An increase in phosphatidic acid in the absence of changes in diacylglycerol in human platelets stimulated with ADP

1. ADP caused an increase in radioactivity of phosphatidic acid but not diacylglycerol in human platelets labelled with [3H]arachidonic acid. 2. The radioactivity of phosphatidic acid was significantly increased 10 sec after adding 10 microM ADP and this increase did not depend on production of thromboxane A2. 3. Thrombin (1 U/ml) caused an increase in both diacylglycerol and phosphatidic acid, the latter being much greater than that caused by ADP. 4. The results confirm that ADP stimulates phosphatidic acid production and suggest that a weak stimulus of the phosphatidyl inositol cycle, such as ADP, does not cause accumulation of diacylglycerol.

Potentiation of ADP-induced aggregation in human platelet-rich plasma by 5-hydroxytryptamine and adrenaline

1. We have used dose-response curves to quantitate the potentiation of adenosine 5'-diphosphate (ADP)-induced aggregation and thromboxane (TXA2) generation by 5-hydroxytryptamine (5-HT) and adrenaline in human citrated platelet-rich plasma. We have also quantitated the inhibition of these responses by aspirin, ketanserin and yohimbine, singly and in pairs. 2. Ketanserin (5 microM) inhibited TXA2 production and the second wave of platelet aggregation induced by a range of concentrations of ADP alone. This indicates that endogenous 5-HT, released from the platelet dense granules, contributes significantly to responses induced by ADP. 3. When 5-HT (10 microM) was added before ADP, a lower concentration of ADP was required to cause 50% aggregation and TXA2 generation. The ratio of ADP concentrations (CR) to cause 50% aggregation in the presence and absence of 5-HT was 2.1 when only added 5-HT was considered, and 5.0 when endogenous 5-HT was also taken into account. 4. Potentiation of ADP-induced aggregation by 5-HT also occurred in the presence of aspirin, resulting in a CR of 2.3. As expected, ketanserin inhibited potentiation by 5-HT in the presence and absence of aspirin. Although aspirin caused substantial inhibition of aggregation induced by ADP and 5-HT (CR 3.4), further inhibition occurred when ketanserin was also present (CR 6.5). 5. A subthreshold concentration of adrenaline (0.25 microM) caused substantial potentiation of ADP-induced aggregation in the absence (CR 4.0) and presence (CR 2.0) of aspirin. As expected, yohimbine (9 microM) inhibited this potentiation.Maximum TXA2 generation induced by ADP increased from 32.5 to 59.4 pg per 106 platelets when adrenaline was present. Aggregation induced by ADP and adrenaline was markedly inhibited by aspirin (CR 5.1) but was further inhibited when yohimbine (9 microM) was also present (CR 10.0).6. Results from this in vitro study show ketanserin and yohimbine have the potential to be used in combination with aspirin as antithrombotic agents in vivo.

Interference caused by acid extraction in the study of diacylglycerol in platelets

A neutral mixture of chloroform and methanol was compared to an acidic mixture of these solvents for the extraction of diacylglycerol from platelets labelled with 3H-arachidonic acid. Using a neutral solvent we found that thrombin caused a rapid increase in the radioactivity of diacylglycerol. With an acidic solvent there was 10 times more background radioactive diacylglycerol, but no increase was detected after stimulation with thrombin. Acidic extraction, but not neutral extraction, caused a small percentage of phosphatidylinositol and phosphatidylcholine to hydrolyse and form diacylglycerol. The extent of hydrolysis accounted for the greater amount of radioactive diacylglycerol found after acidic extraction of radiolabelled platelets. In addition, when platelets were extracted by the acidic solvent a modified form of hydroxy-heptadecatrienoic acid appeared, and thin-layer chromatography in two dimensions was required to separate it from diacylglycerol. It is therefore important to use a neutral extraction method when studying diacylglycerol in platelets.

