Effect of timolol on platelet aggregation in coronary heart disease

The immediate effect of beta-blockers versus placebo in platelet function was studied in ten healthy men one hour after either 40 mg propranolol, 5 mg timolol or placebo was given. Both beta-blockers had similar platelet effects. They increased the ADP-threshold in a platelet aggregation test and reduced the plasma level of beta-thromboglobulin whereas the primary aggregation remained unchanged. Forty-four patients were given timolol 10 mg twice daily or corresponding placebo medication in a secondary prevention trial after a myocardial infarction. At rest and also immediately after a near maximal exercise test the platelet function was similar on timolol and placebo. During exercise a lower ADP-threshold and a higher plasma level of beta-thromboglobulin was observed irrespective of whether the patients received placebo or timolol. Thus, the acute platelet inhibiting effect of timolol observed after a single dose disappeared during chronic timolol treatment. During 24 hours continuous ambulatory ECG monitoring patients on timolol treatment had significantly less arrhythmias than patients on placebo. A general effect on platelets cannot explain this antiarrhythmic effect of timolol during chronic therapy.