Insulin secretion from pancreatic islet β-cells occurs in a pulsatile fashion, with a typical period of ∼5 min. The basis of this pulsatility in mouse islets has been investigated for more than four ...decades, and the various theories have been described as either qualitative or mathematical models. In many cases the models differ in their mechanisms for rhythmogenesis, as well as other less important details. In this Perspective, we describe two main classes of models: those in which oscillations in the intracellular Ca
concentration drive oscillations in metabolism, and those in which intrinsic metabolic oscillations drive oscillations in Ca
concentration and electrical activity. We then discuss nine canonical experimental findings that provide key insights into the mechanism of islet oscillations and list the models that can account for each finding. Finally, we describe a new model that integrates features from multiple earlier models and is thus called the Integrated Oscillator Model. In this model, intracellular Ca
acts on the glycolytic pathway in the generation of oscillations, and it is thus a hybrid of the two main classes of models. It alone among models proposed to date can explain all nine key experimental findings, and it serves as a good starting point for future studies of pulsatile insulin secretion from human islets.
Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in ...metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.
Type 2 diabetes (T2D) is generally thought to result from the combination of 2 metabolic defects, insulin resistance, which increases the level of insulin required to maintain glucose within the ...normal range, and failure of insulin-secreting pancreatic β-cells to compensate for the increased demand. We build on a mathematical model pioneered by Topp and colleagues to elucidate how compensation succeeds or fails. Their model added a layer of slow negative feedback to the classic insulin-glucose loop in the form of a slow, glucose-dependent birth and death law governing β-cell mass. We add to that model regulation of 2 aspects of β-cell function on intermediate time scales. The model quantifies the relative contributions of insulin action and insulin secretion defects to T2D and explains why prevention is easier than cure. The latter is a consequence of a threshold separating the normoglycemic and diabetic states (bistability), which also underlies the success of bariatric surgery and acute caloric restriction in rapidly reversing T2D. The threshold concept gives new insight into “Starling's Law of the Pancreas,” whereby insulin secretion is higher for prediabetics and early diabetics than for normal individuals.
Previously, we have used mathematical modeling to gain mechanistic insights into insulin-stimulated glucose uptake. Phosphatidylinositol 3-kinase (PI3K)-dependent insulin signaling required for ...metabolic actions of insulin also regulates endothelium-dependent production of the vasodilator nitric oxide (NO). Vasodilation increases blood flow that augments direct metabolic actions of insulin in skeletal muscle. This is counterbalanced by mitogen-activated protein kinase (MAPK)-dependent insulin signaling in endothelium that promotes secretion of the vasoconstrictor endothelin-1 (ET-1). In the present study, we extended our model of metabolic insulin signaling into a dynamic model of insulin signaling in vascular endothelium that explicitly represents opposing PI3K/NO and MAPK/ET-1 pathways. Novel NO and ET-1 subsystems were developed using published and new experimental data to generate model structures/parameters. The signal-response relationships of our model with respect to insulin-stimulated NO production, ET-1 secretion, and resultant vascular tone, agree with published experimental data, independent of those used for model development. Simulations of pathological stimuli directly impairing only insulin-stimulated PI3K/Akt activity predict altered dynamics of NO and ET-1 consistent with endothelial dysfunction in insulin-resistant states. Indeed, modeling pathway-selective impairment of PI3K/Akt pathways consistent with insulin resistance caused by glucotoxicity, lipotoxicity, or inflammation predict diminished NO production and increased ET-1 secretion characteristic of diabetes and endothelial dysfunction. We conclude that our mathematical model of insulin signaling in vascular endothelium supports the hypothesis that pathway-selective insulin resistance accounts, in part, for relationships between insulin resistance and endothelial dysfunction. This may be relevant for developing novel approaches for the treatment of diabetes and its cardiovascular complications.