The antiplatelet effect of daily low dose enteric-coated aspirin in man: a time course of onset and recovery

We have studied the onset and recovery of inhibition of platelet function by low dose aspirin. Enteric-coated aspirin 50mg daily was administered to five human volunteers for five weeks and then 100mg daily was given for a further five weeks. We studied platelet aggregation and thromboxane formation in response to a range of stimuli: ADP, adrenaline, arachidonate and collagen, and also measured thromboxane formation after coagulation of whole blood (serum thromboxane). The onset of inhibition of platelet aggregation was progressive over several days for each of the four platelet stimuli, and was synchronous with the inhibition of thromboxane formation. Maximum inhibition occurred by day three for the weak stimuli ADP and adrenaline, by day five for the stronger stimuli arachidonate and collagen, but did not occur until day eight for serum thromboxane. Further inhibitory effects on both aggregation and thromboxane generation were observed after 100mg daily. Two weeks after the cessation of aspirin the responses to collagen and arachidonate and serum thromboxane had returned to normal. Platelet aggregation in response to the weaker stimuli, ADP and adrenaline, still showed detectable inhibition two weeks after cessation of aspirin, but had returned to normal by four weeks. These experiments provided no evidence for an effect of aspirin on platelets separate to its effect on cyclooxygenase. The onset and recovery of inhibition of platelet function by low dose aspirin was dependent on the strength of the stimulus studied.

Effect of aspirin infusions on platelet function in humans

1. The inhibitory effects of aspirin on platelet function in vitro have been shown to be both time (over 3 h) and concentration (1-10 mumol/l) dependent. 2. To determine if these effects occurred in vivo, four volunteers received intravenous infusions on four occasions, to give constant plasma aspirin concentrations of 0, 1, 2 and 4 mumol/l over 3 h. Infusions were performed at intervals of at least 2 weeks. 3. Before and during the infusions, blood was taken for assay of aspirin concentrations, and measurements of platelet aggregation in response to collagen, adenosine 5'-pyrophosphate and arachidonate. Thromboxane generation after stimulated platelet aggregation and whole-blood coagulation was also measured. 4. At each aspirin concentration, both platelet aggregation and thromboxane generation in response to collagen and arachidonate were inhibited progressively over the 3 h infusion period. Greatest inhibition was seen during the 4 mumol/l infusion, which produced maximal or near-maximal inhibition by the third hour. 5. Thromboxane generated during whole-blood coagulation was similarly inhibited in both a time- and concentration-dependent manner throughout all aspirin infusions. 6. The progressive nature of the inhibition of platelet function with these low aspirin concentrations may be due to either slow aspirin transport across the platelet membrane or delayed interaction with cyclo-oxygenase.

Measurement of aspirin concentrations in portal and systemic blood in pigs: effect on platelet aggregation, thromboxane and prostacyclin production

Low doses of enteric-coated aspirin were administered orally to pigs. Plasma aspirin concentrations measured in blood obtained simultaneously from permanent catheters in a systemic artery and portal vein for 6 hours after dosage showed a large variation in the plasma aspirin concentration: time profile between pigs. After 50 mg single dose the ratio of the arterial: portal area under the plasma concentration versus time curve (AUC) was 0.63 +/- 0.08 (mean +/- SE, n = 6). In three pigs which received all three dosage regimens, the arterial: portal AUC ratios were 0.48 +/- 0.05 after 50 mg single dose, 0.52 +/- 0.02 after 100 mg single dose and 0.47 +/- 0.02 after 100 mg daily for 1 week. Platelet aggregation in response to sodium arachidonate (1.65 mM) was completely abolished after chronic aspirin administration of 100 mg daily. Thromboxane production (pg/10(6) platelets) induced by this stimulus decreased from 536 +/- 117 before aspirin to 57 +/- 14 after aspirin (mean +/- SE, n = 4; p = 0.03). Aortic prostacyclin synthesis, measured as 6-keto PGF1 alpha (ng/disc after 10 min incubation), was 1.66 +/- 0.28 (mean +/- SE, n = 4) in untreated pigs and 0.95 +/- 0.25 (n = 5) in treated pigs (p = 0.07). Results from this study support the idea that a difference between aspirin concentrations in the portal and systemic circulations can be achieved. Whether this can be translated into a clinically useful differential effect on the vessel wall compared to the platelet remains to be determined.

