FIBRINOLYTIC ACTIVITY IN THE ORGANISM
Authors: ALBRECHTSEN OLE K. 
Published: December 2008

The purpose of the present monograph is to give an account of the distribution of fibrinolytic components in the organism, with special reference to the tissue activator of plasminogen.

Fibrinolytic activity of tissues.—Various tissues from man and animals can digest fibrin. It has previously been shown that this digestion is referable to an activator present in the tissues; this activator is capable of transforming the fibrinolytic pro‐enzyme, plasminogen, which is present in fibrin, into the active fibrinolytic enzyme, plasmin.

Several tissues also contain components with an inhibitory effect on the fibrinolytic process. The nature of most of these components is unknown. However, it is known that a low‐molecular plasmin inhibitor (pulmin) is present in tissues from the bovine lung and uterus, and it has been shown that no other tissues contain pulmin.

Great difficulties have been encountered in elaborating methods for qualitative and quantitative determinations of the concentration of the tissue activator in fresh tissues, particularly because the tissue activator is firmly bound to the tissue proteins. A review of previously used analytic methods shows that they all had some qualitative or quantitative shortcomings. The elaboration of an analytic method for the quantitative determination of the content of plasminogen activators in the tissues was therefore a prerequisite for the present study. Based on the observation that potassium thiocyanate is a specific solvent for the extraction of the tissue activator it was possible to elaborate such a method, by which the concentration of the tissue activator could be expressed in terms of arbitrary units. The tissue concentration of pulmin may also be determined by this method.

The plasminogen activator of the tissues is relatively stable and differs in that respect from the plasminogen activator present in the blood under certain conditions. Thus, it is not destroyed by heating to 70 or 100°C. at acid reaction, and it is stable at 37 arid 50°C. within a very wide pH range. It is also stable to the action of a large number of chemicals, but is destroyed by formalin. It is very firmly bound to the tissue proteins and can be extracted only by repeated treatments with potassium thiocyanate. The activator is presumably chiefly localised in the microsomes.

A number of studies have shown that the tissue activator presumably reacts stoicheiometrically with plasminogen in an equilibrium process.

Quantitative determinations of the content of tissue activator have revealed considerable variations from organ to organ in the same animal, from animal to animal of the same species, and from one animal species to another. These variations are at present unexplained.

Wide variations in the concentration of the tissue activator have also been revealed in various human tissues, but it is possible to classify the organs into groups with high, moderate and low concentrations. The first group comprises such organs as the uterus, adrenals, lymph nodes, thyroid gland, lungs, prostate and ovaries, whereas liver tissue is practically inactive.

The significance of the tissue activator is not definitely known. It has often been emphasised that it should be of the greatest importance in the redissolution of fibrin deposits and thus be able to counteract the formation of connective tissue. It has further been claimed that a relationship exists between the tissue activator and the occurrence of local bleedings in the tissues; by causing a redissolution of the fibrin deposits formed for the purpose of haemostasis it should be a contributory factor in the persistence of the bleeding. It is unknown whether or not the tissue activator is of importance in the fibrinolytic activity of the blood in vivo or post mortem. Some experimental data are in favour of the assumption that the tissue activator under certain conditions (tissue destruction, cell necrosis) may be liberated from the cells and enter the circulating blood.

Fibrinolytic activity of the uterus.—Both human and animal myometrial tissues contain a plasminogen activator. Thus, the myometrium is the tissue of the human organism which contains the activator in the highest concentration.

Plasminogen activators are also present in the human endometrium. The concentration varies with the various stages of the menstrual cycle; it is only moderate in the proliferative and intermediate stages, but considerable in the secretory stage. The concentration decreases with age; the senile endometrium does not contain tissue activator. Considerable amounts of tissue activator are present in certain pathological endometria (endometrial hyperplasia, etc.). It is reasonable to assume that a relationship exists between the tissue activator of the endometrium and the female sex hormones. Such a relationship has also been observed in animal experiments, in which it has been shown that a decrease in the blood concentration of oestrogens results in an increase in the uterine concentration of the tissue activator.

Based on these observations the theory has been advanced that the tissue activator is of significance in the occurrence of uterine bleedings. It has thus been thought that the decreasing oestrogen concentration in the circulating blood in the premenstrual phase through a constriction of the spiral artery should give rise to incipient cell necrosis in the endometrium and thus provide optimum conditions for a liberation of the tissue activator. This liberation should be a contributory factor in the occurrence of bleeding from the tissue.

