Reversible activation defect of the platelet glycoprotein IIb-IIIa complex in patients with uremia

Patients with chronic renal failure may experience a bleeding tendency and blood loss after surgical procedures or trauma. Altered platelet function has been indicated as the major cause of uremic bleeding, but its pathogenesis remains to be clarified. In two groups of patients with chronic renal disease of various etiology, the receptor function of glycoprotein (GP) Ib and GP IIb-IIIa complex was studied. Glycoprotein Ib was assessed with both 125I-von Willebrand factor (vWF) and 125I-asialo-vWF binding to platelets. Activation-dependent receptor function of the GP IIb-IIIa complex was studied with 125I-fibrinogen and 125I-vWF binding to washed platelets stimulated with adenosine diphosphate plus epinephrine (10 mumol/L each). Flow cytometric analyses on resting and stimulated platelets were performed using an activation-dependent, anti-GP IIb-IIIa monoclonal antibody (PAC1) as well as an activation-independent antibody (LJ-P1). Binding of PAC1 also was assessed in washed and stimulated platelets and in platelet-rich plasma before and after dialysis. We found that the activation-dependent receptor function of the GP IIb-IIIa complex is defective in uremia, as shown by decreased binding of both vWF and fibrinogen to stimulated platelets. Moreover, binding of the activation-dependent anti-GP IIb-IIIa monoclonal antibody, PAC1, was significantly decreased in uremia compared with that of the activation-independent antibody, LJ-P1. Thus, the number of GP IIb-IIIa receptors expressed on the platelet membrane is normal, but their activation is impaired. In contrast to the functional abnormality of GP IIb-IIIa, the vWF-binding activity of GP Ib was normal.(ABSTRACT TRUNCATED AT 250 WORDS)

[Infusion pre-hydration and a humid room in the prevention of postoperative adhesions]

The Authors after an overview on the most important causes of postsurgical adhesions, drove their attention to the tissue injury determined, during abdominal surgery, by exposure of pelvic serosa to the humidity of operating area. The AA tested body temperature in the Douglas pouch and under the liver on 30 patients who were operated by conservative surgery (myomectomy) and on 8 patient who underwent laparoscopy. A decrease of one degree in the Douglas pouch after one hour on the patient who had a Pfannestiel incision has been detected. While on the patients with longitudinal laparotomy no difference was evidenced. Serosal biopsies done at the beginning of surgery after pelvis exposure to the external conditions (temperature, humidity) showed a marked phlogosis in the tissue of the patients not treated at all. These tissue injuries were almost absent in the group treated with saline tissue irrigation and absent in the group that had parenteral prehydration, too. The AA suggested the use of the two techniques in conservative surgery in order to obtain in the open abdomen surgery results similar to those of the endoscopic one.

Thrombotic thrombocytopenic purpura: evidence that infusion rather than removal of plasma induces remission of the disease

Plasma exchange has recently been reported to be more effective than plasma infusion for the treatment of thrombotic thrombocytopenic purpura (TTP). However, in the only available controlled study, the plasma infused during the exchange procedure was three times that given by infusion alone. Here we report the case of a patient with chronic relapsing TTP who had 21 relapsing episodes in the last 3 years. During 18 relapses, infusion of plasma, as infusion alone or in the context of an exchange procedure, invariably induced remission of the disease. By contrast, plasma removal alone (replaced with albumin and saline) was ineffective in three further consecutive relapses so that infusion was eventually necessary to induce remission. We concluded that the effective component of plasma exchange in TTP is infusion, rather than removal of plasma. Unusually large von Willebrand factor (ULvWF) multimers were found during both acute and remission phases, possibly reflecting intravascular leakage from ongoing endothelial cell injury. A relative increase of the 176-kd fragment and a relative decrease of the 225-kd subunit were demonstrated during the acute phase, indicating in vivo proteolytic vWF fragmentation. Since in vitro evidence is available that such fragments of vWF induce platelet aggregation, it is speculated that protease inhibitors of normal plasma help restore normal vWF processing activity in the circulation, which explains remission of the disease associated with the plasma infusion.