Monitoring of heparin in vascular surgery

In patients heparinized for surgery on the infrarenal aorta, the degree of anticoagulation by heparin of stasis blood (taken from below the aortic clamp) was compared with that obtained in circulating blood taken from a forearm artery. A measurement of activated partial thromboplastin time (APTT) was made on a venous blood sample taken from each patient before 5000 units of heparin was administered intravenously. Further measurements of APTT from static blood and from circulating arterial blood were made at 3, 15, 30, and 60 minutes after heparinization. Samples taken below the aortic clamp showed measurements of APTT lower than those from circulating arterial blood at 15, 30, and 60 minutes (p less than 0.05 paired Wilcoxon rank sum test). Current methods for administering and monitoring heparin may not provide an adequate degree of anticoagulation in static blood during vascular surgery. The consequences, if any, of inadequate anticoagulation in vascular surgery need further study.

Inhibition of platelet aggregation and thromboxane production by low concentrations of aspirin in vitro

1. The aspirin concentrations previously reported to inhibit platelet aggregation in vitro (40-500 mumol/l) are much greater than those required in vivo in man (5 mumol/l). 2. Human platelet-rich plasma was incubated with buffer or various aspirin concentrations at 37 degrees C for up to 4.5 h. Platelet aggregation and thromboxane generation were measured in response to collagen (0.4-6.3 micrograms/ml) and adenosine 5'-pyrophosphate (0.5-4 mumol/l). 3. The concentration of aspirin needed to inhibit platelet aggregation in response to a critical concentration of aggregating agent (lowest concentration to cause greater than 50% aggregation) was lower than that required for higher concentrations of aggregating agent. 4. With more prolonged incubation times with aspirin, lower concentrations of aspirin inhibited platelet aggregation. 5. Inhibition of platelet aggregation and thromboxane formation by 10 mumol/l aspirin was maximal by 90 min. There was progressive inhibition by 3 mumol/l aspirin during incubation for 270 min. By the end of this time there was also significant inhibition by 1 mumol/l aspirin. 6. The apparent discrepancy between inhibitory aspirin concentrations in vivo and those observed in vitro in previous studies appears to have been resolved by extending the incubation time of platelets with low aspirin concentrations, thus mimicking the conditions in vivo.

Pharmacokinetics of low-dose oral modified release, soluble and intravenous aspirin in man, and effects on platelet function

The pharmacokinetics of low-dose aspirin and the resulting salicylic acid were studied in 6 healthy volunteers. Each received a single 50-mg dose of (1) oral modified release capsules, (2) oral solution and (3) intravenous solution. The volunteers also received 50 mg modified release capsules daily for 6 days to determine the effect on collagen, ADP and arachidonate induced platelet aggregation and thromboxane production, and to compare the pharmacokinetics after repeated dosing with the parameters obtained after the single dose. The formulation and route of administration profoundly influenced several pharmacokinetic parameters for aspirin: the maximum concentration (Cmax, ng.ml-1) was 221 and 191 after modified release for single and chronic dosing respectively, 1323 after the oral solution and 6000 after intravenous injection; the time to achieve this maximum concentration (tmax, h) was 3.42 and 3.02 after modified release for single and chronic dosing respectively, and 0.29 after the oral solution; the area under the plasma drug concentration versus time curve (AUC, microgram.h.ml-1) was 0.38 and 0.27 after modified release single and chronic dosing respectively, 0.68 after the oral solution and 1.57 after intravenous injection. The elimination of aspirin after the two solutions was at least biphasic. The terminal phase rate constant ranged from 1.52 h-1 after intravenous injection to 1.88 h-1 after the oral modified release form. The absorption of the oral forms of aspirin was complete as reflected by the total recovery of the doses as salicylic acid in urine.(ABSTRACT TRUNCATED AT 250 WORDS)

A dose-ranging study of the antiplatelet effect of enteric coated aspirin in man

Enteric coated aspirin was given to eight human volunteers in escalating doses (20, 40, 60, 80, 100 mg daily), each dose being given over two weeks. In addition, to measure the maximum effect of aspirin, each volunteer was given two single doses of 600 mg of soluble aspirin. At the end of each dosing interval we measured platelet aggregation and thromboxane formation in response to four aggregating agents and to whole blood coagulation. The doses of aspirin required to inhibit platelet aggregation in response to various stimuli were: for collagen 60-80 mg, for adenosine diphosphate and adrenaline 60 mg, and for arachidonate 40 mg. For maximum inhibition of thromboxane formation the doses were: for collagen greater than 100 mg, for adenosine diphosphate and adrenaline 60 mg, for arachidonate 80 mg, and for whole blood coagulation 100 mg. Different aspirin doses are required to inhibit the responses to different stimuli. Furthermore, for some stimuli, inhibition of thromboxane generation may require more aspirin than is required for inhibition of aggregation. The clinical implications of these findings are uncertain since we do not know which stimuli are important in arterial thrombosis in man.

