Hormonal activation of phosphorylase in cockroach fat body trophocytes: A correlation with trans‐membrane calcium flux

Abstract

This study is an investigation of the temporal relationship between transmembrane Ca2+ fluxes, and glycogen phosphorylase activation in dispersed trophocytes from the fat body of the cockroach, Periplaneta americana. Phosphorylase is maximally activated within 5 min after treating the trophocytes with either of the hypertrehalosemic hormones, Pea-HTH-I and Pea-HTH-II. Activation caused by Pea-HTH-II is sustained for a longer period than that produced by Pea-HTH-I. Chelation of extracellular Ca2+ with EGTA blocks the activation of phosphorylase by HTH. Similarly, chelation of intracellular Ca2+ with Quin 2 greatly diminishes the phosphorylase activating effect of both HTHs. The data support the view that an increase in the intracellular Ca2+ concentration is required for the activation of phosphorylase and that extracellular Ca2+ is an essential, although not necessarily sole, source of Ca2+ for this purpose. Using 45Ca2+ to trace the movement of Ca2+ following a challenge with either Pea-HTH-I or -II, it was shown that 45Ca2+influx nearly doubled during the first 30 s. At this time, the trophocytes begin to expel Ca2+ at a rate higher than that of untreated cells and this state persists for approximately 4 min. The Ca2+ fluxes are consistent with its postulated role in the activation of phosphorylase. Arch. Insect Biochem. Physiol. 42:233–244, 1999.© 1999 Wiley-Liss, Inc.