Abstract

Inositol-1,4,5-trisphosphate (IP₃) releases Ca²⁺ from intracellular stores by binding to its receptor (IP₃R), a multigene family of Ca²⁺-release channels consisting of IP₃R1, IP₃R2, and IP₃R3. IP₃R1 is stimulated by low cytoplasmic Ca²⁺ concentrations and inhibited by high concentrations. Discrepant reports appeared about the effect of cytoplasmic Ca²⁺ on IP₃R3, showing either a bell-shaped dependence or only a stimulatory phase with no negative feedback by high Ca²⁺ concentrations. We investigated how calmodulin interfered with the feedback of cytosolic Ca²⁺ on the unidirectional IP₃-induced Ca²⁺ release in permeabilized 16HBE14o- bronchial mucosal cells, where IP₃R3 represents 93% of the receptors at the mRNA level and 81% at the protein level. Calmodulin inhibited the Ca²⁺ release induced by 1.5 μM IP₃ with an IC₅₀ value of 9 μM. This inhibition was absolutely dependent on the presence of cytosolic Ca²⁺. Ca²⁺ inhibited the IP₃R with an IC₅₀ value of 0.92 μM Ca²⁺ in the absence of calmodulin and with an IC₅₀ value of 0.15 μM Ca²⁺ in its presence. It is concluded that: 1) IP₃R3 can be inhibited by calmodulin, 2) IP₃R3 is inhibited by high Ca²⁺ concentrations, and 3) calmodulin shifts the inhibitory part of the Ca²⁺-response curve toward lower Ca²⁺ concentrations.