Which one of the following is not a second messenger in hormonal action?

Sodium is not a secondary messenger.In heart cells, cAMP acts as a secondary messenger which is made in response to adrenaline and it stimulates Ca2+ ions to come out from the sarcoplasmic reticulum of muscle fibres which causes muscle contraction. cGMP (Cyclic Guanosine Monophosphate) which acts as secondary messenger works on the action of acetylcholine, increase in flow of Ca2+ into muscle fibres & hence causes muscle relaxation.