Analysis of the effects of cesium ions on potassium channel currents in biological membranes

Cesium ions block potassium channels in biological membranes in a voltage dependent manner. For example, external cesium blocks inward current with little or no effect on outward current. Consequently, it produces a characteristic N-shaped current-voltage relationship. We have modeled this result by single file diffusion of ions in a narrow channel spanning the membrane with a special blocking site in the channel for cesium ions. The model enables us to make detailed comparisons of the effects of cesium on potassium channels in different types of biological membranes.     

Effects of external cesium and rubidium on outward potassium currents in squid axons

We have studied the effects of external cesium and rubidium on potassium conductance of voltage clamped squid axons over a broad range of concentrations of these ions relative to the external potassium concentration. Our primary novel finding concerning cesium is that relatively large concentrations of this ion are able to block a small, but statistically significant fraction of outward potassium current for potentials less than approximately 50 mV positive to reversal potential. This effect is relieved at more positive potentials. We have also found that external rubidium blocks outward current with a qualitatively similar voltage dependence. This effect is more readily apparent than the cesium blockade, occurring even for concentrations less than that of external potassium. Rubidium also has a blocking effect on inward current, which is relieved for potentials more than 20-40 mV negative to reversal, thereby allowing both potassium and rubidium ions to cross the membrane. We have described these results with a single-file diffusion model of ion permeation through potassium channels. The model analysis suggests that both rubidium and cesium ions exert their blocking effects at the innermost site of a two-site channel, and that rubidium competes with potassium ions for entry into the channel more effectively than does cesium under comparable conditions.     

Negative conductance caused by entry of sodium and cesium ions into the potassium channels of squid axons

Internal Cs⁺, Na⁺, Li⁺, and, to a lesser degree, Rb⁺ interfere with outward current through the K pores in voltage clamped squid axons. Addition of 100 mM NaF to the perfusion medium cuts outward current for large depolarizations about in half, and causes negative conductance over a range of membrane voltages. For example, suddenly reducing membrane potential from +100 to +60 mv increases the magnitude of the outward current. Internal Cs⁺ and, to a small extent, Li⁺, also cause negative conductance. Na⁺ ions permeate at least 17 times less well through the K pores than K⁺, and Cs⁺ does not permeate measurably. The results strongly suggest that K pores have a wide and not very selective inner mouth, which accepts K⁺, Na⁺, Li⁺, Cs⁺, tetraethylammonium ion (TEA⁺), and other ions. The diameter of the mouth must be at least 8 A, which is the diameter of a TEA⁺ ion. K⁺ ions in the mouths probably have full hydration shells. The remainder of the pore is postulated to be 2.6–3.0 A in diameter, large enough for K⁺ and Rb⁺ but too small for Cs⁺ and TEA⁺. We postulate that Na⁺ ions do not enter the narrower part of the pore because they are too small to fit well in the coordination cages provided by the pore as replacements for the water molecules surrounding an ion.     

Voltage clamp studies on the effect of internal cesium ion on sodium and potassium currents in the squid giant axon

Isolated and cleaned giant axons of Loligo pealii were internally perfused with solutions containing cesium sulfate and potassium fluoride. Membrane currents obtained as a function of clamped membrane potentials indicated a severe depression of the delayed outward current component normally attributed to potassium ion movement. Steady-state currents showed a negative slope in the potential range from -45 to -5 mv which corresponded to the negative slope for the peak sodium current relation vs. membrane potential which suggested long duration sodium currents. Using sodium-free sea water externally, sodium currents were separated from total currents and these persisted for longer times than normal. This result suggested that internal cesium ion delays the sodium conductance turnoff. The separated nonsodium currents showed an abnormal rectification as compared with those predicted by the independence principle, such that while potassium permeability appeared normal at the resting potential, its value decreased progressively with increasing depolarization.     

Chemical and ionization interferences in the atomic absorption spectrophotometric measurement of sodium,potassium, rubidium,and cesium

Although chemical and ionization interferences significantly affect the atomic absorption signal of the alkali metals, suitable corrective measures permit accurate analysis of these elements. The observed interferences are affccted in opposite ways by flame temperature, chemical depression of absorption produced by anions decreasing, and ionization enhancement produced by cations increasing with increasing flame temperature. Anionic depression is small in an acetylene-air flame and moderately large in a propane-air flame, increasing in the sequence sulfate < chloride < perchlorate < phosphate. Phosphate affects the cations in the order Na < K < Rb < Cs, with 20 mM phosphate depressing cesium absorbance approximately 40%. Conversely, ionization enhancement by cations is small in a propane-air flame and large in an acetyleneair flame, the effect on rubidium absorbance increasing in the sequence Mg < Ca < Li < Na < K < Cs, with 20 mM cesium producing a twoflod increase in absorbance. This is in the order of decreasing ionization potentials, indicating a direct relationship between ionization potential, degree of ionization, and enhancement produced. From consideration of the over-all effect of flame temperature on various interferences, we conclude that the propane-air flame is probably the most satisfactory for alkali metal analysis, especially for rubidium and cesium. Recovery studies on dialyzed and ashed rat liver microsomes and known controls demonstrate that addition of 15 mM lanthanum, to minimize anionic interferences, and addition of moderate concentrations of cesium, rubidium, or potassium, to minimize cationic enhancement, permit accurate and reliable measurement of the alkali metal cations in biological materials in the presence of potentially interfering cations and anions.     

