The thermostatics and thermodynamics of cotransport

Random-walk model of the sodium-glucose transporter SGLT2 with stochastic steps and inhibition

Random-walk models are frequently used to model distinct natural phenomena such as diffusion processes, stock-market fluctuations, and biological systems. Here, we present a random-walk model to describe the dynamics of glucose uptake by the sodium-glucose transporter of type 2, SGLT2. Our starting point is the canonical alternating-access model, which suggests the existence of six states for the transport cycle. We propose the inclusion of two new states to this canonical model. The first state is added to implement the recent discovery that the Na⁺ion can exit before the sugar is released into the proximal tubule epithelial cells. The resulting model is a seven-state mechanism with stochastic steps. Then we determined the transition probabilities between these seven states and used them to write a set of master equations to describe the time evolution of the system. We showed that our model converges to the expected equilibrium configuration and that the binding of Na⁺and glucose to SGLT2 in the inward-facing conformation must be necessarily unordered. After that, we added another state to implement inhibition in the model. Our results reproduce the experimental dependence of glucose uptake on the inhibitor concentration and they reveal that the inhibitors act by decreasing the number of available SGLT2s, which increases the chances of glucose escaping reabsorption.

Random-walk model of cotransport

We present a statistical mechanical model to describe the dynamics of an arbitrary cotransport system. Our starting point was the alternating access mechanism, which suggests the existence of six states for the cotransport cycle. Then we determined the 14 transition probabilities between these states, including a leak pathway, and used them to write a set of Master Equations for describing the time evolution of the system. The agreement between the asymptotic behavior of this set of equations and the result obtained from thermodynamics is a confirmation that leakage is compatible with the static head equilibrium condition and that our model has captured the essential physics of cotransport. In addition, the model correctly reproduced the transport dynamics found in the literature.

Electrophysiological Actions of Synthetic Cathinones on Monoamine Transporters

Products containing psychoactive synthetic cathinones, such as mephedrone and 3,4-methylenedioxypyrovalerone (MDPV) are prevalent in our society. Synthetic cathinones are structurally similar to methamphetamine, and numerous synthetics have biological activity at dopamine, serotonin, and norepinephrine transporters. Importantly, monoamine transporters co-transport sodium ions along with their substrate, and movement of substrates and ions through the transporter can generate measurable ionic currents. Here we review how electrophysiological information has enabled us to determine how synthetic cathinones affect transporter-mediated currents in cells that express these transporters. Specifically, drugs that act as transporter substrates induce inward depolarizing currents when cells are held near their resting membrane potential, whereas drugs that act as transporter blockers induce apparent outward currents by blocking an inherent inward leak current. We have employed the two-electrode voltage-clamp technique in Xenopus laevis oocytes overexpressing monoamine transporters to determine whether synthetic cathinones found in the so-called bath salts products behave as blockers or substrates. We also examined the structure-activity relationships for synthetic cathinone analogs related to the widely abused compound MDPV, a common constituent in "bath salts" possessing potent actions at the dopamine transporter.

Sodium/calcium exchange and intracellular calcium buffering in ferret myocardium: an ion-sensitive micro-electrode study

Measurements of the intracellular activity of Ca (aiCa), Na (aiNa) and H (pHi) ions have been made with resin-filled ion-sensitive micro-electrodes in ferret ventricular trabeculae. The mean values in quiescent muscle at 30 degrees C were: aiNa, 11.1 +/- 1.0 mM; aiCa, 58.4 +/- 6.4 nM, and pHi, 7.20 +/- 0.11. The relation between aiNa and extracellular Na activity (aoNa) is not linear and is sensitive to temperature: the Q10 for the change in aiNa in normal Tyrode solution is 1.3 +/- 0.5 and rises to 3.5 +/- 0.5 when aoNa is reduced to 1.1 mM. The addition of CN to the bathing fluid causes little or no change in aiNa or aiCa but pHi rises to 7.38 +/- 0.10, yet in some preparations resting tension increases. Similar results are seen with carbonyl cyanide m-chlorophenyl hydrazone. On lowering [Na]o, the fall in aiNa is very much greater than the rise in aiCa and the pHi is generally unchanged. When [Na]o is lowered in the presence of a respiratory inhibitor, the fall in aiNa is reduced, the rise in aiCa and the contracture tension are increased while pHi falls. The apparent coupling ratio for the Na/Ca exchange varies between 3 and 4 depending on the experimental conditions. These results suggest that an intracellular process, with a high Q10 and which depends upon respiration and aiNa, is able to remove Ca2+ from the sarcoplasm and thereby interact with the sarcolemmal Na/Ca exchange. This process could be the increase in the energy-dependent accumulation of Ca2+ within mitochondria that will occur when the Ca efflux from these organelles is progressively inhibited as aiNa falls.