Carotid body (CB) receptor type I cells transduce blood-borne chemical stimuli into electrical signals and release the excitatory neurotransmitter ATP onto afferent terminals that project to the breathing centre located in the brainstem. Within the CB, type I cells are ensheathed by glial-like processes of type II cells. Recently, it was hypothesized that type II cells have a paracrine function in CB chemotransduction by acting as an ATP amplifier and enhancing chemoexcitation (Zhang et al. 2012). Given this recent development, the primary goal of this thesis was to further elucidate the paracrine function of type II cells and characterize the signalling mechanisms involved in type I and type II cell interactions. Ratiometric calcium imaging was used to investigate type II cell sensitivity to two prominent CB neuromodulators, angiotensin II (ANG II) and 5-HT, in rat CB cultures. Both ANG II and 5-HT elicited large rises in intracellular Ca<sup>2+<sup> that were present in the absence of extracellular Ca<sup>2+<sup> and were inhibited by intracellular store depletion agents. ANG II and 5-HT acted on their respective G-protein coupled receptors, AT<sub>1<sub> receptor and 5-HT<sub>2A<sub> receptor, to initiate these Ca<sup>2+<sup> responses presumably via a PLC-IP<sub>3<sub> mediated mechanism. Interestingly, these Ca<sup>2+<sup> responses were required to activate pannexin-1 channels (Panx-1), a channel that has been previously shown to be a conduit for ATP in type II cells (Zhang et al. 2012). We were also interested in determining whether type II cells were capable of indirectly responding to a chemostimulus such that the stimulus would elicit neurosecretion from type I cells and result in a secondary Ca<sup>2+<sup> responses in type II cells. Isohydric hypercapnia and a depolarizing stimulus (30 mM KCl saline) were capable of eliciting indirect Ca<sup>2+<sup> responses in type II cells. These secondary Ca<sup>2+<sup> responses in type II cells were partially inhibited by suramin, a purinergic P2Y2 receptor antagonist, suggesting that ATP was the predominant neurotransmitter responsible for type I to type II crosstalk. Similarly, a selective agonist for type II cells, UTP, evoked indirect Ca<sup>2+<sup> responses in nearby type I cells. Type II to type I cell communication was dependent on Panx-1 channels since the secondary Ca<sup>2+<sup> responses in type I cells were inhibited by the Panx-1 blocker, carbenoxolone (5 µM). UTP-evoked indirect Ca<sup>2+<sup> in type I cells were partially inhibited by adenosine A<sub>2<sub> receptor antagonists suggesting that the neuromodulator, adenosine, governs cross-talk between type II and type I cells. This study elucidates the importance of purinergic signalling in the bi-directional cross-talk between receptor type I cells and glial-like type II cells. / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/17197 |
| Date | 06 1900 |
| Creators | Murali, Sindhubarathi |
| Contributors | Nurse, Colin A., Biology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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