Activation of Na channels in islet cells: metabolic and secretory effects

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

Venom from mammals, amphibians, snakes, arachnids, sea anemones and insects provides diverse sources of peptides with different potential medical applications. Several of these peptides have already been converted into drugs and some are still in the clinical phase. Diabetes type 2 is one of the diseases with the highest mortality rate worldwide, requiring specific attention. Diverse drugs are available (e.g., Sulfonylureas) for effective treatment, but with several adverse secondary effects, most of them related to the low specificity of these compounds to the target. In this context, the search for specific and high-affinity compounds for the management of this metabolic disease is growing. Toxins isolated from animal venom have high specificity and affinity for different molecular targets, of which the most important are ion channels. This review will present an overview about the electrical activity of the ion channels present in pancreatic β cells that are involved in the insulin secretion process, in addition to the diversity of peptides that can interact and modulate the electrical activity of pancreatic β cells. The importance of prospecting bioactive peptides for therapeutic use is also reinforced.

ATP regulates sodium channel kinetics in pancreatic islet beta cells

Pancreatic beta cells act as glucose sensors, in which intracellular ATP ([ATP](i)) are altered with glucose concentration change. The characterization of voltage-gated sodium channels under different [ATP](i) remains unclear. Here, we demonstrated that increasing [ATP](i) within a certain range of concentrations (2-8 mM) significantly enhanced the voltage-gated sodium channel currents, compared with 2 mM cytosolic ATP. This enhancement was attenuated by even high intracellular ATP (12 mM). Furthermore, elevated ATP modulated the sodium channel kinetics in a dose-dependent manner. Increased [ATP](i) shifted both the current-voltage curve and the voltage-dependent inactivation curve of sodium channel to the right. Finally, the sodium channel recovery from inactivation was significantly faster when the intracellular ATP level was increased, especially in 8 mM [ATP](i), which is an attainable concentration by the high glucose stimulation. In summary, our data suggested that elevated cytosolic ATP enhanced the activity of Na(+) channels, which may play essential roles in modulating β cell excitability and insulin release when blood glucose concentration increases.

Na+ channel inactivation: a comparative study between pancreatic islet beta-cells and adrenal chromaffin cells in rat

A comparative study was carried out on the inactivation of Na+ channels in two types of endocrine cells in rats, beta-cells and adrenal chromaffin cells (ACCs), using patch-clamp techniques. The beta-cells were very sensitive to hyperpolarization; the Na+ currents increased ninefold when the holding potential was shifted from -70 mV to -120 mV. ACCs were not sensitive to hyperpolarization. The half-inactivation voltages were -90 mV (rat beta-cells) and -62 mV (ACCs). The time constant for recovery from inactivation at -70 mV was 10.5 times slower in beta-cells (60 ms) than in ACCs (5.7 ms). The rate of Na+-channel inactivation at physiological resting potential was more than three times slower in beta-cells than in ACCs. Na+ influx through Na+ channels had no effect on the secretory machinery in rat beta-cells. However, these 'silent Na+ channels' could contribute to the generation of action potentials in some conditions, such as when the cell is hyperpolarized. It is concluded that the fractional availability of Na+ channels in beta-cells at a holding potential of -70 mV is about 15 % of that in ACCs. This value in rat beta-cells is larger than that observed in mouse (0 %), but is smaller than those observed in human or dog (90 %).

Ouabain suppresses glucose-induced mitochondrial ATP production and insulin release by generating reactive oxygen species in pancreatic islets

We examined the effects of reduced Na(+)/K(+)-ATPase activity on mitochondrial ATP production and insulin release from rat islets. Ouabain, an inhibitor of Na(+)/K(+)-ATPase, augmented 16.7 mmol/l glucose-induced insulin release in the early period but suppressed it after a delay of 20-30 min. Unexpectedly, the ATP content in an islet decreases in the presence of 16.7 mmol/l glucose when Na(+)/K(+)-ATPase activity is diminished by ouabain, despite the reduced consumption of ATP by the enzyme. Ouabain also suppressed the increment of ATP content produced by glucose even in Ca(2+)-depleted or Na(+)-depleted conditions. That mitochondrial membrane hyperpolarization and O(2) consumption in islets exposed to 16.7 mmol/l glucose were suppressed by ouabain indicates that the glycoside inhibits mitochondrial respiration but does not produce uncoupling. Ouabain induced mitochondrial reactive oxygen species (ROS) production that was blocked by myxothiazol, an inhibitor of site III of the mitochondrial respiratory chain. An antioxidant, alpha-tocopherol, also blocked ouabain-induced ROS production as well as the suppressive effect of ouabain on ATP production and insulin release. However, ouabain did not directly affect the mitochondrial ATP production originating from succinate and ADP. These results indicate that ouabain suppresses mitochondrial ATP production by generating ROS via transduction, independently of the intracellular cationic alternation that may account in part for the suppressive effect on insulin secretion.

