Evidence for a Na⁺/Ca²⁺ exchange mechanism in the neuroendocrine cells of the locust corpus cardiacum

The glandular lobe of the locust corpus cardiacum (CC) is a peripheral neuroendocrine gland consisting primarily of neurosecretory cells (NSCs), all of which synthesize, store, and release adipokinetic hormones (AKHs). An influx of extracellular Ca²⁺ into the NSCs provides an essential trigger for initiating AKH release. In this study we demonstrate that the Na⁺ gradient across the plasma membrane of these NSCs has a significant influence on the process of AKH release and on Ca²⁺ fluxes across the membrane. AKHs are released from NSCs when the corpus cardiacum is incubated in Na⁺-free saline or in K⁺-free saline with ouabain. The rate of ⁴⁵Ca²⁺ efflux from the gland is reduced by 50% when glands are incubated in Na⁺-free saline compared to normal saline. The amount of ⁴⁵Ca²⁺ uptake by Na⁺-loaded cells is nearly 100% greater in glands exposed to Na⁺-free saline compared to glands in normal saline. These results are most clearly interpreted by proposing that a Na⁺/Ca²⁺ exchange system is present in the NSCs. This system may provide an essential Ca²⁺ buffering mechanism by extruding Ca²⁺ from the NSCs following a stimulus-secretion cycle.