Prostaglandin Endoperoxide – Thromboxane Synthesis and Dense Granule Secretion as Positive Feedback Loops in the Propagation of Platelet Responses during “The Basic Platelet Reaction”

Platelets respond to a great variety of stimuli by a sequential display of shape change, aggregation, prostaglandin/thromboxane synthesis--dense granule secretion and alpha-granule secretion. It is suggested that these responses are independent of each other, and caused by an increase in the concentration of a second messenger, liberated to the cytoplasm through the interaction between an extracellular agonist and the platelet membrane. The extent of the propagation of responses is determined by the strength of the stimulus. Stimuli can be subdivided into 1) original, applied stimuli and 2) platelet-produced stimuli (substances secreted from dense granules, prostaglandins and thromboxanes); these stimuli may act synergistically. In this way the platelet has two apparently independent means of potentiating their response to external stimuli which act as two separate positive feedback loops.