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

Abstract

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 nmollL, 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, oligomycininsensitive 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 Ca ²⁺ pool following muscarinic receptor activation in the parotid gland.