Glucose-stimulated insulin release depends on depolarisation of the β-cell plasma membrane and the generation of electrical activity. Depolarisation is thought to involve inhibition of KATP channels. However, we have recently reported that addition of a sub-stimulatory concentration of glucose (5 mM) was sufficient to completely inhibit KATP channel activity, and raising the glucose concentration to stimulatory levels (8-20 mM) had no further effect (Best, 2002a). Furthermore, glucose was able to evoke electrical activity in β-cells when KATP channel activity was completely inhibited by 0.5 mM tolbutamide (Best, 2002a). These findings indicate the existence of another ionic mechanism regulated by changes in glucose concentration.
Pancreatic β-cells were prepared from rats (humanely killed) and cultured for 1-14 days. Cell-attached channel recordings were made with a CsCl-rich pipette solution and diazoxide in the bath solution to clamp the membrane potential close to EK. In 20-40 % of patches, an anion-selective channel of approximately 200 pS could be recorded, which conducted an inward current at a pipette potential of 0 mV (Best, 2002b). With 142 mM Cl– in the pipette solution, current reversal occurred at a pipette potential of -52 mV. Channel activity was voltage dependent with a peak at a pipette potential of approximately -60 mV. The channel was activated by a rise in glucose concentration over the range 4-20 mM with a corresponding range of open probability from 0.07 to 0.86. Channel amplitude was not affected by altered glucose concentration. The channel was also activated by methylglyoxal, possibly due to its metabolism to D-lactate, but not by the non-metabolizable glucose analogue 3-O-methyl glucose. The channel was activated by hypotonic cell swelling and was sensitive to inhibition by the anion channel blockers 4,4â-dithiocyanatostilbene-2,2â-disulphonic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, 4-hydroxytamoxifen and DCPIB (4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl) oxobutyric acid).
It is suggested that the channel is the volume-sensitive anion channel previously described in insulin-secreting cells and that activation of this channel by glucose is the principal mechanism leading to depolarisation of the plasma membrane and hence electrical and secretory activity. The KATP channel could be important in hyperpolarizing the β-cell, thus preventing insulin release, during hypoglycaemia.
I should like to thank the NHS Executive North West for financial assistance.