K+ channel and alpha 2-adrenergic effects on glucose-induced Ca2+i surges: aberrant behavior in ob/ob mice

Xanthine-derived KMUP-1 reverses glucotoxicity-activated Kv channels through the cAMP/PKA signaling pathway in rat pancreatic β cells

Hyperglycemia-associated glucotoxicity induces β-cell dysfunction and a reduction in insulin secretion. Voltage-dependent K⁺ (Kv) channels in pancreatic β-cells play a key role in glucose-dependent insulin secretion. KMUP-1, a xanthine derivative, has been demonstrated to modulate Kv channel activity in smooth muscles; however, the role of KMUP-1 in glucotoxicity-activated Kv channels in pancreatic β-cells remains unclear. In this study we examined the mechanisms by which KMUP-1 could inhibit high glucose (25 mM) activated Kv currents (IKv) in pancreatic β-cells. Pancreatic β-cells were isolated from Wistar rats and IKv was monitored by perforated patch-clamp recording. The peak IKv in high glucose-treated β-cells was ∼1.4-fold greater than for normal glucose (5.6 mM). KMUP-1 (1, 10, 30 μM) prevented high glucose-stimulated IKv in a concentration-dependent manner. Reduction of high glucose-activated IKv was also found for protein kinase A (PKA) activator 8-Br-cAMP (100 μM). Additionally, KMUP-1 (30 μM) current inhibition was reversed by the PKA inhibitor H-89 (1 μM). Otherwise, pretreatment with the PKC activator or inhibitor had no effect on IKv in high glucose exposure. In conclusion, glucotoxicity-diminished insulin secretion was due to IKv activation. KMUP-1 attenuated high glucose-stimulated IKv via the PKA but not the PKC signaling pathway. This finding provides evidence that KMUP-1 might be a promising agent for treating hyperglycemia-induced insulin resistance.

Enhanced glucose-induced intracellular signaling promotes insulin hypersecretion: pancreatic beta-cell functional adaptations in a model of genetic obesity and prediabetes

Obesity is associated with insulin resistance and is known to be a risk factor for type-2 diabetes. In obese individuals, pancreatic beta-cells try to compensate for the increased insulin demand in order to maintain euglycemia. Most studies have reported that this adaptation is due to morphological changes. However, the involvement of beta-cell functional adaptations in this process needs to be clarified. For this purpose, we evaluated different key steps in the glucose-stimulated insulin secretion (GSIS) in intact islets from female ob/ob obese mice and lean controls. Obese mice showed increased body weight, insulin resistance, hyperinsulinemia, glucose intolerance and fed hyperglycemia. Islets from ob/ob mice exhibited increased glucose-induced mitochondrial activity, reflected by enhanced NAD(P)H production and mitochondrial membrane potential hyperpolarization. Perforated patch-clamp examination of beta-cells within intact islets revealed several alterations in the electrical activity such as increased firing frequency and higher sensitivity to low glucose concentrations. A higher intracellular Ca(2+) mobilization in response to glucose was also found in ob/ob islets. Additionally, they displayed a change in the oscillatory pattern and Ca(2+) signals at low glucose levels. Capacitance experiments in intact islets revealed increased exocytosis in individual ob/ob beta-cells. All these up-regulated processes led to increased GSIS. In contrast, we found a lack of beta-cell Ca(2+) signal coupling, which could be a manifestation of early defects that lead to beta-cell malfunction in the progression to diabetes. These findings indicate that beta-cell functional adaptations are an important process in the compensatory response to obesity.

Adipose tissue lipolysis is upregulated in lean and obese men during acute resistance exercise

OBJECTIVE

To investigate the effect of acute resistance exercise on adipose tissue triacylglycerol lipase activity (TGLA) in lean and obese men.

RESEARCH DESIGN AND METHODS

Nine lean and eight obese men performed 30 min of circuit resistance exercise. Adipose tissue and blood were sampled during exercise for TGLA, metabolite, and hormone determinations. Respiratory exchange ratio (RER) was measured throughout exercise.

RESULTS

Energy expenditure of exercise relative to body mass was higher in the lean and RER was higher in the obese men, suggesting lower fat oxidation. TGLA increased 18-fold at 5 min of exercise in the lean men and 16-fold at 10 min of exercise in the obese men. The delayed lipolytic activation in the obese men was reflected in serum nonesterified fatty acid and glycerol concentrations. Plasma insulin increased in the obese but did not change in the lean men.

