Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of the blood coagulation factor VIIa-tissue factor complex, as well as a direct inhibitor of factor Xa. Intravenously administered TFPI is rapidly cleared from circulation predominantly via liver. We previously reported that the low density lipoprotein receptor-related protein (LRP), a multifunctional endocytic receptor, mediates the uptake and degradation of TFPI in hepatoma cells. This process is inhibited by a 39-kDa receptor-associated protein which binds to LRP and regulates its ligand binding activity. However, a distinct, low affinity binding site (perhaps heparin sulfate proteoglycans, HSPGs) on the endothelium and liver is thought to be responsible for the majority of TFPI cell surface binding. In the current study, we investigated the role of LRP and this second binding site in the clearance of Tissue factor pathway inhibitor (TFPI) 1 is a serine protease inhibitor that plays a key role in regulating tissue factorinitiated blood coagulation. Human TFPI is a trace 42-kDa plasma glycoprotein consisting of three tandem Kunitz-type domains, followed by a positively charged carboxyl terminus (1). The first Kunitz domain binds to and inhibits factor VIIa, and the second Kunitz domain binds to and inhibits factor Xa (2). Inhibition of tissue factor-induced blood coagulation by TFPI has been postulated to involve the quaternary factor Xa-TFPI-factor VIIa-tissue factor complex (3).Intravenously administered 125 I-TFPI is cleared rapidly from the circulation with a plasma half-life of 2 min in rabbits (4) and Ͻ1 min in rats (5). However, the biology underlying this clearance mechanism has not been elucidated to date. Previously, we demonstrated that the low density lipoprotein receptor-related protein (LRP) mediates the cellular degradation of TFPI in hepatoma cells (6) and that a 39-kDa protein, an inhibitor of all the ligand interactions with LRP (7), inhibits this process. In addition, cell surface heparin sulfate proteoglycans (HSPGs) associated with endothelial cells and liver have been proposed to play a role in the clearance of 125 I-TFPI (8).However, the precise roles of LRP and HSPGs in the plasma clearance of TFPI have yet to be defined.The purpose of the present study was to elucidate the roles of LRP and HSPGs in the catabolism of TFPI both in vivo and in vitro. We took advantage of viral-mediated gene transfer to express the 39-kDa protein in liver in vivo as such an approach has allowed us to define the role of LRP in the clearance of tissue-type plasminogen activator (t-PA) in vivo (9). The current results demonstrate a direct role for LRP as well as HSPGs in the plasma clearance of TFPI and thus suggest strategies for regulation of its catabolism. MATERIALS AND METHODS Reagents-Carrier-free sodium [125 I]iodide was purchased from DuPont NEN. Bovine serum albumin was purchased from Calbiochem Co (La Jolla, CA). Protamine sulfate was purchased from Sigma. Porcine intestinal heparin for intravenous injection was from Elgins-Sinn Inc (Che...

Cyclic analogues of angiotensin II (AII) were synthesized by connecting the side chains of residues 3 and 5 via a disulfide bridge. Appropriate conformational constraints afforded an analogue, [Hcy3,5]AII, having high contractile activity (pD2 = 8.48 vs 8.81 for AII) and excellent binding affinity (IC50 = 2.1 nM vs 2.2 nM for AII). This type of cyclization was also used to prepare a highly potent AII antagonist, [Sar1,Hcy3,5,Ile8]AII (pA2 = 9.09 vs 9.17 for [Sar1, Ile8]AII; IC50 = 0.9 nM vs 1.9 nM for [Sar1,Ile8]AII). Model building suggests that this ring structure is consistent with a receptor-bound conformation having any of a variety of three-residue turns, including a gamma-turn. In contrast, the receptor-bound conformation of AII does not appear to accommodate a beta-turn or an alpha-helix which includes residues 3-5.

