1. That which carries a message.

2. Having message-carrying properties.

First messenger, a hormone that binds to a receptor on the surface cell and, in so doing, communicates with intracellular metabolic processes.

Second messenger, an intermediary molecule that is generated as a consequence of hormone-receptor interaction; e.g., see adenosine 3',5'-cyclic monophosphate; guanosine 3',5'-cyclic monophosphate; calcium; inositide.

(05 Mar 2000)