Effects of proctolin and related analogues on inositol phosphate production in the foregut of the locust Schistocerca gregaria

Abstract

Proctolin (10 nM and 0.5 μM) stimulated dose-dependent release of [ 3H]inositol 1,4,5-trisphosphate ([ 3H]IP 3) and [ 3H]inositol 1,3,4,5-tetrakisphosphate ([ 3H]IP 4) from locust foregut homogenates incubated in Tris buffer containing [ 3H]myo-inositol . Of the other analogues tested for agonist activity in this preparation, the order of potency was proctolin >[ p-F- l-phenylalanine 2] proctolin > [α-methyl- l-tyrosine 2]proctolin > RYT > cycloproctolin. [ N-methyl- l-tyrosine 2] proctolin was unable to induce significant release of [ 3H]inositol phosphates. Incubation of foregut homogenates in Tris buffer containing [α-methyl- l-tyrosine 2] proctolin (10 nM) significantly reduced proctolin-stimulated release of [ 3H]IP 3 and [ 3H]IP 4 by 86 and 63% respectively. Increasing the dose of antagonist to 1 μM did not result in further reduction of proctolin-stimulated [ 3H]IP 3 release although the level of [ 3H]IP 4 production was reduced by 89%. Cycloproctolin and the tripeptide RYT were less potent antagonists of proctolin-stimulated [ 3H]inositol phosphate release than [α-methyl- l-tyrosine 2] proctolin only causing inhibition in excess of 80% at a concentration of 1 μM. These data provide good evidence that the locust foregut contains a proctolin receptor, which when stimulated, causes phosphatidylinositol hydrolysis resulting in the production of the putative second messengers molecules IP 3 and IP 4.