The tissue activator is absent in normal decidual and placental tissues, but present in pathological decidual tissue from spontaneous abortions. It is possible that the tissue activator in this pathological tissue may be a contributory factor in the occurrence of bleeding from the tissue and thus be of importance in the development of spontaneous abortion.

Normal menstrual blood does not clot spontaneously or after addition of thrombin. This is due to the absence of fibrinogen, because the menstrual blood has previously been clotted in the uterine cavity. This coagulation process is presumably due to a liberation of thromboplastin from the endometrium. Subsequent redissolution of the fibrin clot formed in this way is caused by a fibrinolytic process excited by the liberation of the tissue activator from the endometrium. This has been shown by the finding of a stable plasminogen activator of the tissue‐activator type in menstrual blood and by the presence of plasmin and the absence of plasminogen. Thus, the incoagulability of menstrual blood is due to the combined effects of tissue thromboplastin and tissue activator, which have both been liberated from the injured endometrial tissue.

As distinct from the local afibrinogenaemia of menstrual blood, hypo‐ or afibrinogenaemia has often been observed in the circulating blood, especially as an obstetrical complication. It is not known with certainty whether this condition may also be due to a liberation of tissue components (thromboplastin, tissue activator) from the uterus.

Fibrinolytic activity of the prostate and seminal fluid.—Prostatic tissue is fibrinolytically active. It has been shown that this activity is due to the presence of two plasminogen activators in the tissue, of which one corresponds to the stable plasminogen activator of the tissues and the other to the labile plasminogen activator of the blood.

Seminal fluid likewise contains plasminogen activators. It has been shown that the plasminogen activator of seminal fluid corresponds to the plasminogen activator of the tissues and is stable, for which reason it presumably originates from the prostate. However, the experimental results available do not exclude that the seminal fluid also contains plasminogen activators of the labile type.

Whereas the plasminogen activators of the prostate are thus presumably of importance for the fibrinolytic activity of seminal fluid, it is not yet known with certainty whether they may also give rise to the fibrinolytic activity in the circulating blood which has been observed in cases of cancer of the prostate and after prostatectomy.

Fibrinolytic activity of secretions and transudates.—Previous investigations on the fibrinolytic enzyme system of the blood have now rendered it possible to subject the fibrinolytic systems of other body fluids to a closer analysis.

Some secretions (milk, tears and saliva) contain fibrinolytic enzyme systems which differ from that of the blood. They are characterised by the presence of variable amounts of plasminogen activators and large amounts of plasminogen pro‐activators which can be transformed into the activator itself by addition of streptokinase. As distinct from blood, the secretions do not contain trypsin inhibitors. On the basis of these results it is discussed whether the fibrinolytic components originate from the blood or are secretory products from the gland concerned. The former assumption appears to be more likely. Accordingly, the glandular epithelium must be assumed to be selectively permeable to the active components, but capable of retaining the inhibitors.

A number of normal and pathological transudates also contain fibrinolytic enzyme systems largely corresponding to that of the blood. Thus, they contain large amounts of trypsin inhibitors and plasminogen pro‐activators. However, cerebrospinal fluid differs from the other transudates in that it does not contain inhibitors and is thus reminiscent of the secretions. In the present study, special attention was focused on the fibrinolytic enzyme system of amniotic fluid because it may be of significance in the haemorrhagic diatheses which have occasionally been observed in relation to amniotic‐fluid infusions.

SAMMENDRAG

Hensigten med det foreliggende arbejde har været at redegøre for fordelingen af fibrinolytiske komponenter i organismen, idet der specielt er lagt vægt på den i vævene forekommende plasminogen aktivator.

Fibrinolytisk aktivitet i vœv:En række væv fra såvel mennesker som dyr kan opløse fibrin. Det er tidligere vist, at dette skyldes en i vævene tilstedeværende aktivator, som kan omdanne det i fibrinet tilstedeværende fibrinolytiske proenzym, plasminogen, til det aktive fibrinolytiske enzym plasmin.

En række væv indeholder ligeledes komponenter med en hæmmende effekt overfor den fibrinolytiske proces. Naturen af disse er for størstedelen ukendt. Man har imidlertid kendskab til tilstedeværelsen af en lavmolekylær plasmininhibitor (pulmin) i okselungevæv samt i den bovine uterus, og det er vist, at andre væv ikke indeholder pulmin. Pulminets betydning er ukendt.

