Nucleotide formation, platelets

Nucleotide formation from free purine bases is dependent on the availability of phosphoribosylpyrophos-phate (PRPP), the common substrate for both HGPRT and adenine phosphoribosyltransferase (APRT) (4). Vhereas human platelets do contain PRPP, the activity of PRPP synthetase in these cells has not been reported thus far (2). 

Dopamine can thus be added to the list of hormones and neurotransmitters which can stimulate or inhibit cyclic AMP formation, depending upon their tissue of action. Thus, while dopamine stimulates cyclic AMP formation in parathyroid cells, superior cervical ganglia, retina and striatal tissue (27, 58-61), it inhibits the accumulation of the cyclic nucleotide in cells of the intermediate and anterior lobes of the pituitary gland. Opposite effects on the cyclic AMP system are also found with LHRH which stimulates and inhibits cyclic AMP levels in the anterior pituitary gland (62) and ovary (63), respectively. Similarly, alpha-adrenergic agents show opposite effects on cyclic AMP formation in brain (64) and platelets (65). PGE, stimulates cyclic AMP formation in the anterior pituitary gland (62) while it inhibits the same parameter in fat cells (66). 

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