Alternate coupling of receptors to G(s) and G(i) in pancreatic and submandibular gland cells

Many G(s)-coupled receptors can activate both cAMP and Ca2+ signaling pathw ays. Three mechanisms for dual activation have been proposed. One is recept or coupling to both G(S) and G(15) (a G(q) class heterotrimeric G protein) to initiate independent signaling cascades that elevate intracellular level s of cAMP and Ca2+, respectively. The other two mechanisms involve cAMP-dep endent protein kinase-mediated activation of phospholipase C beta either di rectly or by switching receptor coupling from G(S) to G(i). These mechanism swere primarily inferred from studies with transfected cell lines, In nativ e cells we found that two G(S)-coupled receptors (the vasoactive intestinal peptide and beta-adrenergic receptors) in pancreatic acinar and submandibu lar gland duct cells, respectively, evoke a Ca2+ signal by a mechanism invo lving both G(S) and G(i). This inference was based on the inhibitory action of antibodies specific for Get,, Gari, and phosphatidylinositol 4,5-bispho sphate, pertussis toxin, RGS4, a fragment of beta-adrenergic receptor kinas e and inhibitors of cAMP-dependent protein kinase. By contrast, Ca2+ signal ing evoked by G(S)-coupled receptor agonists was not blocked by G(q) class- specific antibodies and was unaffected in G alpha(15) -/- knockout mice. We conclude that sequential activation of G(S) and G(i), mediated by caMP-dep endent protein kinase, may represent a general mechanism in native cells fo r dual stimulation of signaling pathways by G(S)-coupled receptors.