Cross-talk in signal transduction pathways of rat submandibular acinar cells.

The effects of cyclic AMP-generating substances and of the cAMP-dependent protein kinase (PKA) inhibitor H89 on the inositol 1,4,5-trisphosphate (IP3) and Ca2+ responses to acetylcholine (ACh) were examined in rat submandibular acini. Pre-exposure to forskolin (5 microM) and to dibutyryl cyclic AMP (db-cAMP, 1 mM) increased the IP3 formation in response to ACh while H89 reduced it. The enhancement of the IP3 response was not seen, however, in cells pre-exposed to isoproterenol (10 microM) for 45 min. Despite the increase in IP3 formation, pre-exposure to forskolin or db-cAMP inhibited the release of Ca2+ induced by ACh in cells incubated in Ca2+-free solutions. H89 had no effect on the ACh-generated Ca2+ signal. Manipulation of PKA had no effect on the release of Ca2+ induced by thapsigargin. Pre-exposure to test substances caused changes in the rate of Ca2+ influx which paralleled those in Ca2+ release. It is concluded that PKA interacts with IP3/Ca2+-mediated signaling in submandibular cells at two levels, IP3 generation and Ca2+ release from IP3-sensitive stores. Through phosphorylation of target elements, PKA modifies the coupling of the muscarinic receptor with membrane phosphoinositides and the sensitivity of endoplasmic Ca2+ channels to IP3. The effects on these two components of the IP3/Ca2+ signaling pathway depend, however, on the length of exposure to test substances and on the up- or down-regulation of PKA.