In a recent perifusion investigation, we showed that the pineal secretory product melatonin reduces insulin secretion from isolated pancreatic islets of neonate rats stimulated with potassium chloride (KCl), glucose, and forskolin. This effect of melatonin was reproduced with doses ranging from 200 pmol/L to 5 μmol/L. Because it is generally accepted that melatonin exerts some of its biological effects through specific, high‐affinity pertussis‐toxin‐sensitive G‐protein‐coupled receptors, we blocked the putative melatonin receptor of pancreatic islets using both the non‐hydrolyzable guanosine triphosphate analog guanosine 5′‐O‐(3‐thiotriphosphate) (GTPΓS, 30 μmol/L) and the melatonin antagonist luzindole (10 μmol/L). Both GTPΓS and luzindole caused a near normalization of the melatonin‐induced inhibition of the forskolin‐stimulated insulin secretion. To localize putative melatonin receptors within the pancreatic islets autoradiographic studies were additionally carried out. These investigations showed specific binding of 2‐[125I]iodomelatonin, which were in exact correspondence with the localization of the islets. In addition, gray‐level analysis showed that unlabeled melatonin was able to reduce the binding of 2‐[125I]iodomelatonin in a dose‐dependent manner. Concentrations of unlabeled melatonin of 10−9 mol/L produced a 50% reduction in specific binding, whereas concentrations of 10−6 mol/L displaced the binding completely. Likewise, the results of molecular investigations showed that the rat pancreas contains a melatonin receptor, since reverse transcription polymerase chain reaction (RT‐PCR) experiments, using specific primers for the rat melatonin receptor Mel1a, showed that mRNA for this melatonin receptor type is expressed in pancreatic tissue of newborn rats. In summary, it may be said that our functional, autoradiographic, and molecular results indicate that the Mel1a receptor is located on the pancreatic islets, possibly in the beta cells.

Microelectrode wires heated directly in situ by an electric current generate mass as well as heat energy transport phenomena. With continuous heating, a stationary surface temperature is established as a result of thermally promoted convection. The geometry of concentration and temperature profiles under these conditions are investigated by cyclic voltammetry, potential step experiments, and finite element simulation of convection and diffusion processes. It is shown that the Nernst diffusion layer approaches a constant, temperatureindependent thickness if the temperature difference between bulk and electrode surface exceeds ca. 30 K in aqueous solution. At continuously heated wire electrodes, diffusion coefficient values as well as kinetic data can be determined under well-defined temperature conditions, and in a very convenient way.

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The adsorption of O on Ru(0001) at a temperature of 400 K is studied in detail by means of scanning tunnelling microscopy (STM). With increasing O coverage, an ordered p(2 × 2) structure develops, followed by a p(2 × 1) structure. While the p(2 × 2) structure grows via island formation, the p(2 × 1) structure is abruptly formed by a disorder-order phase transition. After completion of the p(2 × 2) structure at a coverage of 0.25 ML, the surface develops a rough structure where the (2 × 2) units remain visible but appear with different heights. As the origin of this phenomenon, we propose additional O-O interactions and/or subsurface O due to the increase in O coverage. At coverages between 0.3 monolayer (ML) and 0.35 ML, different preformations of the p(2 × 1) structure are observed. First, small fragments of p(2 × 1) rows develop, which are randomly distributed over the surface and rotated by 120 • with respect to each other. They grow in one dimension and induce a criss-cross arrangement of linear chains of O atoms. Two-dimensional ordering starts via pairing of the p(2 × 1) rows. At a critical O coverage slightly below 0.40 ML, suddenly large p(2 × 1) domains are formed in three orientations (rotated by 120 • ), which coexist with remnants of the p(2 × 2) structure. At the saturation coverage of O (0.5 ML), the p(2 × 1) domains cover the surface completely.