Kitsize | 96 |
Method | RIA(CT) |
Incubationtime | 1x18h |
Standardrange | 0.8-70ng/mL |
Specimen/Volumes | 100µLserum,plasma |
Substrate/isotope | 125I<300kBq |
RegulatoryStatus: | EU:CE |
RadioimmunoassayfortheinvitroquantitativemeasurementofhumanSomatomedin-C(SM-C)inserumandplasma.
Somatomedin-C(SM-C)orInsulin-likegrowthfactorI(IGF-I)isabasic70aminoacidsinglechainpolypeptide(MW:7649Da)similartoproinsulin(50%sequencehomology),andtotheotherwell-characterizedmemberofthesomatomedinfamily:IGFII(67AA,70%sequencehomologywithIGF-I).SM-Cisthemostimportantfactor,whichmediatesthegrowthpromotingactionsofgrowthhormone,apituitaryhormonewithhighlyfluctuatingbloodlevelsduetopulsatilerelease.ThebloodconcentrationofSM-Cismorestableduetothebindingtocarrierproteins.Theconcentrationofthepredominantbindingprotein(MW53000)aswellastheproductionofSM-C,areregulatedbygrowthhormone.SM-Cisproducedbytheliver,andothertissues,andithasendocrine,paracrineandautocrineactivities.Itstimulatesgrowthandregulatesdifferentiationofvarioustissues,displaysinsulin-likeactivitiesandpromotescartilagegrowth.AlthoughGHisthemostimportantfactorcontrollingSM-Csecretionandconcentration,otherfactorsarealsodeterminant:theage(withapeakatadolescence),thesex,thenutritionalstatus,andotherhormones(oestrogen,thyroxin,prolactin,...).SpecifictrophicstimulimainlycontrolSM-Csecretioninthelocalmicroenvironmentofaparticularorgan(paracrineactivities),whilebloodSM-Cconcentrationisthemostimportantvariableforbalancedsystemicgrowth(endocrineactivities).
Growthretardationmaybeduetoseveralcauses,amongwhichdeficientGHproduction(hypopituitarism),whichisassociatedwithlowSM-Cbloodlevels.BecauseofthedifficultiestogetinterpretableresultsfromGHmeasurements(bydynamicmultipleorstimulationtests),thedeterminationofthestableSM-Cconcentrationinplasmaisoftenconsideredasasimplescreeningtesttoevaluation"GHimpregnation"ofthepatientbeforedecidingmoreextensiveinvestigations.Inseveralclinicalsituationswithimpairedgrowth,lowSM-ClevelsmaybeobserveddespitenormalorhighGHproduction(i.e.malnutrition,chronicdiseasesstates,somegeneticdwarfslikePygmies,...).Interestingly,childrenwithdiscreteGHneuro-secreterydysfunctionmaydisplaylowSM-CvaluesdespitenormalGHlevelsbyconventionaltesting.TheresultsofSM-CassaymustbeinterpretedcautiouslybyconsideringthenormalvariationsofSM-Cduringchildhoodandadolescence(seeRosenfeldetal).
SM-Clevelsareelevatedinacromegaly(excessproductionofGH)andmayserveasanindicatorofdiseaseseverity.Resultsaremorereadilyinterpretedbecausethenormalvaluesaremoreeasilydefinedinadults.SM-Cmeasurementsarealsousefultomonitortreatment.
TheSM-CRIAkitisaninvaluabletooltostudythemodificationsofthisgrowthfactorduringphysiologic(i.e.pregnancy)orpathologic(i.e.diabetes)situations,andthelocalregulationofSM-Cproductioninrelationtoitsparacrineandautocrineactivities(woundhealing,organregeneration,neoplasticgrowth,foetaldevelopment,gonadalregulation,etc).
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