Effect of perchlorate on calcium uptake and insulin secretion in mouse pancreatic islets

Exposure to perchlorate, nitrate and thiocyanate, and prevalence of diabetes mellitus

Background

It is known that perchlorate, nitrate and thiocyanate have the property of inhibiting sodium iodide symporter. Animal studies have suggested that these compounds, especially perchlorate, might also interfere with insulin secretion. However, the association between their exposure and diabetes risk is largely unknown in humans.

Methods

Among 11 443 participants (mean age 42.3 years) from the National Health and Nutritional Examination Survey 2001-14, urinary perchlorate, nitrate and thiocyanate were measured by using ion chromatography coupled with electrospray tandem mass spectrometry. Diabetes was defined as self-reported doctor diagnosis, use of oral hypoglycaemic medication or insulin, fasting plasma glucose ≥ 126 mg/dl or glycated haemoglobin A1c (HbA1c) ≥ 6.5%.

Results

The median (interquartile range) levels of urinary perchlorate, nitrate and thiocyanate were 3.32 (1.84, 5.70) μg/l, 46.4 (27.9, 72.0) mg/l and 1.23 (0.59, 2.78) mg/l, respectively. Higher levels of urinary perchlorate were associated with elevated levels of fasting glucose, HbA1c, insulin and homeostatic model assessment of insulin resistance (all Ptrend < 0.001). After multivariate adjustment including urinary creatinine, smoking status and body mass index (BMI), higher urinary perchlorate, but not nitrate or thiocyanate, was associated with an increased prevalence of diabetes mellitus. Comparing extreme quintiles, the odds ratio (95% confidence interval) of diabetes was 1.53 (1.21, 1.93; Ptrend < 0.001) for perchlorate, 1.01 (0.77, 1.32; Ptrend = 0.44) for nitrate and 0.98 (0.73, 1.31; Ptrend = 0.64) for thiocyanate. When urinary perchlorate, nitrate and thiocyanate were further mutually adjusted, the results did not materially change. Similar results were observed when analyses were stratified by smoking status, as well as by age, gender, kidney function and BMI.

Conclusions

Higher urinary perchlorate levels are associated with an increased prevalence of diabetes mellitus, independent of traditional risk factors. Future prospective studies are needed to confirm these findings.

Magnesium modulates contractile responses of rat aorta to thiocyanate: A possible relationship to smoking-induced atherosclerosis

Thiocyanate anions (SCN-) as the end products of tobacco smoke and found in the blood of cigarette smokers have been implicated in atherogenesis and heart diseases. Magnesium deficiency has also been implicated in the etiology of atherogenesis. The contractile responses of rat aorta to SCN- and the modulation of extracellular magnesium ions ([Mg2+]o) on the effect of SCN- were, therefore, studied in isolated rat aortic rings. SCN- exposure at a range of concentrations (from 10(-5) to 5 x 10(-2) M) induces contractile responses of isolated rat aortic rings with and without endothelium in a concentration-dependent manner. Significant differences in responsiveness to SCN- were found in rat aortic ring segments with and without endothelial cells. Preincubation of these vessels with low [Mg2+]o markedly shifted the contractile concentration-effect curves to the left, and the contractile effects of SCN- in rat aortic rings were potentiated. In contrast to lowering [Mg2+]o, increasing [Mg2+]o to 2.4 mM was found to dramatically attenuate the contractile responses to SCN-. In the absence of extracellular Ca2+ ([Ca2+]o), SCN--induced contractions were, however, almost abolished after exposure to Mg2+-free medium. In order to investigate the mechanisms of [Mg2+]o modulation of SCN--induced contractile response of rat aorta, changes in intracellular Ca2+ ([Ca2+]i) were measured in cultured primary smooth muscle cells isolated from rat aorta. The resting level of [Ca2+]i in the rat aortic smooth muscle cells was 80.6 +/- 6.6 nM. Exposure of these cells to SCN- (5 x 10(-5) to 5 x 10(-3) M) produced rises in [Ca2+]i in a concentration-dependent manner. Preincubation of these cells with low [Mg2+]o (0 or 0.3 mM, the lowest physiological range) for 24 h significantly potentiated increments in [Ca2+]i induced by SCN-. These rises in [Ca2+]i induced by SCN- were completely inhibited by pretreating the cells with 2.4 mM [Mg2+]o for 24 h. These results support a hypothesis whereby cigarette smoking or exposure to smoking can induce cardiovascular diseases, at least partly, probably by causing spasm and thickening of arterial blood vessels as a consequence of large rises in [Ca2+]i in vascular smooth muscle cells. The chronic presence of or exposure to both thiocyanate and low Mg2+ in the blood of smokers can result in rapid flux of Ca2+ into vascular smooth muscle cells, thus accelerating or initiating atherosclerotic processes in smokers.

