RESUMO
Based on excitation emission matrix spectroscopy (EEMs) technology combined with parallel factor analysis (PARAFAC) and ultraviolet-visible (UV-vis) spectra, we analyze the spatial distribution, spectral characteristics, and sources of dissolved organic matter (DOM) in Baiyangdian Lake, China, during a winter freezing period. Results showed that the UV-vis absorption spectrum of DOM had no obvious characteristic peak, and that the variation coefficient of absorption exhibited a significant difference (P<0.05) among different districts of Baiyangdian Lake, but that there was no significant difference between the surface and bottom waters. The changes of E3/E4, E2/E3, and SR showed that DOM had low humic and autochthonous characteristics. Two protein-like substances (C1 and C2) and one humic-like substance (C3) were identified by PARAFAC, with a significant correlation (P<0.001) being found between C1 and C3, and C2 and C3. The total DOM fluorescence intensity and the fluorescence intensity of each component exhibited significant differences (P<0.01) in the distribution among the different districts of Baiyangdian Lake, with the maximum value being associated with a sample from the Tanghe River and the minimum value being associated with a sample from Shaochedian. Moreover, the sum of C1+C2 accounted for the major proportion of DOM. DOM exhibited a strong autochthonous characteristic based on the values of BIX, FI, and HIX. Principle component analysis (PCA) and Adonis analysis showed that the spectral characteristics of DOM exhibited a significant difference (P<0.05) among the different districts. C1, C2, and C3 were significantly correlated (P<0.001) with DOM indices (HIX, BIX, Fn280, and Fn355) and water quality parameters[total nitrogen (TN), permanganate index, and total dissolved phosphorus (TDP)] based on multiple linear regression. Our results contribute to previous investigations and provide findings that can assist in the future management and control of organic carbon pollution sources to Baiyangdian Lake.
RESUMO
The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) activates Ca(2+) influx independent of the release of intracellular Ca(2+) stores. The latter can be negatively regulated by protein kinase C (PKC) through phosphorylation of Thr-888 of the CaR. In this study, we substituted Thr-888 with various amino acid residues or a stop codon to understand how PKC phosphorylation of the CaR inhibits receptor-mediated release of intracellular Ca(2+) stores. Substitutions of Thr-888 with hydrophobic and hydrophilic amino acid residues had various effects on CaR-mediated release of intracellular Ca(2+) stores as well as activation of Ca(2+) influx. Several point mutations, such as T888D, had marked negative effects on CaR-mediated release of intracellular Ca(2+) stores but not on phorbol myristate acetate-insensitive activation of Ca(2+) influx. Presumably, the negatively charged aspartate mimics phospho-threonine. Interestingly, truncating the receptor at 888 had an even more pronounced negative effect on CaR-elicited release of intracellular Ca(2+) stores without significantly affecting CaR-mediated activation of Ca(2+) influx. Therefore, truncation at position 888 of the CaR affects the activity of the receptor in a manner that resembles PKC phosphorylation of the CaR. This in turn suggests that PKC phosphorylation of the CaR prevents G protein subtypes from interacting with the region of the receptor critical for releasing Ca(2+) stores, which is missing in the truncated receptor.