Which among the following is NOT a second messenger?

Calcium is used as second messenger in systems involving G proteins and inositol triphosphate. Second messengers include 3', 5'-cAMP, synthesized from ATP by the enzyme adenylyl cyclase in response to the hormone epinephrine, and Ca2+, which is stored inside the endoplasmic reticulum of most cells. Other second messengers include 3',5'-cGMP, nitric oxide, and the polyphosphoinositols produced by the hydrolysis of inositol phospholipids by hormone-regulated phospholipases. Ref: Kennelly P.J., Rodwell V.W. (2011). Chapter 9. Enzymes: Regulation of Activities. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.