lonized calcium is an important second messenger in a great variety of cellular responses to stimuli, both chemical and electrical.
Review one signaling pathway in which Ca$^{2+}$ acts as a second messenger by correctly completing each sentence below.
1. To begin, cytosolic Ca$^{2+}$ can be increased by the opening of (Click to select) \\
in the plasma membrane or by
receptor activation from a first messenger.
2. With receptor activation, the receptor itself may contain a Ca$^{2+}$ channel, or the receptor may activate a (Click to select) \\
that opens the channel via a second messenger.
3. It is also possible that Ca$^{2+}$ is released from the (Click to select) \\
; this is mediated by (Click to select) or Ca$^{2+}$
entering from the extracellular fluid.
4. When cytosolic Ca$^{2+}$ concentrations are elevated, Ca$^{2+}$ can act as a second messenger. One of the main protein targets
of Ca$^{2+}$ is (Click to select). The resulting Ca$^{2+}$ -calmodulin complex can activate or inhibit a large variety of enzymes and
other proteins, leading to the cellular response.
