Glucose Inhibits Glucagon Secretion from Pancreatic Alpha-Cells by Closure of K ATP-Channels

Abstract

Glucose inhibits glucagon secretion but the mechanisms involved are not known. We performed whole-cell perforated patch membrane potential recordings on alpha-cells in intact mouse pancreatic islets. Alpha-cells are electrically active at low glucose concentrations (<1 mM) and generate overshooting action potentials that peak at +10 mV. These action potentials result from activation of voltage-gated Na++ and A-type K+-channels. The most negative interspike membrane potential is approx. −45 mV. Application of 6 mM glucose reversibly depolarized the alpha-cell by approx. 5 mV and reduced peak voltage of the action potentials by up to 15 mV. We estimated the impact this decrease in action potential amplitude on alpha-cell exocytosis by capacitance measurements of depolarization-evoked exocytosis. We found that exocytosis is reduced by approx. 7% for every mV reduction of spike height. Thus, the glucose-induced reduction of spike height is sufficient to account for the 70-80% inhibition of glucagon secretion measured biochemically. We also measured alpha-cell electrical activity elicited by current injection and found that small depolarizations above the normal interspike voltage mimic the effects of glucose on action potential height. In parallel measurements of glucagon secretion, the inhibitory effect of glucose could be mimicked by tolbutamide (a blocker of KATP-channels) and antagonized by diazoxide (an activator of KATP-channels). We conclude that glucose inhibition of glucagon secretion involves closure of plasmalemmal KATP-channels. The resulting membrane depolarization leads to voltage-dependent inactivation of the ion channels involved in action potential firing and a lowered action potential amplitude. This culminates in reduced activation of the N-type Ca2+-channels mediating the Ca2+ entry required to trigger exocytosis of the glucagon-containing secretory vesicles. Thus, alpha-cells provide an example where reduced K+-conductance is associated with lowered rather than increased (as seen in most other cell types) membrane excitability.