ATP-sensitive potassium (K_ATP) channels of the pancreas play a key role in the regulation of glucose-stimulated insulin secretion (GSIS). They couple the metabolic activity to membrane excitability, preventing insulin secretion when the metabolic state of the cell is low. Although glucose regulation of K_ATP channel function has been well characterised, it is not known whether the metabolic state of the cell has any influence on regulation of channel density within the cell.

To address this, changes in K_ATP channel density were examined in the GSIS responsive INS-1 β-cell line. K_ATP channels were visualised using antibodies raised against each of the two channel subunits SUR1 and Kir6.2 then viewed by confocal microscopy. Cells grown in 3 mM glucose showed much higher K_ATP channel density compared with those in 25 mM glucose. By contrast, cells grown overnight in 25 mM glucose showed reduced expression, but when returned to 3 mM glucose there was rapid increase in K_ATP channel expression. Cells previously incubated in 3 mM glucose when grown in 25 mM showed little change in K_ATP expression. The level of K_ATP expression was also seen to be dependent on glucose concentration. At hypoglycaemic and normal physiological glucose (2-5 mM) K_ATP channel expression is high, whereas in hyperglycaemic glucose (> 11 mM) K_ATP channel expression is decreased.

Low glucose levels stimulate an increase in the expression of the K_ATP channel subunits SUR1 and Kir6.2. To study the rate of K_ATP induction we used [³⁵S]-methionine pulse chase analysis. Cell lysates were collected, subjected to immunoprecipitation with anti-SUR1 antibody and proteins subjected to SDS-PAGE/ autoradiography. Bands of ~260 and ~175 kDa correspond to the expected sizes of mature (glycosylated) and immature forms of SUR1, respectively. In addition, a 44 kDa band corresponding to the size of Kir6.2 was co-immunoprecipitated. Expression of both the mature SUR1 and Kir6.2 increased in parallel, inducing an increase in the channel density of the functional channel (n = 3).

The fact that the increase occurs within minutes of lowering glucose indicates that the change in channel density is likely to have an important physiological role in GSIS. One possibility is that the increased channel numbers together with the increased activity of the channel may help to rapidly terminate insulin secretion and thus protect the animal from any danger of severe hypoglycaemia.

This work was supported by the MRC and the Emma and Leslie Reid Scholarship.