Defective platelet aggregation in response to platelet-activating factor in uremia associated with low platelet thromboxane A2 generation

The bleeding tendency associated with uremia is likely due to a qualitative platelet dysfunction. So far the data available on platelet aggregation are conflicting. Since platelet-activating factor (PAF) plays a role in primary hemostasis, we studied platelet aggregation in response to PAF in 40 patients with chronic uremia on regular hemodialysis and 12 control subjects. Our results showed that in 28 of 40 uremics, platelet aggregation response to PAF was normal, whereas in the remaining 12 it was defective in that no second wave of aggregation was elicited even if the PAF concentrations were increased by a factor of 10,000. This abnormal response was peculiar to PAF and only partially related to factor(s) of plasma origin. The number of platelet PAF receptors and their affinity for the agonist were comparable in controls and "PAF-unresponsive" patients. The defective platelet aggregation in response to PAF was associated with a statistically significant reduction (P less than 0.01) in thromboxane A2 (TxA2) generation in platelet-rich plasma (PRP) challenged with PAF (10 and 100 nmol/L). When PRPs from PAF-unresponsive patients were preincubated with a stable analogue of prostaglandin endoperoxides/TxA2 U-46619, an irreversible platelet aggregation in response to PAF was obtained. Thus in a subpopulation of uremics, platelet aggregation in response to PAF is selectively abnormal as a consequence of a reduced TxA2 generation.

Heterogeneity of plasma von Willebrand factor multimers resulting from proteolysis of the constituent subunit

In this report we demonstrate that proteolytic cleavage of the constituent subunit is one of the causes determining the heterogeneous size distribution of plasma von Willebrand factor (vWf) multimers. As shown by two-dimensional nonreduced/reduced agarose/polyacrylamide gel electrophoresis, the structure of circulating vWf molecules may deviate from that represented by assemblage of a variable number of identical subunits. Indeed, even though the largest multimers in normal plasma appear to be composed predominantly of intact 225-kD subunits, those of intermediate and smaller size contain also 189-, 176-, and 140-kD proteolytic fragments. Different subunit composition patterns are repeated regularly in multimers of increasing molecular mass, yielding series of bands with similar structure. One of these series consists of molecules without evidence of proteolytic fragmentation, and its smallest member appears to be a dimer of 225-kD subunits. Type IIA von Willebrand disease, characterized by absence of the largest multimers, displays a pattern wherein the fragments of 176 and 140 kD are relatively increased, that of 189 kD is markedly decreased or absent, but the composition of individual multimers is otherwise similar to that of species seen also in normal plasma. In contrast to those in the circulation, all normal platelet vWf multimers contain only intact subunit. These results suggest that proteolytic cleavage of plasma vWf subunits occurs after release from cellular sites, whereas platelet vWf stored in alpha-granules is protected from proteolysis. These findings provide information that may be relevant for understanding the normal processing of vWf multimers and for elucidating the pathogenesis of some of the congenital and acquired structural abnormalities of this molecule.

Functional implications of decreased renal cortical atrial natriuretic peptide binding in experimental diabetes

Glomerular hyperfiltration in streptozotocin-induced diabetes mellitus in rats may be mediated by atrial natriuretic peptide (ANP). We wanted to evaluate plasma levels of ANP and plasma volume in relation to renal ANP receptor density and affinity in rats 6 weeks after induction of diabetes. Plasma levels of immunoreactive ANP were significantly higher in hyperglycemic diabetic (75.2 +/- 8.3 pg/ml) than in control animals (34.7 +/- 8.1 pg/ml; p less than 0.01). Administration of insulin to keep diabetic rats normoglycemic normalized plasma levels of immunoreactive ANP (30.5 +/- 5.2 pg/ml). In contrast, plasma volume did not show significant differences among the groups (hyperglycemic diabetes, 46.6 +/- 3.8; normoglycemic diabetes, 42.4 +/- 3.2; controls, 43.2 +/- 2.0 ml/kg body wt). No correlation was found between plasma levels of immunoreactive ANP and plasma volume. By autoradiography a significant reduction in the number of renal cortical ANP receptors was observed in hyperglycemic diabetic rats as compared with controls. At variance, ANP receptor affinity did not change either in the cortex or in the medulla in hyperglycemic diabetics in comparison with control animals. The pathophysiological implication of cortical ANP receptor down-regulation was underscored by the blunted response of glomerular filtration rate to ANP infusion in diabetic animals as compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Blunted excretory response to atrial natriuretic peptide in experimental nephrosis

