PROCTOLIN RECEPTOR IN THE FOREGUT OF THE LOCUST SCHISTOCERCA-GREGARIAIS LINKED TO INOSITOL PHOSPHATE 2ND-MESSENGER SYSTEM

Proctolin caused dose-dependent contraction (EC(50): 0.45+/-0.03 mu M) of the foregut of the locust Schistocerca gregaria which was potentia ted by external Li+ at concentrations ranging from 5 to 100 mM, The ma ximum potentiation (43.8%) occurred at an external Li+ level of 50 mM which corresponded to an internal Li+ concentration of 9.8 mu M. Incub ation of foreguts with the proctolin receptor antagonist [alpha-methyl -L-tyrosine(2)]proctolin (1 mu M) reduced the contractile effects of p roctolin, in the presence of Li+, by 88%, Proctolin caused dose-relate d production of [H-3]glyceroinositol phosphate ([H-3]GIP), [H-3]inosit ol monophosphate ([H-3]IP1), [H-3]inositol bisphosphate ([H-3]IP2), [H -3]inositol trisphosphate ([H-3]IP3) and [H-3]inositol tetrakisphospha te ([H-3]IP4) from tissues preincubated with [H-3]-myo-inositol. Incub ation of tissue homogenates with 1 mu M [alpha-methyl-L-tyrosine(2)]-p roctolin reduced proctolin-stimulated inositol phospholipid production by approximately 80%, Inclusion of 50 mM Li+ in the incubation medium caused significant increases in proctolin-induced production of [H-3] IP3 (44.7%) and [H-3]IP4 (56.5%), Blockade of tissue proctolin recepto rs with 1 mu M [alpha-methyl-L-tyrosine(2)]proctolin, reduced proctoli n-stimulated [H-3]IP3 and [H-3]IP4 production, in the presence of Li+, by 95%, These data suggest that the locust foregut contains a proctol in receptor, which when stimulated, causes tissue contraction via acti vation of an inositol phospholipid pathway resulting in elevated intra cellular levels of IP3 and IP4.