Figure 4.
Mechanism of action of β blockers. The principal agonist at β adrenergic receptors is noradrenaline (norepinephrine) released from sympathetic nerve endings. The heart has both β1 and β2 receptors on the postsynaptic side of the cleft; β2 receptors are activated mainly by circulating adrenaline. There are also α and β presynaptic receptors on the nerve ending, which mediate respectively inhibition and facilitation of noradrenaline release. The presynaptic α2 receptors are of a different class from the postsynaptic α1 receptor, which is responsible mainly for vasoconstriction of arteries. Available β blockers vary in how many of the receptors shown in the diagram are blocked, including some mixed α1 and β blockers.