Det har veret forbundet med betydelige vanskeligheder at foretage kvalitative og kvantitative bestemmelser af vævsaktivatorkoncentrationen i friske væv. Dette skyldes ikke mindst vævsaktivatorens stærke binding til vævsproteinerne. En gennemgang af tidligere analytiske metoder har vist, at de alle frembyder enten kvalitative eller kvantitative mangler. Det har derfor været en forudsætning for det foreliggende arbejde at udarbejde en analysemetode til kvantitativ bestemmelse af vævenes indhold af plasminogen aktivatorer. Baseret på fundet af kaliumrhodanid som et specifikt ekstraktionsmiddel for vavsaktivatoren er det nu lykkedes at udarbejde en sådan metode, ved hjælp af hvilken vævsaktivatorkoncentrationen kan udtrykkes i vævsaktivatorenheder. Denne metode tillader ligeledes bestemmelse af vævenes indhold af pulmin.

Væenes plasminogen aktivator er relativt stabil og adskiller sig herved fra en i blodet under visse forhold tilstedeværende plasminogen aktivator. Den ødelægges således ikke ved opvarmning til 70 og 100 grader ved sur reaktion og er stabil ved 37 og 50 grader over et meget bredt pH spektrum. Den er stabil overfor påvirkning af en lang række kemiske stoffer, men ødelægges dog af formalin. Den er bundet meget kraftigt til vævsproteinerne og kan kun ekstraheres kvantitativt ved gentagne behandlinger med kaliumrhodanid. Aktivatoren er formentlig hovedsagelig lokaliseret til microsomerne.

En række undersøgelser har vist, at vævsaktivatoren formentlig reagerer støkiometrisk med plasminogen i en ligevægtsproces.

Kvantitative bestemmelser af vævsaktivatorindholdet i dyriske organer har vist betydelige variationer fra organ til organ hos samme dyr, fra dyr til dyr indenfor samme art og fra dyreart til dyreart. Disse variationer kan ikke forklares.

Ligeledes er der fundet betydelige variationer i vævsaktivatorkoncentrationen i humane væv. Det har dog væet muligt at skelne mellem organer med et stort, et moderat og et ringe indhold. Den forste gruppe omfatter organer som uterus, binyren, lymfekirtlerne, glandula thyreoidea, lungen, prostata og ovariet, medens levervæv er praktisk talt inaktivt.

Vævsaktivatorens betydning er ikke med sikkerhed kendt. Det er hyppigt fremhævet, at den skulle være af den største betydning for genopløsningen af lokale fibrindannelser i vævene og således i stand til at modvirke bindevævsdannelsen. Man har yderligere villet se en forbindelse mellem vævsaktivatoren og forekomsten af lokale blødninger i vævene, idet den ved at forårsage en genopløsning af i hæmostatisk øjemed dannede fibrinlag kunne have betydning for en blødnings fortsatte beståen. Hvorvidt vævsaktivatoren har betydning for opståen af fibrinolytisk aktivitet i blodet såvel in vivo som post mortem vides ikke. Visse eksperimentelle data taler til gunst for, at vævsaktivatoren under visse bestemte betingelser (vævsødelæggelse, cellenekrose) kan frigøres fra cellerne og gå over i det circulerende blod.

Fibrinolytisk aktivitet i uterus:Myometrievæv fra såvel mennesker som dyr indeholder en plasminogen aktivator. Det humane myometrium er således det mest vævsaktivatorholdige væv i den humane organisme.

Der findes ligeledes plasminogen aktivatorer i det humane endometrium. Koncentrationen af disse varierer med menstruationscyclus og er kun moderat i proliferationsfasen og intermediærfasen, men betydelig i sekretionsfasen. Koncentrationen aftager med alderen, og der findes ingen vævsaktivator i det senile endometrium. Visse pathologiske endometrier (endometriel hyperplasi m. m.) indeholder betydelige maengder vævsaktivator. Det har været naturligt at antage, at der er en forbindelse mellem vævsaktivatoren i endometriet og de kvindelige kønshormoner. En sådan er da også observeret i dyreforsøg, idet det er vist, at en aftagen af østrogenkoncentrationen i blodet medfører en stigning i vævsaktivatorkoncentrationen i uterus.

Disse iagttagelser ligger til grund for den teori, at vævsaktivatoren skulle være af betydning for optræden af uterine blødninger. Man har således ment, at den faldende østrogenkoncentration i det circulerende blod før menstruationen via en kontraktion af a. spiralis skulle forársage en begyndende nekrose af cellerne i endometriet og dermed etablere de bedste betingelser for en frigørelse af vævsaktivatoren. Denne vevsaktivatorfrigørelse skulle være en medvirkende faktor ved optraeden af blødning i vævet.