Storage of platelets for tests of platelet function: effects of temperature on platelet aggregation, platelet morphology and liberation of beta-thromboglobulin

We have studied the effect of temperature on platelets during storage for tests of platelet function. Aliquots of PRP were stored at constant pH at 37 degrees C, room temperature and 4 degrees C. At intervals up to five hours, samples were taken for estimation of platelet shape, plasma levels of beta-thromboglobulin and 14C-serotonin, and assessment of platelet aggregation in response to a range of concentrations of ADP and collagen. When PRP was stored at 37 degrees C there was a gradual decrease in the aggregation response during the period of storage. At room temperature the decrease was slower but the response to ADP often increased dramatically before decreasing; at this temperature there was pronounced liberation of beta TG while there was none at 37 degrees C. Platelets stored at 37 degrees C were smooth and elliptical when examined by electron microscopy, but those stored at room temperature showed partial loss of discoid shape and formation of some pseudopodia. Storage at 4 degrees C was associated with total loss of discoid shape and formation of many large pseudopodia. Light transmission studies also showed loss of discoid shape at room temperature and 4 degrees C. We conclude that storage at 4 degrees C or at room temperature causes platelet activation. To avoid this PRP should be stored at 37 degrees C prior to tests of platelet function.

Intra-arterial streptokinase therapy to relieve acute limb ischemia

One hundred two patients with acute lower limb ischemia were treated with intra-arterial streptokinase. Thirty-seven patients had occluded vascular grafts and sixty-five had had no previous vascular surgery. Eighty-six limbs were acutely threatened. Intra-arterial streptokinase was given as an initial loading dose with a lower maintenance dose given afterward. The mean duration of therapy was 59 hours and hematologic monitoring was meticulous. Indications for intra-arterial streptokinase therapy were contraindication to surgery, anticipation of technically difficult surgery, and multiple occlusions that required separate surgical approaches. Seventy-two legs were saved (71%) and 30 amputated. Morbidity was low and only 1 of the 11 deaths was attributable to streptokinase. No leg was lost that would otherwise have been saved by straightforward surgery and no leg was lost that had not been previously threatened. In 46 patients for whom emergency femorotibial bypass would have been necessary, 35 legs (76%) were saved. Forty-three patients had vascular reconstruction immediately after streptokinase therapy was stopped, to bypass occlusive lesions that had been demonstrated by the thrombolytic therapy in 28 patients, and because streptokinase had produced no response in 15 patients. The advantages of intra-arterial streptokinase in the management of the acutely ischemic leg are that the leg may be saved without surgery, that surgery is not precluded, that the patient can be made as fit as possible for surgery during the streptokinase infusion, and that streptokinase can facilitate surgery by delineating underlying vascular pathologic conditions and clearing distal runoff vessels.

Platelet function in platelet concentrates and in whole blood

Platelet function was studied in CPD whole blood stored at 4 degrees C for one and three days and in platelet concentrates stored at room temperature for the same periods of time. Comparisons were made of platelet shape, nucleotide content, beta-thromboglobulin (beta TG) liberated during storage, and platelet aggregation in response to ADP, collagen, sodium arachidonate and ristocetin. It was found that in whole blood the shape of the platelets was less discoid than in platelet concentrates. However, platelet aggregation in response to ADP, collagen, and sodium arachidonate was preserved better in whole blood than in platelet concentrates. Platelet nucleotides were the same in whole blood as in platelet concentrates, but the plasma levels of beta TG were less in whole blood. The results show that as judged by aggregation, beta TG release and nucleotide content, platelets from whole blood were at least as functional as those from platelet concentrates. However, platelets from whole blood had lost their discoid shape, which suggests that they would have a short survival in the circulation.

