Ion Traffic Across Cellular Membranes

Abstract

The cell membrane protects the delicate internal machinery of the cell and hosts a complex transport system for ion exchange with the environment. Energy is continually expended to maintain a transmembrane electrical potential of about 80 mV. Low extracellular potassium leads to some interesting dynamics that elucidate mechanisms for energy expenditure and survival. As the extracellular potassium concentration, K_e, is lowered, the transmembrane potential hyperpolarizes. But at a certain point in decreasing K_e, a switch to a depolarized state occurs. A switch back to the hyperpolarized state occurs when again increasing the K_e, but this switch occurs at a higher K_e than the one at which the switch to depolarization occurred. So there is an apparent hysteresis. In a system of ion pumps, ion channels, and ion transporters the flows of sodium, potassium, and chloride are tightly coupled. A model is set up involving the most relevant components in the ion transport. The model reproduces the observed hysteresis and can quantitatively account for the change in location and size of the hysteresis loop when an important chloride transporter is blocked or stimulated. The switching points in the hysteresis loop occur when inward rectifying potassium channels (IRKs) close or open. Adding isoprenaline opens other potassium channels and makes the IRK contribution negligible. After also neglecting the role of chloride, the fixed potassium permeability leads to a system that can be analytically solved. Expressions are derived for the position and the size of the hysteresis loop. The expressions agree with experimental data and provide insight into the evolutionary origin of the IRKs. The membrane's freezing temperature is just slightly below physiological temperatures. At the freezing transition the membrane exhibits increased ion permeability. The increased permeability comes in the form of quantized bursts similar to those of ion channel currents. The currents through a pure membrane near the phase transition are measured and analyzed. Power laws are found, and the power spectral density of the current signal appears to follow a 1/<italic>f</italic> pattern. A theoretical basis for this behavior is formulated and a physical explanation is proposed. Biological significance is discussed.