Changes In Uptake Rates Research Articles

Isotope dilution models of uptake and remineralization of ammonium by marine plankton1

The utility of 15N isotope dilution models for the calculation of uptake and remineralization of NH4+ by marine phytoplankton was examined in light of model limitations when applied to field data either when ambient NH4+ levels border on our limit of detection or when there is no statistically significant difference between ambient NH4+ at the beginning and end of an incubation. Through specific examples of field and laboratory data we show that the limitations are a function both of analytical error inherent in the methodology and of changes in rates of uptake and remineralization over the course of a given experiment. We propose modifications to the existing models of NH4+ uptake and remineralization which overcome some of these limitations. The results show that uptake rates have been traditionally underestimated by a factor of ≈2 in routine 15N uptake methodology and that regeneration of NH4+ over relatively brief periods can supply the daily nitrogen requirements of the phytoplankton when there are no losses from the system.

Removal of amino acid during a single passage of water across the gill of marine mussels

AbstractThe clearance of amino acid during a single passage of inhalent water across the gills of marine mussels was studied. Samples of excurrent water were collected and analyzed for their content of 14C‐labeled amino acids and total primary amines. Comparison with concentrations measured in the ambient medium revealed that mussels are capable of removing large fractions of the amino acid present in inhalent water in a single passage across the gill surfaces. At low rates of pumping, Modiolus demissus was capable of removing 95% of the 14C‐glycine from a 1μ M solution. Efficiency of clearance of amino acid was inversely related to pumping rate; absolute rate of uptake was directly related to pumping rate. Concentration related changes in uptake rate and efficiency of clearance indicated that the effective Kt of glycine uptake by intact Mytilus californianus and Modiolus demissus is between 2 and 5μ M. These values are an order of magnitude lower than published figures for Kt of glycine uptake by preparations of isolated gill tissue from these animals. This discrepancy is probably the result of an increase in the effective thickness of unstirred layers on the surfaces of isolated gills. Net clearance of glycine by M. californianus was observed to occur from concentrations as low as 0.28μ M and, from concentrations of 1μ M and less, an average of 20% of the total primary amine offered initially as glycine was removed during a circuit of inhalent water through the animal. Hence, transepidermal transport processes for amino acids in marine mussels are adapted to function efficiently at substrate concentrations found in their normal environment, and can provide a significant supplemental input of reduced carbon and amino nitrogen for these animals.

Variations in the uptake of 3H-norepinephrine during the rat estrous cycle

The rates of 3H-norepinephrine ( 3H-NE) uptake into isolated nuclei-free homogenates from diencephalon were examined in freely cycling female rats of all phases of the estrous cycle. Four time points were investigated in each phase: 12:00, 14:00, 16:00 and 18:00 h. The proestrus and estrus animals displayed no change in uptake rates across the four time points. However, the metestrus and diestrus groups were found to undergo hour to hour variations in uptake rates. In the diestrus group, a significant rise in uptake of 22% was observed from 12:00 to 16:00 h. This increase was followed by a decrease of 21% by 18:00 h as compared to 16:00 h levels. Similarly, a slight increase in uptake was observed in the metustrus animals from 12:00 to 16:00 h. At 18:00 h a 40% decrease from the 16:00 h value was observed. An average of the mean levels of uptake of all phases was calculated for each time point. At 16:00 h an increase in the mean uptake of about 20% was observed when compared to either the 12:00 or the 18:00 h values. Since transmitter reuotake is the major means of terminating receptor activation or inhibition, it is suggested that the above variations in NE uptake may modulate synaptic activity and play a role in the regulation of behavioral and physiological aspects of the estrous cycle.

Responses of Diffusive Conductance to Humidity in a Drought Avoiding and a Drought Resistant (in Terms of tomatal Response) Legume