Destruction of ristocetin cofactor by coagulation at 4 degrees C

Ristocetin cofactor (VIIIR:RCo) and factor VIII-related antigen (VIIIR:Ag) were measured in anticoagulated and non-anticoagulated blood incubated at 4 degrees C, room temperature (RT) or 37 degrees C for 24 hours. A marked decrease in VIIIR:RCo, to almost undetectable levels, and a smaller decrease in VIIIR:Ag occurred when whole blood clotted at 4 degrees C. These changes were slight or absent when blood clotted at RT or 37 degrees C. VIIIR:RCo lost at 4 degrees C was not recoverable by further incubation at 37 degrees C but the less-marked loss of VIIIR:Ag was partially recovered. In blood which had clotted at 4 degrees C there was a change in the electrophoretic profile of VIIIR:Ag on crossed immunoelectrophoresis: there was more anodal migration of the VIIIR:Ag peak, consistent with a decrease in the mean molecular size. Further experiments showed that the decrease in VIIIR:RCo during coagulation at 4 degrees C preceded the decrease in fibrinogen levels. In cell-free plasma VIIIR:RCo also decreased markedly when coagulation occurred at 4 degrees C. The results show that loss of VIIIR:RCo occurs when blood is allowed to clot at 4 degrees C: this is not due to cryoprecipitation and does not require the presence of blood cells. The data suggest that it is probably caused by plasma proteases activated early in the coagulation pathway.

Storage of platelets for tests of platelet function: comparison of two methods of pH control

A comparison was made of two methods to control the pH of platelet-rich plasma (PRP) stored for tests of platelet function. Citrated PRP at 37 degrees C was maintained at pH 7.3-7.4 by incubation either in a controlled CO2/air environment or in a plastic syringe from which all air was expelled. At intervals over 2-5 hours platelet aggregation induced by ADP and collagen was measured. Plasma beta-thromboglobulin (beta TG) was assayed to assess liberation of beta TG from platelets during storage. Platelet aggregation responses were more stable when PRP was stored in a syringe. Liberation of beta TG from platelets did not occur in this system, but did occur in the CO2 system in many experiments. The differences between the two systems were not due to the lower pO2 levels in the syringe, but were probably related to the presence of an air/liquid interface in the CO2 system. The syringe system of storage is a simple method of pH control which offers better preservation of platelet function than a controlled CO2/air environment.

Haematological characteristics of the common marmoset (Callithrix jacchus jacchus)

Blood cell indices and parameters of haemostasis were studied in the common marmoset. The majority of the results were similar to those found in man. Differences from man were that the prothrombin time was shorter in the marmoset, higher concentrations of aggregating stimuli were required to cause platelet aggregation, and marmoset platelets did not aggregate under the influence of adrenalin. There was sexual dimorphism evident in the data for fibrinogen concentration and for platelet count, both of which were higher in females than in males. Marmoset platelets were very similar in ultrastructure to those of man.

Detection of haemophilia carriers: multivariate analysis compared with discriminant analysis using up to five factor VIII variates

Multivariate analysis is potentially superior to the linear discriminant analysis which is commonly used to identify carriers of haemophilia. Our aim was to compare these two statistical methods, and to find which factor VIII variates most effectively partitioned carrier and normal subjects. In this study we assayed one- and two-stage factor VIII coagulant activity, factor VIII related antigen by electroimmunoassay and by fluoroimmunoassay, and ristocetin co-factor in 50 normal females and 50 carriers of haemophilia. From the results we calculated multivariate ellipses which circumscribed the normal and the carrier populations, and we displayed these on the monitor of a microcomputer. These ellipses separated the two populations better than linear discriminants calculated on the same data. Multivariate analysis correctly identified 94% of the carriers whereas discriminant analysis correctly identified only 84%. Discriminant analysis gave poorer results because the statistical assumption of equal variance was breached, whereas the assumption of multivariate normality was upheld. Of the five factor VIII variates, two-stage factor VIII coagulant activity and factor VIII related antigen by electroimmunoassay correctly identified the most subjects. Ristocetin co-factor did not improve the diagnostic ability, either when in lieu of or when added to factor VIII related antigen.

