Tolbutamide stimulation and inhibition of insulin release: Studies of the underlying ionic mechanisms in isolated rat islets

Abstract

Summary. The effects of tolbutamide on insulin release, 45Ca2+ uptake and 86Rb+ efflux were studied in isolated rat islets. At a low glucose concentration (75 mg/dl), tolbutamide (20-500 ~tg/ml) produced a rapid, dose-dependent increase in insulin release from perifused islets. After 30-40 min however, the rate of secretion as well as the potentiating effect of theophylline were inversely related to the concentration of sulphonylurea. The monophasic release of insulin triggered by tolbutamide (100 ,ug/ml) at low glucose could be evoked again by removing and reintroducing the drug, or by temporarily withdrawing calcium or adding cobalt to the medium. Tolbutamide (20 ,ug/ml) accelerated and potentiated the biphasic insulin release in response to a secondary stimulation by glucose (150mg/dl). By contrast, 100 gg/ml tolbutamide reduced the releasing effect of glucose to a slow increase in secretion rates. Theophylline normalized the second phase of release, but did not restore the rapid phase. Tolbutamide stimulated 45Ca 2+ influx (2 rain-uptake) in islet cells; this effect was maximum immediately after addition of the drug and decreased later on, exhibiting a monophasic pattern. Glucose stimulation of Ca 2+ uptake (5 min) was reduced in the presence of 100 ~g/ml tolbutamide. At a low glucose concentration, tolbutamide reversibly reduced 86Rb+ efflux (tracer of K +) from islet cells, without altering the further inhibition of this efflux by a later glucose increase. It is suggested that tolbutamide depolarizes B cells partially by reducing their K § permeability. This depolarization leads to opening of voltagedependent calcium channels and the resulting Ca 2 § influx triggers insulin release. The important and maintained depolarization by high concentrations of tolbutamide may secondarily inactivate these channels and cause a decrease in Ca 2+ influx. This could explain the monophasic release of insulin and the refractoriness of B cells to subsequent glucose stimulation.Key words: Isolated rat islets, tolbutamide, insulin release, glucose metabolism, rubidium efflux, calcium uptake, glucose, theophylline, cobalt, potassium.Sulphonylureas have been used for more than twenty years to treat diabetic patients, but, despite intensive investigation, the mechanisms by which they decrease blood glucose levels are not completely understood. The role of the pancreas in the hypoglycaemic action of sulphonylureas has been established by the pioneer work of Loubati6res [1] and was confirmed by the demonstration that these drugs stimulate insulin release in man [2] and animals [3] by a direct effect on B cells demonstrated in vitro [4]. This insulinotropic action of acute administration of sulphonylureas has since been extensively studied; its characteristics and possible mechanisms have been the subject of several reviews [5][6][7][8]. Many clinical investigations have failed, however, to show a correlation between the lowering of blood glucose and an increase in plasma insulin levels after chronic treatment with sulphonylureas ...

Energy-Dependent Modulation of Glucagon-Like Signaling inDrosophilavia the AMP-Activated Protein Kinase

“…AMPK has been shown to modulate the biophysical properties of the twin pore K + (TWIK) channels, and while it is currently unknown if AKH cells express similar channels, we speculate that AMPK is similarly modulating an unknown channel conductance in AKH cells. There is evidence that AKH cells express the K + ATP channels based on in situ analysis and that dietary introduction of a specific K + ATP channel antagonist, tolbutamide (Henquin 1980), leads to behavioral phenotypes consistent with blocking AKH release (Kim and Rulifson 2004). AMPK has been shown to regulate the activation of this channel subtype (Yoshida et al 2012).…”

Insulinotropic effects of hypoglycaemic and hyperglycaemic sulphonamides: The ionophoretic hypothesis

“…First, sulphonylureas may cause a decrease in K § conductance [9,10] leading to subsequent cell depolarization and gating of voltage-dependent Ca 2+ channels; however, under suitable conditions, tolbutamide is able to stimulate Ca 2+ influx and insulin release without decreasing K + conductance [11]. Second, sulphonylureas may directly interfere with an ionophoretic modality of Ca transport across the B-cell plasma membrane [12][13][14][15].…”

Gymnema sylvestre stimulates insulin release in vitro by increased membrane permeability

“…Sulphonylureas act at a proximal stage in the -cell stimulus-secretion coupling cascade: they close plasma membrane ATP-sensitive K + channels and the consequent decrease in K + efflux depolarises the cells, leading to Ca 2+ influx via voltage-operated Ca 2+ channels (Henquin 1980, Sturgess et al 1985. Increases in intracellular Ca 2+ are sufficient to stimulate insulin release from -cell secretory granules ( Jones et al 1985).…”