Which of the following is a second messenger often activated by calcium ions?

In the context of signaling pathways within cells, calcium ions play a crucial role in the activation of certain second messengers. Inositol trisphosphate (IP₃) is indeed one of the key second messengers that is readily activated by calcium ions.

When certain receptors are stimulated—for instance, G-protein coupled receptors—this can lead to the activation of phospholipase C (PLC). PLC catalyzes the hydrolysis of a specific phospholipid (phosphatidylinositol-4,5-bisphosphate, or PIP₂) in the membrane, producing two important second messengers: diacylglycerol (DAG) and inositol trisphosphate (IP₃).

IP₃ is particularly significant because it diffuses through the cytoplasm to the endoplasmic reticulum (ER) where it binds to specific receptors, leading to the release of calcium ions into the cytosol. This increase in intracellular calcium concentration can further activate various cellular processes including muscle contraction, neurotransmitter release, and other signaling pathways.

In this way, IP₃ serves as a critical relay between the activation of receptors at the cell surface and intracellular responses mediated by calcium ions.