Analysis of cardiac actions of nitroprusside in intact dogs and in isolated atria.

The effects of intravenous administration of sodium nitroprusside to a donor dog on cardiac pacemaker activity and contractility were investigated in a cross-circulated dog heart preparation; i.e., the isolated atrial muscle was perfused with blood from a pentobarbital anesthetized donor dog. Intravenous administration of nitroprusside (1-10 micrograms/Kg) caused a hypotension with tachycardia, and occasionally with bradycardia. This effect was blocked by atropine injections into the donor dog, but no significant chronotropic and inotropic changes were observed in the isolated atria. Larger doses (over 100 micrograms/Kg) of nitroprusside produced positive chronotropic and inotropic effects in isolated atria, which were inhibited completely by propranolol. Moreover, direct intraarterial injection of nitroprusside (10-1,000 micrograms) had no significant chronotropic and inotropic effects in isolated atria, although acetylcholine (0.01-0.1 micrograms) induced negative chronotropic and inotropic effects. From these results, it is concluded that nitroprusside has no direct cardiac effect and that it causes a release of catecholamines in doses which produce a profound hypotension.