Comparison of the properties of the ATP-sensitive K+ channels of pancreatic beta-cells of lean and obese (ob/ob) C57BL/6J mice

Mitral cells of the olfactory bulb perform metabolic sensing and are disrupted by obesity at the level of the Kv1.3 ion channel

Sixty-five percent of Americans are over-weight. While the neuroendocrine controls of energy homeostasis are well known, how sensory systems respond to and are impacted by obesity is scantily understood. The main accepted function of the olfactory system is to provide an internal depiction of our external chemical environment, starting from the detection of chemosensory cues. We hypothesized that the system additionally functions to encode internal chemistry via the detection of chemicals that are important indicators of metabolic state. We here uncovered that the olfactory bulb (OB) subserves as an internal sensor of metabolism via insulin-induced modulation of the potassium channel Kv1.3. Using an adult slice preparation of the olfactory bulb, we found that evoked neural activity in Kv1.3-expressing mitral cells is enhanced following acute insulin application. Insulin mediated changes in mitral cell excitability are predominantly due to the modulation of Kv1.3 channels as evidenced by the lack of effect in slices from Kv1.3-null mice. Moreover, a selective Kv1.3 peptide blocker (ShK186) inhibits more than 80% of the outward current in parallel voltage-clamp studies, whereby insulin significantly decreases the peak current magnitude without altering the kinetics of inactivation or deactivation. Mice that were chronically administered insulin using intranasal delivery approaches exhibited either an elevation in basal firing frequency or fired a single cluster of action potentials. Following chronic administration of the hormone, mitral cells were inhibited by application of acute insulin rather than excited. Mice made obese through a diet of ∼32% fat exhibited prominent changes in mitral cell action potential shape and clustering behavior, whereby the subsequent response to acute insulin stimulation was either attenuated or completely absent. Our results implicate an inappropriate neural function of olfactory sensors following exposure to chronic levels of the hormone insulin (diabetes) or increased body weight (obesity).

Pancreatic islets from hypothalamic obese rats maintain K+ATP channel-dependent but not -independent pathways on glucose-induced insulin release process

One of the main features of obesity is hyperinsulinemia, which is related to insulin oversecretion. Glucose is by far the major physiological stimulator of insulin secretion. Glucose promotes an increase in the ATP/ADP ratio, which inactivates ATP-sensitive K+ channels (K+ATP) and induces beta cell depolarization with consequent calcium influx. Increased intracellular calcium concentration triggers insulin exocytosis. K+ATP channel function is important for K+ATP channel-dependent pathways involved in glucose-stimulated insulin secretion (GSIS). However, K+ATP channel-independent pathway has been identified and it has been found that this pathway sustains GSIS. Both pathways are critical to better GSIS control. GSIS was studied in pancreatic islets from hyperinsulinemic adult obese rats obtained by monosodium L-glutamate (MSG) neonatal treatment. Islets from MSG-obese rats were more glucose responsive than control ones. Diazoxide, a drug which maintains the K+ATP channels open without interfering with cell metabolism, blocked GSIS in islets from both groups. High extracellular potassium concentration plus diazoxide was used to study an alternative to the K+ATP channel pathway; in these conditions islets from MSG-obese rats did not respond, while islets from control animals showed enhanced GSIS. Results indicate that MSG-obese rats oversecreted insulin, even though the K+ATP channel-independent pathway is impaired in their beta cells.

The adipoinsular axis: effects of leptin on pancreatic beta-cells

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic beta-cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on beta-cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.

Leptin suppression of insulin secretion by the activation of ATP-sensitive K+ channels in pancreatic beta-cells

In the genetic mutant mouse models ob/ob or db/db, leptin deficiency or resistance, respectively, results in severe obesity and the development of a syndrome resembling NIDDM. One of the earliest manifestations in these mutant mice is hyperinsulinemia, suggesting that leptin may normally directly suppress the secretion of insulin. Here, we show that pancreatic islets express a long (signal-transducing) form of leptin-receptor mRNA and that beta-cells bind a fluorescent derivative of leptin (Cy3-leptin). The expression of leptin receptors on insulin-secreting beta-cells was also visualized utilizing antisera generated against an extracellular epitope of the receptor. A functional role for the beta-cell leptin receptor is indicated by our observation that leptin (100 ng/ml) suppressed the secretion of insulin from islets isolated from ob/ob mice. Furthermore, leptin produced a marked lowering of [Ca2+]i in ob/ob beta-cells, which was accompanied by cellular hyperpolarization and increased membrane conductance. Cell-attached patch measurements of ob/ob beta-cells demonstrated that leptin activated ATP-sensitive potassium channels (K(ATP)) by increasing the open channel probability, while exerting no effect on mean open time. These effects were reversed by the sulfonylurea tolbutamide, a specific inhibitor of K(ATP). Taken together, these observations indicate an important physiological role for leptin as an inhibitor of insulin secretion and lead us to propose that the failure of leptin to inhibit insulin secretion from the beta-cells of ob/ob and db/db mice may explain, in part, the development of hyperinsulinemia, insulin resistance, and the progression to NIDDM.

KATP channel-dependent and -independent pathways of insulin secretion in isolated islets from fa/fa Zucker rats

We hypothesized that altered insulin secretory patterns in obese (fa/fa) Zucker rats might be caused by changes in downstream stimulus-secretion coupling events, such as ATP-dependent potassium (KATP) channel activity. The functions of KATP-dependent and -independent pathways of insulin secretion were therefore compared in lean and fa/fa Zucker rat isolated islets. KATP channel function was normal in fa/fa rat islets, as assessed by responsiveness to direct channel inactivators glybenclamide and quinine and by the receptor-mediated response to epinephrine and somatostatin. Altered sensitivity to glucose and mannoheptulose were explained by upstream alterations in glucose metabolism documented earlier. Despite normal inactivation of KATP channels by ATP depletion of fa/fa rat islets, glucose-stimulated insulin secretion was not inhibited, leading to studies of a putative KATP-independent pathway. When islets were depolarized by incubating with 30 mM potassium and 0.25 mM diazoxide to bypass KATP channels, glucose elicited a concentration-dependent response in both phenotypes. This response required glucose metabolism and Ca2+, as proven by experiments with nonmetabolizable glucose analogs and calcium chelation, but was only partially inhibited by a glycolytic inhibitor. Intermediates or products of oxidative metabolism are likely involved because alpha-ketoisocaproate also elicited a KATP-independent insulin response. The pattern of responses was similar in lean and fa/fa rat islets, indicating that neither of these pathways explains the insulin secretion by fa/fa rat islets depleted of ATP. In conclusion, phenotype-related differences in KATP channel function were consistent with upstream changes in glucose metabolism in fa/fa rat islets. Further studies are required to understand the basis of insulin secretion in ATP-depleted islets from fa/fa rats.