SummaryGlucagon, secreted by pancreatic islet α cells, is the principal hyperglycemic hormone. In diabetes, glucagon secretion is not suppressed at high glucose, exacerbating the consequences of insufficient insulin secretion, and is inadequate at low glucose, potentially leading to fatal hypoglycemia. The causal mechanisms remain unknown. Here we show that α cell KATP-channel activity is very low under hypoglycemic conditions and that hyperglycemia, via elevated intracellular ATP/ADP, leads to complete inhibition. This produces membrane depolarization and voltage-dependent inactivation of the Na+ channels involved in action potential firing that, via reduced action potential height and Ca2+ entry, suppresses glucagon secretion. Maneuvers that increase KATP channel activity, such as metabolic inhibition, mimic the glucagon secretory defects associated with diabetes. Low concentrations of the KATP channel blocker tolbutamide partially restore glucose-regulated glucagon secretion in islets from type 2 diabetic organ donors. These data suggest that impaired metabolic control of the KATP channels underlies the defective glucose regulation of glucagon secretion in type 2 diabetes.

Leptin plays an important role in the control of food intake, energy expenditure, metabolism, and body weight. This hormone also has a key function in the regulation of glucose homeostasis. Although leptin acts through central and peripheral mechanisms to modulate glucose metabolism, the pancreatic b-cell of the endocrine pancreas is a critical target of leptin actions. Leptin receptors are present in the b-cell, and their activation directly inhibits insulin secretion from these endocrine cells. The effects of leptin on insulin occur also in the long term, since this hormone inhibits insulin gene expression as well. Additionally, b-cell mass can be affected by leptin through changes in proliferation, apoptosis, or cell size. All these different functions in the b-cell are triggered by leptin as a result of the large diversity of signaling pathways that this hormone is able to activate in the endocrine pancreas. Therefore, leptin can participate in glucose homeostasis owing to different levels of modulation of the pancreatic b-cell population. Furthermore, it has been proposed that alterations in this level of regulation could contribute to the impairment of b-cell function in obesity states. In the present review, we will discuss all these issues with special emphasis on the effects and pathways of leptin signaling in the pancreatic b-cell.

Adrenaline is a powerful stimulus of glucagon secretion. It acts by activation of β-adrenergic receptors, but the downstream mechanisms have only been partially elucidated. Here, we have examined the effects of adrenaline in mouse and human α-cells by a combination of electrophysiology, imaging of Ca and PKA activity, and hormone release measurements. We found that stimulation of glucagon secretion correlated with a PKA- and EPAC2-dependent (inhibited by PKI and ESI-05, respectively) elevation of [Ca] in α-cells, which occurred without stimulation of electrical activity and persisted in the absence of extracellular Ca but was sensitive to ryanodine, bafilomycin, and thapsigargin. Adrenaline also increased [Ca] in α-cells in human islets. Genetic or pharmacological inhibition of the Tpc2 channel (that mediates Ca release from acidic intracellular stores) abolished the stimulatory effect of adrenaline on glucagon secretion and reduced the elevation of [Ca] Furthermore, in Tpc2-deficient islets, ryanodine exerted no additive inhibitory effect. These data suggest that β-adrenergic stimulation of glucagon secretion is controlled by a hierarchy of [Ca] signaling in the α-cell that is initiated by cAMP-induced Tpc2-dependent Ca release from the acidic stores and further amplified by Ca-induced Ca release from the sarco/endoplasmic reticulum.