CONCLUSIONS

Resistance exercise upregulated adipose tissue lipolysis and enhanced energy expenditure in lean and obese men, with a delayed lipolytic activation in the obese men.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Postfreeze reduction of locomotor endurance in the freeze-tolerant wood frog, Rana sylvatica

Considerable study has focused on the physiological adaptations for freeze tolerance in the wood frog, Rana sylvatica, a northern species that overwinters within the frost zone, but little attention has been paid to the associated costs to organismal performance. Here we report that freezing causes transient impairment of locomotor endurance and adverse changes in exercise physiology that persist for at least 96 h. Wood frogs frozen at -2 degrees C for 36 h exhibited normal behaviors and hydro-osmotic status and near-normal metabolite (glycogen, glucose, and lactate) levels within 24 h after thawing began. However, when exercised to exhaustion on a treadmill, these frogs showed a 40% reduction in endurance as compared to sham-treated (unfrozen) controls, a reduction that persisted for at least 96 h. Previously frozen frogs exhibited higher rates of lactate accumulation during exercise than controls, suggesting that prior freezing forces greater reliance on the glycolytic pathways of energy production to support exercise. Given that this species breeds in late winter, when subzero temperatures are common, freezing may result in reduced fitness by hampering their ability to reach the pond, avoid predators, and successfully obtain mates.

The multiple actions of GLP-1 on the process of glucose-stimulated insulin secretion

The physiological effects of glucagon-like peptide-1 (GLP-1) are of immense interest because of the potential clinical relevance of this peptide. Produced in intestinal L-cells through posttranslational processing of the proglucagon gene, GLP-1 is released from the gut in response to nutrient ingestion. Peripherally, GLP-1 is known to affect gut motility, inhibit gastric acid secretion, and inhibit glucagon secretion. In the central nervous system, GLP-1 induces satiety, leading to reduced weight gain. In the pancreas, GLP-1 is now known to induce expansion of insulin-secreting beta-cell mass, in addition to its most well-characterized effect: the augmentation of glucose-stimulated insulin secretion. GLP-1 is believed to enhance insulin secretion through mechanisms involving the regulation of ion channels (including ATP-sensitive K(+) channels, voltage-dependent Ca(2+) channels, voltage-dependent K(+) channels, and nonselective cation channels) and by the regulation of intracellular energy homeostasis and exocytosis. The present article will focus principally on the mechanisms proposed to underlie the glucose dependence of GLP-1's insulinotropic effect.

Glycogen repletion following burst activity: a carbohydrate-sparing mechanism in animals adapted to arid environments?

The Western chestnut mouse (Pseudomys nanus ferculinus) is one of several native rodent species adapted to the arid environments of Australia. Since these environments are often associated with a paucity in dietary carbohydrate, the problem arises as to the mechanism whereby these rodents replete their stores of muscle glycogen when recovering from high intensity physical activity. This is an important issue since the maintenance of adequate stores of muscle glycogen is crucial to support the energy demands associated with 'flight or fight' responses. Whilst it is known that food ingestion post-exercise is required for the total repletion of muscle glycogen in rats and humans, our findings indicate that the Western chestnut mouse has the impressive capacity to replete completely its stores of muscle glycogen, even in the absence of food intake. Indeed during recovery from burst activity which results in the massive breakdown of the stores of muscle glycogen, the levels of glycogen return back to pre-exercise levels within only 50 minutes despite the absence of food intake. This capacity is important in the broader context of nutritional adaptation to arid/seasonally-arid regions since it allows muscles to replete their fuel stores even when food is not available. How common is this strategy among desert-adapted mammal species is a question yet to be answered.

Do pancreatic islet cells from neonatal rats have surface receptors or sensors for divalent cations?

