Calmodulin increases the sensitivity of type 3 inositol-1,4,5-trisphosphate receptors to Ca2+ inhibition in human bronchial mucosal cells

Inositol-1,4,5-trisphosphate (IP3) releases Ca2+ from intracellular stores by binding to its receptor (IP3R), a multigene family of Ca2+-release chann els consisting of IP(3)R1, IP(3)R2, and IP(3)R3. IP(3)R1 is stimulated by l ow cytoplasmic Ca2+ concentrations and inhibited by high concentrations. Di screpant reports appeared about the effect of cytoplasmic Ca2+ on IP(3)R3, showing either a bell-shaped dependence or only a stimulatory phase with no negative feedback by high Ca2+ concentrations. We investigated how calmodu lin interfered with the feedback of cytosolic Ca2+ on the unidirectional IP 3-induced Ca2+ 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 Ca2+ release induced by 1.5 mu M IP3 with an IC50 value of 9 mu M. This inhibition was absolutely dependen t on the presence of cytosolic Ca2+. Ca2+ inhibited the IP3R with an IC50 v alue of 0.92 mu M Ca2+ in the absence of calmodulin and with an IC50 value of 0.15 mu M Ca2+ in its presence. It is concluded that: 1) IP(3)R3 can be inhibited by calmodulin, 2) IP(3)R3 is inhibited by high Ca2+ concentration s, and 3) calmodulin shifts the inhibitory part of the Ca2+-response curve toward lower Ca2+ concentrations.