sentence2_structure: Calcium ion<>is<>an example of a second messenger
sentence1: Gating by internal Calcium Internal calcium is a second messenger (see ligand gating above) in many cell types, and thus channels gated by this ion can be characterized as ligand gated channels from the inside.
sentence2: Calcium ion is an example of a second messenger.
question: Calcium ion is an example of what kind of messenger?