Studies of individual pancreatic -cells : Electrophysiological analysis of rhythmic behaviour and development of new techniques
Abstract: The insulin concentration in blood varies periodically, which is believed to prevent down-regulation of the hormone receptors. Loss of the regular insulin oscillations is considered to be an early sign of developing diabetes. The insulin variations are due to pulsatile release of insulin from the pancreatic islets and their β-cells, which occurs in synchrony with slow large amplitude oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i). It was now investigated whether oscillations in metabolism may drive those in [Ca2+]i or if metabolism follows [Ca2+]i. Using the ATP concentration as an indicator of metabolism, the activity of ATP-regulated K+ (KATP) channels was analysed in the cell-attached configuration of the patch clamp technique. At 0-3 mM glucose, a situation with low and stable [Ca2+]i, there were regular slow fluctuations in KATP channel open-time. These variations, with a frequency similar to that of the slow [Ca2+]i oscillations, provide strong evidence that metabolic oscillations are primary events. Ca2+ entry through voltage-activated channels was evaluated as a mechanism for generating the slow [Ca2+]i oscillations. Using Sr2+ and Mn2+ as analogues for Ca2+, glucose stimulation and depolarisation with K+ were found to enhance the influx. Moreover, the [Ca2+]i oscillations were almost perfectly parallel with slow bursts of action currents. Confirming a central role of voltage-dependent Ca2+ entry all these effects were inhibited by the specific Ca2+ channel antagonist nifedipine. During the bursts occasional pronounced [Ca2+]i spikes, due to intracellular release of the ion, temporarily arrested the action currents. It is discussed how these spikes may generate a hyperpolarising current, explaining the generation of the fast oscillatory pattern observed in pancreatic islets. The wavelet transform was found to be an excellent tool for reducing noise in experimental data and for highlighting recurrent electrophysiological patterns. An improved U-tube technique is presented, allowing change of medium around single cells within 60 msec. An engraving tool, producing 1.5 µm broad lines, was constructed and used to label light microscope object and cover glasses with micro-text.
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