Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 108-139).

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In recent decades, many softwater lakes on the boreal shield have experienced significant reductions in aqueous calcium (Ca) concentrations. These declines are a long-term consequence of acid deposition due to the depletion of base cations from watershed soils. There is concern that in some lakes [Ca] may be falling to levels detrimental to the competitiveness of Ca-rich organisms. By examining the crustacean zooplankton remains preserved in lake sediments, this thesis provides field evidence of reduced [Ca] impacting a Ca-sensitive crustacean zooplankton species (Daphnia pulex). Additionally, a 770 lake data set compiled from several Ontario monitoring programs revealed that 62 % (an increase from 35% in the early 1980s) of the lakes were near or below the laboratory-determined Ca threshold (1.5 mg•L-1) for the growth and survival of D. pulex. To determine whether the 1.5 mg•L-1 Ca threshold could be observed in a spatial survey of crustacean zooplankton sedimentary remains, surface sediments from 36 softwater (Ca range 1-3 mg•L-1) Ontario lakes were analyzed. Significant differences in daphniid abundances across the Ca threshold were present; however, only for the D. longispina species complex, indicating differences in Ca tolerances within daphniid species complexes. Extending the analysis to a comparison of modern-day vs. preindustrial assemblages revealed that in the same 36 lakes there have been large declines (by up to 30%) in daphniid relative abundances since preindustrial times coincident with increases in Ca-poor species (i.e. Holopedium gibberum) irrespective of modern day pH. These findings demonstrate that in natural settings, the competitive disadvantages of Ca limitation may occur at a higher [Ca] than previously suspected. Finally, zooplankton sedimentary remains were analyzed from several “pristine” lakes in northwestern Ontario that have also experienced Ca declines in recent years. Reduced abundances of Ca-sensitive taxa and increases in Ca-insensitive fauna provided further evidence of the impacts of Ca decline independent of acid deposition. Collectively these analyses demonstrate the potential importance of Ca as an environmental stressor in softwater regions, as well as the need for further research in order to make better use of the available information preserved in the sediment record. / Thesis (Ph.D, Biology) -- Queen's University, 2011-03-31 16:17:57.86

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Movements of calcium (Ca) across the maternal and fetal aspects of the human placenta were investigated using the isolated placental lobule dually perfused <I>in vitro.</I> Tissues uptakes and releases of calcium were measured and the effects on calcium movements by calcium-protein binding in the perfusion fluids, (associated with extracellular pathways and non-uniform perfusion), evaluated. The effects of ouabain, dinitrophenol (DNP), and cooling on calcium movements were measured and compared to movements of Na⁺ and K⁺. These indicated the presence of active transport of calcium but no evidence was obtained for Ca²⁺/Na⁺ exchange. Cyclic adenosine 3', 5' -monophosphate (cAMP) levels in dually perfused tissue were measured following microwave fixation. This technique was used to measure changes in tissue cAMP production following exposure to forskolin, 3-iso-butyl-l-ethyl-xanthine (IBMX), and various fragments of both bovine parathyroid hormone (bPTH) and human parathyroid hormone-related peptide (hPTHrP). Rises in cAMP were produced by exposure to bPTH(1-34) but not hPTHrP(1-34), hPTHrP(67-86) or hPTHrP (107-138). It is concluded that calcium is actively transported across the placenta but there is no major contribution via a Ca²⁺/Na⁺ exchanger. The patterns of calcium uptake as a function of perfusate calcium concentration support the evidence of other workers that extracellular pathways are present in the syncytiotrophoblast. A significant amount of passive movement of calcium may therefore take place across the perfused placenta. The 1 to 34 region of the PTH molecule stimulates the production of cAMP by the trophoblast, but there is no indication that this has any effect on transplacental Ca transport.

The aim of this study was to investigate the role of Ca²⁺ signalling pathways in the growth, morphogenesis and hyphal reorientation responses of <i>C. albicans</i>. The genes <i>CCH1</i> and <i>MID1</i> were identified in <i>S. cerevisiae</i> as encoding putative Ca²⁺ channels. These genes have since been shown to compose a Ca²⁺ channel complex. Homologues of these genes were identified and cloned from <i>C. albicans</i>. <i>CaMID1</i> was disrupted by the Ura-blaster method, and the resulting mutant characterised. The <i>C. albicans</i> <i>mid1</i> mutant strain was sensitive to the depletion of Ca²⁺ the presence of cell wall perturbing compounds such as SDS and Calcofluor. It formed hyphae more rapidly in the presence of serum, and had a propensity to grow as elongated cells or pseudohyphae in Ca²⁺-depleted medium, on SD, or on medium containing cell wall perturbing compounds. This suggests that depletion of Ca²⁺-uptake perturbs yeast-hypha morphogenesis, perhaps by inducing a nutrient starvation stress response. The <i>mid1</i> mutant and a number of other <i>C. albicans</i> Ca²⁺ signalling mutants were defective in chlamydospore formation, suggesting a role for Ca²⁺ in two morphogenetic genesises: the hyphae and of chlamydospores. The role of Mid1p in the thigmotropic reorientation responses of <i>C. albicans</i> hyphae was investigated. The <i>mid1</i> mutant strain displayed reduced ability to reorientate growth upon contact with ridges on an etched quartz slide. Suggesting that Mid1p may function as stretch-activated Ca²⁺ channel in <i>C. albicans</i>. The ability of <i>C. albicans</i> <i>mid1</i> mutants to respond to an electric field was also attenuated, suggesting that Mid1p may form part of a voltage-sensitive Ca²⁺ channel in <i>C. albicans</i> that plays a central role in the steering mechanism of <i>C. albicans</i> hyphae. The <i>C. albicans </i>kinase Cst20p may function downstream of Mid1p in growth reorientation responses.