Thrombin binds to murine bone marrow-derived macrophages and enhances colony-stimulating factor-1-driven mitogenesis.

The binding and mitogenic properties of thrombin have been established in various transformed cell lines. In such systems, thrombin induces cell division in the absence of exogenous growth factors, and the enzyme is considered to act directly as a mitogen. This study explores thrombin's interaction with nontransformed, growth factor-dependent cells. Binding of 125I-alpha-thrombin to colony-stimulating factor (CSF)-1-dependent bone marrow-derived macrophages is saturable, time-dependent, and displaceable by both unlabeled alpha-thrombin, and esterolytically inactive thrombin. Both dissociation studies of pre-bound radio-labeled thrombin and Scatchard analysis assisted by the program "Ligand" suggest adherence of thrombin-binding data to a multi-site model. There are an estimated 2 x 10(4) high affinity sites (Kd = 7 x 10(-9)M) and 2 x 10(6) low affinity sites (Kd = 9 x 10(-7)M) per cell. Quiescent bone marrow-derived macrophages were cultured with either 10(-8)M thrombin, 1000 units of CSF-1/ml, or both and [3H]thymidine incorporation was determined. Thrombin alone did not induce mitogenesis. CSF-1 induced mitogenesis with peak [3H] thymidine incorporation occurring 24 h after addition of the mitogen. This CSF-1-dependent mitogenic influence was enhanced greater than 2-fold by treatment with thrombin.

Thrombin immobilized to extracellular matrix is a potent mitogen for vascular smooth muscle cells: nonenzymatic mode of action.

Esterolytically inactive diisopropyl fluorophosphate-conjugated thrombin (DIP-alpha-thrombin) stimulated 3H-thymidine incorporation and proliferation of growth-arrested vascular smooth muscle cells (SMCs), similar to native alpha-thrombin. Half-maximal mitogenic response of SMCs was obtained at 1 nM thrombin and was specifically blocked by the leech-derived, high-affinity thrombin inhibitor, hirudin. Native thrombin and a variety of thrombin species that were chemically modified to alter thrombin procoagulant or esterolytic functions were found to induce 3H-thymidine incorporation to a similar extent. Exposure of SMCs to DIP-alpha-thrombin caused a rapid and transient expression of the c-fos protooncogene, determined by Northern blot analysis. These results indicate that thrombin is a potent mitogen for SMCs through a distinct non-enzymatic domain. Binding of 125I-alpha-thrombin to SMC cultures revealed an apparent dissociation constant of 6 nM and an estimated 5.4 x 10(5) binding sites per cell. This binding was inhibited to the same extent by native thrombin and by its nonenzymatic form, DIP-alpha-thrombin. Moreover, the chemotactic fragment of thrombin (CB67-129), which failed to elicit a mitogenic response, competed for 125I-alpha-thrombin binding to SMCs. Cross-linking analysis of 125I-alpha-thrombin to SMCs revealed a specific cell-surface binding site 55 kDa in size. Finally, thrombin immobilized to a naturally produced extracellular matrix retained potent mitogenic activity toward SMCs. These observations lend support to the possibility that in vivo, subendothelial basement membranes sequester thrombin (as well as other bioactive molecules), which may stimulate localized and persistent growth of arterial SMCs. Thrombin may thus be involved directly in progression of atherosclerotic plaque formation.

Anion-binding exosite of human alpha-thrombin and fibrin(ogen) recognition.

Activation of prothrombin to alpha-thrombin generates not only the catalytic site and associated regions but also an independent site (an exosite) which binds anionic substances, such as Amberlite CG-50 resin [cross-linked poly(methylacrylic acid)]. Like human alpha-thrombin with high fibrinogen clotting activity (peak elution at I = 0.40 +/- 0.01 M, pH 7.4, approximately 23 degrees C), catalytically inactivated forms (e.g., i-Pr2P-alpha- and D-Phe-Pro-Arg-CH2-alpha-thrombins) were eluted with only slightly lower salt concentrations (I = 0.36-0.39 M), while gamma-thrombin with very low clotting activity was eluted with much lower concentrations (I = 0.29 M) and the hirudin complex of alpha-thrombin was not retained by the resin. In a similar manner, hirudin complexes of alpha-, i-Pr2P-alpha-, and gamma-thrombin were not retained by nonpolymerized fibrin-agarose resin. Moreover, the ionic strengths for the elution from the CG-50 resin of seven thrombin forms were directly correlated with those from the fibrin resin (y = 0.15 + 0.96x, r = 0.95). In other experiments, the 17 through 27 synthetic peptide of the human fibrinogen A alpha chain was not an inhibitor of alpha-thrombin, while the NH2-terminal disulfide knot (NDSK) fragment was a simple competitive inhibitor of alpha-thrombin with a Ki approximately 3 microM (0.15 M NaCl, pH 7.3, approximately 23 degrees C). These data suggest that alpha-thrombin recognizes fibrin(ogen) by a negatively charged surface, noncontiguous with the A alpha cleavage site but found within the NDSK fragment. Such interaction involving an anion-binding exosite may explain the exceptional specificity of alpha-thrombin for the A alpha cleavage in fibrinogen and alpha-thrombin incorporation into fibrin clots.

