbedEmptying123471-056-049.3Mean=58.871-744.540-2Mean=52-159.5372570Mean=51-248-928-216-0Mean=31-07.412-928-8Mean=16-47.323-516-2Mean=15-75616-6124Mean=11.59.083-013-211-74.57.310-37.43-111-711-08-61-910810-27-61-56-9675.01.93.91-6252-31-72-32-111-223-328-621-06-122-624-617-86-97.4877.73.420-417-013-68-319-18-812-13.02-31-62-31-41-11-11-2761-50.03013-79.31-08-39-19-29.99.4907.3727-818-8709.511-81-52.51-81-92-42-81-82-30.00.00.00*00.50.0030.31-91-00.01-09-8034.95S022-31-93.59.32.53-11-72-42-38-65.25.40-1040.00-2070-6040-61-10.0030O5040709070-61-3090911-413-217-414-013-86-811*810-898994-291-194.797284-376886-179-666586-977.773.467756262-459.949-158255.727-648.540348-827837-634-633.31-15.88-95.32-815-723-213-920-433.513-122-326-632-343-837640-150941-844.372-451559.761-272-262-465466-729-044.050.741-228-355.559847.940.562-863-068-851171-884-069.092-687-171-283-692-776.583-884-394.483-487.688-5'Percentageabsorptionwascalculatedbyassumingthatthematerialnotrecoveredhadbeenabsorbed.2Percentagegastricemptyingistheamountofmaterialnotrecoveredfromthestomach.28RatTimeNo.Killed(hr.)495051525354555657585960616263646566676869 StudiesonthesiteoffatabsorptionTABLEVIPERCENTAGEOF500MG.DOSEOF135I-LABELLEDTRIOLEININSTOMACHANDINTESTINESOFRATSKILLEDATINTERVALSOFQUARTEROFANHOURTOFOURHOURSStomachSmallIntestine12341361036010-011-05s68.58.47.3385.55.5562.7634.92.57.346481.23.19.5461.7231.61-96924832821.512213.024416586167128127146761041093.412010-586127361501043.15.09.55.92317.14611616611312913.621.419210.517020.133017.523.5462481761573827.74.51512899.421619-9830o00o02808040o00408460o0183.5660.03.43.91.50.01.89.710-515-912019712912415-0ColonTotal''Gastric'AbsorbedEmptying0o00o00o00o00o003000-136690o03.50.3060-103200602090o519.40969081461-15.596698.099097.992691798.594.377.782-783-382-279.390777-682-582-576-973-677.785579.373-879.559252.555.855.83.42-01-02-17.48-31I55.722-317-316-718820-79.322-417517-523126-422-314-520-726-220540-847.544-244.255.343.244.448048039.957.348464068.545.559.364.859856760443.969359.354271.865072-069694291790492.1'Percentageabsorptionwascalculatedbyassumingthatthematerialnolrecoveredhadbeenabsorbed.'Percentagegastricemptyingistheamountofmaterialnotrecoveredfromthestomach.alwaysgreaterthanwhendosesof10,50,or100mg.weregiven.Itrosefrom116%ataquarterofanhourtoamaximumof23.5%atoneandahalfhours,andremainedashighas15-7,l51,and19-9%attwo,three,andfourhours(Fig.6,TableVI)whenlessthan3%wasrecoveredfromthissegmentafterthesmallerdoses.Thefourthsegmentcontainedonlysmallamountsofradioactivityuntilthreeandfourhours,atwhichtimesthemeanradioactivityrecoveredfromthissegmentwas12.0and15.0%respectively.Theseamountswerealsogreaterthanwerefoundinthefourthsegmentafterthesmallerdoses.RadioactivityinthecolonInsignificantamountsofradioactivitywererecoveredfromthecolonuntiltwotothreehoursafterthedosesweregiven(Fig.6,TablesIIItoVI).Atthreeandfourhours,thecolonicradioactivitywasinvariablylessthantheamountsexcretedinthefaecesaftersimilardosesweregiventointactrats(TableI).DISCUSSIONAMOUNTSOFFATABSORBEDFROMINCREASINGDOSESTheseresultsshowthatwhenincreasingdosesof1311-labelledfataregiventorats,thereisaprogres-siveincreaseinthepercentageofthedoseexcretedinthefaeces(TableI,Fig.1).Intermsoftheamountoffatabsorbed,however,thereisaprogressiveincreaseinabsorptionasincreasingdosesof1311.labelledtrioleinaregiven(TableII,Fig.2).SITESOFFATABSORPTIONThesitesofabsorptionofthevariousdosesvariedaccordingtotheamountfed.Sinceabsorptioninvariablybegansoonafterfeeding(Fig.4),itseemedlikelythatthelabelledmaterialwasabsorbedfirstfromtheupperintestineregardlessoftheamountoffatgiven.Thiswasconfirmedbythestudiesofthedistributionofradio-activityinthedifferentsegmentsofthesmallintestineduringabsorption(Fig.6).Sincemuchoftheradioactivityintheproximaltwosegmentsdis-appearedduringthefirsthourafterfeedingwithoutsubsequentlyappearinginsimilaramountsinthemoredistalsegments,itisevidentthatthejejunumabsorbedconsiderableamountsoffat.Theimpor-tanceoftheileum(segments3and4)infatabsorptiondependedontheamountoffatgiven,fortheradio-activityreachingthedistaltwosegmentswasproportionaltotheamountoflabelledfatgiven,thegreatestamountsbeingrecoveredfromtheileumafterthelargestdoses(Fig.6).However,theRatTimeNo.Killed(hr.)707172737475767778798081828384858687888990l44.756-855.6Mean=52.0i520i60.1i42-7Mean=516136-0131-5154-5Mean=4071135.2114021143.3Mean=39-6266-123072407Mean=45-8328-2335.0328-0Mean=30445-848-349-6Mean=7-9 C.C.Booth,A.E.Read,andE.Jonesrecoveryofradioactivematerialfromtheileumdidnotnecessarilyindicatethatabsorptionoccurredthere.Althoughasmuchas10%ofadoseof10mg.of131L-labelledtrioleinreachedtheileum(segments3and4,TableIII,Fig.6),itcouldnotbeassumedthatthismaterialwasabsorbedfromthatarea.Themeanfaecalexcretionafteradoseof10mg.offatbyanintactratwas11-8%(TableI,Fig.1),andthe10%recoveredfromtheileumafteradoseof10mg.wasthereforelikelytobemerelyintransittothecolonforsubsequenteliminationinthefaeces.Whenlargeramountsoffatwerefed,however,thepercentageswhichreachedthedistalintestineweregreaterthanwereexcretedinthefaecesofintactratsgivensimilardoses(Figs.1and6),indicatingthatpartoftheselargerdosesmusthavebeenabsorbedintheileum.Anapproximateestimateoftheamountofeachdosewhichwasabsorbedintheileumhasbeenobtainedbysubtractingthepercentageofthedoseexcretedinthefaecesbyintactratsgiventhevariousdoses(TableI,Fig.1)fromthemaximumpercentages_2r-'a_-_s__I.s*..*..rincreasingproportionwasthenabsorbedthere(TableVII).Theileumoftherat,however,appearstobelessefficientatabsorbingfatthanthejejunum,formuchofthelabelledmaterialwhichreachedtheileumpassedonintothecolonandwasexcretedinthefaeces(TablesIandVII).DawsonandIsselbacher(1960),studyingtheesterificationofpalmitate-l-C14byratintestinalmucosainvitro,haveshownthatcell-freehomogenatespreparedfromtheileumareaquartertoafifthasactiveasthosefromthejejunum.Theresultsgiveninthispaperdemonstrateaninterestingparallelinvivototheirobservations.Theobservationthatfatmaybeabsorbedinincreasingamountsfromboththejejunumandtheileumwhenthedietaryloadisincreasedprovidesarationalexplanationfortheunlimitedcapacityofthesmallintestinetoabsorbfat(Fig.2).Theabsorptionofthelargestdosesmayalsobefacilitatedbyslowergastricemptyingaftersuchdoses(Fig.3),whichservestoreducetheamountoffatdeliveredtotheintestineatanyonetime.ofeachdoserecoveredtromtheileumtogetherwithRELATIONSHIPBETWEENRATESANDSITESOFABSORP-thecolonatanyonetime(TablesIIItoVI,Fig.6).TIONIfabsorptionofasubstancebeginsintheIftheamountoffatabsorbedintheileumisthenupperjejunum,thesiteofitsabsorptiondependsonsubtractedfromtheamountshowntobeabsorbedtherelationshipbetweenitsabsorptionrateandthefromtheequivalentdoses(TableIL,Fig.2),aupperintestinalmotility(Parkinsetal.,1960).similarapproximationofjejunalabsorptionmaybeGlucoseoriodide,forinstance,areabsorbedrapidlyderived.Theresultsoftheseestimations,setoutinandtheirabsorptionoccursintheproximalhalfofTableVII,indicatethatwhen10mg.of1311-labelledthesmallintestineoftherat(ReynellandSpray,1956;Parkinsetal.,1960).1311-labelledalbumin,TABLEVIIhowever,isabsorbedmoreslowly;therapidmotilityCALCULATEDABSORPTIONOF1311-LABELLEDTRIOLEINoftheuppersmallintestinepropelsitmoredistallyFROMJEJUNUMANDILEUMbeforeabsorptioniscompleteandsomeabsorptionthereforealsooccursintheileum(Parkinsetal.,AmountFed(mg.)1960).1050100500Thereappearstobeasimilarrelationshipbetween'Meanfaecalexcretionabsorptionrateandthesiteofabsorptionofdifferent(%ofdose)'11-811.016024-2dosesoffat.The10mg.doseoffat,forinstance,Maximummeanradioactivitywasabsorbedrelativelyrapidly(Fig.4)andabsorp-inileumpluscolon(%ofdose)213-121.322-1405tionthereforeoccurredinthejejunum(TableVII).Calculated%ofdose1-310-36.116-3A500mg.dose,however,wasabsorbedmoreabsorptionmg.015161810slowly(Fig.4)andagreaterproportionwasthere-Calculated/g.ofdose86-978777.9S96forepropelledasfarastheileumbeforeabsorptionabsorptionwascomplete.fromjejunummg.8739.477.9298-0'DatafromTableI.'DatafromTablesIIT,IV,V,andVI.fatwasgivenabsorptionwasalmostentirelyinthejejunum(TableVII).Largerdosesoffatwerealsoabsorbedtoagreatextentinthejejunum,whichrespondedtoanincreaseddietaryloadbyanin-creasedabsorption,butatthesametimeincreasingamountsoffatescapedabsorptionintheproximalsegmentsandthereforepassedintotheileum;anPREVIOUSOBSERVATIONSONSITEOFFATABSORPTIONTheresultsgiveninthispaperareinkeepingwithpreviousobservationswhichhavesuggestedthatthejejunumplaysamajorpartinfatabsorption(Borgstrometal.,1957;FavargerandGerlach,1953;Frazer,1943;Turner,1958).Otherworkershaveclaimedthatintheratmaximalabsorptionof1311-labelledoliveoiloccursinthethirdquarterofthesmallintestine,anareasaidtobespecificallyadaptedforthispurpose(Bensonetal.,1956).Fig.630 Studiesonthesiteoffatabsorption31illustratesapossiblereasonforthisconclusion.Theseworkerskilledtheirratsonlyattwohoursandlateraftertheadministrationoflabelledfat,andmeasuredtheradioactivityinthemucosaofdifferentsegmentsofthesmallintestine.Whenratsarekilledattwohoursandlaterafterthetestdoses,mostoftheradioactivematerialislikelytoberecoveredfromtheileum,formostofthejejunalfathasalreadybeenabsorbedinthistime(Fig.6).SUMMARYThefaecalexcretionofradioactivitywasmeasuredinratsgivendosesof10,50,100,or