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

• Published in: Insect biochemistry and molecular biology 1998-05, Vol.28 (5), p.331-343
• Main Authors: Hinton, J.M, Nejad, M, Issberner, J.P, Hancock, J.T, Osborne, R.H
• Format: Article
• Language: English
• Subjects: Acetylcholine; Calcium; Inositol Phosphates; Proctolin; Protein Kinase C; Receptor; Schistocerca gregaria
• ISSN: 0965-1748; 1879-0240
• DOI: 10.1016/S0965-1748(98)00008-3

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 Ca 2+ 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 (IP 3) receptor blocker decavanadate (10 μM–0.5 mM) and ryanodine (1 μM–0.1 mM), an inhibitor of intracellular Ca 2+ 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 Co 2+. These data suggest, that in the locust foregut, stimulation of ACh receptors causes activation of PKC, thereby promoting Ca 2+ entry via the opening of L-type Ca 2+ channels. However, the function of IP 3 produced following activation of tissue muscarinic receptors is unclear. In contrast, activation of proctolin receptors causes verapamil-insensitive entry of extracellular Ca 2+ as well as the generation of IP 3 which causes release of Ca 2+ from intracellular stores to bring about an increase in gut contractility.