Reversible induction of right ventricular atriopeptin synthesis in hypertrophy due to hypoxia.

Right ventricular hypertrophy produced in rats exposed to 10% oxygen for 3 weeks resulted in a ninefold increase in atriopeptin immunoreactivity (APir) and a 160-fold increase in atriopeptin messenger RNA (AP mRNA) in the right ventricular myocardium. A small but significant increase in left ventricular APir and AP mRNA was also present, probably representing the interventricular septum. Right atrial APir was decreased by 50%, but left atrial APir was not different from normoxic controls. Purification of ventricular tissue extracts by high-performance liquid chromatography revealed primarily the high molecular weight prohormone. The development of right ventricular hypertrophy and right ventricular APir content followed a similar time course, each evident at 7 days of hypoxia and reaching a plateau at 14 days. Hypoxia followed by normoxia caused right ventricular APir to fall to control levels within 3 days, despite persistent right ventricular hypertrophy. This data demonstrates that hypoxia can reversibly induce extra-atrial expression of atriopeptin synthesis in the cardiac ventricle.

Fibrinogen-derived peptide B beta 1-42 is a multidomained neutrophil chemoattractant.

The formation and degradation of fibrin play a central role in hemostasis, but other activities have been associated with fibrin(ogen)-derived peptides, which suggests that products of fibrin(ogen) turnover may be involved in inflammation and wound healing. The present study was undertaken to determine whether the plasmic fibrinogen-derived peptide B beta 1-42 has effects on inflammatory cells and fibroblasts (FB). B beta 1-42 was found to be a potent chemotaxin for neutrophils (PMN) and FB, maximally stimulating PMN migration at 10(-9) mol/L peptide. Unlike the chemotactic factors f-Met-Leu-Phe and C5a, B beta 1-42 did not induce the release of lysosomal hydrolases and superoxide anion from PMN, nor did it stimulate directed movement of monocytes (MN). These features of B beta 1-42 resemble the properties of human fibrinopeptide B (hFpB), the 14-reside, thrombin-cleaveable fragment that constitutes the amino terminus of B beta 1-42, and suggested that the chemotactic effects of B beta 1-42 are mediated through its hFpB domain. Against this conclusion, however, were observations that (a) desensitization of PMN with 10(-7) mol/L hFpB ablated chemotaxis to hFpB without affecting chemotaxis to B beta 1-42; (b) antiserum to hFpB, which recognizes the B beta 1-14 sequence both free and bound to larger fragments of the B beta chain, blocked hFpB chemotactic activity but did not affect B beta 1-42-mediated chemotaxis; (c) desensitization of PMN with equimolar amounts of hFpB and beta 15-42 (10(-7) mol/L), the isolated carboxyterminal sequence of B beta 1-42 remaining after the removal of hFpB, completely inhibited B beta 1-42-mediated chemotaxis; and (d) beta 15-42 itself was chemotactic for PMN. These data indicate that PMN recognize several independent domains within the amino terminal region of the human fibrinogen B beta chain and that these biologic effects extend to mesenchymal cells.

Evidence for two functionally different fibrinogen receptors on hemopoietic cells: the glycoprotein IIb-IIIa and the mitogenic fibrinogen receptor.

