Responses of the three-toed sloth, Bradypus tridactylus, to some commonly used pharmacologic agents. I. Autonomic drugs.

1. Sloths are very responsive to epinephrine and norepinephrine; i.v. injection of 1 microgram/kg elevates systolic pressure 80 and 90% respectively. 2. Doses as low as 0.01 microgram/kg of epinephrine as well as norepinephrine raise diastolic pressure. 3. Similarity of effects of these catecholamines can be explained on the basis of the low proportion of skeletal muscle (35 vs 45% in most mammals) and a small liver which reduces the proportion of beta 2 dilators to alpha constrictors responding to epinephrine. 4. Slowness of reflexes allows clear separation of early (0-20 sec), direct accelerating heart rate effect (up 15% with 1 microgram/kg of norepinephrine) and later (20-60 sec), reflex bradycardia (down 30% from control level). 5. Sloths are more sensitive to the vasodilating effects of isoproterenol or less sensitive to beta 1 cardiac stimulating effects than most laboratory mammals; doses as low as 0.1 microgram/kg cause a fall in mean arterial pressure not overcome by increased heart rate.