1. In platelets incubated in vitro, glycogen phosphorylase was activated, and the degree of activation roughly corresponds to the degree of manipulation of the platelets and is greatest in the least physiologic incubation medium.

2. Activation proceeds with an increasing proportion of enzyme which is active in the absence of AMP, suggesting the action of the phosphorylase-activating enzyme, phosphorylase kinase, in platelets.

3. Total enzyme activity (activity with AMP) likewise increases and continues to increase while the AMP-independent fraction begins to decrease again, suggesting more than one activating mechanism.

4. During glycogen breakdown, and during coagulation and clot retraction induced by thrombin, glycogen synthesis was shown to continue in vitro. Clot retraction was found to be more dependent on adequate glycogen stores than was coagulation.

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