We have previously established that the mitogenic effect of fibrinogen on hemopoietic cell lines Raji and JM is mediated via a specific receptor (Levesque, J.-P. et al.: Proc. Natl. Acad. Sci. USA 83:6494-6498, 1986). In this study, we have further characterized the fibrinogen domain involved in the binding to the mitogenic receptor. This binding was not inhibited either by a monoclonal antibody against the C-terminal sequence of the fibrinogen gamma chains or by synthetic peptides containing the Arg-Gly-Asp sequence. Such inhibition is specific of the platelet fibrinogen receptor, the glycoprotein IIb-IIIa complex. Fragments containing the fibrinogen D domain were the only plasmin degradation products of fibrinogen which were mitogenic. These fragments acted via direct binding on the mitogenic receptor with a Kd of 2.24 X 10(-6) M. This value was similar to the KI value of unlabeled fragments D (2.47 X 10(-6) M). Our results suggest the presence of two different functional types of fibrinogen receptors: the glycoprotein IIb-IIIa receptor responsible both for platelet aggregation and leukocyte adhesion and killing, and the mitogenic receptor involved in proliferation control of hemopoietic cells.

Thrombin chemotactic stimulation of HL-60 cells: studies on thrombin responsiveness as a function of differentiation.

Thrombin, a major procoagulant enzyme and growth factor, is also selectively chemotactic for monocytes and macrophages but not for neutrophils. This effect stands in contrast to other well-known chemotactic agents such as fMet-Leu-Phe, C5a fragments, and LTB4, which stimulate directed cell movement in both cell types, and have important physiological implications. The human leukemic cell line HL-60, which is capable of differentiating either along granulocytic or monocytic lineages, was therefore used to explore the development of this selective monocyte/macrophage chemotactic response to thrombin. Esterolytically inactive DIP-alpha-thrombin, as well as the thrombin-derived chemotactic peptide CB67-129, elicits a dose-dependent chemotactic response in HL-60 cells differentiated to monocytelike cells by treatment with 1,25(OH)2D3 (HL-60/mono), whereas no such response is evident in either undifferentiated HL-60 cells or in cells differentiated into granulocytes by treatment with DMSO (HL-60/gran). Similarly, early events which characterize stimulation of inflammatory cells by chemotactic agents are also evident, but only in monocyte-differentiated cells. In HL-60/mono, thrombin selectively stimulates rapid cytosolic Ca2+ elevation as well as rapid cytoskeletal association of cytosolic actin. Following thrombin stimulation, maximal actin association in these cells occurs within 30 sec (declining to basal levels at the end of 5 min), and maximal Ca2+ elevations are also evident within 15-20 sec, suggesting a temporal relationship between these two events. Thus, the events accompanying stimulation of HL-60/mono by thrombin are characteristic of those seen following stimulation of inflammatory cells by chemotaxins, with a major difference being the selectivity of thrombin as a chemotaxin for cells of macrophage/monocytic lineage. The selective chemotactic responsiveness of HL-60/mono to thrombin appears to relate to the development of specific receptors on these cells as part of monocytic differentiation: HL-60/mono (but HL-60/gran nor undifferentiated HL-60) are capable of significant specific 125-I-labeled alpha-thrombin-binding (ka approximately 20 nM), and possess an estimated 400,000 thrombin-binding sites per cell. Our findings further suggest that the thrombin response of HL-60 and particularly the expression of thrombin receptors on these cells may serve as a useful model system for exploring the biology of monocyte/macrophage differentiation.

Biologic activities of nonenzymatic thrombin: elucidation of a macrophage interactive domain.

Our studies to date have clearly demonstrated the existence of a unique cell interactive exosite region in thrombin, which mediates specific biologic effects on cells of monocytic-macrophage lineage. These findings are of potential physiologic significance, since even proteolytically degraded forms of thrombin (beta- and gamma-thrombin) or fragments of thrombin arising due to breakdown of thrombin-thrombomodulin complexes are potentially biologically active and are not subject to inhibition by inhibitors such as antithrombin III. Such degraded thrombin forms and proteolytic fragments are likely to be present at sites of inflammation and wound repair where they may be important modulators of macrophage-monocyte responsiveness. Importantly, as emphasized in this communication, the chemotactic and growth-promoting properties of this site, although overlapping, are clearly dissociable. It is apparent that the chemotactic effects require the presence of most, if not all, of the 338-400 sequence, whereas the mitogenic effects are mediated exclusively through the so-called loop B insertion sequence that is a unique feature of thrombin. Although there are ample precedents for differing functional domains within discrete proteins and the presence of hormonelike regions in proteins (such as fibronectin and immunoglobulin G) has been documented, it is fascinating to speculate how functionally unique regions such as this can arise. Data from Craik and associates suggest that unique amino acid sequences in proteins originate from gene insertions at intron-exon junctions and are characteristically surface expressed. These insertion sequences give rise to unique structural and functional features that characterize the particular protein, and also serve to differentiate it from other related members within its family.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fibrinogen derivatives upon the inflammatory response. Studies with human fibrinopeptide B.

