Dense granule

Dense granules (also known as dense bodies or delta granules) are also specialized secretory organelles. They contain adenine nucleotides (ADP and ATP), ionized calcium, histamine, serotonin, and epinephrine.

It accomplishes this by inducing the conformational change of platelets' GpIIb-IIIa receptor so that they can bind fibrinogen. Fibrinogen acts as a crosslink between the platelets allowing aggregation. ADP also induces other platelets to degranulate potentiating coagulation. Furthermore, ADP in conjunction with thromboxane A2 and thrombin cause platelet contraction and the formation of a secondary hemostatic plug. Endothelial cells express adenosine dephosphatase which acts as an anti-coagulant by breaking down ADP and preventing the actions discussed above.

Other molecules within platelet dense granules include adenosine triphosphate (ATP), ionized calcium which is necessary for several steps of the coagulation cascade, histamine which acts as a vasodilator, seratonin, and epinephrine.

Apicomplexa protozoans have dense granules.

Summary -- dense granules contain: ADP/ATP, calcium, serotonin.