Platelet Function Testing: Platelet Nucleotide Assays
There are two separate nucleotide pools within platelets. 60% is stored within the dense granules and is not metabolically active and the remaning 40% constitutes the metabolic pool and provides the platelet with energy for its various activitie.
There are differences in the relative concentrations of ADP and ATP in these two pools and exchange between the two pools is very slow:
Metabolic Pool: ATP:ADP ratio is 8:1
Dense Granules: ATP:ADP ratio is 2:3
|Dense Granules||ADP [ADP is concentrated in dense granules]
Dense granule release:
When platelets adhere to the damaged vascular endothelium, this leads to:
→ Activation of platelets through various intracellular signaling mechanisms
→ Release of alpha and denise granule contents including ADP and serotonin both of which lead to platelet activation
→ Generation and release of TxA2 [which binds to the Tx receptor]
→ Activation [conformational change in the GpIIb/IIIa receptor] facilitating formation of the ‘tenase’ and ‘prothrombinase’ complexes
→ Exposure of anionic phospholipid - which allows formation of the 'tenase and 'prothrombinase' complexes
→ Generation of procoagulant microvesicles
In Storage Pool Disorders [SPDs] in which there is a reduction or absence of the dense bodies - this leads to a reduction in the total amount of ATP and ADP and a marked increase in the ratio of ATP to total platelet ADP
Storage Pool Disorder
|Dense granule deficiency||Hermansky-Pudlak Syndrome – rare although amongst Puerto Ricans has a prevalence of 1:800
Idiopathic Dense Deficiency
Characteristic Features of dense granule deficiency:
1. Light Transmission Aggregometry:
Low dose ADP: Absent second wave aggregation [i.e. first wave only] with weak agonists. High concentrations of ADP elicit full and irreversible aggregation.
Adrenaline: Absent second wave aggregation [i.e. first wave only] with adrenaline. However this also occurs in 10-15% of healthy individuals.
Collagen: Delayed and impaired response to collagen
Arachadonic Acid/Ristocetin: Impaired response to arachadonic acid and ristocetin
NB. LTA is relatively poor at detecting patients with SPD and measurement of platelet nucleotides, or Lumiaggregometry may be a more sensitive method for detecting these cases.
2. Marked reduction in both the content of ADP and the ADP:ATP ratio or an absence of ATP release [remember dense granule release is a major amplification pathway for platelet activation and is necessary for the sustained activation of the GpIIb/IIIa receptor]
3. Decreased numbers of or absence of dense granules on electron microscopy.
4. Confirmation of dense granule deficiency can also be made rapidly by detection of a reduction in mepacrine-labelled granules in platelets by flow cytometry.
NB. Disorders of platelet secretion and signal transduction can give similar LTA findings to SPD but platelet granules are present in normal numbers in these disorders – the problem lies with the release of the granule contents. Granule release is essential for second wave platelet aggregation and so whether the granules are deficient or not released properly – the effects seen in LTA are the same.
|Acquired Disorders||Myeloproliferative disorders [MPD]
Cardio-pulmonary bypass [CPB]
Thrombotic Thrombcytopenic Purpura [TTP]
Haemolytic Uraemic Syndrome [HUS]
Measuring Platelet Nucleotides
|Lumiaggregometry||Lumiaggregometry is a modification of light transmission aggregometry which measures ATP release from the dense granules, It is based on a bioluminescent determination of ATP in which the ATP reacts with luciferin and luciferase [firefly extracts] resulting in light emission.
For measurement of aggregation, the lumiaggregometer uses an LED which emits light in the infrared range and changes in light transmission are detected by a phototransistor. For the measurement of luminescence resulting from ATP secretion, it uses a photomultiplier tube located at right angles to the light path of the LED.
Luciferin + ATP → Luciferyl Adenylate + Inorganic phosphate [PPi]
Luciferyl Adenylate + O2 → Oxyluciferin + AMP + LIGHT
Measurement of platelet secretion using the luciferin-luciferase reaction can also be evaluated with whole blood aggregometry.
|Electron Microscopy:||Dense granules are normally easily visible on EM and in Dense granule deficiency their absence is obvious .|
|HPLC||Measuring platelet ADP and ATP content: Can be measured in a number of ways including HPLC.|
|Platelet dense granule release||Can also be measured by a 14C serotonin release assay.|
|Total platelet nucleotide content||5.5 - 9.6 nmol/108 platelets|
|ATP content of platelets||3.5 - 5.9 nmol/108 platelets|
|ADP content of platelets||1.9 - 3.8 nmol/108 platelets|
|Ratio||1.3 - 2.0
[>2 = abnormal]
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