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 pathways. Three mechanisms for dual activation have been proposed. One is receptor coupling to both G(s) and G₁₅ (a G(q) class heterotrimeric G protein) to initiate independent signaling cascades that elevate intracellular levels of cAMP and Ca⁺², respectively. The other two mechanisms involve cAMP-dependent protein kinase-mediated activation of phospholipase Cβ either directly or by switching receptor coupling from G(s) to G(i). These mechanisms were primarily inferred from studies with transfected cell lines. In native cells we found that two G(s)-coupled receptors (the vasoactive intestinal peptide and β-adrenergic receptors) in pancreatic acinar and submandibular gland duct cells, respectively, evoke a Ca²⁺ signal by a mechanism involving both G(s) and G(i). This inference was based on the inhibitory action of antibodies specific for Gα(s), Gα(i), and phosphatidylinositol 4,5-bisphosphate, pertussis toxin, RGS4, a fragment of β-adrenergic receptor kinase and inhibitors of cAMP-dependent protein kinase. By contrast, Ca²⁺ signaling evoked by G(s)-coupled receptor agonists was not blocked by G(q) class-specific antibodies and was unaffected in Gα₁₅ -/- knockout mice. We conclude that sequential activation of G(s) and G(i), mediated by cAMP-dependent protein kinase, may represent a general mechanism in native cells for dual stimulation of signaling pathways by G(s)- coupled receptors.