Temperature- and structure-dependent interaction of pyrethroids with the sodium channels in frog node of Ranvier

(1) The interaction of a series of pyrethroid insecticides with the Na+ channels in myelinated nerve fibres of the clawed frog, Xenopus laevis, was investigated using the voltage clamp technique. (2) Out of 11 pyrethroids 9 insecticidally active compounds induce a slowly decaying Na+ tail current on termination of a step depolarization, whereas the Na+ current during depolarization was hardly affected. These tail currents are most readily explained by a selective reduction of the rate of closing of the activation gate in a fraction of the Na+ channels that have opened during depolarization. (3) The rate of decay of the Na+ tail current varies considerably with pyrethroid structure. After alpha-cyano pyrethroids the decay is at least one order of magnitude slower than after non-cyano pyrethroids. The decay always follows a single-exponential time course and is reversibly slowed when the temperature is lowered from 25 to 0 degrees C. Arrhenius plots in this temperature range are linear. (4) These results indicate that the relaxation of the activation gate in pyrethroid-affected Na+ channels is governed by an apparent first order, unimolecular process and that the rate of relaxation is limited by a single energy barrier. Application of transition state theory shows that after alpha-cyano pyrethroids this energy barrier is 9.6 kJ/mol higher than after non-cyano pyrethroids. (5) Differences in rate of decay of the Na+ tail current account for the reported differences in repetitive nerve activity induced by various pyrethroids. In addition, the effect of temperature on the rate of decay explains the increase in repetitive activity with cooling.     

Accelerated transbilayer movement of phosphatidylcholine in sickled erythrocytes. A reversible process

The transbilayer mobility of phosphatidylcholine (PC) molecules in the membrane of homozygous reversible sickle cells (RSCs) was studied using a PC-specific exchange protein from beef liver. In deoxygenated RSCs, all of the PC present in the membrane of the intact cell is rapidly available for exchange, mediated by this protein. Since a substantial amount of the PC is present in the inner membrane leaflet of these cells, this observation implies that the PC molecules in their membranes do experience rapid transbilayer movements. To determine the actual rate of transbilayer movement of the PC, radioactive PC was introduced into the outer monolayer of oxygenated RSCs using the PC-specific exchange protein. Subsequently, the cells were incubated at 37 degrees C under oxy- and deoxygenating conditions to enable the PC to equilibrate within the bilayer. At various time intervals, samples were taken and treated with phospholipase A2, which selectively degrades the PC in the outer monolayer. Analysis of the specific radioactivities of the lyso-PC thus produced, as well as of the residual PC, enabled us to follow the fate of the radioactive PC previously introduced into the outer membrane layer. The half-time value for transbilayer equilibration of the PC in deoxygenated RSCs was determined to be 3.5 h, which is about four times lower than that for oxygenated RSCs. This increased transbilayer mobility of PC, observed in deoxygenated RSCs, is immediately restored to the normal low rate upon reoxygenation of the cells, indicating a complete reversibility of this phenomenon.     

In vivo and in vitro uptake and degradation of acetylated low density lipoprotein by rat liver endothelial, Kupffer, and parenchymal cells