500mg.of1311-labelledtriolein.Asthedosewasincreased,thepercentageofthedoseexcretedinthefaecesbecamegreaterbuttherewasaprogressiveincreaseintheamountoffatwhichwasabsorbed.Thesitesofabsorptionofthe131L-labelledtrioleinvariedaccordingtothedosegiven.Adoseof10mg.wasabsorbedrelativelyrapidlyandabsorptionoccurredinthejejunum.Largerdoses,however,wereabsorbedmoreslowlyandthemotilityoftheupperintestinepropelledthefatmoredistallybeforeabsorptionwascomplete;absorptionthereforealsooccurredintheileumwhenthelargerdosesweregiven.Theincreaseinabsorptionofthelabelledtrioleinwhichoccurredwhenincreasingdosesweregivenwasachievedfirstbyanenhancedjejunalabsorptioninresponsetoanincreaseddietaryload,andsecondlybyanincreasedabsorptionintheileum.WewishtothankMissJanetHeathforherhelpincarry-ingouttheseexperiments.REFERENCESBalint,J.,Pendower,J.,andRamsey,N.W.(1960).Thestabilityofradio-iodinatedoliveoil.Clin.Sci.,19,321-325.Benson,J.A.Jr.,Chandler,G.N.,Vansteenhuyse,F.E.,andGagnon,J.0.(1956).Studiesconcerningthesiteoffatabsorptioninthesmallintestineoftherat.Gastroenterology,30,53-61.Booth,C.C.(1958).AnExperimentalandClinicalStudyoftheIntes-tinalAbsorptionofVitaminB12.M.D.Thesis,St.AndrewsUniversity.andMollin,D.L.(1959).ThesiteofabsorptionofvitaminB,,inman.Lancet,1,18-21.Borgstrom,B.,Dahlqvist,A.,Lundh,G.,andSjovall,J.(1957).Studiesofintestinaldigestionandabsorptioninthehuman.J.clin.Invest.,36,1521-1536.Cummins,A.J.,andJussila,R.(1955).Comparisonofglucoseabsorptionratesintheupperandlowerhumansmallintestine.Gastroenterology,29,982-992.Dawson,A.M.,andIsselbacher,K.J.(1960).Theesterificationofpalmitate-l-C"byhomogenatesofintestinalmucosa.J.ctin.Invest.,39,150-160.Favarger,P.,andGerlach,J.(1953).Variationsdanslemodeder6sorptiondesdifferentesgraissesetacidesgraschezlerat.Helv.physiol.pharmacol.Acta,11,239-250.Frazer,A.C.(1943).Differentiationintheabsorptionofoliveoilandoleicacidintherat.J.Physiol.(Lond.),102,306-312.Kremen,A.J.,Linner,J.H.,andNelson,C.H.(1954).Anexperi-mentalevaluationofthenutritionalimportanceofproximalanddistalsmallintestine.Ann.Surg.,140,439-448.Lewis,S.M.,andPorter,I.H.(1960).Erythrocytesurvivalinrheuma-toidarthritis.Ann.rheum.Dis.,19,54-58.Lubran,M.,andPearson,J.D.(1958).Ascreeningtestforsteator-rhoeausing111I-labelledtriolein.J.*lin.Path.,11,165-169.Parkins,R.A.,Dimitriadou,A.,andBooth,C.C.(1960).Theratesandsitesofabsorptionof"3'I-labelledalbuminandsodium131Iintherat.Clin.Sci.,19,595-604.Reynell,P.C.,andSpray,G.H.(1956).Thesimultaneousmeasure-mentofabsorptionandtransitinthegastro-intestinaltractoftherat.J.Physiol.(Lond.),131,452-462.Turner,D.A.(1958).Theabsorption,transport,anddepositionoffat:applicationofanewmethodforthedeterminationof13'I-lipidactivityindogsandman.Amer.J.dig.Dis.,n.s.,3,594-640,682-708.VanHandel,E.,andZilversmit,D.B.(1958).Limitationofradio-iodineasalabelforfat.J.lab.clin.Med.,52,831-839.Veall,N.,andBaptista,A.M.(1954).Amulti-tubegammacountingapparatusforsmallsamples.Brit.J.Radiol.,27,198-199.Verzar,F.,andMcDougall,E.J.(1936).AbsorptionfromtheIntestine.Longmans,Green,London.