Long Lasting Synchronization of Calcium Oscillations by Cholinergic Stimulation in Isolated Pancreatic Islets

Abstract

Individual mouse pancreatic islets exhibit oscillations in [Ca 21 ] i and insulin secretion in response to glucose in vitro, but how the oscillations of a million islets are coordinated within the human pancreas in vivo is unclear. Islet to islet synchronization is necessary, however, for the pancreas to produce regular pulses of insulin. To determine whether neurohormone release within the pancreas might play a role in coordinating islet activity, [Ca 21 ] i changes in 4-6 isolated mouse islets were simultaneously monitored before and after a transient pulse of a putative synchronizing agent. The degree of synchronicity was quantified using a novel analytical approach that yields a parameter that we call the ''Synchronization Index''. Individual islets exhibited [Ca 21 ] i oscillations with periods of 3-6 min, but were not synchronized under control conditions. However, raising islet [Ca 21 ] i with a brief application of the cholinergic agonist carbachol (25 mM) or elevated KCl in glucose-containing saline rapidly synchronized islet [Ca 21 ] i oscillations for $30 min, long after the synchronizing agent was removed. In contrast, the adrenergic agonists clonidine or norepinephrine, and the K ATP channel inhibitor tolbutamide, failed to synchronize islets. Partial synchronization was observed, however, with the K ATP channel opener diazoxide. The synchronizing action of carbachol depended on the glucose concentration used, suggesting that glucose metabolism was necessary for synchronization to occur. To understand how transiently perturbing islet [Ca 21 ] i produced sustained synchronization, we used a mathematical model of islet oscillations in which complex oscillatory behavior results from the interaction between a fast electrical subsystem and a slower metabolic oscillator. Transient synchronization simulated by the model was mediated by resetting of the islet oscillators to a similar initial phase followed by transient ''ringing'' behavior, during which the model islets oscillated with a similar frequency. These results suggest that neurohormone release from intrapancreatic neurons could help synchronize islets in situ. Defects in this coordinating mechanism could contribute to the disrupted insulin secretion observed in Type 2 diabetes.

Synchronization of mouse islets of Langerhans by glucose waveforms

“…The synchronization index (20) has been used to examine synchronization of ␤-cell [Ca 2ϩ ] i oscillations (25,26) and was used here as one method for quantifying the degree of synchrony between [Ca 2ϩ ] i oscillations in the ϳ20 islets measured in each experiment. This index ranges from 0 (not synchronized) to 1 (complete synchronization) and measures the differences in phase between two oscillators over time.…”

“…There are two main hypotheses to explain this interislet synchronization: periodic stimulation of acetylcholine from intrapancreatic ganglia that entrains islets (4,21,26) or a pancreas-liver feedback loop that produces oscillatory blood glucose levels, which then acts as a global metabolic signal to synchronize islets (3,7,16). There may be redundancy between these mechanisms in vivo, and several pathways may be utilized to synchronize insulin output.…”

Autocrine activation of P2Y1 receptors couples Ca2+ influx to Ca2+ release in human pancreatic beta cells

“…This could result from intragranular ATP accumulation via granule-resident vesicular nucleotide transporter [25]. It should be noted, however, that in intact rodent islets extracellular ATP plays an important role in synchronising the electrical and Ca 2+ responses among beta cells within and between islets through the induction of Ca 2+ release from InsP 3 -sensitive stores [26][27][28], in addition to stimulating exocytosis.…”

Lipotoxicity disrupts incretin-regulated human β cell connectivity

“…Given that various stimuli have been shown to alter the coordinated activity of rodent β cells (46,47), we determined whether lipotoxicity might perturb human islet dynamics to reduce insulin release evoked by glucose or a range of nonfuel secretagogues. FFAs appeared to specifically target incretin action primarily through the disruption of β cell interactivity, since impairment of insulin Figure 11), suggesting that they were likely due to simultaneous depolarization throughout the recorded β cell population.…”