ATP-induced Ca2+ signals in bronchial epithelial cells

Ca2+-dependent Cl- secretion in the respiratory tract occurs physiologically or under pathophysiological conditions when inflammatory mediators are released. The mechanism of intracellular Ca2+ release was investigated in the immortalized bronchial epithelial cell line 16HBE14o-. Experiments on both intact and permeabilized cells revealed that only inositol 1,4,5-trisphosphate (InsP(3)) receptors and not ryanodine receptors are involved in intracellular Ca2+ release. The expression pattern of the three InsP(3) receptor isoforms was assessed both at the mRNA and at the protein level. The level of expression at the mRNA level was type 3 (92.5%) much greater than type 2 (5.4%) > type 1 (2.1%) and this rank order was also observed at the protein level. The ATP-induced Ca2+ signals in the intact cell, consisting of abortive Ca2+ spikes or fully developed [Ca2+] rises and intracellular Ca2+ waves, were indicative of positive feedback of Ca2+ on the InsP, receptors. Low Ca2+ concentrations stimulated and high Ca2+ concentrations inhibited InsP(3)-induced Ca2+ release in permeabilized 16HBE14o- cells. We localized a cytosolic Ca2+-binding site between amino acid residues 2077 and 2101 in the type-2 InsP(3) receptor and between amino acids 2030 and 2050 in the type-3 InsP(3) receptor by expressing the respective parts of these receptors as glutathione S-transferase fusion proteins in bacteria. We conclude that the InsP(3) receptor isoforms expressed in 16HBE14o- cells (mainly type-3 and type-2) are stimulated by Ca2+ and that this phenomenon contributes to the ATP-induced Ca2+ signals in intact 16HBE14o- cells.