Influx Rate Constants Research Articles

Intracellular pH and the kinetics of Rb + uptake by yeast non-carrier versus mobile carrier-mediated uptake

The effect of changes in the intracellular pH upon the concentration dependence of the Rb + uptake by yeast is investigated. It is shown, that the uptake of Rb + can be described by a mechanism in which the total concentration of primary binding sites at the outer side of the membrane is independent of the intracellular ligand composition and of the membrane potential, and the influx rate constants depend upon the intracellular pH and/or upon the membrane potential. It is argued that the involvement of a mobile carrier mechanism is not likely.

Uptake from Sea Water and Clearance of 14C-p,p′-DDT by the Marine Copepod Calanus finmarchicus

It is impossible to measure the uptake rate of p,p′-DDT by copepods from sea water directly since uptake and depuration occur simultaneously. Copepods were found to lose p,p′-DDT exponentially in flowing sea water with a rate constant of kj = 0.048/day; [Formula: see text]. Five experiments were performed to obtain uptake rates with DDT concentrations ranging from 32.5 to 1337 pptr in sea water. Copepod uptake of p,p′-DDT from sea water is amply described by a single-compartment model, [Formula: see text], which accounts for influx and efflux of DDT. Calculated influx rate constants, ki, varied between 1.5 and 2.7 × 103/day. These results enable prediction of DDT levels in nonfeeding copepods if the concentration of DDT in sea water is known.

Active Ca transport of sacroplasmic reticulum during experimental uremia. Changes in kinetics and lipid composition.

The Ca-transport system of sarcoplasmic vesicles of rabbits is altered by experimental uremia. 1. The influx rate constant of the experimental membranes decrease with a resulting decrease of the calcium influx rate. 2. The experimental membranes transport a smaller amount of Ca2+ per mol of ATP split than the controls, i.e. their transport ratio is discussed. 3. The calcium permeability of the experimental membranes increases with a resulting decreased concentrating ability. 4. The phosphatide content but not the cholesterol content of the experimental membranes decreases with a consequent increase of the cholesterol/phosphatide ratio. 5. The fatty acid pattern of total phosphatides of the experimental membranes changes. A relative decrease of palmitic acid and oleic acid occurs and a relative increase of stearic, arachidonic and higher unsaturated fatty acids. 6. The altered lipid composition of the membranes does not change the temperature dependence of the kinetics.

Calcium-calcium and calcium-strontium exchange across the membrane of human red cell ghosts.

Stationary and nonstationary state45Ca fluxes as well as Sr−Ca exchange movements were studied in energy-depleted human erythrocyte ghosts at different intra-and extracellular Ca concentrations. Influx and efflux followed the kinetics of a closed two-compartment system. The influx and efflux rate constants (k in andk out, respectively, fractions of total extra- or intracellular45Ca that move in one direction per unit time) were similar in magnitude. They decreased with increasing Ca concentration on the cisside and increased with increasing Ca concentration on the trans-side of the membrane. Hence, the fluxes in both directions were characterized by saturation kinetics and appeared to be partially caused by an exchange diffusion mechanism. In the presence of a moderate inward (up to 8mm) or outward (up to 2mm) Ca concentration gradient, kin andk out did not vary in the course of an experiment and did not differ significantly from rates which were measured under stationary state conditions. Extracellular Sr induced an outward transport of intracellular Ca against the concentration gradient (counter-transport). The resulting inward Ca concentration gradient (maximal inside-to-outside concentration ratio as 1 to 3) persisted since extra- and intracellular Sr did not equilibrate. Analogous results were obtained studying45Ca−40Ca countertransport. In net flow experiments Ca−Sr exchange proved to occur on a one-for-one basis. Ca−Sr exchange was additive to the noncoupled Ca and Sr net downhill movements. The experimental results suggest that a specific ATP-independent Ca transfer system exists in the erythrocyte membrane which acts symmetrically on the two sides of the membrane and is restricted to a tightly coupled one-for-one exchange diffusion.

The effects of guanidinosuccinic acid and methylguanidine on erythrocyte cation transport.