Generation of nonadrenergic inhibitory junction potentials in the smooth muscle of the guinea-pig gastrointestinal tract after replacing potassium ions with cesium ions

Nonadrenergic inhibitory junction potentials (IJPs), evoked by intramural nerve stimulation, were studied in the smooth muscle of the guinea-pig stomach, cecum, and colon, using a modified sucrose-gap technique. After incubating smooth muscle preparations for 4–9 h in potassium-free Krebs solution, IJPs were abolished, but reappeared when cesium ions (6 mM) were added to the Krebs solution. Under these conditions, in the majority of cases the amplitude of the IJP was half as small, and the latency and duration were significantly longer, than in normal conditions; also ATP, but not adenosine, caused hyperpolarization of the smooth muscle membrane. The amplitude of the IJP depended on the extracellular concentration of cesium. In all types of preparation, in cesium-containing Krebs solution, apamin usually abolished the IJP and responses to ATP. These results are consonant with the purinergic hypothesis of inhibitory neuromuscular transmission. The generation of the IJP in these potassium-free conditions depends on cesium ions, which pass through the small-conductance apamin-sensitive, calcium-dependent potassium channels.A. A. Bogomoletz Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 5, pp. 634–641, September–October, 1990.     

Flow-injection-chemiluminescence method for the determination of penicillin G potassium.

The degradation product of penicillin G potassium can react with potassium permanganate in acidic medium and produce chemiluminescence, which is greatly enhanced by formaldehyde. The optimum conditions for this chemiluminescent reaction were studied in detail using a flow-injection system. The experiments indicated that under optimum conditions, the chemiluminescence intensity was linearly related to the concentration of penicillin G potassium within the range 1.0 x 10(-7)-1.0 x 10(-5) g/mL, with a detection limit (3sigma) of 7 x 10(-8) g/mL. The relative standard deviation was 1.0% for 4.0 x 10(-7) g/mL penicillin G potassium solution (n = 11). This method has the advantages of simple operation, fast response and high sensitivity. The method was successfully applied to the analysis of penicillin G potassium in raw medicines.     

Micro-chemical imaging of cesium distribution in Arabidopsis thaliana plant and its interaction with potassium and essential trace elements

Cesium as an alkali element exhibits a chemical reactivity similar to that of potassium, an essential element for plants. It has been suggested that Cs phytotoxicity might be due either to its competition with potassium to enter the plant, resulting in K starvation, or to its intracellular competition with K binding sites in cells. Such elemental interactions can be evidenced by chemical imaging, which determines the elemental distributions. In this study, the model plant Arabidopsis thaliana was exposed to 1 mM cesium in the presence (20 mM) or not of potassium. The quantitative imaging of Cs and endogenous elements (P, S, Cl, K, Ca, Mn, Fe, and Zn) was carried out using ion beam micro-chemical imaging with 5 microm spatial resolution. Chemical imaging was also evidenced by microfocused synchrotron-based X-ray fluorescence (microXRF) which presents a better lateral resolution (<1 microm) but is not quantitative. Cesium distribution was similar to potassium which suggests that Cs can compete with K binding sites in cells. Cesium and potassium were mainly concentrated in the vascular system of stems and leaves. Cs was also found in lower concentration in leaves mesophyll/epidermis. This late representing the larger proportion in mass, mesophyll/epidermis can be considered as the major storage site for cesium in A. thaliana. Trichomes were not found to accumulate cesium. Interestingly, increased Mn, Fe, and Zn concentrations were observed in leaves at high chlorosis. Mn and Fe increased more in the mesophyll than in veins, whereas zinc increased more in veins than in the mesophyll suggesting a tissue specific interaction of Cs with these trace elements homeostasis. This study illustrates the sensitivity of ion beam microprobe and microfocused synchrotron-based X-ray fluorescence to investigate concentrations and distributions of major and trace elements in plants. It also shows the suitability of these analytical imaging techniques to complement biochemical investigations of metallic stress in plants.