The effects of the hormone 5-hydroxytryptamine, its analogues and its antagonists on the electrical activity and secretion rate of isolated salivary glands of the blowfly Calliphora erythrocephala were investigated. The secretion rate increases linearly with the logarithm of hormone concentration between 10−9 and 10−8M. At >10−8M the transepithelial potential depolarizes and rapidly attains a new stable value. However, at intermediate hormone concentrations, the potential does not maintain a stable intermediate value but displays sustained oscillations. These oscillations are not an artifact resulting from periodic variations in hormone concentration. The frequency of the oscillations increases with hormone concentration and with the concentration of external calcium and hormone analogues. The frequency decreases following the addition of lanthanum to the perfusion medium.
The experimental results suggest that the potential oscillation may be driven by an oscillation in the intracellular concentrations of cyclic AMP and calcium. It is argued that oscillatory control provides a reliable, noise-resistant strategy for controlling secretion rate.