Ion transport across cell membranes

Perhaps the most outstanding advantage of the use of radioisotopes is the opportunity offered to trace dynamic mechanisms. Such biological phenomena as ion transport across cell membranes, turnover, intermediary metabolism, or translocation in plants could, before the advent of radiotracer methods, be approached only indirectly. 

These drugs inhibit Ca2+ ion transport across cell membranes and relax muscle tissues selectively without affecting the working of the heart. Hence high blood pressure can be reduced. Pfizer s amlodipine (Istin or Norvasc ) is a very important drug—it had sales of 1.6 billion dollars in 1996. 

Brevetoxin B, the complex neurotoxin that opened Chapter 9, is a naturally occurring polyether that interferes with Na ion transport across cell membranes. 

Synthetic and naturally occurring macrocyclic compounds complex with metal ions selectively both in aqueous and non-aqueous solvents. In non-aqueous solvents, such complex formation leads to anion desolvation and ion-pair separation. The novel chemistry of these macrocyclic compounds as well as their role in ion transport across cell membranes prompted the vigorous recent activity in this area of research (Christensen et al., 1971 Pedersen and Frensdorff, 1972 Lehn, 1973). 13C spectroscopy is used to obtain structural and configurational information on these systems as well as to investigate their molecular dynamics. 

The protein fraction is responsible for a major part of membrane function. Nonpolar materials can diffuse through the bilayer from one side to the other relatively easily, but polar materials, particularly metal ions such as Na, K", and Ca " cannot. The transport of metal ions is assisted by the membrane proteins. These proteins pick up a metal ion from the aqueous phase on one side of the membrane and shield it from the hydrophobic environment within the membrane while transporting it to the aqueous phase on the other side of the membrane. lonophore antibiotics such as monensin (see Section 16.5, Figure 16.3) disrupt the normal functioning of cells by facilitating metal ion transport across cell membranes. 

The electrophysiological voltage clamp technique is a widely used method to approach mechanisms of ion transport across cell membranes. Basically, the voltage clamp is the application of a rectangular electric field and the measurement of relaxations of electric currents which are frequently rate-controlled by structural changes in the ion transport gating proteins. In a similar manner chemical relaxtion kinetics appears to be the method of... 

Artificial phospholipid vesicles (liposomes) are used to transport vaccines, drugs, enzymes or other substances to target cells or organs. They also make excellent model systems for studying biological ion transport across cell membranes. The vesicles, which are several hundred nanometres in diameter, do not suffer from interference from residual natural ion-channel peptides or ionophores, unlike purified natural cells. For example, the synthetic heptapeptide 5.23 forms pores that promote chloride efflux in vesicle models. Similarly, the ion-pair receptor 2.108 can ferry NaCl from vesicles as an ion-pair ionophore (see Chapter 2, Section 2.6.2), while the hydraphile 5.24 has been shown to transport Na using Na NMR spectroscopy through the bilayer walls of a vesicle model system. 

Lidocaine solution (12 mM), as proposed by Stewart (1981) to remove adherent murine monocytes, activated superoxide dismutase (Gulyaewa et al. 1987). Lidocaine and related local anaesthetics disrupt ion transport across cell membranes through changes in membrane fluidity. Lidocaine... 

Ion transport across cell membranes is a fundamental process of living organisms. It is one of a myriad of subjects in the application of structural research that are of interest to industry, academia, and other research institutions. 

With the adequacy of lipid bilayer membranes as models for the basic structural motif and hence for the ion transport barrier of biological membranes, studies of channel and carrier ion transport mechanisms across such membranes become of central relevance to transport across cell membranes. The fundamental principles derived from these studies, however, have generality beyond the specific model systems. As noted above and as will be treated below, it is found that selective transport... 

Fig. 4.3 Uptake of glutamate is associated with ion transport across neural membranes. Ion transport results in entry of 3 sodium ions and 1 proton in the cell whilst 1 potassium ion is transported out (modified from Attwell, 2000)...

Sodium, potassium and chloride are the primary dietary ions that influence the electrolytic balance and acid-base status, and the proper dietary balance of sodium, potassium and chloride is necessary for growth, bone development, eggshell quality and AA utilization. Potassium is the third most abundant mineral in the body after calcium and phosphorus, and is the most abundant mineral in muscle tissue. It is involved in electrolyte balance and neuromuscular function. The content of potassium in poultry diets is usually adequate. Chloride is present in gastric juice and chlorine is part of the HC1 molecule which assists in the breakdown of feed in the proventriculus. Sodium is essential for nerve membrane stimulation and ionic transport across cell membranes. Signs of sodium, potassium or chloride deficiency include reduced appetite, poor growth, dehydration and increased mortality. 

Sarcoplasmic reticulum Ca2+-transporting ATPase (SR-ATPase) is a member of the ATPase family that couples ion translocation across cell membranes to hydrolysis. There... 

Ions and small molecules may be transported across cell membranes or lipid bilayers by artificial methods that employ either a carrier or channel mechanism. The former mechanism is worthy of brief investigation as it has several ramifications in the design of selectivity filters in artificial transmembrane channels. To date there are few examples where transmembrane studies have been carried out on artificial transporters. The channel mechanism is much more amenable to analysis by traditional biological techniques, such as planar bilayer and patch clamp methods, so perhaps it is not surprising that more work has been done to model transmembrane channels. 

Transient water pores in cellular membranes are involved in several relevant processes, such as maintenance of osmotic balance, drug and antibody delivery into cells, and ion transport across the membrane. Understanding ion transport across membranes is especially important, because membranes strive to maintain a cationic electrochemical gradient used for ATP synthesis. Yet, ions leak through lipid membranes, and understanding the mechanisms associated with ion leakage would allow one to control membrane properties better in related applications. 

In these devices polymer materials containing specific ingredients constitute the backbone of the film covering the electrochemical transducer. Here we deal with a liquid membrane, because the organic solvent provides the medium in which the ions permeate across the membrane. The polymer membrane ion-selective electrodes (ISE) and their ion transport across the membrane function similarly as the ion transport across the membranes of living cells (Figure 8.29). We follow the presentation given by Widmer (1993). 

The electrochemistry of nerves has been the subject of several decades of study. Ion transport across cell walls is a key element in the functioning of nerve cells, and a network of nerves can be viewed as a set of electrochemical, membrane-containing microcells that are coupled by chemical messengers. Interfaces between nerves and... 

Martinelle K, Haggstrom L. 1993. Mechanisms of ammonia and ammonium ion toxicity in animal cells transport across cell membranes. J Biotechnol 30(3) 339-350. 

Valinomycin contains both polar and nonpolar groups. The polar groups bind the ions and the nonpolar -CH3 groups allow the valinomycin molecule to dissolve in the the nonpolar lipid barrier of the cell. Once dissolved in the lipid barrier, the ions transport across the membrane into the cell to offset the ionic balance. 

Given the limitations in fluorescent sensors for lead to date, it is not surprising that relatively little is known about the mechanism by which lead is transported throughout the body. However, those studies that have been conducted to date suggest that Pb(II) can be transported by ion channels, can interact with complex metal transport machineries, and may be passively transported across cell membranes. 

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