The role of extracellular calcium in hormonal activation of glycogen phosphorylase in cockroach fat body

Abstract

Activation of phosphorylase in intact fat body of the cockroach, Periplaneta americana, by trehalagon (hyperglycaemic, hypertrehalosemic hormone) is absolutely dependent on Ca 2+ in the extracellular medium. Maximum activation was attained within 5 min by 1.8 mM Ca 2+ which is within the concentration range found in haemolymph. Mn 2+ could replace Ca 2+ in the salt solution but Mg 2+ was ineffective. In the absence of trehalagon, phosphorylase could be maximally activated by theophylline and cyclic AMP (10 μM). The action of both substances required Ca 2+. Theophylline exhibited an unusual effect in that activation by 1 mM theophylline was totally dependent on Ca 2+ whereas that by 10 mM was only 50% dependent. The calcium ionophore A-23187 is as effective as the hormone in activating phosphorylase but exhibited only a weak stimulatory effect on trehalose output, the rate returning to the basal level after approx 5 min. Activation of phosphorylase by trehalagon was completely blocked by inclusion of 1 mM La 3+ in the salt solution. The same procedure also reduced the rate of trehalose output from trehalagon stimulated tissue to the basal rate. The results suggest that activation of phosphorylase by trehalagon is the result of an increase in phosphorylase kinase activity caused by a rise in intracellular Ca 2+ levels. Cyclic AMP is thought to play a role in regulating the Ca 2+ permeability of the fat body cell membrane.