Glucagon, the body's main hyperglycemic hormone, is produced by α-cells of the islets of Langerhans. A fall in plasma glucose stimulates glucagon release and it acts by increasing hepatic glucose production. How glucose regulates glucagon secretion remains debated and both intrinsic and paracrine mechanisms have been proposed. The fact that glucagon secretion becomes dysregulated in diabetes warrants detailed studies of the cell physiology of its release. We generated mice that express the light-sensitive cation channel Channelrhodopsin-2 (ChR2) under the control of the proglucagon promoter in the α-cells. This allowed us to selectively depolarize the plasma membrane of α-cells within intact isolated islets (using 488 nm laser) and record membrane potential, cytoplasmic Ca2+ ([Ca2+]i) glucagon release in response to the optical activation. At low (1mM) glucose, optoactivation depolarized the α-cells, induced a transient increase in [Ca2+]i, decreased peak voltage of the action potentials (from -2±1mV to -11±1mV, n=5, p<0.01) and reduced glucagon secretion by 50%. At elevated glucose (6-20mM, which inhibited secretion by ∼50% relative that at 1 mM glucose), α-cell action potential firing persisted and optoactivation remained capable of depolarizing the α-cell but now without affecting action potential peal voltage. Under these conditions, optoactivation did not reduce glucagon secretion beyond the inhibition produced by high glucose alone. It was ascertained that optoactivation of α-cells did not affect insulin secretion. The data reinforce earlier reports that membrane depolarization (despite increasing [Ca2+]I and stimulating action potential firing) produces a paradoxical inhibition of glucagon secretion via reduced action potential height. We propose that further optogenetic studies represent a means for non-invasive interrogation of the complex regulation of glucagon secretion in intact pancreatic islets.
C.A. Miranda: None. H. Dou: None. J. Tolö: None. A.I. Tarasov: Employee; Spouse/Partner; Vaccitech Ltd, Oxford, UK. P. Rorsman: None.
Swedish Research Council