Fibrin formation and turnover are intimately associated with inflammation and wound healing. To explore whether fibrin(ogen)-derived peptides exert direct effects upon cells involved in inflammation and tissue repair we examined the capacity of human fibrinopeptide B (hFpB), a thrombin-derived proteolytic cleavage product of the fibrinogen B beta-chain, to stimulate neutrophils (PMN), monocytes, and fibroblasts. hFpB caused directed cell migration of PMN and fibroblasts that was optimal at approximately 10(-8) M. This chemotactic activity was blocked by preincubating hFpB with antiserum to hFpB. hFpB was not chemotactic for monocytes. The chemotactic potency of hFpB for PMN was equivalent to that of anaphylatoxin from the fifth component of human complement (C5a), leukotriene B4 (LTB4), and formyl-methionyl-leucyl-phenylalanine (fMLP), and for fibroblasts its chemotactic activity was comparable to that of platelet-derived growth factor. hFpB did not interact with PMN receptors for C5a, LTB4, or fMLP as (a) desensitization with 10(-7) M hFpB abolished chemotaxis to hFpB but had no effect upon chemotaxis to C5a, LTB4, or fMLP and (b) induction of chemotactic responses to fMLP and LTB4 in neutrophilic leukemic cells (HL-60 cells) by incubation with dimethylsulfoxide did not extend to hFpB. Like fMLP, hFpB caused a rapid, dose-dependent increase in PMN cytoskeletal associated actin, but unlike fMLP, hFpB did not cause PMN aggregation, release of lysosomal enzymes (lysozyme and beta-glucuronidase), or the production of superoxide anion. These results suggest that hFpB may have a role in recruiting PMN and fibroblasts at sites of fibrin deposition and turnover. The capacity of hFpB to cause PMN chemotaxis without causing concurrent release of lysosomal enzymes or the production of superoxide anion is further evidence for the complexity of PMN responses to chemotactic agents.

Identification of a thrombin sequence with growth factor activity on macrophages.

In contrast to fibroblasts, the exposure of G0/G1-arrested J774 cells, a murine macrophage-like tumor cell line, with either active or esterolytically inactive diisopropyl phosphorofluoridate-conjugated alpha-thrombin (the enzymatically active form of thrombin, EC 3.4.21.5) results in a mitogenic response as measured by increased [3H]thymidine incorporation. This response to thrombin is optimal at 10 nM and is specifically blocked by hirudin, a high-affinity thrombin inhibitor. When prethrombin 1 [a single-chain prothrombin derivative lacking fragment 1, resulting from the action of thrombin on prothrombin] is cleaved with cyanogen bromide, a fragment (peptide CB67-129) is produced that, like the parent thrombin molecule, is mitogenic for J774 cells but not for fibroblasts. Limited tryptic digests of this fragment retain the ability to stimulate macrophages--a function that can be mimicked by a synthetic tetradecapeptide homologue of CB67-129 (representing residues 367-380 of the human thrombin B chain sequence) but not by any of a series of well-known growth promoters, including platelet-derived growth factor, epidermal growth factor, nerve growth factor, and fibroblast epidermal growth factor, nerve growth factor, and fibroblast growth factor. The mitogenic effects of this peptide are not limited to J774 cells but can be expressed in other macrophage-like tumor cell lines, including P388D1, RAW, and PU5. In addition to increased [3H]thymidine incorporation, the synthetic B chain peptide stimulates cell proliferation as evidenced by a dose-dependent increase in total protein per culture well and cell number. We conclude that the thrombin molecule contains a macrophage growth factor domain that is separate and distinct from its active center. Thus, thrombin, in addition to its major role in hemostasis and thrombosis, may also have important functions in such basic processes as the inflammatory response and monocytopoiesis.

Hormone-like activity of human thrombin.

