Fig. 5.

Schematic representation of primary circuits containing positive and negative feedback loops present in the GPCR signaling cascade systems reported in: (a) Kummer et al. [59] (phenylephrine (G_αq) and ATP (G_αq)-mediated Ca²⁺-response in hepatocyte cell line), (b) Larsen et al. [60], (c) De Pittà et al. [61] (glutamate (G_αq)-mediated Ca²⁺-response in astrocytes), (d) Giri et al. [54] (norepinephrine (G_αi/o)-mediated Ca²⁺-response in HeLa cell line), (e) Sun et al. [64] (histamine (G_αq)-mediated Ca²⁺-response in HUVEC cells), (f) Potter et al. [65] (ATP (G_αq) mediated Ca²⁺-response in NIH-3 T3/MDA-MB-231 cells), (g) Baillie et al. [66] (β-Arrestin-mediated switching of β-adrenergic receptors from G_αs to G_αi/o in HEK cells), (h) Rinaldi et al. [67] (CHIP mediated inhibition of PKA in forskolin (G_αs)-stimulated mouse embryonic fibroblast), and (i) Abell et al. [68] (Ca²⁺-influx mediated by STIM (G_αq) in drosophila cells). Note that various circuits exhibit different configurations of positive and negative feedback loops. The specific characteristics are motifs with inflow and outflow, fast positive feedback, negative feedback, adaptive negative feedback, and diffusible intermediates.