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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.

To determine the possible role of vascular factors in the bleeding tendency of uraemic patients, three major factors of the haemostatic system normally present in vascular tissues were studied. Factor VIII-related protein (F VIII) was detected on the vascular intima of 13 patients and 10 normal subjects. Comparable values of plasminogen activator (PA) were found in tissue slices from 7 patients and 7 controls. In contrast, prostacyclin like (PGI2) activity, measured as platelet aggregation inhibitory potency, was significantly higher in specimens from 15 patients with either acute or chronic uraemia than in 10 controls. The latter abnormality, leading to impaired platelet-vessel wall interaction, might contribute to the disturbed haemostasis of uraemic patients.

We reported that aspirin (ASA) abnormally prolongs bleeding time (BT) in uremia. The present study was designed to investigate whether the abnormally prolonged post-ASA BT in uremia is due to different ASA pharmacokinetics and bioavailability that might be a consequence of uremic condition, platelet cyclooxygenase is peculiarly sensitive to ASA in uremia, and ASA affects primary hemostasis in uremia by a mechanism independent of cyclooxygenase inhibition. Our results showed that in patients with uremia, but not in normal subjects, ASA markedly prolongs the BT. This effect is transient and depends on the presence of ASA in the blood. The observed differences in ASA kinetic parameters are not an explanation of the exaggerated effect of ASA on primary hemostasis in uremia. The sensitivity of platelet cyclooxygenase to ASA inhibition is comparable in uremics and in normal subjects. The temporal dissociation between ASA-induced prolongation of BT and the effect on platelet thromboxane A2 generation suggests that ASA inhibits platelet function in uremia by a mechanism distinct from cyclooxygenase blocking. This possibility is strengthened by the observation that ibuprofen at a dose that fully inhibits platelet cyclooxygenase activity does not significantly prolong BT.

Uremic patients on long-term hemodialysis show a paradoxical association of hemorrhages on the one hand and thrombotic complications or atherosclerosis on the other. Platelet function has been found to be depressed in some cases but enhanced in others. In 19 patients, both platelet aggregation and prostaglandin formation appeared to be significantly enhanced in response to low concentrations of arachidonic acid but significantly reduced with high concentrations. It is suggested that this double functional abnormality of uremic platelets may contribute to the complex vascular disturbances of hemodialyzed patients.

Specimens of venous tissues from a group of 25 patients with chronic uremia and 7 patients with acute renal failure generated significantly higher PGI2-like (platelet aggregation inhibiting) activity than venous tissues from 30 normal subjects. After repeated washings, when this activity could barely be detected in the controls, pronounced inhibitory activity was still evident in samples containing venous tissues from uremic patients. Both prolonged bleeding times and increased PGI2-like activity returned to normal in 4 acute uremic patients on restoration of their renal function. These findings may be relevant to the pathogenesis of bleeding in renal failure.

A qualitative platelet abnormality and a bleeding tendency are frequently associated with renal failure and uremia. We demonstrated previously that uremic patients display an abnormal platelet aggregation to arachidonic acid and reduced malondialdehyde production in response to thrombin and arachidonic acid. The objectives of this investigation were: (a) to compare platelet prostaglandin (PG) and thromboxane (TX) production in whole blood and in platelet-rich plasma (PRP) of 21 uremic patients and 22 healthy subjects; (b) to evaluate the concentration and activity of platelet PG- and TX-forming enzymes; (c) to assess the functional responsiveness of the platelet TXA(2)/PGH(2) receptor; (d) to explore the hemostatic consequences of partially reduced TXA(2) production.Platelet immunoreactive TXB(2) production during whole blood clotting was significantly reduced, by approximately 60%, in uremic patients as compared to age- and sex-matched controls. Exogenous thrombin (5-30 IU/ml) failed to restore normal TXB(2) production in uremic platelets. Uremic PRP produced comparable or slightly higher amounts of TXB(2) than normal PRP at arachidonate concentrations 0.25-1 mM. However, when exposed to substrate concentrations >2 mM, uremic PRP produced significantly less TXB(2) than normal PRP. To discriminate between reduced arachidonic acid oxygenation and altered endoperoxide metabolism, the time course of immunoreactive TXB(2) and PGE(2) production was measured during whole blood clotting. The synthesis and release of both cyclooxygenase-derived products was slower and significantly reduced, at all time intervals considered. Furthermore, PGI(2) production in whole blood, as reflected by serum immunoreactive 6-keto-PGF(1alpha) concentrations, was significantly reduced in uremic patients as compared with healthy subjects. PGH synthase levels, as determined by an immunoradiometric assay, were not significantly different in platelets from uremic patients as compared to control platelets. A single 40-mg dose of aspirin given to five healthy volunteers reduced their serum TXB(2) to levels found in uremic patients. This was associated with a significant increase of threshold aggregating concentrations of ADP and arachidonic acid and prolongation of bleeding time. Substantially similar threshold concentrations of U46619, a TXA(2) agonist, induced aggregation of normal and uremic platelets. Prostacyclin induced a significant elevation of uremic platelet cyclic AMP, which was suppressed by U46619, further suggesting normal responsiveness of the TXA(2)/PGH(2) receptor. WE CONCLUDE THAT: (a) an abnormality of platelet arachidonic acid metabolism exists in uremia, leading to a reduced TXA(2) production; (b) the characteristics of this abnormality are consistent with a functional cyclooxygenase defect; (c) reduced TXA(2) production may partially explain the previously described abnormality of platelet function in uremia.

Bleeding time, blood loss and platelet retention by glass beads, measured by standardized techniques, were significantly altered in a group of 30 non-thrombocytopenic patients with chronic renal failure undergoing maintenance haemodialysis. Bleeding time or blood loss did not correlate with platelet retention either before or after haemodialysis. No correlation could be found between the above tests and a number of biochemical parameters characterizing the uraemic condition. Haemodialysis only partially corrected the abnormal bleeding time, blood loss and platelet retention. These tests were still significantly different after haemodialysis from those of 30 normal subjects. It is suggested that some non-dialyzable material could play an important role in the aetiology of uraemic bleeding.