Muscarinic receptor-stimulated Ca 2+ signaling and inositol lipid metabolism in avian salt gland cells

Abstract

Activation of muscarinic cholinergic receptors was studied by measuring agonist-stimulated inositol lipid turnover and changes in [Ca 2+] i in dissociated salt gland secretory cells. Carbachol stimulation of quin2-loaded cells results in a sustained 4-fold increase in [Ca 2+] i, while incorporation of [ 32P]P i into phosphatidylinositol (PI) and phosphatidate are similarly increased. [ 3H]Inositol phosphates, measured in the presence of Li +, increased 13-fold. The stimulated increment in [Ca 2+] i required extracellular Ca 2+, whereas [ 3H]inositol phosphate accumulation was independent of external Ca 2+. Dose-response curves for carbachol-induced increments in [Ca 2+] i, PI labeling, and labeled inositol phosphate release are similar, with EC 50 values of 6, 4.5 and 8 μM, 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.