Recently, we have shown that thrombin is a chemotaxin and growth-promoting agent for cells of the mononuclear phagocytic lineage. These activities are independent of thrombin's enzymatic activity. Unlike other chemotactic factors, thrombin is specific for monocytes and does not attract granulocytes. To further explore the cellular specificity we have used a human leukemia cell line HL-60 that is capable of in vitro differentiation toward either monocytes (HL-60/mono) following incubation with 1,25(OH)2D3, or granulocytes (HL-60/gran) following incubation with DMSO. In contrast to undifferentiated HL-60 cells or HL-60/gran, we find that HL-60/mono respond chemotactically to intact human alpha-thrombin, esterolytically inactive iPR2P-alpha-thrombin, and the thrombin-derived peptide CB67-129, previously shown to contain the thrombin chemotactic exosite. In addition, thrombin induces in HL-60/mono association of actin with the cytoskeleton and causes an increase in levels of free cytosolic Ca2+. These phenomena are well characterized as early events occurring concomitant with directed cell movement associated with exposure to chemotactic agents such as FMLP. Furthermore, in contrast to fibroblasts, both iPR2P-alpha-thrombin and the thrombin chemotactic peptide CB67-129 evoke dose-dependent [3H]TdR incorporation, protein synthesis, and cell replication in growth-arrested J-744 cells, a murine macrophage-like cell line. Limited tryptic digests of CB67-129 lose chemotactic activity but retain full mitogenic activity, demonstrating that as with PDGF, the sites on CB67-129 required for chemotaxis and mitogenesis are clearly dissociable. The mitogenic effects of the CB67-129 digest can be mimicked by a synthetic tetradecapeptide analogue of CB67-129 (residues 367-380) that includes the loop B insertion sequence, previously shown to be critical for thrombin's chemotactic effects. From these data, it is apparent that the loop B insertion is critical for thrombin's nonenzymic biological effects on cells, but additional sites are required for stimulation of cell movement.

Growth-promoting effects of esterolytically inactive thrombin on macrophages.

It has been recognized for many years that alpha-thrombin, like other better known mitogens (eg, PDGF, EGF, etc) is capable of initiating proliferation in quiescent cells belonging to the fibroblast family. However, unlike these other peptides, thrombin is a serine protease whose function as a growth stimulator for fibroblasts is intimately linked to its esterolytic activity. Thus, while native alpha-thrombin is capable of evoking DNA synthesis in G0/G1-arrested cells, neither enzymatically inactive thrombin (eg, iPR2P-alpha-thrombin) nor partially degraded thrombin (eg, gamma-thrombin) shares in this capability. Data from our laboratory have shown that thrombin is chemotactic for peripheral blood monocytes and for cells belonging to the monocyte/macrophage family and that this activity is not dependent upon thrombin's enzymatic properties. Our recent findings demonstrate that thrombin also serves as a growth factor for these cells, and this mitogenic capability is independent of esterolytic function and resides in the same region of the molecule as that responsible for chemotaxis. Additionally, by means of techniques such as computer modeling and peptide synthesis, we have now been able to delineate a distinct mitogenic subsite within this chemotactic thrombin sequence. Thus, the sequence in the thrombin B chain that mediates chemotaxis represents a true cell interactive exosite additionally capable of stimulating growth and possibly other biological functions in cells of macrophage/monocyte lineage.

A simple one-step HPLC procedure for the purification of the NH2-terminal plasmin-derived B beta 1-42 peptide of human fibrinogen.

Simplified procedures were developed to produce and isolate the plasmin-derived B beta 1-42 fragment from human fibrinogen. Peptides generated from fibrinogen by plasmin were separated by reverse phase HPLC chromatography. Two distinct peaks were identified as having amino acid compositions identical to B beta 1-42. The average final yield of HPLC-purified B beta 1-42 was 1.2 mg per 100 mg of fibrinogen. Recovery of the peptide (40-45% when compared to the theoretical yield) was significantly higher than yields obtained by open column techniques demonstrating the advantages of this simple one-step HPLC procedure.

Nonrandom catabolism of proteins in the formation of antigenic peptide fragments.

