Cyanide (CN) and dinitrophenol (DNP) rapidly depolarize the cells of oat coleoptiles (Avena sativa L., cultivar Victory) and of pea epicotyls (Pisum sativum L., cultivar Alaska); the effect is reversible. This indicates that electrogenesis is metabolic in origin, and, since active transport is blocked in the presence of CN and DNP, perhaps caused by interference with ATP synthesis, that development of cell potential may be associated with active ion transport. Additional evidence for an electrogenic pump is as follows. (1) Cell electropotentials are higher than can be accounted for by ionic diffusion. (2) Inhibition of potential, respiration, andactive ion transport is nearly maximal, but a potential of -40 to -80 mV remains. This is probably a passive diffusion potential since, under these conditions, a fairly close fit to the Goldman constant-field equation is found in oat coleoptile cells.

A ~ S x 1~ A C X Using cultured cells of the marine alga, Halicystis parvula, we measured the concentrations of 11 inorganic ions in the vacuolar sap and the electrical potential difference (PD) between the vacuole and the external solution. In normal cells under steady-state conditions a comparison of the electrochemical equilibrium (Nernst) potential for each ion with the PD of -8~ mV (inside negative) indicates that Na + and K + are actively transported out of the vacuole whereas all anions are pumped into the cell. Although the [K +] in the vacuole is only 9 raM, the cytoplasmic [K +] is about 420 mM, which suggests that the outwardly directed pump is at the tonoplast. Using large Halicystis cells we perfused the vacuole with an artificial seawater and conducted a short-circuit analysis of ion transport. The short-circuit current (SCC) of 299 peq • cm-2.s -~ is not significantly different from the net influx of CI-. There is a small, but statistically significant net efflux of K + (<1 pmol-cm-2.-t), while the influx and efflux of Na + are not significantly different. Therefore, the SCC is a good measure of the activity of the C1-pump. Finally, we measured the volumetric elastic modulus (e) of the cell wall by measuring the change in cell volume when the internal hydrostatic pressure was altered, The Value of ~ at applied pressures between 0 and 0.4 atm is about 0.6 atm, which is at least 100-fold lower than the values of ~ for all other algae which have been studied,

In the estuarine crab, Callinectes sapidus, net NH4+ efflux was twice as high in animals acclimated to 17% salinity seawater (SW) (0.495 +/- 0.084 mumol . h-1 . g wet wt-1, n = 7) than in animals acclimated to full-strength 35% SW (0.212 +/- 0.028 mumol . h-1 . g-1, n = 7). Amiloride (3 X 10(-4) M) in the external SW reversibly inhibited these effluxes by 63 +/- 6% (n = 6) and 67 +/- 6% (n = 5), respectively. Unidirectional Na+ influx was reversibly inhibited 42 +/- 6% (n = 7) by amiloride in 17% SW-acclimated crabs and 49 +/- 7% (n = 8) in 35% SW-acclimated crabs. This mutual sensitivity to amiloride is evidence for a Na+/NH4+ exchanger. Inhibition of unidirectional Na+ efflux by Na+-free SW indicates the presence of an obligate Na+/Na+ exchange component that accounts for at least 42% of the Na+ flux and is also amiloride sensitive. The lack of inhibition by amiloride of the net H+ efflux does not support the presence of a Na+/H+ exchanger. Acclimation of the crab to dilute SW may involve an increase in the activity of the Na+/NH4+ exchanger in the gills, but this mechanism contributes only a small fraction of the total Na+ transport.

Single cell electropotentials of barley (Hordeum vulgare L., cv. 'Compana') root cortex were measured at different external concentrations of KCl in the presence of Ca2+. The roots were low in salt from seedlings grown on 0.5 mM aerated CaSO4 solution. Thus, the conditions were equivalent to those used to define the dual nmechanisms found with radioactive tracer-labeled ion uptake. In 0.5 mM CaSO4 alone, there is an increase with time of cell negativity from about -65 millivolts 13 minutes after cutting segments to about -185 millivolts in 6 to 8 hours. Two possible hypotheses, not mutually exclusive, are offered to explain this aging effect: that cutting exposes plasmodesmata which are leaky initially but which seal in time, and that some internal factors, e.g., hormones diffusing from the apex, have a regulatory effect on the cell potential, an influence which becomes dissipated in isolated segments and permits the development of a higher potential difference. In any case changes in selective ion transport must be involved. The cell potentials at KCI concentrations above 2.0 mM are more negative than would be expected for a passive diffuision potential. It is suggested that this discrepancy may be due to an electrogenic pump or to a higher K+ concentration in the cytoplasm than in the remainder of the cell, or perhaps to both. Whether there is a clear relationship between cell potential and mechanisms 1 and 2 of cation transport depends upon whether the cell potentials of freshly cut or of aged tissue represent the values relevant to intact roots. low concentrations (Km = 0.02 mM) and is the major process at concentrations below 0.5 mm. The other component, mechanism 2, is identifiable only at concentrations exceeding 0.5 to 1.0 mM. The two components also differ in selectivity for K+ and Na+: mechanism 1 is highly specific for K+ versus Na+, whereas mechanism 2 shows little discrimination between K+ and Na+. The characteristics of the salt absorption isotherm have been explained by assuming that ion transport takes place via specific sites on a carrier. When the sites are saturated, further increase in concentration, within limits, fails to increase the velocity of uptake unless new sites are brought into action. Consequently, it has been held by some workers that independent ion diffusion does not occur, i.e., ions move only in a combined neutral form (5, 34). This view has not been accepted by others (1,4,21,25). Independent diffusion of some ions may be significant in plants inasmuch as the cell interior is electronegative by a value that can be related to external salt concentration by equations based on passive ion diffusion. In oat coleoptiles, for example, increasing the external KCl or NaCl concentration makes the cell potential less negative by an amount expected if the cell were much more permeable to K+ and Na+ than to 15).With passive transport the potential difference, PD3 (or E, in equations), between the cell sap and the external solution is a diffusion potential which assumes a value s...

We have studied the effects of prolonged incubation in low [K+] medium (approximately 0.3 mM) on both K+ and amino acid transport in Chinese hamster ovary (CHO) cells. When incubated in low [K+] medium, CHO cells redressed partially the loss of intracellular K+ after 12 h. After 24 h of incubation, both the activity of Na+-K+-ATPase in crude homogenates, and the transport capacity (Vmax) for ouabain-sensitive (i.e., active) K+ influx approximately doubled. The magnitude of the ouabain-insensitive (i.e., passive) K+ influx decreased by 50%. Thus the regulatory response involves an apparent increase in Na+-K+ pump and a decrease in K+ leak. The transport capacity for the nonmetabolized amino acid, alpha-aminoisobutyric acid (alpha-AIB), also increased after 24 h in low [K+] medium. The Vmax for the Na+-dependent (i.e., active) alpha-AIB influx increased by about 150%, and the magnitude of the Na+-independent influx increased by 20-40%. These changes in alpha-AIB transport result in a twofold greater capacity to accumulate this amino acid. Thus the regulation of K+ and alpha-AIB transport systems appears to be linked and possible mechanisms of this linkage are discussed.