G protein modulation of voltage-sensitive muscarinic receptor signalling in mouse pancreatic acinar cells

Submaximal stimulation of mouse pancreatic acinar cells by acetylcholine (ACh) generates periodic Ca²⁺ responses sensitive to the membrane potential. Monitoring the muscarinic Ca²⁺ responses using patch-clamp whole-cell current recordings, we examined the mechanism of guanine nucleotide-binding protein (G protein)-receptor interaction in terms of the membrane potential. The lowest ACh concentration able to elicit consistent repetitive spikes was 50 nM, in the presence of which hyperpolarization increased and depolarization decreased the spike frequency. The saturating concentration was 10 μM, this induced a sustained response insensitive to voltage. Internal guanosine 5′-tri- and diphosphates (GTP, GDP) depressed and potentiated the voltage sensitivity, respectively, but not for the response to a saturating ACh concentration (10 μM). Internal guanosine 5′-O-(3-thiotriphosphate) (GTPγS) abolished the voltage sensitivity. The results indicate that the ACh-induced Ca²⁺ response is sensitive to the membrane potential and that a close linkage exists between voltage sensitivity and the G protein association/dissociation cycle in the muscarinic receptor.