Signal transduction of transfected and native P2Y receptors

Our understanding of the soft X-ray properties of nearby galaxies has been revolutionised as a result of the Rontgensatellit (ROSAT) mission. This thesis considers a number of aspects of the properties of nearby galaxies on the basis of recent ROSAT PSPC and HRI observations. Firstly, the extragalactic contribution to the X-ray background in the softest accessible X- ray band (0.1 - 0.4 keV) is investigated using the shadows cast by the H I discs of nearby galaxies. A preliminary investigation considers the case of NGC 55, a nearby edge-on galaxy with an extensive H I disc. The technique is then refined and employed on a sample of nearby galaxies, resulting in the best constrained measurement currently available of the extragalactic X-ray background intensity at 0.25 keV of 24.9 5.0 keV cm-2 s-1 sr_1 keV-1. This result has implications for the source populations which compose the extragalactic X-ray background below 1 keV. Detailed studies of the X-ray properties of two nearby galaxies, NGC 55 and NGC 4736, are then presented. The X-ray emission from NGC 55 is dominated by a population of point sources, with some diffuse emission evident in and around its disc. NGC 4736, on the other hand, is dominated by the emission from an extended nuclear source. X-ray data strongly suggests that this region contains a low-luminosity AGN, a conclusion that is supported by observations at other wavelengths. The incidence of low-luminosity AGN in nearby galaxies is probed by correlating ROSATPSPC point source catalogues with an optically-identified sample of nearby galactic nuclei. A sample of 90 "nuclear X-ray sources" is defined, and general trends in their X-ray properties are identified for example, low-luminosity Seyfert nuclei are generally much more X-ray luminous than nuclear H II regions, and soft X-ray hardness ratios are seen to differentiate between optical broad-line and narrow-line AGN