K_V,_Caβ subunit dramatically increases the apparent calcium sensitivity of the α subunit of MaxlK channels when probed in the micromolar [Ca²⁺]_i range. Analysis in a wide range of [Ca²⁺]_i revealed that this functional coupling is exquisitely modulated by [Ca²⁺]_i. Ca²⁺ ions switch MaxiK α+β complex into a functionally coupled state at concentrations beyond resting [Ca²⁺]_i. At [Ca²⁺] ≤ 100 nM, MaxiK activity becomes independent of Ca²⁺, is purely voltage-activated, and its functional coupling with its β subunit is released. The functional switch develops at [Ca²⁺]_i that occur during cellular excitation, providing the molecular basis of how MaxiK channels regulate smooth muscle excitability and neurotransmitter release.