EFFECTS OF THROMBIN ON THE POTASSIUM AND ATP CONTENT OF PLATELETS

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Size-controlled synthesis of granular polyphosphate nanoparticles at physiologic salt concentrations for blood clotting

Size-controlled granular polyphosphate (PolyP) nanoparticles were synthesized by precipitation in aqueous solutions containing physiological concentrations of calcium and magnesium. We demonstrate using dynamic light scattering (DLS) that the solubility is correlated inversely with PolyP chain length, with very long chain PolyP (PolyP1000+, more than 1000 repeating units) normally found in prokaryotes precipitating much more robustly than shorter chains like those found in human platelet dense granules (PolyP80, range 76-84 repeating units). It is believed that the precipitation of PolyP is a reversible process involving calcium coordination to phosphate monomers in the polymer chain. The particles are stable in aqueous buffer and albumin suspensions on time scales roughly equivalent to catastrophic bleeding events. Transmission electron microscopy images demonstrate that the PolyP nanoparticles are spherical and uniformly electron dense, with a particle diameter of 200-250 nm, closely resembling the content of acidocalcisomes. X-ray elemental analysis further reveals that the P/Ca ratio is 67:32. The granular nanoparticles also manifest promising procoagulant effects, as measured by in vitro clotting tests assaying contact pathway activity.

A patch-clamp study of mammalian platelets and their voltage-gated potassium current

1. Mammalian platelets were freshly isolated from human, rabbit, or rat blood. The whole-cell and cell-attached voltage-clamp variations of the patch-clamp technique were employed to study the passive electrical properties and ion channels of unstimulated platelets. 2. The input capacitance of a platelet measured by the phase-sensitive detection method was about 128 fF, the input resistance of a platelet was about 59 G omega and the resting membrane potential was about -50 mV which was directly measured by a whole-cell recording in the current-clamp mode. 3. The predominant ion channel was a voltage-gated K+ channel resembling the delayed rectifier K+ channel of nerve, muscle and T-lymphocyte. There was no indication of any inward current in the platelet membrane. The activation of the K+ current could be fitted by n4 kinetics, and was half-maximal at about -35 mV. 4. The time constant of K+ current inactivation was virtually independent of voltage and varied from cell to cell. Recovery from inactivation was slow and dependent on the size and duration of the preceding conditional voltage step. Steady-state inactivation was half-maximal at about -50 mV and was complete at positive potentials. 5. The predominant single-K+-channel conductance was 9 pS and the estimated number of K+ channels per platelet was about 325, corresponding to a density of 25/micron2 apparent membrane area.

Elemental composition of the dense bodies of rat platelets determined by electron probe x-ray microanalysis of freeze-substituted sections

A semiquantitative x-ray microanalytical study was made of dense bodies in rat and human blood platelets prepared by freeze substitution. After the freeze substitution, electron dense bodies containing 5-hydroxytryptamine(5-HT) were well preserved. Electron probe x-ray microanalyses clearly defined species differences in the cations of dense bodies of rat and man. Direct calculation of elemental concentration ratios demonstrated a high level of Mg, with lesser amounts of K and Ca in rat, while there was a negligible level of Mg in man but a high amount of Ca. These results indicate that freeze-substitution is a suitable preparation for the study of diffusible electrolytes which may open the way to quantitative analysis using appropriate standards for analytical electron microscopy of resin sections.

Separation of platelet-rich plasma by modified centrifugal elutriation

The Beckman centrifugal elutriation (CE) system is modified for the separation of platelet-rich plasma (PRP) from human and rabbit blood. The Beckman separation chamber is found inadequate for this purpose, and a new chamber of conical shape has been developed. Optimal flow rate for separating PRP from whole blood with the new chamber of 11.3 ml capacity is 3.5 ml/min while centrifuging at 2500 rpm or 700 g. Collection process takes about 4 minutes. This new process is based on the principle of centrifugal counterflow displacement and filtration, and is different from the CE process which is based on counterflow centrifugation and differential elution. About 90% of total platelets are recovered with this new process. The collected samples are free of leucocytes and contained only a few erythrocytes. Platelets collected by either differential centrifugation (DC) or new procedures are found to be similar in morphological characteristics, both being discoidal. Other characteristics such as aggregation response induced by ADP or epinephrine, serotonin-14C secretion, and survival of autologous platelets in rabbits are also found to be similar. However, ATP release induced by ADP is consistently higher from platelets prepared by our procedure than from those prepared by the DC procedure.

