Page "Active transport" Paragraph 16

Many cells also possess a calcium ATPase, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger.

But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger.

The exchanger comes into service when the calcium concentration rises steeply or " spikes " and enables rapid recovery.

This shows that a single type of ion can be transported by several enzymes, which need not be active all the time ( constitutively ), but may exist to meet specific, intermittent needs.