Direct stomatal responses to humidity have been well documented in the past decade (e.g. Schulze et al., 1975a, b). They appear to increase water use efficiency by reducing the T/P ratio (Schulze et al ., 1975a) and also reduce total water use (Schulze et al ., 19756). The stomatal response to humidity is a response (probably hydroactive) to water stress in the stomatal apparatus (paper in preparation). It was thought that a species which avoids physiological water stress by stomatal closure at relatively high bulk leaf water potentials might also show a greater stomatal humidity response than one in which stornata are less responsive to changes in the bulk leaf water potential. Two leguminous species, Macroptilium atropurpureum (DC.) Urb., which avoids physiological drought by stomatal closure (Dr M. M. Ludlow, pers. comm.), and Desmodium uncinatum DC., which has stornata relatively unresponsive to changes in bulk leaf potential (Dr M. M. Ludlow, pers. comm.), and which both grow in the same region of semi-arid tropical Australia (in Queensland), were chosen to investigate the above possibility. Single leaves of each species were excised under water from glasshouse-grown plants (five replicates for each species); the petioles were connected to a Ganong potometer and the leaves placed, individually, in a leaf chamber (Sheriff and McGruddy, 1976). Air (C02 concentration 300 p.p.m.) was passed through the chamber at a rate which would completely replace the chamber air every 30 s. Leaves were kept at 26*5 °C (±0-5 °C) and illuminated at an intensity of 940 ju E m~2 s1 by a Philips HPLR700W mercury vapour lamp. Water uptake rate from the potometer was measured approximately every 60 min, immediately before changing chamber humidity. Changes in uptake rate were assumed to reflect changes in transpiration rate under equilibrium conditions. Equilibrium was considered to have been established if two consecutive readings, a few minutes apart, were the same. Approximately half of the replicates were carried out with step increases, and half with step decreases, in humidity. From measurements of water uptake leaf diffusive conductances were calculated using the method of Cowan (1977), and expressed as cmol of water lost per unit leaf area per second. Leaf conductance was plotted against the concentration difference between the air and the evaporation sites (AH), assuming these to be saturated with water at leaf temperature (Fig. 1.). Lines of best fit for the conductances of each species were computer plotted as fourth-order polynomials. This was done because the variation in conductances with AH has been found (in this lab.) to fit a polynomial curve fairly closely and a fourthorder polynomial produced the best fit for these data.

The uptake of bases and their incorporation into RNA during the cell cycle of Schizosaccharomyces pombe in normal growth and after a step-down

The uptake and incorporation of adenine and uracil during the cell cycle of the fission yeast Schizosaccharomyces pombe in normal growth was measured by grain counting in autoradiographs and by bulk counting in synchronous cultures. The following cellular properties increased steadily and in proportion through the cycle: initial rate of uptake, rate of incorporation, size of the preexisting precursor pool and, with adenine, the size of the expanded pool. Similar patterns of incorporation and uptake were shown for guanine, cytosine, guanosine and cytidine. Both ribosomal RNA and rapidly labelled nuclear RNA were synthesised at an increasing rate through the cycle and followed an approximately exponential curve of synthesis. There was no indication of changes in the base ratios. The incorporation rates did not appear to be distorted by changes in uptake rate or pool size. The uptake and incorporation into step-down RNA was followed by autoradiography. It differed from the other two RNA's in showing a marked shift in the ratio of adenine/uracil incorporated at the end of the cycle. This is probably due to a change in the base composition of the RNA and not to changes in uptake rate or pool size. About half the step-down RNA was found in the cytoplasm after a short pulse label.

The uptake of valine and cytidine by sea-urchin embryos and its relation to the cell surface.

ABSTRACT The rate of uptake of [14C] valine and [3H]cytidine into the metabolic pool of early embryos of the sea urchin Paracentrotus lividus was measured by giving 5-min pulses of these precursors. The uptake rate rose sharply after fertilization, reached a maximum before first cleavage and then stayed constant until at least the fifth interphase. There were no changes in uptake rate at cell division. A longer-term experiment with valine showed that the uptake rate remained approximately constant until the pluteus stage, and was not altered for the first 10 h by a actinomycin D. Continuous labelling experiments indicated that the uptake rate may be controlled by the level of precursor in the pool. The following experiments indicated that the uptake was by specific transport mechanisms. Both precursors showed an uptake inhibited by competitors, an apparent concentration within the cells, and no back-exchange. In addition, the uptake of valine was inhibited by dinitrophenol and azide, and showed a concentration response with Michaelis-Menten kinetics. Valine uptake was not inhibited by puromycin added at fertilization, though it was somewhat affected if the eggs were pretreated. It is suggested that the transport mechanisms are mediated by specific carrier molecules at the cell surface. These mechanisms are activated at fertilization through links with metabolism. There is no change in the number of carriers per embryo throughout early development. Since the number of cells and the total cell surface are increasing, this implies that the number of carriers per cell or per unit of surface is decreasing. There is a possible reason for this differentiation of the cell surface.

A continuous flow method for the determination of glucose uptake by excised rat diaphragm.

A technique is described for the determination of the rate of uptake of glucose by the excised rat diaphragm preparation at intervals as short as 20 seconds. The rate is estimated from the fall in glucose concentration which occurs when a suitable medium flows over the muscle at constant rate. When insulin is added to the perfusion fluid, the rate of glucose uptake rises to a value about 50 per cent higher than that established before the introduction of insulin. Under the conditions described the change in uptake rate requires about 10 to 12 minutes to reach completion, being three-quarters complete in about 7 minutes. Although the major part of this delay must be ascribed to the time required for the washing out of insulin-free medium from the apparatus, and for the diffusion of insulin into the interstitial space, it is suggested that some of the delay may be due to processes occurring between the arrival of insulin at the fibre surface and the exertion of its characteristic effect on glucose uptake, although neither the time-course nor the nature of this induction phase is established.