Do presynaptic opiate receptors and alpha-adrenoceptors alter acetylcholine release from a sympathetic ganglion by a similar mechanism?
The present experiments tested the possible involvement of a calcium-sensitive mechanism in the alpha-adrenoceptor- and opiate receptor-mediated inhibition of acetylcholine release from the cat superior cervical ganglion. First, the calcium-dependence of evoked acetylcholine release was measured in the presence and absence of the alpha-adrenoceptor agonist noradrenaline or of the opiate receptor agonist [Met5]enkephalin-Arg6-Phe7. When ganglia were perfused with Krebs medium containing [Ca2+] = 2.4, 1.2, 0.6, 0.2 mM, evoked release of acetylcholine was depressed by both agonists and the inhibition increased with reduced levels of extracellular Ca2+; this was especially evident when calcium in the medium was reduced to 0.2 mM. Second, the effects of both noradrenaline and [Met5]enkephalin-Arg6-Phe7 on calcium influx into presynaptic nerve endings was determined by measuring the accumulation of 45Ca into ganglia in the presence and absence of either drug. Both agonists reduced the stimul