Effect of timolol on the sympathetic nervous system in coronary occlusion in cats

Clinical studies have shown that treatment with chronic timolol after a myocardial infarction decreases the incidence of reinfarction and mortality. It also limits the size of infarction when it is given IV during the acute phase of an infarction and followed by chronic oral dosing. It has been postulated that beta blockers work not only by a direct mechanism on the heart, but by altering neural discharge to the heart and depressing the sympathetic overactivity seen in 35% of patients immediately after an infarction. Our study examined the effect of timolol on sympathetic cardiac neural discharge, blood pressure, heart rate, and rhythm during acute coronary occlusion produced by ligation of the left anterior descending artery 10 to 14 mm below its origin in alpha-chloralose-anesthetized cats. Timolol or normal saline 5 mg/kg IV was given five minutes post coronary occlusion. The infusion of timolol significantly decreased the mean arterial blood pressure and heart rate. Sympathetic cardiac neural discharge in the group treated with timolol five minutes post coronary occlusion did not differ from that in the saline group. Thus there was a nonuniform (an increase, a decrease, or no change) sympathetic cardiac neural discharge associated with the production of occlusion-induced arrhythmia. These data suggest that the action of timolol on the sympathetic cardiac neural discharge is not its major mode of protection.