Selected aspects of presynaptic modulation of noradrenaline release from the heart.

This article presents selected aspects of the presynaptic modulation of noradrenaline release from the heart. Several presynaptic effector systems that mediate either inhibition or facilitation of noradrenaline release are discussed. In particular, the potencies of muscarinic agonists and antimuscarinic drugs in muscarine receptor-mediated inhibition are reviewed. Evidence is presented that presynaptic inhibitory muscarine receptors can apparently not be differentiated from postsynaptic muscarine receptors in the heart. Presynaptic inhibitory alpha-adrenoceptors belong to the alpha 2 subtype. The physiological role of autoinhibition by the endogenous transmitter in the heart is discussed in relation to the influence that important clearance mechanisms have to reduce the concentration of noradrenaline in the synaptic cleft under physiological conditions. Inhibitory presynaptic dopamine receptors differ from postsynaptic receptors, but evidence of a physiological role for presynaptic receptors is lacking. The significance of transsynaptic modulation of noradrenaline release from the heart by prostaglandins and adenosine is briefly discussed. Opioid receptors appear to inhibit noradrenaline release in the heart to some extent. Finally, several other receptor systems, possibly involved in the regulation of transmitter release from sympathetic nerves in the heart, are mentioned.