Contribution of neural intrapancreatic non-cholinergic non-adrenergic mechanisms to glucose-induced insulin release in the isolated rat pancreas

In the isolated rat pancreas the effect of intrapancreatic non-adrenergic non-cholinergic nerves was examined upon insulin, glucagon and somatostatin release during perturbations of perfusate glucose. Elevation of glucose from 1.6 to 8.3 mmol/l increased insulin and somatostatin secretion and inhibited glucagon release. The first phase of insulin secretion was significantly reduced by the neurotoxin tetrodotoxin to 55% of the controls (p < 0.05). The somatostatin response was attenuated by tetrodotoxin while the change of glucagon remained unaffected. In contrast the combined adrenergic and cholinergic blockade with atropine, phentolamine and propranolol (10(-5) mol/l) did not modify the insulin, glucagon and somatostatin response. When glucose was changed from 8.3 to 1.6 mmol/l, the reduction of insulin and somatostatin release was not modified by tetrodotoxin, but stimulation of glucagon was significantly attenuated by 60-70% (p < 0.03), which was similar to the effect of combined adrenergic and cholinergic blockade. Subsequently, the effect of neural blockade was examined during more physiological perturbations of perfusate glucose levels. When glucose was changed from 3.9 to 7.2 mmol/l, tetrodotoxin also attenuated first phase insulin response by 40% while cholinergic and adrenergic blockade had no effect. The nitric oxide synthase inhibitor NG-Nitro-L-arginine-methyl-ester (L-NAME) did not alter the glucose-induced insulin response indicating that nitric oxide is not involved in this mechanism. It is concluded that neural non-adrenergic non-cholinergic mechanisms contribute to the first, but not second phase of glucose-induced insulin release. Non-adrenergic non-cholinergic effects do not participate in regulation of glucagon and somatostatin secretion under the conditions employed.(ABSTRACT TRUNCATED AT 250 WORDS)

Na channels and two types of Ca channels in rat pancreatic B cells identified with the reverse hemolytic plaque assay

The reverse hemolytic plaque assay (RHPA) was used to study the secretory properties of single rat pancreatic B cells, and to identify insulin-secreting cells for patch-clamp experiments. In secretion studies using the RHPA, we find that the percentage of secreting B cells and the amount of insulin secreted per B cell increase as the glucose concentration is raised from 0 to 20 mM. Using the whole-cell variation of the patch-clamp technique, we find that identified B cells have three types of channels capable of carrying inward current: (a) tetrodotoxin-sensitive, voltage-dependent Na channels, which are nearly completely inactivated at -40 mV, (b) fast deactivating (FD) Ca channels, and (c) slowly deactivating (SD) Ca channels. We have shown that Na channels are functionally significant to the B cell, because tetrodotoxin partially inhibits glucose-induced insulin secretion. The properties of FD and SD Ca channels differ in several respects. FD channels deactivate at -80 mV, with a time constant of 129 microseconds, they are half-maximally activated near +15 mV, they do not inactivate during 100 ms, they conduct Ba2+ better than Ca2+, and they are very sensitive to washout during intracellular dialysis. SD channels, on the other hand, deactivate with a time constant of 2.8 ms, they are half-maximally activated near -5 mV, they inactivate rapidly, they conduct Ba2+ and Ca2+ equally well, and they are insensitive to washout.

