Adipokinetic hormone is dependent on extracellular Ca2+ for its stimulatory action on the glycogenolytic pathway in locust fat body in vitro

Abstract

Inclusion of glucose or trehalose in the medium during the incubation of locust fat body in vitro leads to a reduction of the relative amount of active (AMP-independent) glycogen phosphorylase. The presence of adipokinetic hormone (AKH I) results in a rapid activation of phosphorylase, reaching a maximum within 5 min. This AKH effect is highly dependent on added Ca²⁺, and requires ⩾ 1 mM Ca²⁺ for maximal enzyme activation. Ca²⁺ alone has no effect on phosphorylase activity, but it does activate the enzyme when the ionophore A23187 is also included in the medium. In a cell-free system from locust fat body the activation of endogenous phosphorylase by phosphorylase kinase is stimulated by Ca²⁺. Activity of the latter enzyme can be increased further by high doses of calmodulin. Both in the presence and in the absence of external calmodulin, the calmodulin antagonist trifluoperazine has an inhibitory effect on phosphorylase kinase. Results are discussed in relation to the possible mechanisms underlying hormonal control of glycogenolysis.