Characterization, localization and axial distribution of Ca2+ signalling receptors in the rat submandibular salivary gland ducts

Spacial compartmentalization of Ca2+ signaling complexes in pancreatic acini

Imaging [Ca2+]i at high temporal resolution and measuring the properties of Ca2+ signaling in streptolysin O (SLO)-permeabilized cells were used to study the spacial organization of signaling complexes. Sequential stimulation of single cells within pancreatic acini with several Ca2+-mobilizing agonists revealed an agonist-specific pattern and propagation rate of Ca2+ waves in the same cells, with CCK8 stimulating the fastest and bombesin the slowest waves. More importantly, each agonist initiated the wave in a different region of the same cell. On the other hand, repetitive stimulation with the same agonist induced Ca2+ waves of the same pattern that were initiated from the same region of the cell. The agonist-specific Ca2+ signaling does not appear to be the result of coupling to different G proteins as infusion of an anti-Galphaq antibody into the cells through a patch pipette equally inhibited Ca2+ signaling by all agonists. Further evidence for compartmentalization of signaling complexes was developed in permeabilized cells. The time-dependent loss of Ca2+ signaling due to SLO permeabilization occurred in an agonist-specific manner in the sequence cabachol > bombesin > cholecystokinin. Signaling by all agonists could be completely restored with as low as 2 micro guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). At this low concentration GTPgammaS recoupled inositol 1,4,5-trisphosphate production and Ca2+ release, rather than enhancing phospholipase C activity. Priming of Ca2+ signaling by GTPgammaS was agonist-specific. Guanosine 5'-O-(thio)diphosphate (GDPbetaS) uncoupled the ability of signaling complexes to release Ca2+ much better than stimulating inositol 1,4,5-trisphosphate production. The uncoupling of Ca2+ signaling by GDPbetaS was also agonist-specific. The combined findings of agonist-specific initiation sites of the Ca2+ wave and differential access of guanine nucleotides to signaling complexes suggest spacial compartmentalization of Ca2+ signaling complexes. Each complex must include a receptor, G protein, and phospholipase C that are coupled to a specific portion of the Ca2+ pool.

Gbetagamma transduces [Ca2+]i oscillations and Galphaq a sustained response during stimulation of pancreatic acinar cells with [Ca2+]i-mobilizing agonists

A central unresolved question in agonist-evoked [Ca2+]i signaling is the pathway by which [Ca2+]i oscillations and a sustained response are transduced. We show here that activation of Gbetagamma signal [Ca2+]i oscillations and activation of Galphaq signal a sustained response during stimulation by a number of Ca2+-mobilizing agonists. Thus, infusion of purified Gbetagamma into pancreatic acinar cells through a patch pipette evokes [Ca2+]i oscillations by Ca2+ release from internal stores, which were inhibited by two independent scavengers of Gbetagamma, the beta-adrenergic receptor kinase fragment, and a mutated Galphai1G203A. These proteins, as well as an inhibitory antibody against Galphaq/11, prevent [Ca2+]i oscillations and the sustained response when applied before cell stimulation, possibly by preventing the dissociation of Gq into its subunits. After cell stimulation and dissociation of Gq into Gbetagamma and Galphaq, scavenging Gbetagamma stabilized the sustained response and inhibited reassociation of the subunits on termination of cell stimulation with antagonist, whereas scavenging Galphaq inhibited the sustained response and uncovered the Gbetagamma-dependent oscillations. These findings provide a general mechanism by which Ca2+-mobilizing agonists can control the type of [Ca2+]i signal to be transduced to the cell interior.