http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:11410 Background and purpose: Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca²⁺ store in uterine activity. Experimental approach: We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca²⁺ concentration ([Ca²⁺]c) in longitudinal smooth muscle of the mouse uterus. Key results: The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca²⁺]c or depolarization/transient hyperpolarization and transient increase in [Ca²⁺]c for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca²⁺]c. CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca²⁺]c. Application of a range of K⁺ channel blockers indicated a role of Ca²⁺-activated K⁺ (KCa) channels in the CCCP- and CGP37157-induced actions. Conclusions and implications: Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca²⁺]c and/or Ca²⁺ influx. 2012-08-29T04:05:20.116Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9917 This study presents a theoretical analysis of the role of store Ca²⁺ uptake on sinoatrial node (SAN) cell pacemaking. Two mechanisms have been shown to be involved in SAN pacemaking, these being: 1) the membrane oscillator model where rhythm generation is based on the interaction of voltage-dependent membrane ion channels and, 2) the store oscillator model where cyclical release of Ca²⁺ from intracellular Ca²⁺ stores depolarizes the membrane through activation of the sodium-calcium exchanger (NCX). The relative roles of these oscillators in generation and modulation of pacemaker rate have been vigorously debated and have many consequences. The main new outcomes of our study are: 1) uptake of Ca²⁺ by intracellular Ca²⁺ stores increases the maximum diastolic potential (MDP) by reducing the cytosolic Ca²⁺ concentration [Ca²⁺]c and hence decreasing the NCX current; 2) this hyperpolarization enhances recruitment of key pacemaker currents (e.g. the hyperpolarization-activated HCN current (If) and T-type Ca²⁺ current (IT-Ca)); 3) the resultant enhanced Ca²⁺ entry during the pacemaker depolarization increases [Ca²⁺]c causing advancement of the store Ca²⁺ release cycle and increased NCX current. In overview, the novel feature of our study is an investigation of the role of store Ca²⁺ uptake on SAN pacemaking. This occurs during the early diastolic period and causes enhanced If, IT-Ca and store release (and hence INCX) during the later diastolic period. There is thus a symbiotic interaction between the two pacemaker “clocks” over the entire diastolic period, this providing robust and highly malleable SAN pacemaking. Accounting for store Ca²⁺ uptake also provides insight into hitherto unexplained SAN behaviour, as we exemplify for the sinus bradycardia exhibited in catecholaminergic polymorphic ventricular tachycardia (CPVT). 2012-02-08T01:20:04.698Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:6901 Research Doctorate - Doctor of Philiosphy (PhD) 2012-01-30T05:01:56.833Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9515 1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle. 2. At present, there is strong agreement that interstitial cells of Cajal (ICC) are the pacemaker cells that generate slow waves. What has been less clear is the relative role of primary types of ICC, including the network in the myenteric plexus (ICC-MY) and the intramuscular network (ICC-IM). It is concluded that both ICC-MY and ICC-IM are likely to serve a major role in slow wave generation and propagation. 3. There has been long-standing controversy as to how slow waves ‘propagate’ circumferentially and down the gastrointestinal tract. Two mechanisms have been proposed, one being action potential (AP)-like conduction and the other phase wave-based ‘propagation’ resulting from an interaction of coupled oscillators. Studies made on single bundle gastric strips indicate that both mechanisms apply with relative dominance depending on conditions; the phase wave mechanism is dominant under circumstances of rhythmically generating slow waves and the AP-like propagation is dominant when the system is perturbed. 4. The phase wave mechanism (termed Ca²⁺ phase wave) uses cyclical Ca²⁺ release as the oscillator, with coupling between oscillators mediated by several factors, including: (i) store-induced depolarization; (ii) resultant electrical current flow⁄depolarization through the pacemaker cell network; and (iii) depolarization- induced increase in excitability of downstream Ca²⁺ stores. An analogy is provided by pendulums in an array coupled together by a network of springs. These, when randomly activated, entrain to swing at the same frequency but with a relative delay along the row giving the impression of a propagating wave. 5. The AP-like mechanism (termed voltage-accelerated Ca²⁺ wave) propagates sequentially like a conducting AP. However, it is different in that it depends on regenerative store Ca²⁺ release and resultant depolarization rather than regenerative activation of voltage-dependent channels in the cell membrane. 6. The applicability of these mechanisms to describing propagation in large intact gastrointestinal tissues, where voltagedependent Ca²⁺ entry is also likely to be functional, is discussed. 2011-11-29T02:40:03.853Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4460 Guinea pig mesenteric lymphatic vessels exhibit rhythmic constrictions induced by action potential (AP)-like spikes and initiated by entrainment of spontaneous transient depolarizations (STDs). To characterize STDs and the signaling mechanisms responsible for their occurrence, we used intracellular microelectrodes, Ca²⁺ imaging, and pharmacological agents. In our investigation of the role of intracellular Ca²⁺ released from Ca²⁺ stores, we observed that intracellular Ca²⁺ transients accompanied some STDs, although there were many exceptions where Ca²⁺ transients occurred without accompanying STDs. STD frequency and amplitude were markedly affected by activators/inhibitors of inositol 1,4,5-trisphosphate receptors (IP₃Rs) but not by treatments known to alter Ca²⁺ release via ryanodine receptors. A role for Ca²⁺-activated Cl⁻ (ClCa) channels was indicated, as STDs were dependent on the Cl⁻ but not Na⁺ concentration of the superfusing solution and were inhibited by the ClCa channel blockers niflumic acid (NFA), anthracene 9-carboxylic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid but not by the volume-regulated Cl⁻ blocker DIDS. Increases in STD frequency and amplitude induced by agonist stimulation were also inhibited by NFA. Nifedipine, the hyperpolarization-activated inward current blocker ZD-7288, and the nonselective cation/store-operated channel blockers SKF-96365, Gd³⁺, and Ni²⁺ had no or marginal effects on STD activity. However, nifedipine, 2-aminoethoxydiphenyl borate, NFA, SKF-96365, Gd³⁺, and Ni²⁺ altered the occurrence of spontaneous APs. Our findings support a role for Ca²⁺ release through IP₃Rs and a resultant opening of ClCa channels in STD generation and confirm the importance of these events in the initiation of lymphatic spontaneous APs and subsequent contractions. The abolition of spontaneous APs by blockers of other excitatory ion channels suggests a contribution of these conductances to lymphatic pacemaking. 2010-04-27T04:52:07.072Z ]]>