Inhibition of Na/Ca exchange stimulates insulin release from isolated rat pancreatic islets

Summary— Na/Ca exchange was recently shown to regulate cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) in the pancreatic B-cell. The aim of the present study was to provide direct evidence that inhibition of the activity of the exchange may also increase insulin release. In the presence of extracellular Na+, caffeine stimulated ⁴⁵Ca outflow but did not increase insulin release from islets perifused in the presence of 2.8 mM glucose. By contrast, in the absence of extracellular Na+, caffeine almost failed to increase ⁴⁵Ca outflow and reversibly stimulated insulin release despite the fact that the absence of extracellular Na+ per se reduced basal insulin release. Similar findings were observed in islets perifused at a higher glucose concentration (8.3 mM) except that, in the presence of extracellular Na+, caffeine more markedly increased ⁴⁵Ca outflow and stimulated insulin release. Our data provide direct evidence that inhibition of Na/Ca exchange with resulting blockade of Ca²⁺ outflow may increase insulin release from the pancreatic B-cell under suitable experimental conditions.     

Isolated abducens nerve palsy following lumbar puncture: case report and review of the mechanism of action

Isolated abducens nerve palsy following lumbar puncture is a very rare condition. In this case we discussed the probable causes of abducens nerve palsy and review the mechanism of action in anatomical relevant literature. A 53-year old hypertensive female patient with a saccular aneursym on the left middle cerebral artery (MCA) bifurcation underwent emergency operation. Before the operation lumbar puncture (LP) was performed to the patient lying on the right lateral position to facilitate cerebral relaxation intraoperatively. The left MCA bifurcation aneurysm was clipped successfully with a left pterional-transsylvian approach. Postoperatively, she complained of visual diplopia and postural headache. In her neurological examination, isolated abducens nerve palsy was found on the left eye. The patient was treated with intravenous hydration, bed rest and non - steroid anti-inflamatuary drugs (NSAID) for postural headache. Her postural headache was resolved in the postoperative fifth day, but her abducens nerve palsy was present in the postoperative sixth month follow-up. Many classic textbooks have attributed the vulnerability of the abducens nerve to its long intracranial course, but it is now known that abducens nerves angulation points are the vulnerable parts of the nerve. We hypothesize that the petroclival dural entrance point is the entrapment point and lateral type abducens nerve, if present may be a factor that facilitates the injury of the nerve by lumbar puncture (LP).     

Children with autism spectrum disorder spontaneously use scene knowledge to modulate visual object processing

Several studies have investigated contextual influences on visual object processing in individuals with autism spectrum disorder (ASD) and failed to find reduced context effects. However, these studies did not properly account for local inter-object effects and/or the influence of post-perceptual processes, leaving it unclear whether individuals with ASD display equally large global top-down effects of context, or whether they rely on a more local mechanism producing the same result. In this study, an eye-movement paradigm was used to investigate implicitly induced context effects on visual object processing in children with ASD compared to matched controls. To find out whether the context effects in the ASD group were, to the same extent, due to early top-down influences on object recognition, we also examined the interaction between context effects and the ease of object recognition. Both groups displayed equally large context effects and congruent contextual information facilitated object recognition to the same extent in both groups. This indicates that the context effects in the ASD group did not result from the operation of a more local, less top-down mechanism. These findings contradict predictions based on the weak central coherence account. However, a good alternative to explain all inconsistencies is currently lacking.     

Cognitive flexibility in autism spectrum disorder: Explaining the inconsistencies?

The Wisconsin Card Sorting Task (WCST) is the only cognitive flexibility task that has consistently shown deficits in individuals with an autism spectrum disorder (ASD). As this is the only task characterized by limited explicit task instructions and a high degree of disengagement required to perform the switch, we hypothesized that cognitive flexibility deficits of individuals with ASD might only become apparent in situations fulfilling these requirements. However, the WCST involves various additional cognitive processes besides switching, making it uncertain whether difficulties are indeed due to cognitive flexibility impairments. The aim of this study was to investigate whether individuals with ASD show cognitive flexibility impairments on a more controlled task-switching variant of the WCST, still fulfilling both requirements. We therefore developed such a task and administered it to 40 high-functioning children with ASD and 40 age- and IQ- matched typically developing controls. As predicted, individuals with ASD made more perseveration errors and had a significantly higher switch cost than typically developing controls, but they performed equally well on the control measures.     

Overexpression of the Na/Ca exchanger shapes stimulus-induced cytosolic Ca(2+) oscillations in insulin-producing BRIN-BD11 cells.

