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Biomedical subjects

F Markwardt

Publications and source records attributed to F Markwardt.

At least 55 records · Page 3Linked to original sources
PubMed1991

Hirudin as anticoagulant in experimental hemodialysis.

After genetically engineered recombinant DNA desulfatohirudin (r-hirudin) had been investigated as to its pharmacokinetic behavior and blood level course in nephrectomized dogs, the compound was studied for its capability to prevent thrombus formation in the extracorporeal circulation. Beagle dogs underwent bilateral functional nephrectomy followed by experimental hemodialysis. r-Hirudin content in blood, fibrinogen level as well as platelet count were determined before and during the dialysis. Furthermore, the blood loss during the experiment was measured as well as the mean arterial pressure and the pressure in the blood line system. Intravenous administration of the thrombin inhibitor resulted in initial distribution in the extracellular space (distribution phase 90 min) followed by retarded decrease of the r-hirudin blood level (t1/2 beta approximately 6-8 h) which is due to the missing renal excretion of the inhibitor. This caused a long-lasting, dose-dependent anticoagulant effect, which is not only characterized by the prevention of increasing pressure before the capillary dialyzer but also by the reduced drop in fibrinogen and platelets during hemodialysis. The required dose of r-hirudin (0.5 mg/kg) is within a range where bleeding complications will not occur yet.

Animals↗
PubMed1991

Anticoagulant and antithrombotic action of the synthetic thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginine nitrile.

Anticoagulant and antithrombotic effects of a synthetic thrombin inhibitor of the tripeptide type, D-phenylalanyl-L-prolyl-L-arginine nitrile (1), were studied in vitro and in vivo. The anticoagulant action was investigated in common test assays such as thrombin time, activated partial thromboplastin time, prothrombin time and thrombelastography. In human plasma 1 caused a concentration-dependent prolongation of coagulation times and influenced the recalcification measured by thrombelastographic recording. In different models of experimental thrombosis in rats the antithrombotic effectiveness of 1 was shown. I.v. infusion of the thrombin inhibitor reduced or prevented the formation of stasis-induced venous thrombi and of arterial thrombi after electrically induced damage of the vessel wall as well as prolonged the time of a thrombotic occlusion of an extracorporeal arterio-venous shunt. At low doses 1 was also effective in thrombin-induced microthrombosis.

Animals↗
PubMed1991

Studies on toxicity and pharmacokinetics of the synthetic thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginine nitrile.

The tripeptide-type synthetic thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginine nitrile (1) was studied with respect to its toxicity and pharmacokinetics in mice, rats and rabbits. In mice the LD50 after i.v. injection was determined to be about 30-40 mg/kg. After i.v. injection and infusion in rats 1 caused a decrease in blood pressure up to 70-80% of initial values. The dose of 2 mg/kg.min led to a final outcome after about 15 min accompanied by a sharp decrease in blood pressure. The bleeding time after standardized incision of the rat tail was not significantly prolonged at doses which were tolerated. In pharmacokinetic studies the biologic half-life of 1 after i.v. injection was estimated to be about 12 min. After s.c. injection measurable plasma levels were obtained up to 5 h. Oral or intraduodenal administration of high doses of the inhibitor did not give effective plasma levels. Biliary excretion seems to be an important route of elimination. Cumulative excretion of 1 with the bile amounted to 31% of the dose within 240 min.

Administration, Oral↗
PubMed1991

Isolation and characterization of a thrombin inhibitor from the tick ixodes ricinus.

The secretion of the salivary glands of ticks, Ixodes ricinus, contains anticoagulant substances. Some years ago an antithromboplastin, ixodin, was described. The present paper refers the isolation and characterization of a second anticoagulant substance of the ticks named ixin. Ixin proved to be a relatively stable and specific thrombin inhibitor. The multi step procedure for isolation results in a 850-fold purification. The preparation obtained has a specific activity of 250 antithrombin units/mg. It is not homogeneous and still contaminated. The substance was assumed to be a miniprotein.

Amides↗
PubMed1990

Hirudin in haemodialysis.

The usability of recombinant hirudin as anticoagulant agent in haemodialysis was studied in nephrectomized dogs. To this end, we examined the capability of recombinant hirudin to penetrate the membranes of different capillary dialyzers used. Furthermore we investigated the pharmacokinetic behaviour of recombinant hirudin in nephrectomized dogs as well as its capability to prevent the activation of the clotting system and fibrin deposition during haemodialysis. The present results evidence the efficiency of recombinant hirudin in preventing thrombus formation in experimental haemodialysis and hence its suitability as anticoagulant in such extracorporeal circulation.

Animals↗
PubMed1990

Comparison of the anticoagulant and antithrombotic effects of synthetic thrombin and factor Xa inhibitors.

