Physiological levels of adrenaline fail to stop pancreatic beta cell activity at unphysiologically high glucose levels

Abstract

Adrenaline inhibits insulin secretion from pancreatic beta cells to allow an organism to cover immediate energy needs by unlocking internal nutrient reserves. The stimulation of α2-adrenergic receptors on the plasma membrane of beta cells reduces their excitability and insulin secretion mostly through diminished cAMP production and downstream desensitization of late step(s) of exocytotic machinery to cytosolic Ca2+ concentration ([Ca2+]c). In most studies unphysiologically high adrenaline concentrations have been used to evaluate the role of adrenergic stimulation in pancreatic endocrine cells. Here we report the effect of physiological adrenaline levels on [Ca2+]c dynamics in beta cell collectives in mice pancreatic tissue slice preparation. We used confocal microscopy with a high spatial and temporal resolution to evaluate glucose-stimulated [Ca2+]c events and their sensitivity to adrenaline. We investigated glucose concentrations from 8-20 mM to assess the concentration of adrenaline that completely abolishes [Ca2+]c events. We show that 8 mM glucose stimulation of beta cell collectives is readily inhibited by the concentration of adrenaline available under physiological conditions, and that sequent stimulation with 12 mM glucose or forskolin in high nM range overrides this inhibition. Accordingly, 12 mM glucose stimulation required at least an order of magnitude higher adrenaline concentration above the physiological level to inhibit the activity. To conclude, higher glucose concentrations stimulate beta cell activity in a non-linear manner and beyond levels that could be inhibited with physiologically available plasma adrenaline concentration.

The effect of forskolin and the role of Epac2A during activation, activity, and deactivation of beta cell networks

“…Although forskolin is a potent adenylyl cyclase activator that successfully evoked oscillations of [Ca 2+ ] IC in 6 mM glucose, the effect of forskolin in 6 mM glucose seems to be significantly weaker compared to the stimulation with 12 mM glucose, since we observed a much shorter activation delay in 12 mM glucose. This supports previously published data showing progressively shorter activation delay with increasing glucose concentrations ( 43 , 57 , 81 , 88 ), a phenomenon most probably resulting from the amount of metabolized glucose needed to activate beta cells or direct cAMP production ( 93 , 94 ). Furthermore, while in 12 mM glucose most beta cells within an islet responded to stimulation, when stimulated with 6 mM glucose + forskolin, only a minority of beta cells became active (data not shown), demonstrating large differences in their metabolic activity and therefore their sensitivity to glucose also under stimulation by forskolin.…”

Collective biological computation in metabolic economy

“…Furthermore, tissue slice preparation helped us to uncover new evidence regarding the function of intracellular Ca 2+ receptors underlying fast changes of the [Ca 2+ ] c in addition to previously described sources [22,23]. Based on these novel data, the concept congruent with all previously described mechanisms, including two independent mechanisms with sequent activation of inositol-1,4,5-trisphosphate (IP 3 ) and ryanodine intracellular Ca 2+ receptors, and incorporating several time scales of Ca 2+ -induced-Ca 2+ release (CICR) has been demonstrated [22][23][24]. These two mechanisms support a robust asynchronous initial activation, as well as a long-term activity stimulated with a repetitive or prolonged exposure to glucose.…”

Molecular determinants and intracellular targets of taurine signalling in pancreatic islet β‐cells

“…52 Supplementing extra Ca 2+ expectedly enhanced the triggering stimulus, whereas elevating cytosolic cAMP by GLP-1 potentiated the granule fusion during exocytosis (Figures 3B and 8B). The relatively smaller effect of forskolin may reflect the adverse metabolic impact of labdane, 53 which is likely to have a critical role in the late stages of granule fusion. 54 The surprising lack of insulin exocytosis potentiation by the activation of PKC (Figure 3B), a recognized modulator of TauT, 48 recorded for the homologs tested acutely (Figure 3B) and in chronic taurine cultures (Figure 4), suggests that these compounds, at least in part, could be implementing their insulinotropic effects via the PKC pathway.…”