Function and expression of the epithelial Ca2+ channel family: comparison of mammalian ECaC1 and 2

Abstract

The epithelial Ca²⁺ channel (ECaC) family represents a unique group of Ca²⁺-selective channels that share limited homology to the ligand-gated capsaicin receptors, the osmolarity-sensitive channel OTRPC4, as well as the transient receptor potential family. Southern blot analysis demonstrated that this family is restricted to two members, ECaC1 and ECaC2 (also named CaT1). RT-PCR analysis demonstrated that the two channels are co-expressed in calbindin-D-containing epithelia, including small intestine, pancreas and placenta, whereas kidney and brain only express ECaC1 and stomach solely ECaC2. From an electrophysiological point of view, ECaC1 and ECaC2 are highly similar channels. Differences concern divalent cation permeability, the kinetics of Ca²⁺-dependent inactivation and recovery from inactivation. Ruthenium red is a potent blocker of ECaC activity. Interestingly, ECaC2 has a 100-fold lower affinity for ruthenium red (IC₅₀ 9 ± 1 μm) than ECaC1 (IC₅₀ 121 ± 13 nm). ECaCs are modulated by intracellular Mg²⁺ and ATP. ECaC1 and ECaC2 activity rapidly decay in the absence of intracellular ATP. This effect is further accelerated at higher intracellular Mg²⁺ concentrations. In conclusion, ECaC1 and ECaC2 are homologous channels, with an almost identical pore region. They can be discriminated by their sensitivity for ruthenium red and show differences in Ca²⁺-dependent regulation.