http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:2746 Activation of skeletal muscle ryanodine receptors (RyRs) by suramin and disulfonic stilbene derivatives (Diisothiocyanostilbene-2',2'-disulfonic acid (DIDS), 4,4'-dibenzamidostilbene-2,2'-disulfonic acid (DBDS),and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS)) was investigated using planar bilayers. One reversible and two nonreversible mechanisms were identified. Ka for reversible activation (∼100 μM) depended on cytoplasmic [Ca²⁺] and the bilayer composition.Replacement of neutral lipids by negative phosphatidylserine increased Ka fourfold, suggesting that reversible binding sites are near the bilayer surface. Suramin and the stilbene derivatives adsorbed to neutral bilayers with maximal mole fractions between 1–8% and with affinities ∼100 μM but did not adsorb to negative lipids. DIDS activated RyRs by two nonreversible mechanisms, distinguishable by their disparate DIDS binding rates (10⁵ and 60 M⁻¹ s⁻¹) and actions. Both mechanisms activated RyRs via several jumps in open probability, indicating several DIDS binding events. The fast and slow mechanisms are independent of each other, the reversible mechanism and ATP binding. The fast mechanism confers DIDS sensitivity∼ 1000-fold greater than previously reported, increases Ca²⁺ activation and increases Ki for Ca²⁺ /Mg²⁺ inhibition 10-fold. The slow mechanism activates RyRs in the absence of Ca²⁺and ATP, increases ATP activation without altering Ka, and slightly increases activity at pH 6.5. These findings explain how different types of DIDS activation are observed under different conditions. 2010-04-27T07:00:33.271Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3089 Evolutionary computation algorithms are increasingly being used to solve optimization problems as they have many advantages over traditional optimization algorithms. In this paper we use evolutionary computation to study the trade-off between pleiotropy and redundancy in a client-server based network. Pleiotropy is a termused to describe components that perform multiple tasks, while redundancy refers to multiple components performing one same task. Pleiotropy reduces cost but lacks robustness, while redundancy increases network reliability but is more costly, as together, pleiotropy and redundancy build flexibility and robustness into systems. Therefore it is desirable to have a network that contains a balance between pleiotropy and redundancy. We explore how factors such as link failure probability, repair rates, and the size of the network influence the design choices that we explore using genetic algorithms. 2010-04-27T06:40:10.409Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1802 In resting muscle, cytoplasmic Mg²⁺ is a potent inhibitor of Ca²⁺ release from the sarcoplasmic reticulum (SR). It is thought to inhibit calcium release channels (RyRs) by binding both to low affinity, low specificity sites (I-sites) and to high affinity Ca²⁺ sites (A-sites) thus preventing Ca²⁺ activation. We investigate the effects of luminal and cytoplasmic Ca²⁺ on Mg²⁺ inhibition at the A-sites of skeletal RyRs (RyR1) in lipid bilayers, in the presence of ATP or modified by ryanodine or DIDS. Mg²⁺ inhibits RyRs at the A-site in the absence of Ca²⁺, indicating that Mg²⁺ is an antagonist and does not simply prevent Ca²⁺ activation. Cytoplasmic Ca²⁺ and Cs⁺ decreased Mg²⁺ affinity by a competitive mechanism. We describe a novel mechanism for luminal Ca²⁺ regulation of Ca²⁺ release whereby increasing luminal [Ca²⁺] decreases the A-site affinity for cytoplasmic Mg²⁺ by a noncompetitive, allosteric mechanism that is independent of Ca²⁺ flow. Ryanodine increases the Ca²⁺ sensitivity of the A-sites by 10-fold, which is insufficient to explain the level of activation seen in ryanodine-modified RyRs at nM Ca²⁺, indicating that ryanodine activates independently of Ca²⁺. We describe a model for ion binding at the A-sites that predicts that modulation of Mg²⁺ inhibition by luminal Ca²⁺ is a significant regulator of Ca²⁺ release from the SR. We detected coupled gating of RyRs due to luminal Ca²⁺ permeating one channel and activating neighboring channels. This indicated that the RyRs existed in stable close-packed rafts within the bilayer. We found that luminal Ca²⁺ and cytoplasmic Mg²⁺ did not compete at the A-sites of single open RyRs but did compete during multiple channel openings in rafts. Also, luminal Ca²⁺ was a stronger activator of multiple openings than single openings. Thus it appears that RyRs are effectively "immune" to Ca²⁺ emanating from their own pore but sensitive to Ca²⁺ from neighboring channels. 2010-04-27T06:11:00.204Z ]]>