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 mu M) 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 preexposed to isoproterenol (10 mu M) for 45 min. Despite the increase in IF, 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 IF,. 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.