The anticoagulant effect of selected synthetic inhibitors of thrombin and factor Xa was studied in vitro in commonly used clotting assays. The concentrations of the compounds doubling the clotting time in the various assays were mainly dependent on their thrombin inhibitory activity. Factor Xa inhibitors were somewhat more effective in prolonging the prothrombin time compared to the activated partial thromboplastin time, whereas the opposite was true of thrombin inhibitors. In vivo, in a venous stasis thrombosis model and a thrombo-plastin-induced microthrombosis model in rats the thrombin inhibitors were effective antithrombotically whereas factor Xa inhibitors of numerically similar Ki value for the respective enzyme were not effective at equimolar dosage. The results are discussed in the light of the different prerequisites and conditions for inhibition of thrombin and factor Xa in the course of blood clotting.

Animals↗
PubMed1990

Antithrombotic effects of recombinant hirudin in experimental angioplasty and intravascular thrombolysis.

The effect of recombinant desulphatohirudin CGP 39393 (rH) on arterial thrombus formation and especially on thrombotic reocclusion after experimental angioplasty as well as after thrombolysis was investigated in rabbits. In the femoral artery thrombi were induced after endothelial damage of the vessel wall by a balloon catheter and following stasis. After removing the thrombus by angioplasty or after lysing it by streptokinase reocclusion of the artery was observed within a relatively short period of time. Subcutaneous injection of rH reduced the incidence of both primary thrombus formation and reocclusion in dependence on the dose administered. After a dose of rH of 2 mg/kg s.c. arterial thrombus formation was completely prevented and after administering 4 mg/kg s.c. thrombotic reocclusion did also not occur. Comparative studies with heparin showed that similar antithrombotic effects were only achieved at doses of 12 mg heparin/kg s.c. The results obtained suggest a clear potential of rH for prevention of thrombotic reocclusion in clinical states.

Angioplasty, Balloon↗
PubMed1990

Pharmacological modification of mechanical and electrical responses of frog heart to thrombin.

The pharmacological modification of the thrombin effect on the mechanical and electrical responses of frog heart was examined in the Straub heart preparation and in single ventricular cells. Associated with the positive inotropic action thrombin increases voltage and duration of action potentials of isolated frog ventricular cells. As found by the patch-clamp technique in the cell-attached mode, thrombin stimulates single L-type Ca2+ channels, presumably mediated by a second messenger. The enhancement of contractility by thrombin depends on the proteolytic activity of the enzyme because enzymatically inactivated thrombin has no effect on frog hearts. The positive inotropic effect of thrombin as well as its stimulation of Ca2+ channel currents were inhibited by the protein kinase C blocker 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H7). However, phorbol 12-myristate 13-acetate (PMA), a known stimulator of protein kinase C, was ineffective in stimulating the inotropic action of thrombin. The inhibition of the thrombin-induced enhancement of contractility by indometacin indicates an involvement of arachidonic acid in the action of thrombin on frog heart.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1990

Effects of thrombin on single calcium channels in frog ventricular cells.

Single calcium channel (Ca channel) currents were measured using the patch-clamp technique in isolated ventricular myocytes of the frog (Rana esculenta). Sodium was used as the charge carrier. After formation of cell-attached patches, the proteolytic enzyme thrombin was added to the bath solution, where it increased the amplitude of the averaged currents more than twofold, by decreasing the number of empty sweeps and reducing the time constant of the slow exponential term of the shut-time histogram. Single channel conductance was not changed by thrombin. If the activation kinetics of the Ca channels are described by the commonly used C1-C2-O model, where C1 and C2 indicate closed states 1 and 2 respectively and O denotes the open state, thrombin increases the open-state probability in the non-empty sweeps by increasing the rate constant (k1) for the transition from C1 to C2. It is shown that thrombin acts via an H-7 blockable pathway.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1990

Platelet functions in recombinant hirudin-anticoagulated blood.

The influence of genetically engineered recombinant hirudin (r-hirudin) on platelet functions was studied. Depending on the concentration, r-hirudin inhibits the thrombin-induced aggregation and 14C-serotonin secretion to the same extent as native hirudin. Comparative studies in blood anticoagulated by r-hirudin, heparin or citrate show a significantly lower spontaneous platelet aggregation in r-hirudinized blood. The extent of the ADP-induced aggregation is nearly the same in r-hirudinized, heparinized or citrated plasma. In r-hirudinized plasma, however, aggregation is reversible. In contrast to heparinized or citrated plasma, adrenaline causes only a very slight aggregation in r-hirudinized plasma. ADP- or adrenaline-induced secretion of 14C-serotonin do not occur in r-hirudinized plasma. The collagen- as well as the PAF-induced aggregation and 14C-serotonin release in hirudinized plasma do not differ significantly from those in heparinized or citrated plasma. r-Hirudin is a suitable anticoagulant for studying platelet functions because it does not produce any alterations in platelet reactions and does not provoke any changes in the ionized calcium concentration in blood.

