Sulfonylurea Stimulation of Insulin Secretion Peter Proks,1 Frank Reimann,2 Nick Green,1 Fiona Gribble,2 and Frances Ashcroft1 Sulfonylureas are widely used to treat type 2 diabetes smooth, and skeletal muscle, and some brain neurones. In because they stimulate insulin secretion from pancre- all these tissues, opening of KATP channels in response to atic ␤-cells. They primarily act by binding to the SUR metabolic stress leads to inhibition of electrical activity. subunit of the ATP-sensitive potassium (KATP) channel Thus they are involved in the response to both cardiac and and inducing channel closure. However, the channel is cerebral ischemia (2). They are also important in neuronal still able to open to a limited extent when the drug is regulation of glucose homeostasis (3), seizure protection bound, so that high-affinity sulfonylurea inhibition is not complete, even at saturating drug concentrations. (4), and the control of vascular smooth muscle tone (and, thereby, blood pressure) (5). KATP channels are also found in cardiac, skeletal, and smooth muscle, but in these tissues are composed of The KATP channel is a hetero-octameric complex of two different SUR subunits that confer different drug different types of protein subunits: an inwardly rectifying sensitivities. Thus tolbutamide and gliclazide block Kϩ channel, Kir6.x, and a sulfonylurea receptor, SUR (6,7). ␤ ϩ channels containing SUR1 ( -cell type), but not SUR2 Kir6.x belongs to the family of inwardly rectifying K (cardiac, smooth muscle types), whereas glibenclamide, (Kir) channels and assembles as a tetramer to form the glimepiride, repaglinide, and meglitinide block both types of channels. This difference has been exploited channel pore (Fig. 1A). Binding of ATP to the intracellular to determine residues contributing to the sulfonyl- domains of this subunit produces channel inhibition (8). urea-binding site. Sulfonylurea block is decreased by SUR is a member of the ABC transporter family, with 17 mutations or agents (e.g., phosphatidylinositol bisphos- transmembrane helices (TMs), arranged as one group of 5 phate) that increase KATP channel open probability. We TMs, and two repeats each of 6 TMs followed by a large now propose a kinetic model that explains this effect in cytosolic loop containing consensus sequences for nucle- terms of changes in the channel open probability and in the transduction between the drug-binding site and the otide binding and hydrolysis. Interaction of Mg nucleotides channel gate. We also clarify the mechanism by which with the nucleotide-binding domains (NBDs) mediates acti- MgADP produces an apparent increase of sulfonylurea vation of the KATP channel (9–12). SUR also endows Kir6.2 efficacy on channels containing SUR1 (but not SUR2). with sensitivity to certain drugs, such as the inhibitory Diabetes 51 (Suppl. 3):S368–S376, 2002 sulfonylureas and the stimulatory Kϩ channel openers (6,7). More than one isoform exists for both Kir6.x (Kir6.1, Kir6.2) and SUR (SUR1, SUR2A, SUR2B). In most tissues, ulfonylureas stimulate insulin secretion from Kir6.2 serves as the pore-forming subunit, but it associates pancreatic ␤-cells and are widely used to treat with different SUR subunits; for example, it associates type 2 diabetes. Their principal target is the ATP-sensitive potassium (K ) channel, which with SUR1 in the pancreas and brain; SUR2A in heart and S ATP skeletal muscle; and SUR2B in brain and smooth muscle plays a major role in controlling the ␤-cell membrane (13–15). In vascular smooth muscle, the KATP channel is potential. Inhibition of KATP channels by glucose or sulfo- nylureas causes depolarization of the ␤-cell membrane; in composed of Kir6.1 in association with SUR2B. Variation 2ϩ turn, this triggers the opening of voltage-gated Ca chan- in the subunit composition of the KATP channel accounts 2ϩ 2ϩ nels, eliciting Ca influx and a rise in intracellular Ca for the different metabolic and drug sensitivities of KATP which stimulates the exocytosis of insulin-containing se- channels in different cells. cretory granules (1). KATP channels are also found at high Inhibitors of KATP channel activity fall into two groups: density in a variety of other cell types, including cardiac, those that interact with Kir6.2 and those that interact with SUR. Imidazolines (e.g., phentolamine, cibenzoline) and antimalarials (e.g., quinine, mefloquine) block K chan- From the 1University Laboratory of Physiology, Oxford University, Oxford, ATP U.K., and the 2Department of Clinical Biochemistry, University of Cambridge, nels by binding to Kir6.2 (16–18). In contrast, sulfonyl- Addenbrooke’s Hospital, Cambridge, U.K. ureas (e.g., tolbutamide, gliclazide, glimepiride), and Address correspondence and reprint requests to Prof. Frances M. Ashcroft, University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, U.K. benzamido derivatives (e.g., meglitinide) close KATP chan- E-mail: [email protected]. nels by binding with high affinity to SUR (19–21). Sulfo- Received for publication 12 March 2002 and