title: The role of inhibition of nitric oxide synthesis in the aggregation of platelets due to the stimulated production of thromboxane A2.
abstract: The aggregation of platelets by ADP is reported to be mediated through prostaglandin synthesis. In contrast, nitric oxide is known to inhibit platelet aggregation through the synthesis of cyclic AMP and cyclic GMP. Studies were conducted to determine the role of ADP, if any, on the synthesis of nitric oxide in platelets. Both normal male and female volunteers between the ages of 30 and 45 years participated in the study. Thromboxane A2 (TXA2) was measured as thromboxane B2 by ELISA. Nitric oxide was measured by methhaemoglobin method. It was found that the treatment of platelet-rich plasma (PRP) with different concentrations of ADP (0-8.0 μmol/l) resulted in increased platelet aggregation, and at 8.0 μmol/l ADP, the basal nitric oxide level was found to be maximally decreased from 0.3 ± 0.10 nmol/10 platelets to 0 nmol/10 platelets in PRP (P < 0.0001; n = 10). Line-weaver-Burk plot of nitric oxide synthase (NOS) activity in the presence of 2.0 μmol/l ADP reduced the Vmax from 6.662 to 2.22 nmol nitric oxide/h per mg protein compared with control. Inhibition of nitric oxide synthesis by N-methyl-L-arginine acetate ester (L-NAME), an inhibitor of NOS, was found to aggregate platelets due to the reduction of platelet nitric oxide level (Pearson's coefficient of correlation, r =  -0.986, P < 0.001, n = 10). The treatment of PRP to L-NAME was found to increase TXA2 synthesis to 1.679 ± 0.05 from 0 pmol/10 platelets. These results suggested that inhibition of NOS in platelets resulted in platelet aggregation through TXA2 synthesis in PRP through a novel pathway.
authors.affiliation: aDepartment of Physiology, Rammohan College (University of Calcutta) bSinha Institute of Medical Science & Technology, Kolkata, West Bengal, India.
authors.forename: Debipriya
authors.initials: D
authors.lastname: Banerjee
authors.forename: Sahana
authors.initials: S
authors.lastname: Mazumder
authors.forename: Asru K
authors.initials: AK
authors.lastname: Sinha
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000328:Adult; D001792:Blood Platelets / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000096:biosynthesis; D006152:Cyclic GMP / Q000096:biosynthesis; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009569:Nitric Oxide / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D010974:Platelet Aggregation / Q000187:drug effects; D013928:Thromboxane A2 / Q000819:agonists / Q000096:biosynthesis*
chemical_list: D004791:Enzyme Inhibitors; D009569:Nitric Oxide; D013928:Thromboxane A2; D000244:Adenosine Diphosphate; D000242:Cyclic AMP; D019001:Nitric Oxide Synthase; D006152:Cyclic GMP; D019331:NG-Nitroarginine Methyl Ester