Platelets: a critical link between inflammation and microvascular dysfunction

Inflammation is an underlying feature of a variety of human diseases. An important manifestation of this pathophysiological response is microvascular dysfunction, which includes the activation of vascular endothelial cells, and circulating leucocytes and platelets. While endothelial cells and leucocytes are widely accepted as critical players in the microvascular alterations induced by inflammation, recent attention has focused on the modulatory role of platelets, which act both as effector and target cells in inflamed microvessels. Evidence is presented to demonstrate the capacity for 'cross-talk' between platelets and other cells (endothelial cells, leucocytes) that contribute to an inflammatory response, and to illustrate the pathophysiological consequences of these interactions of platelets with other cells within the microvasculature.

Aprotinin inhibits local platelet trapping and improves tissue destruction in hepatic cryosurgery

BACKGROUND

During the last decade, cryosurgery became an interesting alternative in the treatment of nonresectable liver neoplasms. The freeze-thaw procedure, however, may be associated with life-threatening thrombocytopenia due to local platelet trapping, and success of neoplasm ablation may be compromised by inadequate parenchymal cell destruction.

METHODS

Because aprotinin is capable of inhibiting the initiation of both coagulation and fibrinolysis, we studied-by whole body scintigraphy of Indium-111-labeled platelets and histomorphology in a porcine model of hepatic cryosurgery-whether this serine protease inhibitor is effective in attenuating platelet trapping and in improving tissue destruction.

RESULTS

Fifteen minutes of cryotherapy (-168 degrees C at the tip of the cryoprobe) induced a 30 +/- 4 cm(3) cryolesion, which presented with massive platelet trapping (14.0 +/- 1.7% cryolesion activity/whole body activity) and incomplete parenchymal cell destruction (0.9 +/- 0.3; score of hepatocyte nuclear destruction within the margin of the cryolesion). Aprotinin treatment with 500,000 IU initial bolus injection and additional 500,000 IU infusion over 3 hours did not affect the size of the cryolesion (29 +/- 3 cm(3)) but reduced local platelet activity (1.9 +/- 1.9%; P<.001) and induced hepatocyte nuclear destruction (3.0 +/- 0.0; P<.001).

CONCLUSIONS

Thus, our study indicates that aprotinin inhibits cryoablation-associated platelet trapping and improves tissue destruction. The serine protease inhibitor may represent a valuable adjunct in cryosurgery of hepatic neoplasms.

Aprotinin: Antithrombotic and Vasoactive Mechanisms of Action

Aprotinin is a serine protease inhibitor that has been in clinical use since the late 1980s to reduce blood loss in patients undergoing cardiopulmonary bypass surgery. Its hemostatic mechanism of action is mediated predominantly through inhibition of plasmin, thus exerting a net antifibrinolytic effect. Compared to other antifibrinolytics, however, aprotinin provides an additional patient benefit at the level of improved platelet function and diminished inflammatory response to bypass. Recent work on platelets has identified a cell-associated target for aprotinin: the thrombin-receptor, protease-activated receptor 1. Selective blockade of the protease-activated receptor 1 limits thrombin-induced activation and consequent “exhaustion” of platelets in the bypass circuit, while maintaining the hemostatic activity of platelets in the pericardial cavity in response to nonproteolytic agonists, such as collagen, adenosine diphosphate and epinephrine. While no specific cellular receptors have as yet been identified to explain the antiinflammatory and vasoactive properties of aprotinin, awareness is growing that serine protease-sensitive receptors belonging to the protease-activated receptor family (1-4) may represent important aprotinin targets, since these receptors are expressed by all major cells of the vasculature and act as sensors of the coagulation, inflammatory and vasoactive pathways activated by major surgery or trauma. The possibility is discussed that endothelial protease-activated receptor 2, whose natural ligands are trypsin, tryptase and the ternary tissue factor-Vlla-Xa complex, may be targeted by aprotinin.

Permeability of endothelial monolayers to albumin is increased by bradykinin and inhibited by prostaglandins

Using monolayers of bovine aortic endothelial cells (BAEC) in modified Boyden chambers, we examined the role of prostaglandins (PGs) in the bradykinin (BK)-induced increase of albumin permeability. BK induced a concentration-dependent increase of the permeability of BAEC, which reached 49.9 +/- 1% at the concentration of 10(-8) M. Two inhibitors of the prostaglandin G/H synthase, indomethacin (2.88 microM) and ibuprofen (10 microM), potentiated BK-induced permeability 1.8- and 3.9-fold, respectively. Exogenously administered PGE2 and iloprost, a stable analog of prostacyclin, attenuated the effect of BK in a concentration-dependent manner. Butaprost equally reduced the effect of BK, suggesting the participation of the EP2 receptor in this phenomenon. However, the EP4-selective antagonist AH-23848 did not significantly inhibit the protective effect of PGE2. The inhibitory effect of PGE2 was reversed by the adenylate cyclase inhibitor MDL-12330A (10 microM). These results suggest that BK-induced increase of permeability of BAEC monolayer to (125)I-labeled albumin is negatively regulated by PGs. This postulated autocrine activity of PGs may involve an increase in the intracellular level of cAMP.

Mechanisms mediating Porphyromonas gingivalis gingipain RgpA-induced oral mucosa inflammation in vivo

Suffusion of gingipain RgpA (GRgpA) elicited a significant concentration-dependent increase in the clearance of macromolecules from in situ hamster cheek pouch which was attenuated by NPC 17647, a selective bradykinin B(2) receptor antagonist. Leupeptin and a mixture of proteinase inhibitors also attenuated GRgpA-induced responses. These data indicate that GRgpA elicits plasma exudation from in situ oral mucosa in a catalytic site-dependent fashion by elaborating bradykinin.

Subtilisin increases macromolecular efflux from the oral mucosa

The purpose of this study was to determine whether subtilisin, a potent serine proteinase derived from Bacillus species contaminating smokeless tobacco, increases macromolecular efflux from the oral mucosa and, if so, whether local elaboration of bradykinin mediates this response. Using intravital microscopy, I found that suffusion of subtilisin elicits significant, concentration-dependent leaky site formation and an increase in the clearance of fluorescein isothiocyanate-labeled dextran (molecular mass, 70 kDa) from the in situ hamster cheek pouch (P<0.05). Heat-inactivated subtilisin had no significant effects on macromolecular efflux. Subtilisin-induced responses were significantly attenuated by Hoe 140 and NPC 17647, two structurally distinct selective bradykinin B(2) receptor antagonists, but not by des-Arg(9)-[Leu(8)]bradykinin, a selective bradykinin B(1) receptor antagonist, or CP-96,345, a selective neurokinin-1 receptor antagonist. Aprotinin, but not leupeptin, significantly attenuated subtilisin-induced increase in macromolecular efflux. Indomethacin had no significant effects on subtilisin-induced responses. Collectively, these data indicate that subtilisin increases the macromolecular efflux from the in situ hamster cheek pouch in a catalytic-site-dependent fashion through local elaboration of bradykinin. This response does not involve the stimulation of local afferent nerves or the production of prostaglandins.