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Ionic mechanisms involved in the regulation of insulin secretion by muscarinic agonists
S Bordin1, A C Boschero, E M Carneiro
1Laboratory of Cell Biology and Genetics, National Institute of Diabetes Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0840, USA.
Abstract:
The effects of the muscarinic agonist oxotremorine-m (oxo-m) on insulin secretion, K(+)-permeability and electrical activity from isolated mouse pancreatic islets were studied. Oxo-m potentiated glucose-induced insulin secretion in a dose-dependent manner, saturating at ca. 10 microM. At 11.2 mM glucose, oxo-m (0.1 and 10 microM) had two distinct effects on beta-cell electrical activity. Both concentrations increased the steady-state burst frequency, however, at 10 microM an initial and transient polarization was measured, and the subsequent activity was accompanied by a slight depolarization. The polarizing effect of oxo-m was almost completely suppressed by charybdotoxin (ChTX), a blocker of the large conductance (maxi) [Ca2+]i-activated potassium channel (K(Ca)). In the presence of 11.2 mM glucose, oxo-m (50 microM) provoked a significant and transient increase in the 86Rb efflux from perifused islets. This effect was inhibited by ChTX. ChTX also potentiated oxo-m stimulated insulin secretion in the presence of glucose. Finally, the balance between the polarizing and depolarizing effects of oxo-m was variable in different islets and depended on glucose concentration. Insulin secretion stimulated by oxo-m in the presence of glucose was more closely correlated to the agonist induced increase in burst frequency than to an increase in plateau fraction. We conclude that muscarinic stimulation has at least two effects on beta-cell electrical activity, an initial hyperpolarization, owing to activation of K(Ca) channels, followed by depolarization and high-frequency bursts, proposed to reflect the activation of a current sensitive to the depletion of intracellular Ca2+ stores (CRAC).
Insights
Muscarinic agonist oxotremorine-m enhances glucose-stimulated insulin secretion by affecting pancreatic beta-cell electrical activity. It initially hyperpolarizes cells via K(Ca) channels, then depolarizes them, increasing insulin release.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Physiology
Background:
- Muscarinic receptors play a role in regulating insulin secretion.
- Pancreatic beta-cell electrical activity is crucial for glucose-induced insulin release.
- The specific mechanisms by which muscarinic agonists influence beta-cell function require further elucidation.
Purpose of the Study:
- To investigate the effects of the muscarinic agonist oxotremorine-m (oxo-m) on insulin secretion.
- To analyze the impact of oxo-m on pancreatic beta-cell electrical activity and ion permeability.
- To determine the role of specific potassium channels in mediating oxo-m's effects.
Main Methods:
- Isolated mouse pancreatic islets were used to study insulin secretion and electrical activity.
- Measurements included glucose-induced insulin secretion, 86Rb efflux (a marker for K+ permeability), and beta-cell electrophysiology.
- The effects of oxo-m were examined alone and in combination with charybdotoxin (ChTX), a K(Ca) channel blocker.
Main Results:
- Oxo-m potentiated glucose-induced insulin secretion in a dose-dependent manner.
- Oxo-m induced distinct changes in beta-cell electrical activity: initial hyperpolarization followed by depolarization and increased burst frequency.
- The hyperpolarizing effect was mediated by large conductance calcium-activated potassium (K(Ca)) channels, while depolarization was linked to a CRAC current.
Conclusions:
- Muscarinic stimulation by oxo-m has dual effects on beta-cell electrical activity, involving K(Ca) channels and potentially CRAC channels.
- These electrical changes contribute to the enhanced insulin secretion observed in the presence of glucose.
- Charybdotoxin partially inhibited oxo-m's effects on ion flux but potentiated insulin secretion, suggesting complex channel interactions.