Factors Affecting the Uptake of Glibenclamide in Microdissected Pancreatic Islets Rich in β-Cells

Abstract

The uptake of radioactively labelled glibenclamide was studied in microdissected pancreatic islets of obese-hyperglycemic mice. The steady-state concentration reached was linearly related to the medium concentration of glibenclamide. This is in contrast to the short-term uptake, for which there was evidence for saturation. The amounts of glibenclamide incorporated increased by more than 100 % when the plasma membranes were made permeable to sucrose by reducing the osmolality of the incubation medium. No binding of glibenclamide in excess of sucrose could be detected after increasing the albumin concentration to 10 mg/ml. The steady-state concentrations reached were approximately the same in the presence of different monovalent cations. The observation of a reduced uptake of glibenclamide in the presence of cysteine raises the question whether sulfonylureas stimulate insulin release by reacting with sulf-hydryl groups in the /3-cell membrane.

Antidiabetic sulfonylurea stimulates insulin secretion independently of plasma membrane KATPchannels

“…This is consistent with the K d for calcium of rhod-2 (500 -700 nM; Ref. 25) and that stimulated ␤-cell intracellular calcium generally transits to within twofold on either side of these values. With one-to-one binding stoichiometry of calcium and rhod-2, 0.90 saturation would occur at ϳ5 M, an order of magnitude above the peak ␤-cell calcium level.…”

“…BSA was used at 0.1%, where it blocks nonspecific binding sites for insulin. BSA, however, also is well known to bind glibenclamide (25,26). Therefore, for perifusion, we titrated glibenclamide to the minimum final total concentration in the superfusate (4 M) that rapidly achieved secretory response rate amplitudes that were comparable to those achieved by 20 mM glucose.…”

“…Glibenclamide is distinguished from other antidiabetic sulfonylureas, not only by its superior secretagogue potency but also by its exceptional ability to be internalized within ␤-cells (23,25,26). Furthermore, glibenclamide has been shown to localize to high-affinity sites of the insulin secretory granule membrane (7,34), which recently have been identified as K ATP channel subunits (21,47,50).…”

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

“…Repolarization of B cells (by withdrawal of tolbutamide) or temporary displacement of calcium from the channel (by omission of the cation or addition of its antagonist, cobalt) reactivate the calcium channels in B cells and in axons [32], and restore the releasing properties of tolbutamide. It is unlikely that reactivation of the calcium channels results from a reduction in tolbutamide binding, since binding of sulphonylureas to islet cells is unaffected by Ca 2+ omission from a salt-balanced medium [33]. It seems rather that calcium itself, together with depolarization, is involved in the inactivation of the Ca 2+ uptake mechanism [34]; involvement of Ca 2+ in the depolarizing effect of tolbutamide is also possible.…”

Binding of hypoglycaemic sulphonylureas to an artificial phospholipid bilayer

“…The molecular mechanism responsible for the change in K § permeability is not known [7]. Radioisotopic studies of sulphonylurea uptake by isolated islets suggest that these hypoglycaemic agents are bound to the plasma membrane of pancreatic B cells [8][9][10], which could be equipped with sulphonylurea receptors [111. The cell boundary [12] could therefore represent the primary site of action of hypoglycaemic sulphonylureas.…”