ACIDIFICATION OF SEROTONIN-CONTAINING SECRETORY VESICLES INDUCED BY APLASMA-MEMBRANE CALCIUM RECEPTOR

Parafollicular (PF) cells secrete B-hydroxytryptamine in response to i ncreased extracellular Ca2+ (up arrow[Ca2+](e)). This stimulus causes Cl- channels in PF secretory vesicles to open, leading to vesicle acid ification. PF cells express a plasmalemmal heptahelical receptor (CaR) that binds Ca2+, Gd3+, and Ba2+. We now report that the CaR mediates vesicle acidification, Ca2+, Gd3+, and Ba2+ induced vesicle acidificat ion, which was independent of channel-mediated Ca2+ entry. Agonist-ind uced vesicle acidification was blocked by pertussis toxin, inhibitors of phosphatidylinositol-phospholipase C, calmodulin, NO synthase, guan ylyl cyclase, or protein kinase G. PF cells contained NO synthase immu noreactivity, and vesicles were acidified by NO donors and dibutyryl c GMP. [Ca2+]e, and Gd3+ mobilized thapsigargin-sensitive internal Ca2stores. [S-35]G(alpha i) and [S-35]G(alpha q) were immuno-precipitated from PF membranes incubated with agonists in the presence of [S-35]ad enosine 5'-O-(thiotriphosphate). Labeling of G(alpha i) but not G(alph a q) was antagonized by pertussis toxin. Vesicles acidified in respons e to activation of protein kinase C; however, protein kinase C inhibit ion blocked calcium channel- but not CaR-dependent acidification. We p ropose the following signal transduction pathway: CaR --> G(i) --> pho sphatidylinositol-phospholipase C --> inositol 1,4,5-trisphosphate --> up arrow [Ca2+](i) --> Ca2+/calmodulin --> NO synthase --> guanylyl c yclase --> cGMP --> protein kinase G --> opens vesicular Cl- channel.