We have come a long way since 1960 when Hellem observed that a small molecule lost from red cells caused platelets to adhere to glass [1]. During the next year, Ollgaard [2] showed that this small molecule caused platelet aggregation, and it was identified as adenosine diphosphate (ADP) in Owren’s laboratory [3]. At that time, Marjory Zucker was grading ADP-induced aggregation visually as 1–4 after shaking citrated platelet-rich plasma by hand for 1 min [4]. She had been carrying out research on platelets since 1947 and was already a major figure in the field. The key development of the aggregometer in Gustav Born’s laboratory in 1962 provided a light transmission technique for assessing and recording the rate and extent of aggregation that is still widely used today (light transmission aggregometry [LTA]) [5]. In the next few years, Born and his colleagues used the aggregometer in detailed investigations of the changes in platelets during ADP-induced aggregation, and inhibitors of this process [6]. Much of the early work was well reviewed in the classic book The Physiology of Blood Platelets, subtitled Recent Biochemical, Morphologic and Clinical Research, that was written by Aaron Marcus and Marjorie Zucker in 1965 [7]. In 1970, another major review summarized the developments in the 1960s [6]. It had become obvious that ADP plays an important role in hemostasis and thrombosis. The much more recent findings that platelets possess two P2Y receptors (P2Y1 and P2Y12) for ADP, and a P2X1 receptor for ATP have made it possible to understand the reactions responsible for many of the early observations [8]. Our present knowledge about ADP-induced platelet activation is attributable to the work of thousands of investigators and this historical review can mention only some of them. In the late 1950s and early 1960s, several groups of investigators carried out in vitro experiments showing that thrombin or collagen caused platelet aggregation and that ADP was released during this process [9–13]. In vivo, ADP and ATP do not normally circulate in the plasma, but they are stored in the dense granules of the platelets. During the formation of hemostatic plugs or arterial thrombi, platelets are stimulated by collagen and thrombin to release the contents of these platelet storage granules. In vitro, at a normal platelet count of 250,000/μl, the concentrations of ATP and ADP in plasma immediately after release of granule contents induced by collagen or thrombin have been reported in the ranges of 4–7 μM for ATP and 3–4 μM for ADP [14, 15]. The released ADP adds to the response of platelets to the other aggregating agents. In addition to causing aggregation, the effects of ADP on platelets include shape change, refractoriness, potentiation of the effects of other aggregating agents, inhibition of platelet adenylyl cyclase, increase in cytosolic free calcium, and activation of specific receptors that stimulate intracellular signaling pathways that converge on the cytoplasmic domain of the integrin αIIbβ3 (glycoprotein (GP) IIb–IIIa), leading to its becoming able to bind extracellular fibrinogen and von Willebrand factor [16, 17].
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Jan 1, 2013
Manoj Kumar Jaiswal 
Open Access
