A novel bioelectronic glucose sensor to process distinct electrical activities of pancreatic beta-cells.

Glucose sensors have improved and facilitated therapy for type 1 diabetes. However, they are still not capable to sense all physiological signals and to act in a closed-loop. Pancreatic ß-cells have been shaped during evolution as biological sensors and offer the advantage to integrate all physiological signals in addition to glucose. Moreover, biosensors based on these cells may also serve for non-invasive and continuous long-term characterization of ß-cells, drug research, tissue engineering and pre-transplantation quality control. ß-cells alter their electrical activity upon exposure to glucose and physiological hormones and we have used these properties to design a biosensor. To this end signals were recorded extracellularly from islet cells kept on multi-electrode arrays. Slow and rapid oscillations were observed, both modulated by glucose. Especially slow oscillations are very robust and have an excellent signal/noise ratio. Signal processing functions were designed to separate the two activities to extract and analyze relevant parameters. These parameters correlate very well with either increasing or decreasing glucose concentrations. An electronic device is under construction, based on an embedded FPGA capable of processing multiple channels in parallel. In the future, such a device shall be used as a portable real-time biosensor regulating insulin delivery from a pump.

Nimodipine inhibits AP firing in cultured hippocampal neurons predominantly due to block of voltage-dependent potassium channels.

L-type calcium channels (LTCC) are important functional elements of hippocampal neurons contributing to processes like memory formation and gene expression. Mice lacking the Ca(V)1.2 channel in hippocampal pyramidal cells exhibited defects in spatial memory (Moosmang et al. 2005) and lowered frequency of repetitive action potential (AP) firing (Lacinova et al. 2008). We tested the contribution of LTCC to AP firing of cultured rat neonatal hippocampal neurons using the dihydropyridine channel blocker nimodipine. Ionic currents and APs were recorded in the whole cell patch clamp configuration. A prolonged depolarizing current pulse activated the firing of a series of APs. The presence of 10 µM nimodipine blocked all but the first AP in series. This concentration, which is potent enough to completely block LTCC, inhibited about 35-50% of the total calcium current. In addition, nimodipine blocked about 50% of both calcium-dependent and voltage-dependent potassium currents whereas the sodium current was not affected. We suggest that nimodipine suppressed the firing of APs in cultured neonatal rat hippocampal neurons due to inhibition of both calcium and potassium currents.

Ca(v)1.3 and BK channels for timing and regulating cell firing.

L-type Ca(2+) channels (LTCCs, Ca(v)1) open readily during membrane depolarization and allow Ca(2+) to enter the cell. In this way, LTCCs regulate cell excitability and trigger a variety of Ca(2+)-dependent physiological processes such as: excitation-contraction coupling in muscle cells, gene expression, synaptic plasticity, neuronal differentiation, hormone secretion, and pacemaker activity in heart, neurons, and endocrine cells. Among the two major isoforms of LTCCs expressed in excitable tissues (Ca(v)1.2 and Ca(v)1.3), Ca(v)1.3 appears suitable for supporting a pacemaker current in spontaneously firing cells. It has steep voltage dependence and low threshold of activation and inactivates slowly. Using Ca(v)1.3(-/-) KO mice and membrane current recording techniques such as the dynamic and the action potential clamp, it has been possible to resolve the time course of Ca(v)1.3 pacemaker currents that regulate the spontaneous firing of dopaminergic neurons and adrenal chromaffin cells. In several cell types, Ca(v)1.3 is selectively coupled to BK channels within membrane nanodomains and controls both the firing frequency and the action potential repolarization phase. Here we review the most critical aspects of Ca(v)1.3 channel gating and its coupling to large conductance BK channels recently discovered in spontaneously firing neurons and neuroendocrine cells with the aim of furnishing a converging view of the role that these two channel types play in the regulation of cell excitability.