Acetylcholine Promotes Ca2+and NO-Oscillations in Adipocytes Implicating Ca2+→NO→cGMP→cADP-ribose→Ca2+ Positive Feedback Loop - Modulatory Effects of Norepinephrine and Atrial Natriuretic Peptide

Purpose This study investigated possible mechanisms of autoregulation of Ca 2+ signalling pathways in adipocytes responsible for Ca 2+ and NO oscillations and switching phenomena promoted by acetylcholine (ACh), norepinephrine (NE) and atrial natriuretic peptide (ANP). Methods Fluorescent microscopy was used to detect changes in Ca 2+ and NO in cultures of rodent white adipocytes. Agonists and inhibitors were applied to characterize the involvement of various enzymes and Ca 2+ -channels in Ca 2+ signalling pathways. Results ACh activating M 3 -muscarinic receptors and G βγ protein dependent phosphatidylinositol 3 kinase induces Ca 2+ and NO oscillations in adipocytes. At low concentrations of ACh which are insufficient to induce oscillations, NE or α1, α2-adrenergic agonists act by amplifying the effect of ACh to promote Ca 2+ oscillations or switching phenomena. SNAP, 8-Br-cAMP, NAD and ANP may also produce similar set of dynamic regimes. These regimes arise from activation of the ryanodine receptor (RyR) with the implication of a long positive feedback loop (PFL): Ca 2+ → NO→cGMP→cADPR→Ca 2+ , which determines periodic or steady operation of a short PFL based on Ca 2+ -induced Ca 2+ release via RyR by generating cADPR, a coagonist of Ca 2+ at the RyR. Interplay between these two loops may be responsible for the observed effects. Several other PFLs, based on activation of endothelial nitric oxide synthase or of protein kinase B by Ca 2+ -dependent kinases, may reinforce functioning of main PFL and enhance reliability. All observed regimes are independent of operation of the phospholipase C/Ca 2+ -signalling axis, which may be switched off due to negative feedback arising from phosphorylation of the inositol-3-phosphate receptor by protein kinase G. Conclusions This study presents a kinetic model of Ca 2+ -signalling system operating in adipocytes and integrating signals from various agonists, which describes it as multivariable multi feedback network with a family of nested positive feedback.