The effects of immune challenge on phenoloxidase activity in locust salivary glands in vitro

Homogenates of salivary glands from Locusta migratoria possess phenoloxidase (PO) activity. This study investigates the activation of prophenoloxidase (PPO) in these glands in vitro. When freshly dissected salivary glands from L. migratoria are incubated with the immunogen laminarin, and then homogenized, a 4-fold increase in PO activity (expressed as a percentage of the total PO) can be measured within 20 min. Addition of laminarin to the incubation medium is best made prior to addition of salivary gland tissue, because when laminarin is added 10 min after the addition of tissue, the response to laminarin is reduced by 50%. When salivary glands are incubated in Ca2þ-free Ringer, the response to laminarin cannot be demonstrated. Addition of a calcium ionophore to the incubation in normal Ringer does not initiate a response on its own, but does augment the response to laminarin. Addition of phorbol ester to an incubation containing normal Ringer has no effect on PO activity, and does not augment the response to laminarin. In contrast, addition of okadaic acid to normal Ringer has no effect on its own, but does augment the response to laminarin. Activation of PPO in response to laminarin is therefore calcium-dependant, possibly involving an influx of extracellular Ca2þ, and modulated by protein phosphatases. Future work should aim to clarify the function of salivary glands in the immune defence of the locust and to investigate the exact source of the PO associated with the glands.