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

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Abstract
The effects of i.v. 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. I.v. administration of nitroprusside (1-10 .mu.g/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 (> 100 .mu.g/kg) of nitroprusside produced positive chronotropic and inotropic effects in isolated atria, which were inhibited completely by propranolol. Direct intraarterial injection of nitroprusside (10-1000 .mu.g) had no significant chronotropic and inotropic effects in isolated atria, although acetyl-choline (0.01-0.1 .mu.g) induced negative chronotropic and inotropic effects. Apparently, nitroprusside has no direct cardiac effect it causes a release of catecholamines in doses which produce a profound hypotension.