Vasopressin (VP) is an antidiuretic hormone that is synthesized and released by osmosensitive magnocellular neurosecretory cells (MNCs) to regulate water homeostasis in the body. The rate and firing pattern of MNCs determines the amount of VP release, which is elevated during physiological stress particularly dehydration. During acute osmotic changes the MNCs shrink and swell due to hypertonic and hypotonic stimuli, respectively. In contrast to hippocampal neurons, which display regulatory volume increases (RVI) and regulatory volume decreases (RVD) in response to hypertonic and hypotonic stimuli, MNCs do not have compensatory mechanisms. The MNCs undergo hypertrophy as a part of their physiological structural and functional plasticity during chronic dehydration. These changes are thought to be important during long term osmotic changes for the sustained and high level releases of hormone. However, the mechanism of hypertrophy is still unclear and it is difficult to address this issue in vivo. We therefore undertook studies on acutely isolated MNCs to test hypertrophy in MNCs. We observed that acutely isolated MNCs treated with hyperosmolar solution (325 mOsmol kg-1) for 150 minutes in vitro showed hypertrophy (a 9% increase in CSA) and recovered their original size when returned to isotonic solution (295 mOsmol kg-1) for another 60 minutes. Whole cell patch clamp experiments showed a 34% increase in cell membrane capacitance following treatment with hypertonic solution for 90-150 minutes. The osmotically-evoked hypertrophic response was blocked by using a TAT (human immunodeficiency virus transactivator of transcription) peptide (TAT-NSF700) that prevents SNARE-mediated exocytotic fusion by blocking the function of NSF (N-ethylmaleimide-sensitive factor). The hypertrophic response did not appear to be altered by a scrambled version of the peptide, showing that osmotically-evoked hypertrophy depends on SNARE-mediated exocytotic fusion. The VP and OT-MNCs exposed to hyperosmolar solution for two hours showed increased immunofluorescence for L-type Ca²⁺ channels (both Cav1.2 and Cav1.3). Our data suggest that the osmotically-evoked hypertrophy is associated with an increase in the total membrane surface area due to the exocytotic fusion of intracellular granules with the plasma membrane and with increased expression of L-type Ca2+ channels. This study will be helpful in understanding of the adaptation that MNCs undergo during long term dehydration and pathological conditions that lead to increased plasma osmolality.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:SSU.etd-05262011-102523 |
| Date | 20 June 2011 |
| Creators | Mumtaz, Naima |
| Contributors | Mulligan, Sean, Desautels, Michel, Fisher, Thomas E., West, Nigel, Muir, Gillian |
| Publisher | University of Saskatchewan |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://library.usask.ca/theses/available/etd-05262011-102523/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Saskatchewan or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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