Modulation of noradrenaline release through activation of presynaptic beta-adrenoreceptors.

On peripheral noradrenergic nerve endings there exist beta-adrenoreceptors activation of which results in an enhanced release of noradrenaline in response to nerve stimulation. These presynapatic beta-adrenoreceptors do not appear to be activated by neuronally-released noradrenaline. However, adrenaline may be a physiological activator during enhanced adrenomedullary secretion. Adrenaline can also be incorporated into the noradrenergic transmitter stores and be released as a co-transmitter. Under these conditions presynaptic beta-adrenoreceptors may be activated by neuronally-released adrenaline, thus forming a 'positive feedback loop'. The release of adrenaline from the adrenal medullae may also be modulated through facilitatory beta-adrenoreceptors, but the release of noradrenaline from noradrenergic nerves in the central nervous system is not. The facilitatory presynaptic beta-adrenoreceptors appear to be in the main of the beta 2-subtype although precise receptor characterization has not been carried out. Increased activation of presynaptic beta-adrenoreceptors by adrenaline may be implicated in the development of essential hypertension. Part of the antihypertensive action of beta-adrenoreceptor blocking drugs may be due to blockade of these facilitatory presynaptic beta-adrenoreceptors.