Adriamycin (ADR) nephrosis and a model of unilateral ADR-induced proteinuria were produced in Sprague-Dawley (S.D.) rats to investigate the mechanism of sodium retention by the nephrotic kidney. Plasma volume, as measured by the dilution principle using radioiodinated serum albumin, was significantly higher in nephrotic animals than in control ones (NS: 69.61 +/- 15.02: control: 47.05 +/- 5.32 ml/kg: P less than 0.01). Similarly plasma levels of immunoreactive ANP (iANP) were significantly higher in nephrotic animals compared to controls (NS 104.22 +/- 36.41: control 59.94 +/- 20.88 pg/ml; P less than 0.05). Using the unilateral model we found a markedly reduced diuretic and natriuretic response to the infusion of synthetic rat atrial natriuretic peptide (ANP 1-28) in proteinuric kidney but not in contralateral kidney, despite a comparable increase in glomerular filtration rate. To explain the blunted diuresis and natriuresis in the presence of normal glomerular response to ANP, we investigated the possibility of an abnormality at post-glomerular level by studying ANP receptor density and affinity of the inner stripe of outer medulla and the inner medulla in ADR-and vehicle-treated rats. The inner stripe of outer medulla and the inner medulla receptor density and affinity were not significantly different in ADR rats as compared to animals given the vehicle alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Catecholamine receptor binding in rat kidney: effect of aging

Binding to several receptors was compared in kidneys from 3- and 24-month-old rats. In crude membrane preparations of aged rat kidneys, the number of beta 2-adrenergic receptors was significantly reduced but the number of total beta-adrenoceptors was unchanged. The high-affinity alpha 1-adrenoceptor component was significantly reduced in old rats, whereas the low-affinity component was unchanged. The number of alpha 2-adrenoceptors showed a non-significant decrease. 3H-spiperone binding sites were similar in young and old rats. For each receptor binding the KD values were the same in young and old animals. The D1 dopamine receptor was significantly reduced in old rats. In our experiments, age-related changes of specific binding sites in the kidney were selective for some receptors studied and did not seem to be due to general aging-induced membrane modifications. Moreover, the renal and central receptors were sensitive to aging differently in the same animal model.

Role of platelet-activating factor in primary hemostasis

To determine whether platelet-activating factor (PAF) has a physiological role in the process of primary hemostasis in the rabbit, we measured skin bleeding times in animals given orally a specific PAF-receptor antagonist L-652,731. One hour after the administration of L-652,731 (20 or 40 mg/kg), a significant prolongation of bleeding time was observed. Parameters known to interfere with the process of primary hemostasis were not altered by PAF-receptor antagonist. In addition, no changes were observed in platelet count and vessel wall arachidonic acid metabolism, as revealed by serum thromboxane B2 levels, vascular 6-ketoprostaglandin F1 alpha generation, and by aspirin experiment. The fact that aspirin alone did not induce prolongation of bleeding time in rabbits is probably due to the simultaneous inhibition of platelet thromboxane A2 and vascular prostacyclin. [3H]PAF binding to washed platelets of rabbits given L-652,731 showed a significant reduction in maximum number of detectable binding sites in comparison with values found in the same animals before the L-652,731 administration. The results suggest that the bleeding-time prolongation observed after L-652,731 administration results from its ability to selectively inhibit PAF bioactivity. Thus locally released PAF appears to exert a physiological role in the process of platelet plug formation that follows a skin incision in the rabbit.