Anticoagulants↗
PubMed1990

Preparation of dextran-bound recombinant hirudin and its pharmacokinetic behaviour.

Recombinant desulphatohirudin was bound via lysine residues to oxidized dextrans. The hirudin-dextran conjugates inhibit thrombin like free hirudin. The Ki-values are in the same range. In rabbits and rats the pharmacokinetic behaviour following i.v. administration of these conjugates was examined in comparison to that of hirudin. The hirudin-dextran conjugates were more slowly eliminated than hirudin, the distribution volumes in steady-state were significantly reduced, and, in comparison to hirudin, 5 to 15 times larger areas under the plasma concentration-time-curves were obtained.

Animals↗
PubMed1990

Changes of calcium channel inactivation during run-down.

Run down of calcium channel currents carried by Ba2+ was studied in enzymatically isolated ventricular cells from guinea-pig hearts using the suction pipette method. The decay of the Ba currents elicited by 100 ms voltage steps from -60 to 0 mV could be best described biexponentially. The peak current amplitude declined monotonously during dialysis of cell interior, whereas the inactivation showed a biphasic behaviour. After start of intracellular perfusion, the time constants of the fast and slow exponential component decreased up to the 6th and 10th minute, respectively. Thereafter the inactivation decelerated. These changes of the inactivation behaviour could be described by a model of current dependent inactivation. It was assumed, that in the first phase the decay is accelerated due to predominating of washout of phosphorylating agents and in the second phase the reduced conductance of the cell membrane for Ba2+ due to progressive irreversible closure of Ca channels causes slowing down of inactivation.

Animals↗
PubMed1990

Pharmacological studies on the low molecular weight heparin derivative CY 216.

In animal experiments pharmacological properties of the low molecular weight heparin derivative CY 216 were determined. The heparin fragment studied caused concentration-dependent anticoagulant effects in vitro. Investigations in vivo demonstrated the antithrombotic potency of the drug in experimental models of venous and arterial thrombosis in rats. In dependence on the dose administered CY 216 reduced the formation of stasis-induced thrombi of the jugular vein and prevented the thrombotic occlusion of the carotid artery after electrically induced damage of the vessel wall. An influence on primary haemostasis measured by prolongation of bleeding time after standardized incision of the rat tail could only be observed at doses which were higher than that required for antithrombotic effects.

Animals↗
PubMed1990

Antithrombin effects of native and recombinant hirudins.

Three recombinant variants of hirudin with the most evident difference in amino acid position 47 (Lys-47, Arg-47, Asn-47) were studied for their selectivity and affinity for the target enzyme thrombin in comparison to native hirudin. Native hirudin and the recombinant hirudins inhibit selectively the clotting enzyme thrombin. The affinity of native hirudin does not differ significantly from that of recombinant hirudin Lys-47 whereas a distinctly lower affinity for thrombin is found for recombinant hirudin Arg-47 and recombinant hirudin Asn-47. All hirudins investigated have the same potency to inhibit thrombin-induced coagulation. Changes in the affinity of hirudins for thrombin become evident from the inhibition of thrombin-induced platelet aggregation only.

Animals↗
PubMed1990

[I-125 labeling of human, rat, and bovine fibrinogen].

Human, rat and bovine fibrinogen were radioiodinated using a modified chloramine-T method under mild labelling conditions and with a satisfactory labelling efficiency. The 125I-human, rat and bovine fibrinogen preparations obtained met the requirements with respect to clottability, low content of 125Iodide as well as stability in vitro and in vivo. SDS-PAGE mobility demonstrated that the integrity of the fibrinogen molecule was not affected by the labelling procedure.

Animals↗
PubMed1989

A new assay for the determination of factor XII in plasma using a chromogenic substrate and a selective inhibitor of plasma kallikrein.

A new chromogenic assay for factor XII in plasma was designed by the use of the soluble activator Kalliplastin, the substrate H-D-HHT-Gly-Arg-pNA and a synthetic inhibitor of plasma kallikrein (Pefabloc PK). The assay was carried out in one and the same cuvette and did not require factor XII-deficient plasma. There was a good correlation between F XII coagulant and amidolytic assays.

Benzamidines↗
PubMed1989

Studies on antithrombotic effects of recombinant hirudin.

Recombinant desulfato hirudin (rH) was studied with regard to its effects on various models of thrombosis, its pharmacokinetic behaviour as well as its influence on primary haemostasis in rats. The pharmacological behaviour of rH proved to be similar to that of native hirudin. Depending on the different models of experimental thrombosis different dosages of rH were required to prevent thrombus formation. This corresponds to rH plasma concentrations of 0.5-1.5 micrograms/ml. Primary haemostasis is influenced only after excess dosages and plasma concentrations of 4 micrograms/ml and above.

Animals↗