Sodium plasma membrane conductivity

Transducin, cyclic GMP, and phosphodiesterase. Tire essential consequence of light absorption is an alteration in the membrane potential in the vicinity of the absorbed photon with the resulting propagation of a nerve impulse down the plasma membrane to the synapse by cable conduction (Chapter 30). Tire type of potential change that is transmitted differs among vertebrates and invertebrates.522 In the case of mammalian photoreceptors the rod outer segment is permeable to sodium ions so that a large... 

A mixture of lentil lectin-reactive glycoproteins from pig lymphocyte-plasma membrane was isolated by lentil lectin-Sepharose chromatography of sodium deoxycholate-solubilized membranes.445 Eighty-seven percent of the protein applied (17% of hexose) passed through unretarded, and 13% of the applied protein (83% of hexose) was bound, and eluted with methyl a-D-glucopyranoside solution. Recovery was 95% of the material applied, in contrast to the recovery in similar experiments conducted on con A-Sepharose columns (80% recovery).850 The eluate from the lentil column, which contained at least ten glycoproteins, blocked lymphocyte transformation induced by lentil or kidney-bean lectins.445... 

In the myocardium, automaticity is the ability of the cardiac muscle to depolarize spontaneously (i.e., without external electrical stimulation from the autonomic nervous system). This spontaneous depolarization is due to the plasma membrane within the heart that has reduced permeability to potassium (K+) but still allows passive transfer of calcium ions, allowing a net charge to build. Automaticity is most often demonstrated in the sinoatrial (SA) node, the so-called pacemaker cells. Abnormalities in automaticity result in rhythm changes. The mechanism of automaticity involves the pacemaker channels of the HCN (Hyperpolarization-activated, Cyclic Nucleotide-gated) family14 (e.g., If, "funny" current). These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium ions. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. 

The striking compartmentalization of potassium in intracellular fluid, and of sodium in extracellular fluid, is a condition which is established and maintained by active transport across all plasma membranes. In the absence of active transport pumps, cotransporters and conductance channels, a directional, selective, rapid and regulated movement of potassium (or sodium) through the cell membranes would be impossible. The major molecular pathways of potassium permeation through plasma membranes are Na, K- ATPase, H-K-ATPase, Na-2C1-K-transporter, and potassium conductance channels (Peterson 1997). 

Sodium channels spontaneously close and potassium channels begin to open at about this time (see Figure 13.19B). Consequently, potassium ions flow outward, and so the membrane potential returns to a negative value. The resting level of —60 mV is restored in a few milliseconds as the K conductance decreases to the value characteristic of the unstimulated state. Only a very small proportion of the sodium and potassium ions in a nerve cell, of the order of one in a million, flows across the plasma membrane during the action potential. Clearly, the action potential is a very efficient means of signaling over large distances. 

In primary cultures of cerebellar neurons, palytoxin also caused a rapid intracellular acidification. Previous work had shown palytoxin-induced intracellular acidification in chick ventricular cells and osteoblasts. The acidification induced by the toxin was reported to be dependent on the opening of proton-conductive pathways in the plasma membrane or to be a secondary effect of the interaction of the toxin with the sodium pump, respectively. In primary neuronal cultures, palytoxin caused a fast and irreversible intracellular acidification. The decrease in intracellular pH (pHj) caused by palytoxin was already maximal at toxin concentrations of 1 nM (Figure 31.4) at this concentration, palytoxin decreased intracellular pH by 0.6 units. 

It is noteworthy that the inactivation of the Heliothis RyR at millimolar [Ca ] was prevented at all flubendiamide concentrations tested. This could plausibly explain the insecticidal mechanism since deactivation of calcium release channels at high [Ca ] would be essential to terminate the intracellular calcium transient (27). According to this hypothesis, ryanodine receptors would be fixed in the (sub)conductance conformation leading to calcium store depletion and, possibly, to subsequent activation of capacitative calcium entry through plasma membrane channels. This would override compensatory calcium removal mechanisms such as the sarcoplasmic Ca-ATPase (SERCA) activity and the sodium-calcium exchanger (NCX) in the plasma membrmie. The sustained high intracellular [Ca would finally lead to muscle contraction paralysis that is consistently observed in flubendiamide-affected lepidopteran larvae. 

The ubiquitous role of chloride (as dissolved sodium chloride) in animal and human cells and blood plasma is manifold as just discussed, it provides Cl for organosynthesis and to control the electrolytic properties such as osmosis (a process where water molecules move through a semipermeable membrane from a dilute solution into a more concentrated solution), for nutrient and waste transport, as well as providing electrical gradients (based on conductivity) for information transfer through neurons. 

High cost and sharp decline in its conductivity above lOO C are the drawbacks of Nafion membranes, and many attempts have been made to improve these (Fatyeyeva et al. 2011). One such attempt is the fabrication of MMMs with surface-treated particles. Laponite clay was plasma-treated followed by the post-grafting of sulfonic monomer, sodium salt of p-styrene sulfonic acid, or 1,3-propane sul-tone. Plasma-processed clay particles were treated by PEG 1500 and freeze-dried... 

---