Inhibitory and excitatory muscarinic receptors modulating the release of acetylcholine from the postganglionic parasympathetic neuron of the chicken heart

Summary The effects of muscarinic receptor antagonists on ACh release were studied in the absence or presence of cholinesterase (ChE) inhibition using the isolated perfused chicken heart. Presynaptic inhibitory muscarinic autoreceptor were characterized by determining the potency of various antagonists to enhance ³H]-ACh release evoked by field stimulation (3 Hz, 1 min). The order of potencies was: (±)-telenzepine > atropine > 4-DAMP > silahexocyclium > pirenzepine > hexahydro-siladifenidol > AF-DX 116. The comparison with known pA₂ values for M₁-, M₂- and M₃-receptors revealed that the presynaptic autoreceptor meets the criteria of an M₁-receptor. Basal, not electrically evoked overflow of unlabelled ACh into the perfusate was caused by leakage release (non-exocytotic), as it was independent of extracellular Ca²⁺ . Muscarinic receptor antagonists failed to enhance basel overflow. In contrast, when ChE activity was inhibited by 10^–6M tacrine or pretreatment with 10^–4M DFP, the ACh overflow was partially Ca²⁺-dependent and was reduced by tetrodotoxine. Moreover, block of the inhibitory muscarinic autoreceptors by (±)-telenzepine or pirenzepine caused a several-fold enhancement of the ACh release. The potencies of these antagonists were identical to those found for the electrically evoked ³H]-ACh release. The rate of ACh release enhanced by ChE inhibition plus telenzepine corresponds to about 12% of the total ACh pool per min, which is about the maximum amount of ACh that is available for any kind of stimuli. The release was dependent on the presence of exogenous choline. Hence elevation of ACh release led to a correspondingly enhanced ACh synthesis. The dramatic enhancement of ACh release by the ChE inhibition in combination with a block of presynaptic muscarinic autoinhibition was not inhibited by (+)-tubocurarine but by atropine (10^–9 to 10^–7 M) or 10^–6 M telenzepine. It is concluded that basal release of ACh in the heart was due to non-exocytotic leakage release. Inhibition of ChE led to a marked stimulation of excitatory muscarinic receptors of the intrinsic parasympathetic neuron with a consecutive postganglionic release of ACh. The strong postganglionic excitation was obvious when the inhibitory muscarinic autoreceptors were selectively blocked. Of the two described muscarinic receptors found in the parasympathetic postganglionic neuron of the chicken heart only the inhibitory was classified as being M₁, whereas the subtype of the excitatory one is unlike M₁ and remains to be identified.Preliminary results have been presented at the Spring meeting of the German Pharmacological Society in 1992 (Brehm and Lindmar 1992) Correspondence to R. Lindmar at the above address