Caffeine does not inhibit substance P-evoked intracellular Ca2+ mobilization in rat salivary acinar cells

We used the Ca2+-sensitive fluorescent dye fura 2, together with measuremen ts of intracellular D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3], to assess the inhibitory effects of caffeine on signal transduction via G prot ein-coupled receptor pathways in isolated rat mandibular salivary acinar ce lls. ACh, norepinephrine (NE), and substance P (SP) all evoked substantial increases in the intracellular free Ca2+ concentration ([Ca2+](i)). Respons es to ACh and NE were markedly inhibited by prior application of 20 mM caff eine. The inhibitory effect of caffeine was not reproduced by phosphodieste rase inhibition with IBMX or addition of cell-permeant dibutyryl cAMP. In c ontrast to the ACh and NE responses, the [Ca2+](i) response to SP was unaff ected by caffeine. Despite this, SP and ACh appeared to mobilize Ca2+ from a common intracellular pool. Measurements of agonist-induced changes in Ins (1,4,5)P-3 levels confirmed that caffeine inhibited the stimulus-response c oupling pathway at a point before Ins(1,4,5)P-3 generation. Caffeine did no t, however, inhibit [Ca2+](i) responses evoked by direct activation of G pr oteins with 40 mM F-. These data show that caffeine inhibits G protein-coup led signal transduction in these cells at some element that is common to th e muscarinic and alpha-adrenergic signaling pathways but is not shared by t he SP signaling pathway. We suggest that this element might be a specific s tructural motif on the G protein-coupled muscarinic and alpha-adrenergic re ceptors.