Regulation of adipokinetic hormone release from locust neuroendocrine tissue: participation of calcium and cyclic AMP

In the locust, cyclic adenosine monophosphate (cAMP) mediates at least part of the effects of octopamine, the neurotransmitter which regulates the release of two adipokinetic hormones (AKHs) from the glandular lobe of the corpus cardiacum (CC). We have examined the requirement for extracellular Ca²⁺ in the process of AKH release mediated by octopamine and by agents which artificially elevate intracellular cAMP levels. Octopamine and the adenylate cyclase activator forskolin elevate the cAMP content of the glandular lobe in normal saline, in normal saline with the Ca²⁺ channel blocker, methoxyverapamil, and in Ca²⁺ -free saline during 10-min exposure periods. Octopamine, forskolin, and 8-bromo cAMP mediate release of AKHs in vitro in normal saline, but release is prevented in the absence of extracellular Ca²⁺. When glands are exposed to these agents in normal saline in the presence of methoxyverapamil, AKH release is curtailed in a similar manner. Lanthanum and EGTA dramatically reduce cAMP production elicited by octapamine and forskolin, and lanthanum prevents octopamine-mediated release of AKHs. The phosphodiesterase inhibitor, IBMX, elevates cAMP content in the presence and absence of extracellular Ca²⁺, and stimulates normal release of AKHs both in the presence and absence of extracellular Ca²⁺. However, following extensive washing in Ca²⁺ -free saline, IBMX fails to evoke AKH release. Methoxyverapamil has no effect on IBMX-mediated secretion. These results suggest that IBMX may mobilize intracellular stores of Ca²⁺ to induce release. Extracellular Ca²⁺ is apparently required for the process of neurotransmitter-evoked release, as has been shown for release of other peptide hormones. Cyclic AMP is intimately associated with Ca²⁺ in mediating this process. The release of AKHs is more dependent upon extracellular Ca²⁺ than is cAMP production under the conditions examined in this study. Ca²⁺ may provide the signal which initiates the secretory response, although cAMP may modulate this signal or the cells' responsiveness to this signal in some way. Support for this hypothesis is provided by experiments with the Ca²⁺ ionophore, A23187. This agent provokes release of AKHs in a Ca²⁺ -dependent manner, probably by elevating intracellular Ca²⁺ levels. A23187 does not elevate cAMP levels in the glandular lobe, indicating that cAMP elevation is not a preprequisite for secretion.