The pancreatic islets of Langerhans are biomedically important because they are home to the beta cells that secrete insulin and are hence important for understanding diabetes. They are also an ...important case study for the mechanisms of bursting oscillations and how these oscillations emerge from the electrical coupling of highly heterogeneous cells. Early work has pointed to a voting/democratic paradigm, where the islet properties are a nonlinear average of the cell properties, with no ‘conductor leading the orchestra’. Recent experimental work has uncovered new facets of this heterogeneity, and has identified small world networks dominated by a small subset of cells with a high degree of functional connectivity, assessed via correlations of calcium oscillations. It has also been suggested that these connectivity hubs act as pacemakers necessary for islet oscillations. We reviewed modeling studies that have confirmed the existence of small worldness, and we did not find evidence for obligatory pacemakers. We conclude that democracy rather than oligarchy remains the most likely organizing principle of the islets.
The timing of increase in 1-hour PG and its utility as an earlier predictor of both prediabetes (PreDM) and type 2 diabetes (T2D) compared to 2-hour PG (2 h-PG) are unknown. To evaluate the timing of ...crossing of the 1 h-PG ≥ 155 mg/dl (8.6 mmol/L) for PreDM and 209 mg/dl (11.6 mmol/L) for T2D and respective current 2 h-PG thresholds of 140 mg/dl (7.8 mmol/L) and 200 mg/dl (11.1 mmol/L).
Secondary analysis of 201 Southwest Native Americans who were followed longitudinally for 6–10 years and had at least 3 OGTTs.
We identified a subset of 43 individuals who first developed PreDM by both 1 h-PG and 2 h-PG criteria during the study. For most (32/43,74%), 1 h-PG ≥ 155 mg/dl was observed before 2 h-PG reached 140 mg/dl (median IQR: 1.7 -0.25, 4.59 y; mean ± SEM: 5.3 ± 1.9 y). We also identified a subset of 33 individuals who first developed T2D during the study. For most (25/33, 75%), 1 h-PG reached 209 mg/dl earlier (median 1.0 -0.56, 2.02 y; mean ± SEM: 1.6 ± 0.8 y) than 2 h-PG reached 200 mg/dl, diagnostic of T2D.
1 h-PG ≥ 155 mg/dl is an earlier marker of elevated risk for PreDM and T2D than 2 h-PG ≥ 140 mg/dl.
The standard model for Ca2+ oscillations in insulin-secreting pancreatic β cells centers on Ca2+ entry through voltage-activated Ca2+ channels. These work in combination with ATP-dependent K+ ...channels, which are the bridge between the metabolic state of the cells and plasma membrane potential. This partnership underlies the ability of the β cells to secrete insulin appropriately on a minute-to-minute time scale to control whole body plasma glucose. Though this model, developed over more than 40 years through many cycles of experimentation and mathematical modeling, has been very successful, it has been challenged by a hypothesis that calcium-induced calcium release from the endoplasmic reticulum through ryanodine or inositol trisphosphate (IP3) receptors is instead the key driver of islet oscillations. We show here that the alternative model is in fact incompatible with a large body of established experimental data and that the new observations offered in support of it can be better explained by the standard model.
Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP‐sensitive potassium channel (KATP) links glucose metabolism to islet electrical activity in these cells by responding to ...increased cytosolic ATP/ADP. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell KATP locally produces the ATP that inhibits KATP activity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside‐out patches inhibited KATP. To test the relative contributions of local vs. mitochondrial ATP production, we recorded KATP activity using mouse beta cells and INS‐1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of KATP activity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well‐known action of protons to inhibit KATP. In cell‐attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on KATP channel closure by glucose, further suggesting that PK is not an important regulator of KATP. In contrast, addition of mitochondrial inhibitors robustly increased KATP activity. Finally, by measuring the ATP/ADP responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise ATP/ADP even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of KATP in pancreatic beta cells.
Key points
Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit KATP activity in excised patches. PEP solutions only inhibit KATP activity if the pH is unbalanced.