The effects of extracellular divalent cations on the intracellular Ca2+ concentration ([Ca2+]i) in neonatal rat islet cells were investigated to determine whether these cells, like several others, have signal-generating surface cation sensors. Raising the external Ca2+ concentration by 1 mM increments triggered either sustained increases in [Ca2+]i or large sharp [Ca2+]i spikes followed by return to a suprabasal level. The external Ca(2+)-triggered [Ca2+]i responses were abolished by treating the cells with the inhibitor of inositol phospholipid hydrolysis, neomycin (1.5 mM), but not by another phospholipase C inhibitor, U-73,122 (2.5 microM), or the voltage-sensitive Ca2+ channel blockers nifedipine (20 microM) and methoxyverapamil (D600; 50 microM). [Ca2+]i responses were also triggered by barium (Ba2+; 1 mM) and cobalt (Co2+; 1 mM). The Ba2+ responses were also inhibited by neomycin and unaffected by nifedipine or D600 and the Co2+ response required external Ca2+. Therefore, neonatal rat pancreatic islet cells may display divalent cation receptors/sensors on their surfaces. Activation of these putative receptors, which are coupled to neomycin-sensitive, voltage-independent, dihydropyridine-insensitive channels, by Ca2+, Ba2+ or Co2+ would trigger [Ca2+]i responses by opening these channels to admit external Ca2+ into the cell. The physiological function(s) of such cell-surface divalent cation receptors/sensors and the [Ca2+]i surges they generate in pancreatic islet cells is not known.

ATP-sensitive K+ channels mediate alpha 2D-adrenergic receptor contraction of arteriolar smooth muscle and reversal of contraction by hypoxia

Evidence in rat skeletal muscle suggests that local metabolic control of blood flow is facilitated by the reliance on alpha 2D-adrenergic receptors (ARs) for constriction of arterioles, together with the strong sensitivity of this constriction to inhibition by hypoxia. The present study examined the role of ATP-sensitive K+ (KATP) channels in the selective interaction between alpha 2D-ARs and hypoxia. Arterioles from rat cremaster muscle that possess both alpha 1D (alpha 1A/D)- and alpha 2D-AR subtypes were microcannulated, pressurized, and isolated in a tissue bath for measurement of changes in lumen diameter. Three studies first examined whether stimulation of alpha 2D- and alpha 1D-ARs involves inhibition of the KATP channel. Concentration-dependent constriction by the KATP antagonists glibenclamide (GLB, 0.01 to 10 mumol/L) and disopyramide (0.001 to 1 mmol/L) were abolished during alpha 2D stimulation but unaffected during alpha 1D stimulation. Activation of the KATP channel by cromakalim inhibited alpha 2D constriction with greater potency than alpha 1D (EC50, 7.0 +/- 0.2 versus 6.3 +/- 0.1). Finally, GLB (0.5 mumol/L) abolished dose-dependent alpha 2D constriction, whereas alpha 1D was unaffected. These data suggest that alpha 2D but not alpha 1D stimulation is "coupled" with closure of the KATP channel, leading to depolarization and contraction of vascular smooth muscle. In a second series, hypoxic (PO2, 6 mm Hg) inhibition of intrinsic smooth muscle tone was completely reversed by 0.1 mumol/L GLB, concentration-dependent GLB constriction was enhanced during hypoxia, and hypoxia reversed GLB constriction. These data confirm reports by others that hypoxia potentiates the activation of KATP channels, leading to hyperpolarization and relaxation. Finally, GLB constriction, which was abolished by concomitant alpha 2D stimulation, was completely restored by simultaneous activation of KATP channels with hypoxia. These findings suggest that the sensitivity of alpha 2D-AR constriction to inhibition by hypoxia arises through "antagonistic coupling" between these two stimuli, by which the alpha 2D-AR inhibits and hypoxia activates KATP channels.

Insulin secretion and intracellular Ca2+ rises in monolayer cultures of neonatal rat beta-cells

Glucose-induced insulin release, glucose-induced rises in intracellular free Ca2+ concentration ([Ca2+]i), and voltage-dependent Ca2+ channel activity were assessed in monolayer cultures of beta-cells from 3-5-day-old rats. The glucose-stimulated insulin secretory responses and [Ca2+]i rises were like those in adult rat beta-cells rather than fetal rat beta-cells. Voltage-dependent Ca2+ channel antagonists decreased glucose-induced insulin secretion, aborted the [Ca2+]i rise and, like deprivation of extracellular Ca2+, prevented the glucose-induced rise in [Ca2+]i when added before the glucose challenge. The presence of nifedipine-sensitive, voltage-dependent Ca2+ channels was demonstrated directly by measuring Ca2+ currents using the whole-cell configuration of the patch-clamp technique and indirectly by measuring [Ca2+]i after membrane depolarization by 45 mM K+ or 200 microM tolbutamide. Thus, in cultured beta-cells of 3-5-day-old rats the coupling of glucose stimulation to Ca2+ influx is essentially mature, in contrast to what has been reported for fetal or very early neonatal cells.