The murburn precepts for cellular ionic homeostasis and electrophysiology

Starting from the basic molecular structure and redox properties of its components, we build a macroscopic cellular electrophysiological model. We first present a murburn purview that could explain ion distribution in bulk-milieu/membrane-interface and support the origin of trans-membrane potential (TMP) in cells. In particular, the discussion focuses on how cells achieve disparity in the distribution of monovalent and divalent cations within (K⁺  > Na⁺  > Mg²⁺  > Ca²⁺ ) and outside (Na⁺  > K⁺  > Ca²⁺  > Mg²⁺ ). We explore how TMP could vary for resting/graded/action potentials generation and project a model for impulse conduction in neurons. Outcomes based on murburn bioenergetic equilibriums leading to solubilization of ion-pairs, membrane's permittivity, protein channels' fluxes, and proteins' innate ability to bind/adsorb ions selectively are projected as the integral rationale. We also provide experimental modalities to ratify the projections.

Interactions among diameter, myelination, and the Na/K pump affect axonal resilience to high-frequency spiking

The reliability of spike propagation in axons is determined by complex interactions among ionic currents, ion pumps, and morphological properties. We use compartment-based modeling to reveal that interactions of diameter, myelination, and the Na/K pump determine the reliability of high-frequency spike propagation. By acting as a “reservoir” of nodal Na⁺ influx, myelinated compartments efficiently increase propagation reliability. Although spike broadening was thought to oppose fast spiking, its effect on spike propagation is complicated, depending on the balance of Na⁺ channel inactivation gate recovery, Na⁺ influx, and axial charge. Our findings suggest that slow Na⁺ removal influences axonal resilience to high-frequency spike propagation and that different strategies may be required to overcome this constraint in different neurons.

Axons reliably conduct action potentials between neurons and/or other targets. Axons have widely variable diameters and can be myelinated or unmyelinated. Although the effect of these factors on propagation speed is well studied, how they constrain axonal resilience to high-frequency spiking is incompletely understood. Maximal firing frequencies range from ∼1 Hz to >300 Hz across neurons, but the process by which Na/K pumps counteract Na⁺ influx is slow, and the extent to which slow Na⁺ removal is compatible with high-frequency spiking is unclear. Modeling the process of Na⁺ removal shows that large-diameter axons are more resilient to high-frequency spikes than are small-diameter axons, because of their slow Na⁺ accumulation. In myelinated axons, the myelinated compartments between nodes of Ranvier act as a “reservoir” to slow Na⁺ accumulation and increase the reliability of axonal propagation. We now find that slowing the activation of K⁺ current can increase the Na⁺ influx rate, and the effect of minimizing the overlap between Na⁺ and K⁺ currents on spike propagation resilience depends on complex interactions among diameter, myelination, and the Na/K pump density. Our results suggest that, in neurons with different channel gating kinetic parameters, different strategies may be required to improve the reliability of axonal propagation.

Physiological Effects of the Electrogenic Current Generated by the Na+/K+ Pump in Mammalian Articular Chondrocytes

Background: Although the chondrocyte is a nonexcitable cell, there is strong interest in gaining detailed knowledge of its ion pumps, channels, exchangers, and transporters. In combination, these transport mechanisms set the resting potential, regulate cell volume, and strongly modulate responses of the chondrocyte to endocrine agents and physicochemical alterations in the surrounding extracellular microenvironment. Materials and Methods: Mathematical modeling was used to assess the functional roles of energy-requiring active transport, the Na⁺/K⁺ pump, in chondrocytes. Results: Our findings illustrate plausible physiological roles for the Na⁺/K⁺ pump in regulating the resting membrane potential and suggest ways in which specific molecular components of pump can respond to the unique electrochemical environment of the chondrocyte. Conclusion: This analysis provides a basis for linking chondrocyte electrophysiology to metabolism and yields insights into novel ways of manipulating or regulating responsiveness to external stimuli both under baseline conditions and in chronic diseases such as osteoarthritis.

Depolarization of the plasma membrane of Neurospora during active transport of glucose: evidence for a proton-dependent cotransport system

Intracellular microelectrodes were used to measure the effects of glucose transport on membrane voltage and membrane resistance in Neurospora crassa. Sudden activation of glucose uptake, via the high-affinity, derepressible system II, results in a very large depolarization of the plasma membrane. At saturating concentrations of glucose, the depolarization averages 120 mV; it is diphasic in time, with an initial shift at rates of 100-200 mV/sec followed by a slow, spontaneous, partial repolarization. Changes in intracellular ATP concentration are small and could account for only 10 mV of the initial depolarization, while the rest appears to depend upon the transport process itself. A plot of peak depolarization against the extracellular glucose concentration gives a saturation curve which is half-maximal at 42 muM, in good agreement with the K(1/2) reported for glucose transport via system II. The nonmetabolized analogue 3-O-methyl-D-glucose also causes depolarization, and in addition leads to a pulsed alkalinization of the medium occurring at approximately the same rate as 3-O-methyl-D-glucose uptake. The membrane resistance falls only slightly during glucose depolarization, a fact which requires the transport system itself to have a high internal resistance, or the membrane current-voltage relationship in glucose-starved cells to be quite nonlinear. All of the data support Mitchell's notion that sugar and hydrogen ions are contransported under the influence of the membrane potential, and lead to values for H(+):glucose stoichiometry of 0.8 to 1.4.