To examine the role of protein catabolism in the formation of antigenic peptide fragments, human fibrinopeptide-immune guinea pig T cells were stimulated with the large native molecule, human fibrinogen. Two different systems were tested. In the first, we determined responses by human fibrinopeptide B (hFPB)-immune T cells, to which strain (St.) 2 guinea pigs are responders and St. 13 are nonresponders, and by human fibrinopeptide A (hFPA)-immune T cells to which St. 13 are responders and St. 2 are nonresponders. Of interest in this comparison is that both hFPA and hFPB are amino terminal peptides on the A and B chain of fibrinogen, respectively, and are readily cleaved by thrombin during fibrin formation and by other trypsin-like enzymes, leaving a carboxyl terminal Arg. Thus, if fibrinogen catabolism occurred, both antigenic peptides should be equally represented for availability in T cell responses. It was found that hFPB-immune St. 2 T cells responded to fibrinogen, but no response was observed with hPFA-immune St. 13 T cells cultured with fibrinogen. To rule out that there was a general catabolic defect in St. 13 antigen-presenting cells, fibrinogen was presented by (2 X 13)F1 macrophages to fibrinopeptide-immune parental T cells. Again it was found that F1 macrophages could present fibrinogen to hFPB-immune T cells but failed to present hFPA. In another comparison, responses with fibrinogen were also determined with des-ARg-hFPB, which lacks the carboxyl terminal Arg of hFPB, to which St. 13 are responders and St. 2 are nonresponders. The advantage of this comparison is that both antigenic determinants are contained within the same small peptide. St. 13 des-Arg-hFPB-immune T cells failed to respond in vitro by culture with human fibrinogen, suggesting that these antigenic determinants are not produced from larger peptides or proteins containing those determinants. To rule out the possibility that this was only an in vitro phenomenon, guinea pigs were immunized with the larger protein, the B chain of fibrinogen, and the immune T cells were examined for responses to fibrinopeptides derived from the B chain. Immune St. 2 T cells responded to hFPB but not to des-Arg-hFPB, whereas St. 13 T cells remained unresponsive with both peptides. These results indicate that proteolysis of larger proteins to form small antigenic peptides is not a random event and that not all potential antigenic determinants contained in a protein are produced during antigen processing.

Fibrin-mediated vascular injury. Identification of fibrin peptides that mediate endothelial cell retraction.

The deposition of fibrin, a ubiquitous component of acute and chronic inflammatory reactions, has been implicated by a number of recent studies as playing an active role in inflammation. In particular, fibrin deposition has been implicated in the development of tissue edema. As the "gateway" through which intravascular-to-extravascular movement of fluid, nutrients, and cells must pass, the vascular endothelial cells play a crucial regulatory role in this process. In support of this concept, recent studies in this laboratory have demonstrated that endothelial cells retract not only in the presence of fibrin but also in the presence of low molecular weight cleavage products of fibrinogen. It was further shown that this reaction was 1) specific for both vascular and corneal endothelial cells, 2) nontoxic, and 3) completely reversible. The present work examined the physiochemical nature of these endothelial-cell reactive factors. It was demonstrated by the use of enzymatically derived and synthetic fibrinogen peptides, that the active soluble fibrinogen-derived factor was associated with the amino-terminal end of the B chain of fibrinogen. The active factor has been tentatively identified as the B beta peptides, which is a primary plasmin cleavage product of fibrinogen and contains the thrombin-generated fibrinopeptide B. It is thus suggested that soluble, endothelial-cell-reactive peptides are released during both fibrinogenesis and fibrinolysis and, as such, modulate endothelial cell functions in vivo.

Lack of elevation of platelet factor IV in plasma from patients with myocardial infarction.

Platelet factor IV and beta-thromboglobulin are protein constituents of platelet granules. Elevated levels of these proteins in plasma have been used as sensitive indicators of platelet degranulation. Clearance of platelet factor IV is much faster than that of beta-thromboglobulin after release of the proteins in vivo. Although increases of platelet factor IV have been observed in patients with infarction, the implication that they reflect pathogenetic phenomena such as coronary thrombosis has not been assessed explicitly. Accordingly, plasma samples obtained serially from 52 patients with acute myocardial infarction under rigorous conditions verified to minimize platelet degranulation in vitro were evaluated prospectively. Correlative studies were performed to detect left ventricular mural thrombus, and coronary thrombosis was assessed independently in selected patients with indium-111 platelet scintigraphy. Platelet factor IV was normal at the time of admission in patients with infarction, averaging 6.3 +/- 3.3 ng/ml, similar to values in 44 other patients with chest pain without subsequent infarction (5.7 +/- 2.7 ng/ml) and in 25 normal subjects (4.3 +/- 1.6 ng/ml). Platelet factor IV generally did not increase during hospitalization in patients with infarction despite recurrent chest pain, development of left ventricular thrombus or documented recurrent infarction. However, platelet factor IV increased consistently after invasive procedures, accounting for 104 of the total of 110 increases due to platelet activation in vivo as reflected by persistence of elevated levels of beta-thromboglobulin. Thus, platelet factor IV values generally remain normal despite acute myocardial infarction. Rare increases that occur reflect platelet degranulation in vitro due to sampling artifact or perturbations of platelets in vivo due to invasive procedures.(ABSTRACT TRUNCATED AT 250 WORDS)