Muscarinic receptor-stimulated Ca2+ signaling and inositol lipid metabolism in avian salt gland cells.

Activation of muscarinic cholinergic receptors was studied by measuring agonist-stimulated inositol lipid turnover and changes in [Ca2+]i in dissociated salt gland secretory cells. Carbachol stimulation of quin2-loaded cells results in a sustained 4-fold increase in [Ca2+]i, while incorporation of [32P]Pi into phosphatidylinositol (PI) and phosphatidate are similarly increased. [3H]Inositol phosphates, measured in the presence of Li+, increased 13-fold. The stimulated increment in [Ca2+]i required extracellular Ca2+, whereas [3H]inositol phosphate accumulation was independent of external Ca2+. Dose-response curves for carbachol-induced increments in [Ca2+]i, PI labeling, and labeled inositol phosphate release are similar, with EC50 values of 6, 4.5 and 8 microM, respectively. Dissociation constants for atropine vs. the quin2 and phospholipid responses are 0.59 +/- 0.3 nM and 0.48 +/- 0.28 nM, respectively. These cells thus provide a model system for the study of non-exocytotic secretion as a consequence of stimulated inositol lipid turnover.