Effects of monensin on metabolism of isolated rat islets of Langerhans

Monensin, a univalent ionophore, is a carboxylic acid produced by Streptomyces cinnamonensis. It will complex various alkali-metal ions, but most readily binds Na+. Because of interest in the possible role of Na+ in the regulation of insulin secretion, we examined its effects on several aspects of the metabolism of isolated rat islets of Langerhans. The ionophore inhibited glucose-stimulated insulin release in a concentration-dependent manner, completely inhibiting secretion evoked by 20 mM-glucose at concentrations as low as 0.1 microM in static incubations. In perifusion experiments, both phases of insulin release were equally affected. Monensin (0.1 microM) had no significant effect on glucose oxidation as measured by the generation of 14CO2 from [14C]glucose. Monensin increased the rate of 22Na+ efflux from preloaded islets and net 22Na+ uptake over 30 min, in the absence of changes in islet volume or extracellular space. The ionophore increased the Rb+/K+ permeability of islet cells, as shown by its inhibition of 86Rb+ retention and stimulation of 86Rb+ efflux. At 0.1 microM, monensin abolished glucose-stimulated 45Ca2+ uptake by islets during 5 min incubations, and stimulated 45Ca2+ efflux from preloaded islets perifused with Ca2+-free medium, even in the complete absence of extracellular Na+. Studies of the uptake of 14C-labelled 5,5-dimethyloxazolidine-2,4-dione showed that 0.1 microM-monensin increased net intracellular pH from 7.05 to 7.13. 7 Monensin has widespread, complex, effects on the secretory responses and ion handling by the B cells, which are difficult to interpret in terms solely of actions as a Na+ ionophore.

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in single alpha and beta cells during glucose perifusion of rat islets

To study the relationship between islet hormonal secretion and intracellular content of five elements, a rat islet perifusion technique was used in 24 paired experiments. Control and experimental chambers each containing 100 islets, received 2.8 and 16.7 mM D-glucose, respectively. Effluent was collected frequently for hormone measurements. At eight different time intervals form 0--30 min islets were fixed and prepared for scanning electron microscopy. Over 900 unobscured alpha and beta cells were selected by size and shape criteria. Energy dispersive x-ray analysis was applied to each single cell to determine relative content of calcium (Ca), potassium (K), sodium (Na), chlorine (Cl), and phosphorus (P). Experimental chambers exhibited typical acute (0--9 min) and second phase (10--30 min) insulin secretion in association with suppression of glucagon release after 10 min. At 2 min an abrupt upward K spike in both alpha and beta cells was followed at 3--4 min with a 1.5- to 2-fold rise of Ca and a reciprocal decrease in K, Na, Cl, and P. From 3 to 30 min biphasic insulin secretion. Reduced alpha cell calcium after 6 min preceded suppression of glucagon secretion. After 2 min K related inversely to Ca content in both alpha and beta cells. These results could not be reproduced when D-galactose was substituted for D-glucose. We conclude that sequential changes of Ca content that are reciprocally related to K are predictive of beta cell insulin release and suppression of alpha cell glucagon secretion.

Inhibition by somatostatin of bovine growth hormone secretion following sodium channel activation

1. Growth hormone secretion, exchangeable cellular sodium and calcium concentrations measured by 22Na and 45Ca incorporation, and efflux of 45Ca were studied in dispersed bovine anterior pituitary cells. 2. Addition of veratridine (100 microM), an activator of sodium channels, increased exchangeable sodium and calcium concentrations in the cells, the efflux of 45Ca from prelabelled cells, and caused a biphasic stimulation of the rate of growth hormone secretion. Secretion of growth hormone was not stimulated when the extracellular calcium was decreased below 0.1 mM. 3. The increases in growth hormone secretion, exchangeable calcium concentration and 45Ca efflux from prelabelled cells caused by veratridine were abolished by addition of the calcium antagonist verapamil (20 microM). Verapamil also reduced the rise in exchangeable sodium caused by veratridine and increased the resting exchangeable sodium concentrations. 4. The peptide somatostatin (1 micrograms/ml) prevented veratridine-stimulated growth hormone secretion but did not inhibit the increases in exchangeable sodium and calcium caused by veratridine. The peptide itself elicited a transient increase in 45Ca efflux and subsequently partially inhibited veratridine-stimulated 45Ca efflux. 5. The data suggest that anterior pituitary cells possess voltage-sensitive sodium channels. Activation of these channels by veratridine may lead to depolarization and increased entry of calcium via potential-dependent calcium channels, which contributes to a rise in cytoplasmic calcium concentration and the subsequent stimulation of growth hormone secretion. We conclude that the calcium antagonist verapamil may also interact with sodium channels, and that the peptide somatostatin may act on growth-hormone-secreting cells either to prevent the rise in cytoplasmic calcium by hyperpolarizing the cells or to decrease the affinity of a population of calcium binding sites in the cells.