A further study on constrictor responses of dog coronary arterial strips to acetylcholine under potassium-contracture.

Experiments were designed to clarify what mechanisms are involved in the contraction of dog coronary artery induced by acetylcholine. Helical strips cut from the left circumflex coronary artery were suspended in a bath filled with oxygenated Krebs-Ringer solution at 37 degrees C, and tension developments were recorded isometrically. During potassium-contracture acetylcholine produced further contractions of the strips, which were significantly depressed by atropine together with phentolamine or atropine together with d-tubocurarine. In the strips prepared from dogs in which 6-hydroxydopamine was administered 2 hrs before isolation of hearts, acetylcholine-induced contraction was suppressed by atropine alone. Norepinephrine produced contractions in these strips, which were blocked by phentolamine. Results indicate that acetylcholine may directly constrict the artery through a muscarinic action and simultaneously produce norepinephrine release from sympathetic nerve endings by acting on presynaptic nicotinic receptors resulting in the contraction of the artery.