Mobilization of Intracellular Calcium by Methacholine and Inositol 1,4,5-Trisphosphate in Rat Parotid Acinar Cells

In the rat parotid acinar cell, methacholine caused an increase in [Ca2+]i as determined by quin-2 fluorescence. The increase in [Ca2+]i was initially independent of, and subsequently dependent on, the presence of extracellular Ca2+, indicating mobilization of intracellular Ca2+, as well as activation of Ca2+ entry. Methacholine mobilization of the internal Ca2+ pool and stimulation of the initial transient phase of K+ efflux have similar concentration dependencies; the EC50 value for Ca2+ mobilization is 80 nmol/L, the EC50 value for K+ efflux is 200 nmol/L. In a permeable parotid cell preparation, inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, and inositol 4,5-bisphosphate were able to release Ca2+ from an ATP-dependent, oligomycin-insensitive pool. These observations, when taken with the previous finding that methacholine stimulates Ca-independent inositol trisphosphate formation, support the view that inositol 1,4,5-trisphosphate acts as a second messenger mediating the release of an intracellular Ca2+ pool following muscarinic receptor activation in the parotid gland.