Calcium dependent serotonin release from blood platelets: a model system for neurosecretion.

In contrast to most neurosecretory systems, blood platelets are capable of serotonin secretion in the absence of extracellular calcium. However, the agents which trigger Ca²⁺-independent release are also strong aggregating agents. Under conditions where platelet aggregation is minimized, we observe a strong dependence of the release of serotonin on extracellular calcium. We have controlled aggregation through the use of creatine phosphokinase, which reduces the level of extracellular adenosine 5′-diphosphate, and Concanavalin A as secretogogue, a treatment which has minimal aggregating activity. Optimal conditions for release are 100μg/ml Concanavalin A and 1.0?5.0 mM Ca²⁺. Titration studies and experiments using radiolabeled Concanavalin A imply that the interaction between Ca²⁺ and Concanavalin A on release does not reflect altered binding of the lectin to the platelets. Ionic species which have been shown to affect other Ca²⁺-dependent release systems have similar effects on the system described.These results indicate that, in the absence of aggregation, blood platelets possess a release system analogous to that found in other secretory cells. This, combined with the structural simplicity of the platelet and the homogeneity of platelet preparations, suggests that platelets warrant consideration as a model system for biochemical studies of the release process.