Identification of G protein-coupled signaling pathways in cardiac fibroblasts: cross talk between Gq and Gs

Abstract

Cardiac fibroblasts (CFs) are an important cellular component of myocardial responses to injury and to hypertrophic stimuli. We studied G protein-coupled receptors to understand how CFs integrate signals that activate Gq, Gs, and Gi. We predicted that the second messenger pathways present in CFs were distinct from those in cardiac myocytes and that unique signaling interactions existed in the CFs. ANG II, bradykinin, ATP, and UTP stimulated inositol phosphate (IP) production 2.2- to 7-fold. Each of these agonists elevated intracellular Ca2+ concentration ([Ca2+]i) via release from the intracellular Ca2+ storage compartment. Endothelin-1 (ET-1), carbachol, and norepinephrine failed to increase either IP production or [Ca2+]i. Although agonists that activated IP and Ca2+ transients had no effect on cAMP production when administered alone, these agents potentiated the β2-adrenergic response two- to fourfold. Hormones known to inhibit adenylyl cyclase activity in cardiac myocytes, such as ET-1 and carbachol, failed to lower the β-adrenergic response in fibroblasts. Order of potency and inhibitor data indicate that the functional receptor subtypes in these cells are β2, P2Y2, and AT1 for isoproterenol, ATP, and ANG II, respectively. We conclude that CFs express functional G protein-linked receptors that couple to Gq and Gs, with little or no coupling to Gi. The expression of receptors and their coupling to Gq- but not to Gi-linked responses distinguishes the signaling in CFs from that in myocytes. Furthermore, agonists that activate Gq in CFs potentiate stimulation of Gs, an example of signaling cross talk not observed in adult myocytes. These data suggest that G protein-mediated signaling in CFs is unique and may contribute to the specificity of hormone and drug action on individual cell types within the heart.