The control of enzyme secretion from fly salivary glands

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Abstract
Stimulation of fluid secretion from fly [Calliphora erythrocephala] salivary glands by 5-hydroxytryptamine (5-HT) is known to involve Ca and cyclic[c]AMP. Isolated salivary glands were used to investigate the role of these 2nd messengers in the control of enzyme (sucrase) secretion. The protein component of secretion from isolated glands treated with 5-HT appears identical to that of saliva secreted by flies during feeding. Stimulation of fluid secretion by 5-HT follows a definite dose-response curve, but there is no consistent relationship between the rate of enzyme secretion and the stimulating concentration of 5-HT. Exogenous cAMP causes secretion of enzymes and fluid, mimicking the action of 5-HT. The phosphodiesterase inhibitor theophylline enhances the rate of 5-HT-stimulated enzyme secretion. Removal of Ca from the bathing medium enhances enzyme secretion in response to 5 or 10 nM-5-HT but has no effect on enzyme secretion stimulated by 100 nM-5-HT or by cAMP. Addition of 0.1 mM-La to medium containing 2 mM-Ca mimics the effect of Ca-free solution on 5-HT-stimulated enzyme secretion. The ionophore A 23187 cuses secretion of both fluid and enzyme. The secretory rate is initially high but soon declines and ceases after about 40 min. Enzyme secretion in response to 5-HT or to cAMP is progressively inhibited as the concentration of K is increased from 10 to 80 mM. Secretion in response to A 23187 is initially inhibited by 80 mM-K but then partially recovers. The rate of enzyme secretion appears affected by intracellular concentrations of Ca and cAMP. Probably, the rate of enzyme secretion increases as the intracellular Ca concentration rises, until the optimal Ca concentration is reached when further increase in the Ca level progressively inhibits secretion. The optimal Ca concentration for enzyme secretion is lower than that for fluid secretion, and 5-HT normally causes maximal fluid secretion and submaximal enzyme secretion.

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