Ischemia in type 2 diabetes: tissue selectivity of sulfonylureas and clinical implications

Selectivity of repaglinide and glibenclamide for the pancreatic over the cardiovascular K(ATP) channels

AIMS/HYPOTHESIS

Sulfonylureas and glinides close beta cell ATP-sensitive K(+) (K(ATP)) channels to increase insulin release; the concomitant closure of cardiovascular K(ATP) channels, however, leads to complications in patients with cardiac ischaemia. The insulinotrope repaglinide is successful in therapy, but has been reported to inhibit the recombinant K(ATP) channels of beta cells, cardiocytes and non-vascular smooth muscle cells with similar potencies, suggesting that the (patho-)physiological role of the cardiovascular K(ATP) channels may be overstated. We therefore re-examined repaglinide's potency at and affinity for the recombinant pancreatic, myocardial and vascular K(ATP) channels in comparison with glibenclamide.

METHODS

K(ATP) channel subunits (i.e. inwardly rectifying K(+) channels [Kir6.x] and sulfonylurea receptors [SURx]) were expressed in intact human embryonic kidney cells and assayed in whole-cell patch-clamp and [(3)H]glibenclamide binding experiments at 37 degrees C.

RESULTS

Repaglinide and glibenclamide, respectively, were >or=30 and >or=1,000 times more potent in closing the pancreatic than the cardiovascular channels and they did not lead to complete inhibition of the myocardial channel. Binding assays showed that the selectivity of glibenclamide was essentially based on high affinity for the pancreatic SUR, whereas binding of repaglinide to the SUR subtypes was rather non-selective. After coexpression with Kir6.x to form the assembled channels, however, the affinity of the pancreatic channel for repaglinide was increased 130-fold, an effect much larger than with the cardiovascular channels. This selective effect of coexpression depended on the piperidino substituent in repaglinide.

CONCLUSIONS/INTERPRETATION

Repaglinide and glibenclamide show higher potency and efficacy in inhibiting the pancreatic than the cardiovascular K(ATP) channels, thus supporting their clinical use.

Human ether-a-go-go-related (HERG) gene and ATP-sensitive potassium channels as targets for adverse drug effects

Torsades de pointes (TdP) arrhythmia is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT interval prolongation and TdP arrhythmias is blockade of the rapid component of the cardiac delayed rectifier K+ current (IKr), which is encoded by human ether-a-go-go-related gene (HERG). Over 100 non-cardiovascular drugs have the potential to induce QT interval prolongations in the electrocardiogram (ECG) or TdP arrhythmias. The binding site of most HERG channel blockers is located inside the central cavity of the channel. An evaluation of possible effects on HERG channels during the development of novel drugs is recommended by international guidelines. During cardiac ischaemia activation of ATP-sensitive K+ (KATP) channels contributes to action potential (AP) shortening which is either cardiotoxic by inducing re-entrant ventricular arrhythmias or cardioprotective by inducing energy-sparing effects or ischaemic preconditioning (IPC). KATP channels are formed by an inward-rectifier K+ channel (Kir6.0) and a sulfonylurea receptor (SUR) subunit: Kir6.2 and SUR2A in cardiac myocytes, Kir6.2 and SUR1 in pancreatic beta-cells. Sulfonylureas and glinides stimulate insulin secretion via blockade of the pancreatic beta-cell KATP channel. Clinical studies about cardiotoxic effects of sulfonylureas are contradictory. Sulfonylureas and glinides differ in their selectivity for pancreatic over cardiovascular KATP channels, being either selective (tolbutamide, glibenclamide) or non-selective (repaglinide). The possibility exists that non-selective KATP channel inhibitors might have cardiovascular side effects. Blockers of the pore-forming Kir6.2 subunit are insulin secretagogues and might have cardioprotective or cardiotoxic effects during cardiac ischaemia.

The impact of ATP-sensitive K+ channel subtype selectivity of insulin secretagogues for the coronary vasculature and the myocardium

Insulin secretagogues (sulfonylureas and glinides) increase insulin secretion by closing the ATP-sensitive K+ channel (KATP channel) in the pancreatic beta-cell membrane. KATP channels subserve important functions also in the heart. First, KATP channels in coronary myocytes contribute to the control of coronary blood flow at rest and in hypoxia. Second, KATP channels in the sarcolemma of cardiomyocytes (sarcKATP channels) are required for adaptation of the heart to stress. In addition, the opening of sarcKATP channels and of KATP channels in the inner membrane of mitochondria (mitoKATP channels) plays a central role in ischemic preconditioning. Opening of sarcKATP channels also underlies the ST-segment elevation of the electrocardiogram, the primary diagnostic tool for initiation of lysis therapy in acute myocardial infarction. Therefore, inhibition of cardiovascular KATP channels by insulin secretagogues is considered to increase cardiovascular risk. Electrophysiological experiments have shown that the secretagogues differ in their selectivity for the pancreatic over the cardiovascular KATP channels, being either highly selective (approximately 1,000x; short sulfonylureas such as nateglinide and mitiglinide), moderately selective (10-20x; long sulfonylureas such as glibenclamide [glyburide]), or essentially nonselective (<2x; repaglinide). New binding studies presented here give broadly similar results. In clinical studies, these differences are not yet taken into account. The hypothesis that the in vitro selectivity of the insulin secretagogues is of importance for the cardiovascular outcome of diabetic patients with coronary artery disease needs to be tested.

Clonal endothelial cells produce humoral factors that inhibit osteoclast-like cell formation in vitro

Angiogenesis and bone remodeling are closely associated, and vascular endothelial cells may have potential roles for osteoclastic bone resorption. We examined whether clonal endothelial cells established from bone, aorta and brain of Balb/c mice influenced osteoclast-like cell formation in vitro. As low as 1% conditioned media of those endothelial cells inhibited osteoclast-like cell formation in bone marrow cultures induced by 1,25-dihydroxyvitamin D3, and did so in spleen cell cultures in the presence of soluble receptor activator of nuclear factor-kappaB ligand (RANKL), M-CSF and prostaglandin E2. The level of osteoprotegerin (OPG), a decoy receptor for RANKL, secreted by endothelial cells was not high enough to inhibit osteoclastogenesis. These observations suggest that endothelial cells derived from various tissues secrete factor(s) that inhibits precursors to differentiate into osteoclasts even in the presence of optimal stimulators for osteoclastogenesis. Hence, endothelial cells in bone may inhibit recruitment of fresh osteoclasts, and those in tissues other than bone may be involved in prohibiting ectopic osteoclastogenesis.