Der findes ikke vævsaktivator i normalt decidua‐ og placentarvæv, men derimod i pathologisk deciduavæv fra spontane aborter. Det er muligt, at vævsaktivatoren i dette pathologiske væv kan være en medvirkende faktor ved optræden af blødning i vævet og således have betydning for abortens opståen.

Normalt menstruationsblod kan ikke koagulere hverken spontant eller ved tilsætning af trombin. Dette skyldes en mangel på fibrinogen, idet menstruationsblodet tidligere har været koaguleret i uterincaviteten. Denne koagulationsproces skyldes formentlig en frigorelse af tromboplastin fra endometriet. Til grund for den senere indtrædende genopløsning af det således dannede fibrinkoagel ligger en fibrinolytisk proces, fremkaldt ved frigørelse af vævsaktivatoren fra endometriet. Dette er vist ved fundet af en stabil plasminogen aktivator af vævsaktivatortypen i menstruationsblodet samt ved fundet af plasmin og mangelen på plasminogen. Menstruationsblodets manglende koagulationsevne skyldes således en kombineret effekt af vævstromboplastin og vævsaktivator, begge frigjort fra det læderede endometrievæv.

I modstning til den lokalt optrædende afibrinogenæmi i menstruationsblodet er der ofte observeret hypo‐ eller afibrinogenæmi i det circulerende blod, navnlig som obstetrisk komplication. Hvorvidt denne ligeledes skyldes en frigørelse af vvskomponenter fra uterus (tromboplastin, vævsaktivator) vides ikke med sikkerhed.

Fibrinolytisk aktivitet i prostata og sperma:Prostatavæv er fibrinolytisk aktivt. Det er vist, at denne aktivitet skyldes tilstedeværelsen af to plasminogen aktivatorer i vævet, hvoraf den ene svarer til vævenes stabile plasminogen aktivator og den anden til blodets labile plasminogen aktivator.

Sperma indeholder ligeledes plasminogen aktivatorer. Det er vist, at plasminogen aktivatoren i sperma svarer til vævenes plasminogen aktivator og er stabil, hvorfor den formentligt må stamme fra prostata. Dog udelukker de foreliggende forsøg ikke, at der samtidig findes plasminogen aktivatorer af den labile type i sperma.

Medens plasminogen aktivatorerne i prostata saledes formentlig er af betydning for den fibrinolytiske aktivitet i sperma, vides det endnu ikke med sikkerhed, hvorvidt de ligeledes kan forårsage fibrinolytisk aktivitet i det circulerende blod, således som det er observeret ved tilfælde af cancer prostatae og efter prostatectomier.

Den fibrinolytiske aktivitet i sekreter og transsudater:Tidligere undersøgelser af blodets fibrinolytiske enzymsystem har nu muliggjort en nærmere analyse af de fibrinolytiske systemer i andre legemsvædsker.

En række sekreter (mælk, tåæer og spyt) indeholder fibrinolytiske enzymsystemer, som adskiller sig fra blodets enzymsystem. De er karakteriseret ved tilstedeværelsen af varierende mængder plasminogen aktivatorer og store mængder plasminogen proaktivatorer, som kan omdannes til selve aktivatoren ved tilsætning af streptokinase. Der findes, i modsætning til hvad der er tilfældet i blodet, ingen trypsininhibitorer i sekreterne. På grundlag af disse resultater har man diskuteret, hvorvidt de fibrinolytiske komponenter stammer fra blodet eller er sekretionsprodukter fra kirtlen. Den første teori forekommer mest sandsynlig, hvorfor kirtelepithelet må antages at være selektivt permeabelt for de aktive komponenter, men i stand til at tilbageholde inhibitorerne.

En række transsudater og pathologiske transsudater indeholder ligeledes fibrinolytiske enzymsystemer, som stort set svarer til enzymsystemet i blodet. Der findes således store mængder trypsininhibitorer og plasminogen proaktivatorer. Imidlertid adskiller cerebrospinalvædsken sig fra de øvrige transsudater ved ikke at indeholde inhibitorer og minder således om sekreterne. Interessen i det foreliggende arbejde har navnlig koncentreret sig om amnionvwdskens fibrinolytiske enzymsystem på grund of dettes eventuelle betydning for de haemorrhagiske diatheser, der i flere tilfælde er observeret i tilslutning til amnioninfusioner.