no clinical relevance. Nevertheless, in vitro experiments indicate that the maximum amount of TXB2 that can be produced may vary. For example, exogenous thrombin will further increase the amount of TXB2 produced.566 Moreover, when human platelets are treated with thrombin to induce the release of their alpha and dense granule constituents, additional TXB2 can be formed in response to other platelet agonists, albeit less than by platelets not depleted of their granule contents.567 In contrast, arachidonic acid induced TXA2 formation does not differ between throm bin-treated platelets and non-treated platelets,567 and the addition of extra thrombin produces significantly less TXB2 by thrombin-treated platelets than by control
samples.568 Furthermore, collagen stimulation leads to reduced TXB2 formation in degranulated platelets, but more TXB2 is produced than when thrombin is used as the agonist with the same platelets.567 Pretreatment of platelets with thrombin does not lead to recovery with respect to thrombin inducible TXB2 synthesis when plate lets are incubated in the presence or absence of exogenous arachidonic acid.568 Indeed, impaired thromboxane synthesis in pre-activated platelets may result from an agonist-specific, irreversible desensitisation to thrombin.568 These experiments in dicate that the production of thromboxane is, in part, dependent on prior exposure to platelet agonists, in particular thrombin.
Therefore it would appear that substrate availability and/or degree of agonist stimu lation, and not the enzyme system, is the limiting factor in the production of TXB2 in serum. The observations demonstrate the very real potential for platelet agonists and metabolic substrates of thromboxane to influence the ability of platelets to form TXB2 leading to variability in TXB2 formation in association with clotting.
4.5.1.2 Thromboxane Synthesis in Different Diseases. Diabetics may have an altered capacity to generate TXB2 in clotting whole blood.569 Subjects with non-insu lin dependent (NIDDM) and insulin dependent diabetes mellitus (IDDM) have both been found to have a reduced capacity to synthesise TXB2 in response to endog enous stimuli released during whole blood clotting with the synthesis of TXB2 being influenced by the degree of diabetic control.569 Others however, have found that platelets obtained from subjects with NIDDM synthesise significantly higher amounts of TXB2 than those with IDDM and matched controls.570 Yet again, in another study, the ability to generate TXB2 did not differ between diabetics and controls.571 The platelets from diabetics with microangiopathy or from diabetics taking oral hypoglycaemic agents produced reduced amounts of TXB2 during clot ting.571 Additionally, a wide variability is present in all the groups studied, and there is a significant overlap in values for individuals.
Reduced platelet TXB2 formation has been noted in patients with renal failure.566,572. In uraemic patients, there may be up to a 60% decrease in TXB2 formation in clot ting whole blood.566 Exogenous thrombin failed to restore normal TXB2 production in uraemic patients, whereas TXB2 production increased upon the addition of thrombin to serum in normal subjects. These characteristics are consistent with a functional defect of the enzyme cyclooxygenase. In another study, patients with chronic renal failure produced less serum TXB2 than age- and sex-m atched controls.572
An important influence on the production of TXB2 in serum appears to be the other cell constituents in blood. For example, the reduction in TXB2 synthesis observed in association with renal impairment occurred in the presence of a lower He and a different platelet count.572 This study indicates that the decrease in TXB2 production may not be due to a cyclooxygenase defect, but rather to differences in other blood parameters which could influence TXB2 production, including He and platelet count.572 Indeed, there is evidence to suggest that TXB2 formed during clotting should be divided by the platelet count.572,573 It would appear that there is a depend ence of TXB2 production in PRP on the platelet count.573 The TXB2 production in clotting whole blood in normal subjects also appears to be related to the He and the
platelet count.572
Soon after an AMI, male patients with no other diseases present, when com pared to an age- and sex- matched control group, show a large decrease in platelet cyclooxy genase end-products formed during clotting of whole blood.574
4.5.1.3 Intra-individual variability in normals. Some studies suggest that in norm al subjects there is no clinical or statistically significant correlation betw een serum TX B 2 levels and age, BMI, BP, tobacco use, cholesterol level, triglyceride level, o r blood glucose lev el.504,575 H ow ever, as indicated above, it w ould appear th a t th e se ru m T X B 2 le v e l is r e la te d to th e n u m b e r o f p la te le ts in w h o le blood.504,572,575 Furthermore, repeat blood sampling over successive days in healthy subjects dem onstrates an in tra-su b ject variability in serum TX B 2 levels o f 14+/- 5%.576
4.5.1.4 Influence of extrinsic factors on serum TXB2. The formation of TXA2 in w hole blood, m easured as its stable m etabolite TX B2, reflects the capacity o f platelets to form TXA2 in response to thrombin and other endogenous platelet ago nists. TXA2 is the main product o f cyclooxygenase metabolism in platelets,577,578 but T X A 2 is also pro d u ced by poly m o rp h o n u clear leu k o cy tes579 and m o n o cy tes.580 H ow ever, throm bin formed during blood coagulation preferentially stimulates plate lets com pared to monocytes,581 and the amount of TXB2 produced directly correlates with the whole blood platelet count in normal individuals.504,572
Serum TXB2 levels have been used as an ex vivo measure o f platelet activation in a n u m b er o f m ed ical c o n d itio n s.566,569 H ow ever, the cap acity o f p latelets to form TX B 2 ex vivo (m easured as the levels o f TX B2 in clo tted w hole blood) m ay not differ in various clinical conditions associated with platelet activation ex v/vo.563,582 If the m axim um am ount of TXB2 form ed in clotted whole blood is fixed and inde pendent o f the biological milieu and clinical conditions, it would be an inappropriate m easure for the investigation o f differences in platelet function betw een different clinical groups.
4.5.1.5 Summary. W hether the differences in serum TX B2 levels betw een the