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A series of 5-[1-[4-[(4,5-disubstituted-1H-imidazol-1-yl)methyl]- substituted]-1H-pyrrol-2-yl]-1H-tetrazoles and 5-[1-[4-[(3,5-dibutyl-1H-1,2,4-triazol-1-yl)methyl]-substituted]- 1H-pyrrol-2-yl]-1H-tetrazoles were investigated as novel AT1-selective angiotensin II receptor antagonists. Computer-assisted modeling techniques were used to evaluate structural parameters in comparison to the related biphenyl system. New synthetic procedures have been developed to prepare the novel compounds. The best antagonists in this series had IC50 values (rat uterine membrane receptor binding) in the 10(-8) M range and corresponding pA2 in isolated organ assay (rabbit aorta rings). Structure-activity relationships indicate some similarities with the finding in the biphenyl system. Substitution on the pyrrole ring modulates activity. Compound 5 antagonized angiotensin-induced blood pressure increase when administered to conscious rat at 30 mg/kg per os.

Oxytocin (10 nM) stimulated the phosphorylation of the 20,000 mol wt myosin light chain in rat mammary myoepithelial cells from a basal level of 0.17 to 0.85 mol phosphate/mol light chain within 30 sec. Of the smooth muscle stimulants tested, oxytocin appears to be the only normal physiological stimulus for myosin phosphorylation in these cells. The roles of cAMP, cGMP, and calcium ions were investigated in the mode of action of oxytocin and the regulation of myosin phosphorylation. Although oxytocin had no effect on cGMP metabolism, there was an increase in the cAMP content of the treated myoepithelial cells. Further investigation suggested that the increase in cAMP levels in response to oxytocin was not directly involved in the regulation of myosin phosphorylation. Various agents known to interfere with calcium ion transport were used to study the role of calcium ions in the action of oxytocin and the regulation of myosin phosphorylation. The results indicate that the duration of the cellular response to oxytocin depends on an influx of extracellular calcium through calcium-specific channels in the plasma membrane.

Atriopeptin (AP) 24, containing amino acids Serlo3 -Tyr126 of the carboxy-terminal portion of the atrial natriuretic peptide prohormone, was degraded rapidly by rabbit kidney brush border membranes. The rate of degradation of AP24 measured by the loss of vasorelaxant activity followed a similar time course to the decrease in peptide peak area measured by high-performance liquid chromatography. Inactivation of AP24 produced peptide fragments which were separated by HPLC. The major products were purified individually and their peptide sequences determined. Results indicate that AP24 was proteolytically cleaved at three peptide bonds: Ser103-Ser'04, C y~'~' -P h e '~~ and S e~-'~~-P h e ' 24. des-Serlo3-AP24 had similar vasorelaxant activity to AP24, while AP24 cleaved at C y~'~' -P h e ' '~ was inactive. Regarding the proteolytic cleavage at Ser123-Phe'24, there was an accumulation of the C-terminal tripeptide, Phe-Arg-Tyr, only at the later time points of the incubation. Degradation experiments were repeated with an amino-and carboxy-terminal protected peptide, acetyl-AP24-amide. Peptide sequence analysis of the major degradation products of this peptide revealed that the critical peptide bond cleaved was C y~'~' -P h e '~~. We conclude that the Cys-Phe peptide bond renders atrial peptides highly susceptible to proteolysis by renal brush border membranes, resulting in inactivation.Atrial peptides are factors which have been isolated from mammalian heart atria and effect natriuresis, diuresis and vasorelaxation [I, 21. Atriopeptin 24 (also named atriopeptin 111) consists of 24 amino acids with a disulfide bridge between the cysteine residues in positions 105 and 121, the ring structure of the peptide being important for bioactivity [3]. It has been reported for rat [4], dog [5] and human [6] that exogenously administered atrial peptides have a short biological half-life, of the order of a few minutes. Recent reports have implicated the kidney as a major site for clearance of these peptides from the circulation [7,8]. A number of peptide hormones, such as angiotensin, glucagon and lutcinizinghormone-releasing hormone [9] are believed to be proteolytically degraded at the brush border of the renal proximal tubule which is rich in hydrolases [I 01.In this study, we report that atriopeptin 24 is rapidly degraded by renal brush border membranes. From peptide sequence analysis of the degradation products from AP24 and acetyl-AP24-amide, we have determined that there is a single peptide bond which renders atrial peptides susceptible to endoproteolytic cleavage, resulting in inactivation.