Enrique Saldı́var

Platelet aggregation contributes to arresting bleeding at wound sites, but may cause occlusion of atherosclerotic vessels, thus curtailing blood flow to vital organs. According to current dogma, the integrin alphaIIbbeta3 plays an exclusive role in linking platelets to one another through interactions with fibrinogen or vWf. We demonstrate here that, depending on shearing flow conditions, this process may require vWf binding to glycoprotein Ibalpha, even when alphaIIbbeta3 is competent to bind adhesive ligands. Platelet activation induced solely by high shear stress is initiated by glycoprotein Ibalpha interaction with vWf, but results in aggregation only if the latter can bind concurrently to alphaIIbbeta3. In contrast, platelets exposed to high shear rate after activation by exogenous agonists such as ADP and epinephrine can aggregate when fibrinogen is the alphaIIbbeta3 adhesive ligand, yet only if vWf binding to glycoprotein Ibalpha can also occur. Thus, the latter interaction appears to provide a bond with biomechanical properties necessary to overcome the effects of high shear rate and initiate interplatelet cohesion. These findings highlight the distinct function of two adhesive receptors mediating platelet aggregation under varying fluid dynamic conditions, and modify the current interpretation of a crucial event in hemostasis and thrombosis.

Aggregation of blood platelets contributes to the arrest of bleeding at sites of vascular injury, but it can occlude atherosclerotic arteries and precipitate diseases such as myocardial infarction. The bonds that link platelets under flow conditions were identified using confocal videomicroscopy in real time. Glycoprotein (GP) Ibalpha and von Willebrand factor (vWF) acted in synergy with alphaIIbbeta3 and fibrinogen to sustain platelet accrual at the apex of thrombi where three-dimensional growth resulted in increasing shear rates. The specific function of distinct adhesion pathways in response to changing hemodynamic conditions helps to explain hemostatic and thrombotic processes.

Mechanisms mediating tumor cell attachment to the vessel wall under flow conditions are largely unknown. Therefore we analyzed the ability of human melanoma cells to adhere to an immobilized matrix during blood flow and determined the role of platelets in this process. In a parallel plate flow chamber, M21 melanoma cells were suspended in human blood and perfused over a collagen I matrix at a wall shear rate of 50 s-1 (2 dynes/cm2) to simulate venous flow over a thrombogenic surface. Melanoma cell interaction with the matrix or blood cells and platelets was monitored and quantified by fluorescence and confocal laser microscopy. Despite their ability to adhere to collagen I under static conditions, M21 cells failed to attach directly to this matrix during blood flow. However, they associated with adherent thrombi, and this resulted in stable melanoma cell arrest. Inhibition of platelet activation or platelet integrin alphaIIbbeta3 function abolished M21 cell attachment. Melanoma cell interaction with thrombi was specific and required beta3 integrin expression. M21-L cells which lack integrin alphavbeta3 failed to associate with thrombi and to arrest during blood flow. Transfection of these cells with the integrin subunits alphav or alphaIIb resulted in variants expressing alphavbeta3, as in the wild type, or alphaIIbbeta3. Both variants were able to associate with thrombi and to arrest during blood flow. Therefore, beta3 integrin-mediated binding to activated platelets represents an efficient mechanism for melanoma cell arrest under flow, and this may contribute to the role of platelets in hematogenous metastasis.

Microcirculatory changes in the window chamber preparation in Syrian golden hamsters, secondary to chronic hypoxia adaptation, are presented herein. Adaptation was attained by keeping animals in a 10% oxygen environment for 1 wk and 5% the following week. The following groups were studied: group 1, adapted to chronic hypoxia and kept in a 5% oxygen environment throughout the experiment; group 2, adapted to chronic hypoxia and kept in a 21% oxygen environment 24 h before and during the experiment; and group 3, control. Adaptation caused venule enlargement and hematocrit increase (68.6 +/- 2.44 in group 1, 70 +/- 2.66 in group 2, and 43.27 +/- 2.30 in group 3; P < 0.05). Whereas heart rate decreased in adapted animals, blood pressure remained constant. Group 1 presented alkalosis, hypocapnia, and hypoxemia. The adapted groups had decreased blood flow velocity in arterioles and veins. We found no difference in microvasculature oxygen tension between groups 2 and 3; however, the number of capillaries with flow was markedly reduced in group 1 but significantly increased in group 2. Our findings suggest that, as an adaptation to hypoxia, erythropoiesis may prove beneficial by increasing blood viscosity and shear stress, leading to vasodilatation, in addition to the increase in oxygen-carrying capacity. Calculations show that oxygen extraction in the tissue of the window chamber model was significantly lowered in adapted animals breathing 5% oxygen, but was unchanged from the control when breathing 21% oxygen, even though blood hemoglobin content was increased from 14.5 +/- 0.07 g/dl at control to 21.04 +/- 1.24 g/dl in the adapted animals (P < 0.05).