The Sodium-Potassium Exchange Pump: II. Analysis of Na+-Loaded Frog Sartorius Muscle

A model for the Na-K exchange pump was applied to data on Na⁺-loaded frog sartorius muscle, and was used to relate the rate of adenosine triphosphate (ATP) hydrolysis to the electrical properties of the cell membrane. Membrane hyperpolarization was considered to arise from an electrical current which was produced by the hydrolysis reaction coupled to ion movements, and which flowed across the membrane. The reaction rate, as calculated from hyperpolarization, agreed with direct measurements of ATP hydrolysis and with the rate estimated from Na⁺ tracer efflux studies. Although Na⁺ is actively extruded, the model showed that K⁺ is inwardly transported if the potassium permeability of the membrane is less than about 6.6 × 10^-6 cm/sec, as is suggested by resistance data. Calculations indicated that the reaction conductance L_rr was relatively constant when compared with the reaction rate and reaction free energy for large changes in internal and external ionic concentrations. Its value agreed with the value obtained from the dependence of Na⁺ tracer efflux on external K⁺. A set of experiments was suggested which would provide a more complete interpretation of the data.     

Intracellular Potassium : A Determinant of the Sodium-Potassium Pump Rate

Normal human red cells which have had their intracellular sodium (Na_c) reduced have a diminished Na-K pump rate, but only if intracellular potassium (K_c) is high. If most of the K_c is replaced by tetramethylammonium or choline, both ouabain-sensitive Na efflux and K influx are significantly increased even with Na_c below normal. Cells with reduced Na_c and high K_c have an unchanged Na efflux if external potassium (K_ext) is removed. In contrast, low-Na, low-K cells have a large ouabain-sensitive Na efflux which shows a normal response to removal of K_ext. Neither low-K nor high-K cells have an altered ouabain-sensitive K efflux. Measurement at constant low Na_c and varying K_c shows the pump Na efflux to be an inverse function of K_c. Thus, in low-Na cells, K_c appears to act as an inhibitor of the pump. Inhibition by high K_c can be seen even when Na_c is normal. The effects attributed to K_c are distinguished experimentally from other variables such as cell volume, adenosine triphosphate concentration, effects of the replacement cations, and the method used to alter intracellular cation concentrations. A role is proposed for K_c, in cooperation with Na_c, in regulating the pump rate of normal human red cells.     

Polyamine Metabolism and Osmotic Stress : I. Relation to Protoplast Viability

Cereal leaves subjected to the osmotica routinely used for protoplast isolation show a rapid increase in arginine decarboxylase activity, a massive accumulation of putrescine, and slow conversion of putrescine to the higher polyamines, spermidine, and spermine (HE Flores, AW Galston 1984 Plant Physiol 75: 102). Mesophyll protoplasts from these leaves, which have a high putrescine:polyamine ratio, do not undergo sustained division. By contrast, in Nicotiana, Capsicum, Datura, Trigonella, andVigna, dicot genera that readily regenerate plants from mesophyll protoplasts, the response of leaves to osmotic stress is opposite to that in cereals. Putrescine titer as well as arginine and ornithine decarboxylase activities decline in these osmotically stressed dicot leaves, while spermidine and spermine titers increase. Thus, the putrescine:polyamine ratio in Vigna protoplasts, which divide readily, is 4-fold lower than in oat protoplasts, which divide poorly. We suggest that this differing response of polyamine metabolism to osmotic stress may account in part for the failure of cereal mesophyll protoplasts to develop readily in vitro.     

The Sodium-Potassium Pump Controls the Intrinsic Firing of the Cerebellar Purkinje Neuron

In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na⁺/K⁺ pump activity sets the Purkinje cell''s operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na⁺/K⁺ pump. The model can replicate these recordings. We propose that Na⁺/K⁺ pump activity controls the intrinsic firing mode of cerbellar Purkinje cells.     

Cation Activation of the Basolateral Sodium-Potassium Pump in Turtle Colon

The current generated by electrogenic sodium-potassium exchange at the basolateral membrane of the turtle colon can be measured directly in tissues that have been treated with serosal barium (to block the basolateral potassium conductance) and mucosal amphotericin B (to reduce the cation selectivity of the apical membrane). We studied the activation of this pump current by mucosal sodium and serosal potassium, rubidium, cesium, and ammonium. The kinetics of sodium activation were consistent with binding to three independent sites on the cytoplasmic side of the pump. The pump was not activated by cellular lithium ions. The kinetics of serosal cation activation were consistent with binding to two independent sites with the selectivity Rb > K > Cs > NH₄. The properties and kinetics of the basolateral Na/K pump in the turtle colon are at least qualitatively similar to those ofthe well-characterized Na/K-ATPase of the human red blood cell .     

Electrical Sizing of Particles in Suspensions: I. Theory

The processes involved during the passage of a suspended particle through a small cylindrical orifice across which exists an electric field are considered in detail. Expressions are derived for the resulting change in current in terms of the ratios of particle to orifice volume and particle to suspending fluid resistivity, and particle shape. Graphs are presented of the electric field and of the fluid velocity as functions of position within the orifice, and of the shape factor of spheroids as a function of their axial ratio and orientation in the electric field. The effects of the electric and hydrodynamic fields on the orientation of nonspherical particles and on the deformation of nonrigid spheres is treated, and the migration of particles towards the orifice axis is discussed. Oscillograms of current pulses produced by rigid, nonconducting spheres in various orifices are shown and compared with the theoretical predictions.     