SummaryCellular abnormalities observed in uremic patients, have been ascribed to toxins, hormones, or malnutrition. Guanidine derivatives, which accumulate in uremia, possess some toxic properties for platelets and erythrocytes. Accordingly in vitro effects of several guanidine compounds on cation transport in human erythrocytes were studied. Guanidinosuccinic acid (GSA) and methylguanidine (MGUA), in varying concentrations were incubated with normal human red blood cells. After incubations of 48 hr, the sodium and potassium content of erythrocytes exposed to these compounds did not differ from control cells. Sodium outflux and influx rate constants, determined with sodium 22, were unaffected by GSA and MGUA. We conclude that, under the conditions of these experiments, GSA and MGUA did not alter the steady state intracellular sodium and potassium content or the bidirectional sodium fluxes of human erythrocytes.

Sodium movements in the human red blood cell.

Measurements were made of the sodium outflux rate constant, (o)k(Na), and sodium influx rate constant, (i)k(Na), at varying concentrations of extracellular (Na(o)) and intracellular (Na(c)) sodium. (o)k(Na) increases with increasing [Na(o)] in the presence of extracellular potassium (K(o)) and in solutions containing ouabain. In K-free solutions which do not contain ouabain, (o)k(Na) falls as [Na(o)] rises from 0 to 6 mM; above 6 mM, (o)k(Na) increases with increasing [Na(o)]. Part of the Na outflux which occurs in solutions free of Na and K disappears when the cells are starved or when the measurements are made in solutions containing ouabain. As [Na(o)] increases from 0 to 6 mM, (i)k(Na) decreases, suggesting that sites involved in the sodium influx are becoming saturated. As [Na(c)] increases, (o)k(Na) at first increases and then decreases; this relation between (o)k(Na) and [Na(c)] is found when the measurements are made in high Na, high K solutions; high Na, K-free solutions; and in (Na + K)-free solutions. The relation may be the consequence of the requirement that more than one Na ion must react with the transport mechanism at the inner surface of the membrane before transport occurs. Further evidence has been obtained that the ouabain-inhibited Na outflux and Na influx in K-free solutions represent an exchange of Na(c) for Na(o) via the Na-K pump mechanism.

Ion metabolism in a potassium accumulation mutant of Escherichia coli B. I. Potassium metabolism.

A potassium transport mutant of Escherichia coli is described which is deficient in the intake of potassium. The phenotype of this mutant is characterized by (i) failure to grow in K(+)-deficient medium, (ii) failure to accumulate K(+) in K(+)-deficient medium, (iii) a steady-state intracellular K(+) that varies sigmoidally with the medium K(+) concentration, (iv) a signoidally shaped rate-concentration curve and a curved reciprocal plot for net K(+) uptake kinetics, and (v) a low steady-state flux of potassium associated with a reduced influx rate constant. The data are discussed in terms of the present day models of cation transport. These models have led to four possible explanations of the mutant's phenotype: (i) a selectivity reversal such that intracellular cation binding sites bind another cation instead of K(+); (ii) a structural alteration of cation binding cell proteins so that K(+) is bound by "cooperative binding" (sigmoid isotherm) instead of by simple adsorption (hyperbolic isotherm); (iii) conversion of an enzyme in intermediate metabolism that rate-limits K(+) uptake to an allosteric protein; (iv) conversion of the "carrier protein" for K(+) to an allosteric protein.

Studies of the kinetics of amino acid transport, incorporation into protein and oxidation in kidney-cortex slices

The present report describes an approach to the study of the steady-state kinetics of amino acid transport and protein synthesis in rat-kidney-cortex slices, using multi-compartment models. The following conclusions seem justified from the data presented: 1.1. Using inulin and the non-metabolizable amino acid, α-aminoisobutyric acid, amino acid uptake could be characterized by a three-compartment “parallel” model representing the medium, extracellular space and intracellular space. Appropriate influx and efflux rate constants were calculated.2.2. 2,4-Dinitrophenol and incubation at 27° significantly effected the rate of efflux of α-aminoisobutyric acid from the intracellular space, as well as the rate of influx, suggesting that efflux phenomena are metabolically linked.3.3. Kinetic studies of combined transport and protein synthesis with glycine and l-lysine indicated that equilibration of exogenous amino acid with the intracellular pool need not occur before incorporation into protein takes place.4.4. The kinetics of 14CO2 evolution from labeled l-lysine indicated that the rate of amino acid oxidation reflected the buildup of the intracellular lysine pool.5.5. The results suggest that the mathematical approach, used in the analysis of the experimental data, provides means of quantitating and understanding membrane phenomena and the relationship between rates of membrane transfer and subsequent intracellular utilization.