How do calcium ions act as second messengers in hormone signaling?

In hormone signaling, calcium ions ($C{a}^{2+}$) function as second messengers that transmit signals from cell-surface receptors to intracellular targets. While hormones act as "first messengers" outside the cell, $C{a}^{2+}$ ions facilitate rapid responses within the cytoplasm.1

Cells typically maintain a very low resting concentration of cytoplasmic $C{a}^{2+}$ by actively pumping the ions out of the cell. When a signaling event is triggered, specific channels in the cell membrane open, allowing $C{a}^{2+}$ to rush into the cytoplasm.1

Once inside, these ions act as signaling molecules by binding to specialized proteins, such as calmodulin. This binding induces a conformational change in the protein, which then regulates the activity of various calcium-dependent enzymes to mediate specific cellular processes, such as muscle fiber contraction or metabolic adjustments. By rapidly increasing in concentration, $C{a}^{2+}$ effectively propagates the hormonal signal to downstream effectors, enabling the cell to respond quickly to external stimuli.21