Abstract
The mechanisms of glucagon secretion and its suppression by glucose are presently unknown. This study investigates the relationship between intracellular calcium levels ([Ca(2+)](i)) and hormone secretion under low and high glucose conditions. We examined the effects of modulating ion channel activities on [Ca(2+)](i) and hormone secretion from ex vivo mouse pancreatic islets. Glucagon-secreting α-cells were unambiguously identified by cell specific expression of fluorescent proteins. We found that activation of L-type voltage-gated calcium channels is critical for α-cell calcium oscillations and glucagon secretion at low glucose levels. Calcium channel activation depends on K(ATP) channel activity but not on tetrodotoxin-sensitive Na(+) channels. The use of glucagon secretagogues reveals a positive correlation between α-cell [Ca(2+)](i) and secretion at low glucose levels. Glucose elevation suppresses glucagon secretion even after treatment with secretagogues. Importantly, this inhibition is not mediated by K(ATP) channel activity or reduction in α-cell [Ca(2+)](i). Our results demonstrate that glucose uncouples the positive relationship between [Ca(2+)](i) and secretory activity. We conclude that glucose suppression of glucagon secretion is not mediated by inactivation of calcium channels, but instead, it requires a calcium-independent inhibitory pathway.