Muscarinic acetylcholine and proctolin receptors in the foregut of the locust Schistocerca gregaria: Role of inositol phosphates, protein kinase C and calcium in second messenger effects

Acetylcholine (ACh) induced dose-dependent contraction of the locust (Schistocerca gregaria) foregut was antagonised by 4-DAMP (1,1-dimethyl-4- diphenylacetoxypiperidinium; 10 nM-10 μM) and Li⁺ (50- 100 mM). The inhibition by Li⁺, of ACh-induced gut contraction suggests that this muscarinic ACh receptor is linked to phosphatidylinositol second messenger systems. ACh (1 μM and 0.5 mM) stimulated concentration-dependent production of [³H]-inositol phosphates, including [³H]-inositol trisphosphate, from foregut homogenates incubated in Tris buffer containing [³H]-myo-inositol. Pre-incubation of homogenates with Li⁺ (50 mM) or atropine (10 μM) reduced ACh-stimulated inositol phosphate production by 95%. The contractile effects of ACh, 1-oleoyl-2-sn-glycerol (OAG) and phorbol 12,13-dibutyrate (PDBu), but not those of proctolin, on the foregut, were reduced by the protein kinase C (PKC) inhibitor H7 (10 nM-10 μM) and the L-type Ca₂₊ channel blocker verapamil (1 μM-0.1 mM). In comparison, the contractile effects of proctolin, but not those of ACh, OAG and PDBu, were potentiated by H7 (10 nM- 10 μM) and attenuated by the inositol 1,4,5-trisphosphate (IP₃) receptor blocker decavanadate (10 μM-0.5 mM) and ryanodine (1 μM-0.1 mM), an inhibitor of intracellular Ca²⁺ release. Decavanadate and ryanodine had no effect on ACh-induced tissue contraction. Proctolin-induced contraction was unaffected by the presence of verapamil at concentrations as high as 1 mM but was abolished by 10 mM Co²⁺ . These data suggest, that in the locust foregut, stimulation of ACh receptors causes activation of PKC, thereby promoting Ca²⁺ entry via the opening of L-type Ca²⁺ channels. However, the function of IP₃ produced following activation of tissue muscarinic receptors is unclear. In contrast, activation of proctolin receptors causes verapamil-insensitive entry of extracellular Ca²⁺ as well as the generation of IP₃ which causes release of Ca²⁺ from intracellular stores to bring about an increase in gut contractility.