Modulating pyruvate kinase has minimal effects on KATP activity.
Mitochondrial inhibition, in contrast, robustly potentiates KATP activity in cell‐attached patches.
Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close KATP.
Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the KATP activity of pancreatic beta cells.
figure legend The intracellular mechanisms that control KATP channel activity in insulin‐secreting pancreatic beta cells were investigated. From inside‐out and cell‐attached electrophysiological recordings, we found that KATP activity was not significantly affected by ATP synthesized by pyruvate kinase. However, in intact beta cells perifused with high glucose concentrations, we observed a dramatic increase in KATP channel activity when mitochondrially produced ATP was inhibited by NaN3 or rotenone (two inhibitors of the electron transport chain). These results confirm that the ATP synthesized by oxidative phosphorylation is the main factor responsible for KATP modulation in beta cells. This mechanism was also supported by simulations of the Integrated Oscillator Model for beta cells. The top trace in the figure corresponds to a cell‐attached recording of KATP obtained from mouse beta cells under the indicated conditions.
Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP-sensitive potassium channel (K
) links glucose metabolism to islet electrical activity in these cells by responding to ...increased cytosolic ATP/ADP. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell K
locally produces the ATP that inhibits K
activity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside-out patches inhibited K
. To test the relative contributions of local vs. mitochondrial ATP production, we recorded K
activity using mouse beta cells and INS-1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of K
activity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well-known action of protons to inhibit K
. In cell-attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on K
channel closure by glucose, further suggesting that PK is not an important regulator of K
. In contrast, addition of mitochondrial inhibitors robustly increased K
activity. Finally, by measuring the ATP/ADP responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise ATP/ADP even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of K
in pancreatic beta cells. KEY POINTS: Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit K
activity in excised patches. PEP solutions only inhibit K
activity if the pH is unbalanced. Modulating pyruvate kinase has minimal effects on K
activity. Mitochondrial inhibition, in contrast, robustly potentiates K
activity in cell-attached patches. Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close K
. Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the K
activity of pancreatic beta cells.
The mammalian pituitary gland is a complex organ consisting of hormone‐producing cells, anterior lobe folliculostellate cells (FSCs), posterior lobe pituicytes, vascular pericytes and endothelial ...cells, and Sox2‐expressing stem cells. We present single‐cell RNA sequencing and immunohistofluorescence analyses of pituitary cells of adult female rats with a focus on the transcriptomic profiles of nonhormonal cell types. Samples obtained from whole pituitaries and separated anterior and posterior lobe cells contained all expected pituitary resident cell types and lobe‐specific vascular cell subpopulations. FSCs and pituicytes expressed S100B, ALDOC, EAAT1, ALDH1A1, and VIM genes and proteins, as well as other astroglial marker genes, some common and some cell type‐specific. We also found that the SOX2 gene and protein were expressed in ~15% of pituitary cells, including FSCs, pituicytes, and a fraction of hormone‐producing cells, arguing against its stem cell specificity. FSCs comprised two Sox2‐expressing subclusters; FS1 contained more cells but lower genetic diversity, while FS2 contained proliferative cells, shared genes with hormone‐producing cells, and expressed genes consistent with stem cell niche formation, regulation of cell proliferation and stem cell pluripotency, including the Hippo and Wnt pathways. FS1 cells were randomly distributed in the anterior and intermediate lobes, while FS2 cells were localized exclusively in the marginal zone between the anterior and intermediate lobes. These data indicate the identity of the FSCs as anterior pituitary‐specific astroglia, with FS1 cells representing differentiated cells equipped for classical FSC roles and FS2 cells exhibiting additional stem cell‐like features.
Main Points
Pituitary folliculostellate cells (FSCs) were studied with scRNAseq and immunohistofluorescence.
Expression profiles of FSCs and pituicytes confirmed their astroglial identity.
FSC subtypes included pituitary stem‐like cells and differentiated FSCs.
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