Insulinotropic action of white lupine seeds (Lupinus albus L.): Effects of ion fluxes and insulin secretion from isolated pancreatic islets

Abstract

Debittered seeds of Lupinus albus L. are generally considered to have antidiabetic properties and are even prescribed by some physicians to counter hyperglycemia. We have assessed the insulin releasing action of an aqueous lupine extract (ALE) on isolated rat pancreatic islets and investigated the underlying ionic mechanism. Transmembrane Ca2+ and K+ fluxes were assessed radioisotopically from rat islets preloaded with 45Ca2+ and 86Rb+. Mouse β-cell membrane potential was recorded via high-resistance microelectrodes. ALE had a pronounced albeit transient (∼15 min) stimulatory effect on insulin secretion in the presence of 7 mM glucose, and minimal effect at 3 mM glucose. ALE increased 45Ca2+ efflux from preloaded islets both in the presence and absence of extracellular Ca2+. ALE decreased 86Rb+ efflux from preloaded islets in the presence of 3 mM glucose while increasing it in high glucose (16.7 mM). ALE depolarized the β-cell membrane in the absence of glucose and changed the regular bursting pattern of electrical activity into a continuous firing pattern in the presence of 11 mM glucose. It is concluded that seeds of Lupinus albus L. contain water-soluble substances with capacity to potentiate glucose-induced insulin release. The lupine extract appears to contain at least two active principles, one of them causing Ca2+ release from intracellular stores and the other a reduction in β-cell K+ permeability, presumably via inhibition of ATP-sensitive K+ channels. White lupines represent therefore an hitherto unexplored source for the isolation of new oral hypoglycemic agents, potentially useful for the treatment of type 2 diabetes.