Platelets and Clot Retraction

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Abstract
The effect of Ca2+, Mg2+, Mn2+ and some drugs known to influence smooth muscle behaviour on human and rat whole blood and PRP clot retraction was studied. The force of the retraction was measured by a dynographic apparatus using electrical transduction. Clot retraction was studied by thrombelastographic method as well. The results obtained by using EDTA and EGT A show that both Ca2+ and Mg2+ are necessary for the normal development of clot retraction. They suggest the coexistence in the platelets of an accessible calcium fraction and a fraction not readily accessible to chelating agents, which is released following the calcium chelation in the first fraction. MnCl2 inhibits clot retraction and induced a quick, complete and irreversible relaxation of fully contracted clots. It is concluded that there are some analogies between blood clots and smooth muscle. The absence of any effect of adrenaline, noradrenaline and ADP, which strongly modify smooth muscle contraction, is not surprising. This is probably because clotting caused by thrombin-triggered platelets has already produced a maximal clot retraction.