Cation Metabolism in Relation to Cell Size in Synchronously Grown Tissue Culture Cell

In randomly grown tissue culture cells (mouse leukemic lymphoblast, L5178Y) the number, volume, and Na⁺ and K⁺ content increase as an exponential function with a doubling time of 11.3 hr. In synchronously grown cells the volume increase of the population and of single cells follows the same exponential function as in randomly grown cells. In contrast, the cation content fluctuates during a single cell cycle. About 1½ hr after the cell division burst (at the beginning of the S period), a net loss of K⁺ occurs for a period of about 1 hr amounting to about 20% of the total K. Over the next 5 to 6 hr, the deficit in K⁺ is eliminated. The Na⁺ content shows a double fluctuation. It falls during the cell division burst, rises when the K⁺ content decreases, falls again when K⁺ content rises, and then increases again before the next cell division burst. The net fluxes of both Na⁺ and K⁺ are very small compared to the unidirectional fluxes (less than 5%), thus small changes in the balance of influx and efflux account for the changes in cation content during the growth cycle. Both unidirectional fluxes increase dramatically (by a factor of two) about 2 hr after the cell division burst, and then remain constant until after the next cell division. The pattern of electrolyte regulation during cell division does not follow a simple function such as cell number, cell surface, or cell volume, but must be related to specific internal events in the cell.     

The Interval-Strength Relationship in Mammalian Atrium: A Calcium Exchange Model: I. Theory

A model is developed for predicting the interval-strength relationship in mammalian atrium. The postulates underlying the model relate the intracellular and transmembrane calcium fluxes to changes in contractility. The predictions of the model agree qualitatively with the behavior of atrium for the following patterns of stimulation: (a) constant interval between stimuli, (b) a rest, or period with no stimuli, after the attainment of a steady-state force level, (c) a sudden change in the interval between stimuli, and (d) paired pulse stimulation. The effects of varying several parameters of the model on both the contraction staircases, after a rested-state contraction, and the steady-state interval-strength relationship are examined. Additional considerations are made: (a) estimates are made of the tissue calcium content available for contraction; (b) the physical meaning of the rested-state contraction is discussed; and (c) estimates are made of the proportionality constant between the maximum value of the contractile tension and the amount of calcium released before a contraction.     

Cell-density-dependent Changes in the Metabolism of Chloronema Cell Cultures: I. Relationship between Cell Density and Enzymic Activities

In the growing chloronema cell suspension cultures of the moss Funaria hygrometrica Hedw., activities of several enzymes have been found to be cell-density-dependent. Cyclic nucleotide phosphodiesterase (cNPDE), nitrate reductase (NR), and protein kinase showed highest activity at a low cell density (1 to 2 milligrams per milliliter) while indoleacetic acid (IAA) oxidase and peroxidase were highest at a high cell density (>10 milligrams per milliliter). 3′-Nucleotidase and the glycolytic enzymes (aldolase, hexokinase, phosphofructokinase, phosphoglucoisomerase, pyruvate kinase, and triose phosphate isomerase) showed no significant dependence on the cell density. Alternatively, if the NR and peroxidase activities were determined as a function of time in batch cultures, their levels were maximal 60 to 70 and 320 hours after subculture, respectively, the corresponding cell densities being 1 to 2 and 23 milligrams per milliliter. The relationship between cell density and NR and peroxidase activities is the same, whether these enzymes are measured in batch cultures during a growth cycle or in the cells cultured at different initial inoculum densities for a constant time. Conventionally enzymic changes have been correlated with growth phases; however, it is felt that the pattern of enzymic activities can also be interpreted as cell-density-dependent.     

Passive Electrical Properties of Microorganisms: III. Conductivity of Isolated Bacterial Cell Walls

The dielectric properties of isolated Micrococcus lysodeikticus cell walls have been studied to establish more firmly the view that wall-associated ions play a major role in the conduction of low frequency electric current by intact bacterial cells. The conductivity of isolated walls was found to be about 0.40 mho/m. If counterions associated with fixed, ionized groups in the wall have average mobilities equal to that of sodium ions in free solution, the fixed charge concentration required to account for the measured conductivity is between 75 and 95 meq/liter of wet wall volume. Estimates of the numbers of titratable amino and carboxyl groups in wall polymers indicate that conductivity is more closely related to net wall charge than to total wall charge. The measured wall conductivity was used to predict a value of 0.15 ± 0.03 mho/m for whole cell conductivity. This prediction is close to the measured value of 0.25 ± 0.05 mho/m and it is thought that much of the disparity in values is related to changes in wall structure and composition during the isolation procedures.     

The Effect of Temperature Treatments on Growth and the Metabolism of Ribonucleic Acid in Relation to Cell Division and Cell Elongation of Pisum sativum Roots

We observed the effects of a range of temperatures on root elongation, cell production, total RNA in the root tip, and changes in various nucleic acid fractions in dividing and elongating areas of pea roots. High temperatures affected elongation of roots more adversely than cell production. In the tip section of the root, growth was positively associated with a decrease in the first soluble RNA peak that eluted from a MAK column, and with an increase in ribosomal RNA. There was no similar change in these RNA fractions in the elongating area of the pea root. In pulse type tests sRNA-1 labeled first and the isotype was depleted upon subsequent growth. The specific activity of sRNA-1 and the depletion of the radioactivity was not nearly so great in the elongating as in the dividing portion of the pea root.