Pancreatic β-cell ATP-sensitive potassium (KATP) channels are critical links between nutrient metabolism and insulin secretion. In humans, reduced or absent β-cell KATP channel activity resulting from loss-of-function KATP mutations induces insulin hypersecretion. Mice with reduced KATP channel activity also demonstrate hyperinsulinism, but mice with complete loss of KATP channels (KATP knockout mice) show an unexpected insulin undersecretory phenotype. Therefore we have proposed an “inverse U” hypothesis to explain the response to enhanced excitability, in which excessive hyperexcitability drives β-cells to insulin secretory failure without cell death. Many patients with type 2 diabetes treated with antidiabetic sulfonylureas (which inhibit KATP activity and thereby enhance insulin secretion) show long-term insulin secretory failure, which we further suggest might reflect a similar progression.
Methods and Findings
To test the above hypotheses, and to mechanistically investigate the consequences of prolonged hyperexcitability in vivo, we used a novel approach of implanting mice with slow-release sulfonylurea (glibenclamide) pellets, to chronically inhibit β-cell KATP channels. Glibenclamide-implanted wild-type mice became progressively and consistently diabetic, with significantly (p < 0.05) reduced insulin secretion in response to glucose. After 1 wk of treatment, these mice were as glucose intolerant as adult KATP knockout mice, and reduction of secretory capacity in freshly isolated islets from implanted animals was as significant (p < 0.05) as those from KATP knockout animals. However, secretory capacity was fully restored in islets from sulfonylurea-treated mice within hours of drug washout and in vivo within 1 mo after glibenclamide treatment was terminated. Pancreatic immunostaining showed normal islet size and α-/β-cell distribution within the islet, and TUNEL staining showed no evidence of apoptosis.
These results demonstrate that chronic glibenclamide treatment in vivo causes loss of insulin secretory capacity due to β-cell hyperexcitability, but also reveal rapid reversibility of this secretory failure, arguing against β-cell apoptosis or other cell death induced by sulfonylureas. These in vivo studies may help to explain why patients with type 2 diabetes can show long-term secondary failure to secrete insulin in response to sulfonylureas, but experience restoration of insulin secretion after a drug resting period, without permanent damage to β-cells. This finding suggests that novel treatment regimens may succeed in prolonging pharmacological therapies in susceptible individuals.
In a mouse study aiming to understand why long-term treatment for type 2 diabetes with sulfonylureas eventually fails, Colin Nichols and Maria Remedi suggest that slow restoration of insulin secretion may be possible after a drug-resting period.
Diabetes is an increasingly common chronic disease characterized by high blood sugar (glucose) levels. In normal people, blood sugar levels are controlled by the hormone insulin. Insulin is released by β-cells in the pancreas when blood glucose levels rise after eating (glucose is produced by the digestion of food). In fasting people, membrane proteins called ATP-sensitive potassium (KATP) channels keep the β-cell in a “hyperpolarized” state in which they do not secrete insulin. After a meal, glucose enters the β-cell where its chemical breakdown converts ADP into ATP (the molecule that provides the energy that drives cellular processes). The increased ratio of ATP to ADP closes the KATP channels, “depolarizes” the β-cells, and allows the entry of calcium ions, which trigger insulin release. The released insulin then “instructs” insulin-responsive muscle and fat cells to take up glucose from the bloodstream. In type 2 diabetes, the commonest type of diabetes, the muscle and fat cells gradually become nonresponsive to insulin and consequently blood glucose levels rise. Over time, this hyperglycemia increases the risk of heart attacks, kidney failure, and other life-threatening complications. On average, people with diabetes die 5–10 y younger than people without diabetes.
Why Was This Study Done?
People with type 2 diabetes are often initially treated with drugs called sulfonylureas (for example, glibenclamide). Sulfonylureas help to reduce blood glucose levels by inhibiting (in effect, closing) the KATP channels, which enhances insulin secretion. Unfortunately, after patients have been treated for several years with sulfonylureas, their β-cells often stop secreting insulin and the patients then have to inject insulin to control their blood sugar levels. The mechanism by which chronic sulfonylurea treatment affects β-cell behavior is poorly understood, which means that it is hard to improve this antidiabetes treatment. Mice that have been genetically altered so that they have no KATP channels (KATP knockout mice) also rapidly lose their ability to secrete insulin, although they secrete unusually large amounts at birth. This suggests that permanent membrane depolarization (β-cell hyperexcitability) may cause insulin secretory failure. In this study, the researchers investigate whether this mechanism might be responsible for sulfonylurea-induced loss of insulin secretion.
What Did the Researchers Do and Find?
The researchers implanted slowly releasing pellets of glibenclamide into wild-type mice and then monitored their blood glucose levels and glucose tolerance (the speed of glucose removal from the blood after a glucose “meal”) for up to 128 d; the pellets released drug for 90 d. The glibenclamide-implanted mice progressively developed diabetes, lost the ability to secrete insulin in response to glucose and, after 1 wk of treatment, were as glucose intolerant as adult KATP knockout mice. Compared to freshly isolated β-cells from untreated wild-type mice, glucose-stimulated insulin secretion by β-cells isolated from glibenclamide-treated wild-type mice and from KATP knockout mice was reduced to a similar degree. However, the secretory capacity of β-cells isolated from the glibenclamide-treated wild-type mice was restored to normal within hours of drug washout and was normal in β-cells isolated from treated mice 1 mo after exhaustion of the slow-release pellets. Consistent with this result, there was no obvious β-cell death in the glibenclamide-treated mice.
What Do These Findings Mean?
Although findings from animal studies do not always reflect what happens in people, these findings suggest that insulin secretion might sometimes fail in people who take sulfonylureas for a long time, because these drugs cause β-cell hyperexcitability. The finding that the secretory failure caused by sulfonylurea treatment is reversible is important because it suggests that short-acting sulfonylureas might be re-evaluated to see whether they could delay sulfonylurea-induced failure of the insulin secretory response by providing the pancreatic β-cells with periods when they are not depolarized. This finding (and the absence of β-cells death in the glibenclamide-treated mice) also suggests that there may be a way to reverse the loss of the insulin secretory response in patients who have taken sulfonylureas for a long time. Both approaches could help patients with diabetes delay or even avoid the need for insulin injections.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050206.
This study is further discussed in a PLoS Medicine Perspective by Renstrom and colleagues
The MedlinePlus encyclopedia provides information for patients about diabetes (in English and Spanish)
The US National Diabetes Information Clearinghouse provides information on all aspects of diabetes (in English and Spanish)
The International Diabetes Federation also provides comprehensive information about diabetes
Wikipedia has pages on KATP channels and on sulfonylurea drugs (note: Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)