Levels of cAMP and cGMP in platelets treated with NO, adenosine, or milrinone. Effect of milrinone on the thrombin-induced shape change. (A) [8-¹⁴C]guanine-prelabeled or [U-¹⁴C]adenine-prelabeled, aspirin-treated, and gel-filtered human platelets in the presence of CP/CPK were exposed to NO (2.1 × 10^-10 mol NO/mL/min) or adenosine (10 μM) for 1 minute, and to the phosphodiesterase inhibitor milrinone (10 μM) for 5 minutes. The agents were present alone or in combinations. Values are presented as means ± SEM (n = 3-11). Determination of cyclic nucleotides is described in “Materials and methods.” (B) The shape change induced by thrombin (0.01 U/mL) was studied as described in the legend to Figure 1, except that various concentrations (0, 1, 2, 5, and 10 μM) of milrinone were present for 5 minutes before the addition of thrombin or were added afterward as indicated. ^*P < .05 versus control (or as indicated). †P = .07 versus control.