accepted in revised form 8 May 2002. nylureas also interact with Kir6.2, but with low affinity. All IC50, half-maximal inhibitory concentration value; KATP channel, ATP- drugs that block K channels stimulate insulin secretion, ϩ ATP sensitive potassium channel; Kir channel, inwardly rectifying K channel; but only those that interact with the SUR subunit are used NBD, nucleotide-binding domain; PO, open probability; SUR, sulfonylurea receptor; TM, transmembrane helix. therapeutically to treat type 2 diabetes. Thus, in this P.P., F.G., and F.A. receive honoraria from Servier for speaking engage- review, we focus on how studies of recombinant KATP ments. F.A. is a paid consultant for NovoNordisk. The symposium and the publication of this article have been made possible channels have helped to elucidate the molecular mecha- by an unrestricted educational grant from Servier, Paris. nism of sulfonylurea inhibition. S368 DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002 P. PROKS AND ASSOCIATES inhibit the Kir6.2 subunit are much higher than those found in the plasma of patients treated with the drugs. Thus, in this review, we focus on the high-affinity drug- binding site. STOICHIOMETRY Because four SUR subunits contribute to the KATP channel complex, the question arises as to how many subunits must bind drug to close the channel. This topic is currently controversial. Do¨ rschner et al. (22) found that the affinity of sulfonylurea binding to Kir6.2/SUR1 or Kir6.2/SUR2B channels is between three- and sevenfold lower than their potency on channel inhibition (Table 1). Consequently, they concluded that occupation of a single binding site is sufficient to induce channel closure. In contrast, Russ et al. (23) found that the concentration-response relation for [3H]glibenclamide binding to Kir6.1/SUR2B lies to the left of that for glibenclamide block of channel activity, so that the Kd is sevenfold lower than the half-maximal inhibitory concentration on channel activity (IC50) (Table 1). They therefore suggested that glibenclamide can bind indepen- dently to four identical sites on the channel and that occupation of all four sites is required for channel closure (23). Interestingly, the different proposed stoichiometries arise from the different binding affinities measured by these groups (Table 1). It is not clear why the affinities were found to be so different, but possible confounding factors include the nature of the Kir6.x subunit (Kir6.2 or FIG. 1. A: Schematic of the transmembrane topology of Kir6.2 and SUR1. B: Mean relation between gliclazide concentration and inhibi- Kir6.1) and the nucleotide concentrations used. The con- -flicting results suggest that conclusions based on compar ؍ tion of the Kir6.2/SUR1 current, measured in inside-out patches (n 5–8). The conductance in the presence of gliclazide (G) is expressed as ing binding affinities with potencies of channel inhibition a fraction of its amplitude in the absence of the drug (Gc). The line is .؉ ؊ ؉ h1 ؋ ؉ may not be reliable ؍ fit to (equation 2) G/Gc (L (1 L)/(1 ([S]/IC50–1) )) (1/(1 ؍ ؍ ؍ h2 ([S]/IC50–2) )), where IC50–1 51 nmol/l, h1 1.0, IC50–2 3.1 mmol/l, ;S] is the concentration of sulphonylurea] ;(19) 0.40 ؍ and L ,1 ؍ h2 IC50–1 and IC50–2 are the half-maximal inhibitory concentrations at the SULFONYLUREA-BOUND CHANNELS CAN STILL OPEN high- and low-affinity sites, respectively; h1 and h2 are the Hill coefficients for the high- and low-affinity sites, respectively; and L is In the absence of added nucleotides, high-affinity inhibi- the fraction of current that remains unblocked when all high-affinity tion of the KATP current by sulfonylureas is not complete, sites are occupied. but reaches a maximum of 60–80% (Fig. 1B). There are several possible explanations for this finding. One possi- SULFONYLUREAS BLOCK KATP CHANNELS AT TWO bility is that some percentage of the channels are in a state SITES in which they do not bind drug with high affinity. An When measured in excised patches in the absence of alternative possibility is that all channels bind drug with nucleotides, sulfonylurea block of KATP channels is best high affinity, but they are still able to open when drug is described by a two-site model (Fig. 1B). The low-affinity bound, albeit with lower probability. Single-channel cur- site is independent of SUR, as a similar block is seen when rents recorded at sulfonylurea concentrations that satu- Kir6.2 is expressed in the absence of SUR (20). The rate the high-affinity site (but do not interact with Kir6.2) high-affinity site lies on SUR, as it is only present when can be used to distinguish between these possibilities. If SUR is coexpressed with Kir6.2. The presence of both low- some channels exist in a low-affinity state and others in a and high-affinity sites for sulfonylureas on the KATP chan- high-affinity state, and transitions between these states are nel introduces important considerations when interpreting relatively slow, there should be two kinetically distinct experiments with these drugs.