In response to glucose, mouse beta-cells display slow oscillations of the membrane potential and cytosolic free Ca(2+) concentration ([Ca(2+)](i)), whereas rat beta-cells display a staircase increase in these parameters. Mouse and rat islet cells differ also by their level of Na/Ca exchanger (NCX) activity. The view that the inward current generated by Na/Ca exchange shapes stimulus-induced electrical activity and [Ca(2+)](i) oscillations in pancreatic beta-cells was examined in insulin-producing BRIN-BD11 cells overexpressing the Na/Ca exchanger. BRIN-BD11 cells were stably transfected with NCX1.7, one of the exchanger isoforms identified in the beta-cell. Overexpression could be assessed at the mRNA and protein level. Appropriate targeting to the plasma membrane could be assessed by microfluorescence and the increase in Na/Ca exchange activity. In response to K(+), overexpressing cells showed a more rapid increase in [Ca(2+)](i) on membrane depolarization as well as a more rapid decrease of [Ca(2+)](i) on membrane repolarization. In response to glucose and tolbutamide, control BRIN cells showed large amplitude [Ca(2+)](i) oscillations. In contrast, overexpressing cells showed a staircase increase in [Ca(2+)](i) without such large oscillations. Diazoxide-induced membrane hyperpolarization restored large amplitude [Ca(2+)](i) oscillations in overexpressing cells. The present data confirm that Na/Ca exchange plays a significant role in the rat beta-cell [Ca(2+)](i) homeostasis, the exchanger being a versatile system allowing both Ca(2+) entry and outflow. Our data suggest that the current generated by the exchanger shapes stimulus-induced membrane potential and [Ca(2+)](i) oscillations in insulin-secreting cells, with the difference in electrical activity and [Ca(2+)](i) behavior seen in mouse and rat beta-cells resulting in part from a difference in Na/Ca exchange activity between these two cells.     

Na/Ca exchanger overexpression induces endoplasmic reticulum-related apoptosis and caspase-12 activation in insulin-releasing BRIN-BD11 cells

Ca(2+) may trigger programmed cell death (apoptosis) and regulate death-specific enzymes. Therefore, the development of strategies to control Ca(2+) homeostasis may represent a potential approach to prevent or enhance cell apoptosis. To test this hypothesis, the plasma membrane Na/Ca exchanger (NCX1.7 isoform) was stably overexpressed in insulin-secreting tumoral cells. NCX1.7 overexpression increased apoptosis induced by endoplasmic reticulum (ER) Ca(2+)-ATPase inhibitors, but not by agents increasing intracellular calcium concentration ([Ca(2+)](i)), through the opening of plasma membrane Ca(2+)-channels. NCX1.7 overexpression reduced the rise in [Ca(2+)](i) induced by all agents, depleted ER Ca(2+) stores, sensitized the cells to Ca(2+)-independent proapoptotic signaling pathways, and reduced cell proliferation by approximately 40%. ER Ca(2+) stores depletion was accompanied by the activation of the ER-specific caspase (caspase-12), and the activation was enhanced by ER Ca(2+)-ATPase inhibitors. Hence, Na/Ca exchanger overexpression, by depleting ER Ca(2+) stores, triggers the activation of caspase-12 and increases apoptotic cell death. By increasing apoptosis and decreasing cell proliferation, overexpression of Na/Ca exchanger may represent a new potential approach in cancer gene therapy. On the other hand, our results open the way to the development of new strategies to control cellular Ca(2+) homeostasis that could, on the contrary, prevent the process of apoptosis that mediates, in part, beta-cell autoimmune destruction in type 1 diabetes.     

Novel inhibitors of the sodium-calcium exchanger: benzene ring analogues of N-guanidino substituted amiloride derivatives

A series of N-guanidino substituted 2,4-diamino-5-carbonylguanidine molecules related to amiloride were synthesised and evaluated for their ability to inhibit the sodium-calcium exchanger in rat insulinoma cells (RINm5F) and human platelets. Specific chemical pathways were used to prepare the benzene derivatives designed as bioisosteric analogues of the pyrazine derivatives of amiloride. Several so-called 'simplified analogues', where some substituents of amiloride were omitted or replaced, were also prepared and included in the biological evaluation. The inhibitory potency of the sodium-calcium exchanger was screened on both cell types by measuring their effect on 45Ca(2+) uptake. Among the most active compounds, N-(2-amino-5-chloro-4-nitrobenzoyl)-N'-(1-naphtylmethyl)guanidine (IC(50)=3.4 microM) was found more active than amiloride (IC(50)=690 microM) and 3,4-dichlorobenzamil (IC(50)=15.2 microM), the reference inhibitor.