Pancreatic β-cells are electrically active cells and release insulin in response to glucose through marked changes in the cell membrane potential. The resting membrane potential is determined by the Na⁺,K⁺-ATPase pump and K_ATP channels. Following glucose metabolism, K_ATP channel closure facilitates a depolarisation of the membrane, leading to the opening of voltage-gated calcium channels, and insulin is released as a consequence of Ca²⁺ influx. Delayed rectifier potassium (Kv) channels play a crucial role in determining β-cell electrical activity since they repolarise action potentials and thereby limit Ca²⁺ channel activity. Since over-expression of Kv channels in β-cells leads to diabetes in transgenic animals, we have investigated the expression of Kv channel mRNAs and their function in insulin-secreting cells isolated from patients with hyperinsulinism.

All experiments were carried out in vitro using tissues obtained from control donors (n = 6), and patients with hyperinsulinism in infancy (HI) (n = 11) or insulinoma (n = 1). RNA was extracted from isolated tissues by standard protocols and, with specific oligonucleotide primers, RT-PCR was used to document the expression of α-subunit (Kv1.1 to 1.6, Kv2.1, 2.2, Kv3.1, 3.3, 3.4, Kv4.1 and 4.2) and β-subunit (Kvβ1.1, 2.1 and 3.1) mRNAs for human Kv channels. Functional data were obtained using the whole-cell patch-clamp techniques under standard conditions; Kv currents were evoked by holding the cells at -70 mV and applying a series of 10 mV step depolarisations of 500 ms duration from -60 mV to +80 mV. Results are expressed as means ± S.E.M. and the Mann-Whitney rank sum test was used to test for statistically significant differences.

Analysis of Kv channel mRNAs was undertaken and revealed the differential loss of several Kv channel subunit mRNAs in HI islets (n = 3). Specifically, we found that all HI islet preparations failed to express Kv3.3 and the Kvβ2.1 subunit, which is used as a chaperone protein for Kv channel α-subunits. Additionally, islets from one patient with a contiguous gene deletion of Ch11p15 also failed to express Kv3.1 mRNA. In control (n = 40 experiments) and patient tissues defined as either focal or diffuse HI (n = 10 patients, 56 experiments), Kv channel currents were readily recorded and exhibited similar peak current values in both tissues: 133 ± 9 pA pF^-1 vs. 164 ± 12 pA pF^-1 (n.s.) in control and HI, respectively. However, in one patient with an atypical form of HI in which K_ATP channel function was preserved, the magnitude of Kv currents was markedly reduced with respect to control values: 24 ± 6 pA pF^-1 (n = 6, P < 0.001).

In summary we have documented the mRNA expression profiles and function of Kv channels in control human insulin-secreting cells and in tissues isolated from patients with hyperinsulinism in infancy. In typical HI β-cells, Kv channels are operational despite altered expression of mRNAs, but in those patients with non-typical disease where HI arises without K_ATP channel defects, our data suggest that defects in Kv channels may be related to the pathogenesis of HI.