Modulation of noradrenaline release by activation of presynaptic beta-adrenoceptors in the cardiovascular system.

Peripheral sympathetic nerve terminals in many tissues, but not all, are endowed with beta-adrenoceptors. Activation of these result in an enhancement of noradrenaline release evoked by electrical nerve stimulation. These so-called presynaptic beta-adrenoceptors are possibly located on the outer surface of the varicosity of the noradrenergic nerves. A postsynaptic location, however, is also a possibility. The presynaptic beta-adrenoceptors appear to be of the beta 2-adrenoceptor subtype. However, specific classification is lacking. The stereospecificity of the beta-adrenoceptors is controversial. These receptors are not activated by noradrenaline released from sympathetic nerves. Adrenaline derived from the adrenal medulla may be the physiological activator. Either circulating adrenaline or adrenaline taken up by sympathetic nerve terminals and then released as a cotransmitter with noradrenaline activates the presynaptic beta-adrenoceptors. In the latter case, a "positive" feedback" loop may be formed.