Acetylcholine acts on the different components of the coronary arterial wall by 1) initiating endothelium-dependent relaxation of the smooth muscle cells; 2) inhibiting the exocytotic release of norepinephrine (NE), which could result in either vasodilator or vasoconstrictor effects depending on whether the main action of NE is alpha- or beta-adrenergic, respectively; and 3) activating the contractile process of the smooth muscle cells. These different effects of the cholinergic transmitter are muscarinic in nature. Their relative importance varies among species, or when acetylcholine is given exogenously rather than released from cholinergic nerves.