Movement of sodium into human platelets induced by ADP

1. In normal human platelets the concentrations of Na+ and K+ were 42.1 +/- 4.3 and 98.8 +/- 3.7 mequiv/l of platelet water respectively (mean +/- S.E. of 22 samples). 2. When platelet-rich plasma was incubated with 22Na+ at 37 degrees C for 2-3 h an increase in platelet Na+ concentration was found which was significant after 210 min. Platelet K+ concentration did not change significantly. The platelet 22Na+ radioactivity increased faster than did the total Na+ suggesting a Na+o-Na+ exchange process in unactivated platelets. 3. Addition of ADP to platelet-rich plasma resulted in platelet aggregation and a rapid rise (within seconds) in 22Na+-radioactivity within the platelets and after 300 s this increase diminished toward control levels. 4. Under the same experimental conditions, ADP did not bring about an increase of 36Cl- in the platelets. 5. Ouabain (10-(6) M) added to platelet-rich plasma induced an increase in Na+ concentration and 22Na+ radioactivity in the platelets, as well as a decrease in K+ concentration. ADP produced a further increase in 22Na+, which did not return toward control values, in the presence of ouabain. 6. The association of an increase in 22Na+ but not of 36Cl- accompanying aggregation by ADP suggests a selective mechanism for the movement of Na+ into platelets rather than a movement of NaCl together with water under an osmotic gradient.

Platelets in early hyperacute allograft rejection in kidneys and their modification by sulfinpyrazone (Anturan) therapy. An experimental study

Experiments were carried out on mongrel dogs to study the early ultrastructural changes in hyperacute rejection of kidney. Renal allografts were performed on dogs presensitized by three to five skin grafts. The animals were grouped in pairs, control and treated with sulfinpyrazone, a platelet inhibitor. The electron microscopic studies showed that the earliest change in the kidney was aggregation of platelets mainly in the peritubular capillaries and a few glomeruli. Later, platelets showed degranulation with vascular damage leading to rejection of kidney. Treated dogs showed less tendency for platelets to aggregate, and graft survival was prolonged. The role or platelets in the hyperacute rejection is discussed.

Effect of thrombin on the radioactive nucleotides of human washed platelets

Radioactive ATP and ADP were found in platelets after incubation of human platelet-rich plasma with either [8-(14)C]adenosine or [8-(14)C]ADP. Treatment of the labelled and washed platelets with thrombin indicated that, though considerable amounts of ATP and ADP were released to the supernatant, radioactive ATP and ADP remained predominantly in the cellular fraction. Breakdown of radioactive ATP took place to form mainly IMP and hypoxanthine, the latter compound appearing in the supernatant. The results indicate the presence of at least two pools of nucleotide in platelets. Evidence is given that the two pools contain approximately the same amounts of ATP plus ADP, and that the ratio of ATP to ADP in the pool released to the supernatant by the action of thrombin is about 0.7-0.8.

The effect of 5-hydroxytryptamine on the potassium exchange of human platelets

1. The concentration of potassium in platelets freshly isolated from nine persons was 74 +/- 4.8 mumole/10(11) platelets (mean +/- S.E.M.).2. The influx of potassium into platelets from citrated plasma at 37 degrees C, estimated with the use of (42)K, was 18.3 +/- 1.6 n-mole/sec. 10(11) platelets.3. When plasma contained 5-hydroxytryptamine (5-HT) at a concentration of about 5 mug/ml., the influx of potassium into platelets was increased for as long as they took up 5-HT. This effect was absent when the concentration of 5-HT was either lower (2 mug/ml.) or much higher (50 mug/ml.).4. The concentration of 5-HT in the platelets did not affect their potassium flux.