The effects of age, gene source and familial severity on Factor VIII in normals and haemophilia carriers: analysis by multiple regression

There are conflicting views on the effects of age, gene source and familial severity on levels of factor VIII in carriers of haemophilia. Different workers have found that factor VIII increases with age, is higher in paternal carriers, and is higher in carriers from families with more severe haemophilia. Other workers have disagreed with these findings. In this study we explored some of the causes of this conflict. We measured factor VIII related antigen and factor VIII coagulant activity on 40 normal females and 48 carriers, and analysed the results by multiple regression and analysis of covariance. Our results indicated that both factor VIII coagulant activity and factor VIII related antigen increased with age, but were unaffected by the familial severity of haemophilia or whether the defective gene came from the mother or the father. We found that the conflicting reports of previous authors were due to high inter-correlations of the studied variables.

Aspirin kinetics and platelet aggregation in man

Our aims were (1) to determine the effect of six commercially available aspirin (ASA) preparations on in vitro platelet aggregation, and (2) to relate changes in platelet function to ASA kinetics. Each of six subjects took a single dose of one of the following preparations--600 mg Asproclear, 600 mg Bufferin, 600 mg Bi-prin, 600 mg compressed ASA, 650 mg Ecotrin, or 650 mg S.R.A.--in random order every 3 wk. Venous blood was drawn before and at 2, 4, 6, and 24 hr after ASA dosage to measure platelet aggregation in response to collagen and adenosine diphosphate and, at more frequent intervals, to characterize ASA kinetics. Asproclear, Bufferin, Bi-prin, and compressed ASA yielded peak plasma ASA levels of 28 to 56 mumol/l (5 to 10 mg/l) within 15 to 60 min and peak salicylic acid (SA) levels of 72 to 290 mumol/l (10 to 40 mg/l) within 2 hr. Ecotrin and S.R.A. yielded plasma SA levels of 14 to 87 mumol/l (2-12 mg/l) within 4 to 24 hr and no measurable ASA at any time after dosing. Platelet aggregation was inhibited to an equal extent by all preparations. The time course for this inhibition was the same for all preparations but Ecotrin (which led to a more delayed effect). There was significant recovery of collagen-induced platelet aggregation at 24 hr with all preparations but Ecotrin. With Ecotrin and S.R.A. there was inhibition of platelet aggregation in the absence of measurable circulating ASA. We postulate that this was due to acetylation of cyclooxygenase in the portal circulation and that inhibition of peripheral cyclooxygenase may be spared.

Storage of platelets for tests of platelet function: effects of pH on platelet aggregation and liberation of beta-thromboglobulin

Platelet aggregation responses are influenced by conditions of storage of platelet-rich plasma (PRP). The aim of the present study was to further define the necessity for pH control during storage of PRP for tests of platelet function. Aliquots of citrated PRP were maintained at different pH levels by alteration of the CO2 content of the atmosphere in an incubation chamber. At intervals over 2-2 1/2 hours, plasma beta-thromboglobulin and 14C-serotonin were measured as well as platelet aggregation induced by ADP and collagen. At each time a dose response curve was studied for aliquots stored at each pH level. When two aliquots were maintained at different pH levels in the range 6.85-7.90, there was a significant increase in aggregation at the higher pH, even when the pH difference was as small as 0.2 units. In this range, pH did not influence the rate of deterioration of the aggregation response, but when pH was above 8.0, there was marked deterioration of the response. Increased pH was associated with an increase in plasma levels of beta-thromboglobulin and 14C-serotonin, which was more marked when pH was above 8.0. It appears that increases in pH are harmful to platelets and even small pH changes should be avoided during storage of platelet-rich plasma for tests of platelet function.

Effect of EDTA and PGE1 on beta-thromboglobulin liberation from platelets

The aim of this study was to evaluate the effect of PGE1 and EDTA on liberation of beta-thromboglobulin (beta TG) from platelets in vitro. Liberation of beta TG was followed in citrated blood at room temperature for 120 minutes after venesection. PGE1 reduced beta TG liberation, and maximal inhibition was attained by concentrations greater than 2 X 10(-6)M. EDTA induced the efflux of beta TG. This EDTA-induced efflux was delayed but not prevented by PGE1 and by citrate; it was not found at 0-4(0)C. Therefore the use of EDTA to prevent beta TG liberation during sampling for in vitro or in vivo studies depends heavily on modifying factors such as PGE1 and low temperature, and on the time taken to process samples. Its effectiveness must be in some doubt where the platelets may be sufficiently stimulated to overcome these modifying influences, or where handling of samples is less than optimal.