Mechanisms of facilitation and muscarinic or alpha-adrenergic inhibition of acetylcholine and noradrenaline secretion from peripheral nerves.

The muscarinic acetylcholine (ACh) receptor from rat brain has been partially purified and characterized. The molecular weight of the smallest subunit of the receptor protein obtained by a chaotropic agent is 30,000 dalton, while the protein found in sodium dodecyl sulphate polyacrylamide gels has a molecular weight of 80,000 dalton. The isoelectric point is in the range 4.5 to 5.1. The low affinity dissociation constant for agonists of the alpha-adrenoceptor from rat and guinea-pig brain, and of the muscarinic ACh receptor from guinea-pig ileum longitudinal muscle-myenteric plexus preparation, is increased 2-3-fold at potassium concentrations of 80-100 mM. The transmitter stores of noradrenergic nerves of guinea-pig vas deferens were labelled by preincubation with 3H-noradrenaline (3H-NA), and those of cholinergic nerves of guinea-pig ileum myenteric plexus were labelled by preincubation with 3H-choline. The mechanisms underlying frequency dependent facilitation and presynaptic receptor mediated depression of transmitter secretion were studied. The secretion evoked by electrical stimulation required both invasion of the nerve terminals and activation of the depolarization-secretion coupling in varicosities, while that evoked by high potassium (40-110 mM) was due to direct depolarization varicosities. Blockade of presynaptic alpha-adrenergic and muscarinic autoreceptors by phentolamine and atropine, respectively, caused 4-5-fold greater increase in the secretion of labelled transmitter evoked by electrical than by potassium stimulation from noradrenergic and cholinergic terminals. This suggests that depression of transmitter secretion is exerted mainly by control of the invasion of nerve terminals, and only to a minor extent by control of depolarization-secretion coupling in invaded terminals. Autoinhibition of transmitter secretion involves regulation of calcium availability and is antagonized by facilitation. Endogenous cyclic nucleotides are not critically involved in either facilitation, or alpha-adrenergic or muscarinic autoreceptor mediated control of transmitter secretion in these nerves.