Presynaptic muscarinic receptor subtypes involved in the inhibition of acetylcholine and noradrenaline release in bovine cerebral arteries.
Experiments were performed in bovine cerebral arteries preincubated with [3H]-choline or [3H]-noradrenaline to analyze the presynaptic muscarinic receptors involved in inhibition of acetylcholine and noradrenaline release induced by electrical stimulation (4 Hz, 200 mA, 0.3 ms, 1 min). For this purpose, the actions of several muscarinic receptor antagonists on the 3H overflow and on the carbachol-induced inhibition of this overflow were assessed. The evoked [3H]-acetylcholine release and [3H]-noradrenaline release were markedly reduced by the presence of tetrodotoxin, Ca(2+)-free medium, and the inhibitor of both choline transport and choline acetyltransferase, AF64A. Chemical sympathetic denervation with 6-hydroxydopamine (6-OHDA) decreased the uptake of [3H]-noradrenaline, and AF64A reduced mainly the uptake of [3H]-choline, but also of [3H]-noradrenaline. Carbachol reduced the evoked [3H]-noradrenaline and [3H]-acetylcholine release; the IC50 values were 0.37 and 0.43 mumol/l, respectively. Atropine and 4-DAMP, but not AF-DX 116, methoctramine or pirenzepine, increased the evoked [3H]-acetylcholine release. However, these muscarinic antagonists failed to modify the evoked [3H]-noradrenaline release. Carbachol inhibited the release of both acetylcholine and noradrenaline. The inhibition was blocked by the antagonists. The rank orders of potency (based on plC50 values) were, in the case of [3H]-acetylcholine release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to pirenzepine greater than or equal to methoctramine, and, in the case of [3H]-noradrenaline release, atropine greater than 4-DAMP greater than AF-DX 116 greater than or equal to methoctramine greater than or equal to pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)