Calmodulin increases the sensitivity of type 3 inositol-1,4, 5-trisphosphate receptors to Ca(2+) inhibition in human bronchial mucosal cells.

Inositol-1,4,5-trisphosphate (IP(3)) releases Ca(2+) from intracellular stores by binding to its receptor (IP(3)R), a multigene family of Ca(2+)-release channels consisting of IP(3)R1, IP(3)R2, and IP(3)R3. IP(3)R1 is stimulated by low cytoplasmic Ca(2+) concentrations and inhibited by high concentrations. Discrepant reports appeared about the effect of cytoplasmic Ca(2+) on IP(3)R3, showing either a bell-shaped dependence or only a stimulatory phase with no negative feedback by high Ca(2+) concentrations. We investigated how calmodulin interfered with the feedback of cytosolic Ca(2+) on the unidirectional IP(3)-induced Ca(2+) release in permeabilized 16HBE14o- bronchial mucosal cells, where IP(3)R3 represents 93% of the receptors at the mRNA level and 81% at the protein level. Calmodulin inhibited the Ca(2+) release induced by 1.5 microM IP(3) with an IC(50) value of 9 microM. This inhibition was absolutely dependent on the presence of cytosolic Ca(2+). Ca(2+) inhibited the IP(3)R with an IC(50) value of 0.92 microM Ca(2+) in the absence of calmodulin and with an IC(50) value of 0.15 microM Ca(2+) in its presence. It is concluded that: 1) IP(3)R3 can be inhibited by calmodulin, 2) IP(3)R3 is inhibited by high Ca(2+) concentrations, and 3) calmodulin shifts the inhibitory part of the Ca(2+)-response curve toward lower Ca(2+) concentrations.