Thiocyanate induces vasoconstriction in rat-tail artery primed with norepinephrine

The vasoregulating effects of thiocyanate (SCN-) were studied by means of a new rat tail artery perfusion model that uses constant driving pressure (1000 mm H2O). When a 30 mm long artery was perfused with 20 micromol/L verapamil, the flow rate increased 6%. Norepinephrine (10 to 1000 nmol/L) caused a dose-dependent flow inhibition. SCN- alone (0.05 to 5 mmol/L) had a slight, if any, effect on the arterial flow. However, when the artery was pretreated with norepinephrine (1000 nmol/L) for 10 minutes, followed by a basal-medium washout that left a 13% norepinephrine-induced flow inhibition, 0.05 to 5 mmol/L SCN- caused a marked flow rate reduction of between 20.4% +/- 9.8% and 28.0% +/- 21.8%, as calculated for the entire test perfusion. The decrease in flow rate correlated with the SCN- concentration (p < 0.05) and showed a clear reversibility. A similar SCN(-)-induced (0.05 mmol/L) vasoconstriction (p < 0.05) was seen when the artery was given a basal tone by continuous perfusion with low-dose norepinephrine, 10 nmol/L. Nifedipine (100 nmol/L) abolished the effect of 0.05 mmol/L SCN- but did not affect the norepinephrine priming. We conclude that SCN- amplifies the norepinephrine-induced vascular smooth muscle tone and that this may be caused by an altered calcium channel activity.

Comparison of the effects of perchlorate and Bay K 8644 on the dynamics of cytoplasmic Ca2+ concentration and insulin secretion in mouse beta-cells

Non-inbred ob/ob mice were used to study the dynamics of cytoplasmic Ca2+ concentration ([Ca2+]i) in isolated pancreatic beta-cells using microfluorimetry with fura 2/AM as probe, and the dynamics of insulin secretion in isolated pancreatic islets. D-Glucose (20 mM) caused a transient peak increase in [CA2+]i which changed to either an oscillating or a flat, elevated phase. The lag-time before the first peak increase in [Ca2+]i was markedly shortened by 12 mM ClO4- and the glucose-stimulated level of [Ca2+]i after the first peak was clearly elevated by the anion. ClO4- did not change the basal [Ca2+]i at 3 mM glucose. Extracellular Ca2+ deficiency abolished the effect of high glucose and ClO4- on [Ca2+]i. This suggests that ClO4- acts as an amplifier of transmembrane Ca2+ inflow. The L-type Ca2+ channel agonist, Bay K 8644 (0.01-1.0 microM), strictly reproduced all the effects of perchlorate on the glucose-stimulated beta-cell [Ca2+]i. Both phases of insulin release (20 mM glucose) were markedly enhanced by ClO4- (12 mM) or Bay K 8644 (1.0 microM). The lag-time for glucose-stimulated insulin release was shortened by both agents. Taken together, these data strengthen the idea that perchlorate amplifies the glucose-stimulation of [Ca2+]i and insulin release by directly modifying the function of the L-type Ca2+ channel. This effect can induce both a more prompt onset of and an amplified level of beta-cell secretory activity.

Interaction between perchlorate and nifedipine on insulin secretion from mouse pancreatic islets

In order to elucidate the mechanisms responsible for the stimulatory effect of perchlorate (ClO4-) on insulin secretion, we have investigated the interaction between this chaotropic anion and the organic calcium antagonist nifedipine. This drug, known as a blocker of L-type calcium channels, was chosen as a tool to test the idea that ClO4- acts on insulin secretion by stimulating the gating of voltage-controlled Ca2+ channels. ClO4- amplified the stimulatory effect of D-glucose on insulin release from perfused pancreas (first and second phases) as well as from isolated islets incubated in static incubations for 60 min. This indicates that ClO4- amplifies physiologically regulated insulin secretion. Nifedipine reduced D-glucose-induced (20 mM) insulin release in a dose-dependent manner with half-maximum effect at about 0.8 microM and apparent maximum effect at 5 microM nifedipine. In the presence of 20 mM D-glucose, the inhibitory effects of 0.5, 1 or 5 microM nifedipine were only slightly, if at all, counteracted by perchlorate. When 12 mM ClO4- and 20 mM D-glucose were combined, calculation of the specific effect of ClO4- revealed that nifedipine produced almost maximum inhibition already at 0.05 microM. Thus, the perchlorate-induced amplification of D-glucose-stimulated insulin release shows higher sensitivity to nifedipine than the D-glucose-effect as such. This supports the hypothesis that perchlorate primarily affects the voltage-sensitive L-type calcium channel in the beta-cell.