CARDIAC SLOWING EFFECTS OF CLONIDINE (ST-155) IN DOGS

¹ Merck Institute for Therapeutic Research, West Point, Pennsylvania

Clonidine was previously reported to lower blood pressure and heart rate in animals and man, presumably by a central action. In this report evidence is presented that in dogs clonidine is likely to reduce heart rate by a peripheral action. At 10 µg/kg i.v., clonidine was shown to reduce heart rate in dogs with the spinal cord sectioned at the level of the 2nd cervical vertebra. At 3 and 10 µg/kg i.v., clonidine antagonized pressor and positive chronotropic effects of a muscarinic ganglionic stimulant, McN A-343, and blocked cardiac acceleration caused by low-frequency electrical stimulation of right postganglionic cardiac sympathetic nerves. At a single dose of 0.1 µg administered into the sinus node artery, clonidine slowed the heart rate in open-chest dog preparations. At doses up to 50 µg, clonidine had no effect on the rate of contractions of isolated perfused dog hearts.

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