Dual Mode of Action of Acetylcholine on Cytosolic Calcium Oscillations in Pancreatic Beta and Acinar Cells In Situ

Abstract

Cholinergic innervation in the pancreas controls both the release of digestive enzymes to support the intestinal digestion and absorption, as well as insulin release to promote nutrient use in the cells of the body. The effects of muscarinic receptor stimulation are described in detail for endocrine beta cells and exocrine acinar cells separately. Here we describe morphological and functional criteria to separate these two cell types in situ in tissue slices and simultaneously measure their response to ACh stimulation on cytosolic Ca2+ oscillations [Ca2+]c in stimulatory glucose conditions. Our results show that both cell types respond to glucose directly in the concentration range compatible with the glucose transporters they express. The physiological ACh concentration increases the frequency of glucose stimulated [Ca2+]c oscillations in both cell types and synchronizes [Ca2+]c oscillations in acinar cells. The supraphysiological ACh concentration further increases the oscillation frequency on the level of individual beta cells, inhibits the synchronization between these cells, and abolishes oscillatory activity in acinar cells. We discuss possible mechanisms leading to the observed phenomena.

High resolution analysis of the cytosolic Ca2+events in beta cell collectives in situ

“…These results, together with the previously published data[27], suggest that pharmacological activation of both RYR or IP 3 intracellular release channel could produce [Ca 2+ ] c events in beta cells that would be regularly observed during the 8 mM glucose stimulation. Both stimulations generated compound [Ca 2+ ] c events through inter-molecular CICR and presented two distinct kinetic components for intracellular release of Ca 2+ and distinct regimes of intercellular coordination.…”

“…In addition to support sufficient Ca 2+ load in the ER, glucose-dependent effects in beta cells provide all key substrates, like ATP and cAMP, to directly trigger and modulate the activation of intracellular Ca 2+ release channels, even in the absence of plasma membrane depolarization that would increase the opening probability of VACCs. Included in these stimuli is the parasympathetic release of ACh binding to muscarinic ACh receptors (mAChRs) and induce insulin release via the production of IP 3 and Ca 2+ release from the intracellular stores[27, 55–57]. Previous studies, including our own, systematically underestimated the role of RYR and IP 3 receptors due to pre-emptying of intracellular Ca 2+ stores in too low (0-3 mM) extracellular glucose.…”

“…Specific pharmacological modulation of RYR in intact beta cell collectives stimulated with 8 mM glucose and recorded with a unique spatio-temporal resolution demands an updated model of beta cell activation and bursting activity. According to our model, multiple time scales of events are generated with a progressive temporal superposition of ultra-short events, which represent CICR of either IP 3 [27] or RYR Ca 2+ release channels. Both intracellular channels can be directly stimulated by glucose.…”

“…Increased frequency of [Ca 2+ ] c events was also recorded in our experiment using low extracellular Ca 2+ concentration. We demonstrate that glucose around the threshold (6-7 mM glucose) supports ER Ca 2+ release through IP 3 [27] and RYR release channels. In addition to support sufficient Ca 2+ load in the ER, glucose-dependent effects in beta cells provide all key substrates, like ATP and cAMP, to directly trigger and modulate the activation of intracellular Ca 2+ release channels, even in the absence of plasma membrane depolarization that would increase the opening probability of VACCs.…”

Calcium imaging in intact mouse acinar cells in acute pancreas tissue slices

“…From the functional point of view, ACh and CCK are potent triggers for the stimulus secretion cascade (SSC) that results in secretion of enzymes. Previous studies demonstrated that acinar cells respond to these agonists with [Ca 2+ ] i oscillations, but with different temporal patterns [ 91 , 98 , 99 ], that there is typically a delay between the [Ca 2+ ] i increase in the apical and the basal pole ( S1 Fig ) [ 25 ], and that high concentrations induce a biphasic response lacking oscillations [ 90 , 100 – 102 ]. Scarce and partially conflicting data are available on how acinar cells transduce differences in ACh and CCK concentration into differences in [Ca 2+ ] i .…”