Isolation and separation of rat liver cells into endothelial, Kupffer, and parenchymal cell fractions were performed at different times after injection of human 125I-acetyl low density lipoproteins (LDL). In order to minimize degradation and redistribution of the injected lipoprotein during cell isolation, a low temperature (8 degrees C) procedure was applied. Ten min after injection, isolated endothelial cells contained 5 times more acetyl-LDL apoprotein per mg of cell protein than the Kupffer cells and 31 times more than the hepatocytes. A similar relative importance of the different cell types in the uptake of acetyl-LDL was observed 30 min after injection. For studies on the in vitro interaction of endothelial and Kupffer cells with acetyl-LDL, the cells were isolated with a collagenase perfusion at 37 degrees C. Pure endothelial (greater than 95%) and purified Kupffer cells (greater than 70%) were obtained by a two-step elutriation method. It is demonstrated that the rat liver endothelial cell possesses a high affinity receptor specific for the acetyl-LDL because a 35-fold excess of unlabeled acetyl-LDL inhibits association of the labeled compound for 70%, whereas unlabeled native human LDL is ineffective. Binding to the acetyl-LDL receptor is coupled to rapid uptake and degradation of the apolipoprotein. Addition of the lysosomotropic agents chloroquine (50 microM) or NH4Cl (10 mM) resulted in more than 90% inhibition of the high affinity degradation, indicating that this occurs in the lysosomes. With the purified Kupffer cell fraction, the cell association and degradation of acetyl-LDL was at least 4 times less per mg of cell protein than with the pure endothelial cells. Although cells isolated with the cold pronase technique are also still able to bind and degrade acetyl-LDL, it appeared that 40-60% of the receptors are destroyed or inactivated during the isolation procedure. It is concluded that the rat liver endothelial cell is the main cell type responsible for acetyl-LDL uptake.     

A dynamometer for the measurement of the extension torque of the lower leg during static and dynamic contractions of the quadriceps femoris muscle

A dynamometer which makes an angular movement is described. The dynamometer enables the measurement of the extension torque of the lower leg at different knee angles during static and slow concentric and eccentric contractions of the quadriceps femoris muscle. The influence of gravity on the measured torque signal can be compensated for by another signal representing the angular movement. The application of the dynamometer is demonstrated by giving an example of measurement.     

Acidification and alkalinization of soils

Acidification or alkalinization of soils occurs through H⁺ transfer processes involving vegetation, soil solution and soil minerals. A permanent change in the acid neutralizing capacity of the inorganic soil fraction (ANC_(s)),i.e. soil acidification (ΔANC<0) or soil alkalinization (ΔANC>0), results from an irreversible H⁺ flux. This irreversible H⁺ flux can be caused either by direct proton addition or depletion, by different mobility of components of the ANC_(s) or by a permanent change in redox conditions. The contributions of (a) acidic atmospheric deposition, (b) nitrogen transformations, (c) deprotonation of CO₂ and of organic acids and protonation of their conjugate bases, (d) assimilation of cations and anions by the vegetation, (e) weathering or reverse weathering of minerals and (f) stream output to changes in the ANC_(s) are illustrated by means of H⁺ budgets for actual soils and watersheds.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

In a typical Dutch polder landscape the effects of nutrient transport from cultivated grassland to mesotrophic fen communities were studied. In a comparative approach, biomass production and nutrient (N, P and K) uptake were determined monthly in four fens and a hayfield differeing in productivity and species composition. The interstitial ground water was sampled every two weeks for determinations of inorganic nutrient concentrations.The differences in productivity between the fens were clearly reflected in the amount of N, P and K taken up in the above-ground vegetation. N and P proved to be limiting plant growth in the fens, whereas K was the main limiting factor in the hayfield. The ground water welling up from the sandy bottom into the fens proved to be rich in ammonia (3–5 ppm). There are strong indications that this continual seepage leads to a considerable input of N into the fens but not to a higher productivity, as the ammonia is absorbed by the lowermost peat layers covering the sand.At this moment, the differences in productivity between the fens must be caused by differences in the rates of mineralization of the superficial peat layer. The degree of fixation of the floating vegetation mat, determining whether or not low water levels lead to an aerated soil top layer, is important in this respect. Within a period of decades, however, the continuous inflow of ammonia may eventually cause an increase in the productivity and a change in the species composition of the fens.     

Simulation of marked root hair curling in Rhizobium-legume symbiosis

The soil bacterium Rhizobium infects its leguminous host plants in temperate regions of the world mostly by way of the growing root hairs. Root hair curling is a prerequisite for root hair infection, although sidelong root hair infections occasionally have been observed. The processes underlying Rhizobium -induced root hair curling are unknown.
Computer simulation of root hair growth indicates that one-sided tip growth inhibition by Rhizobium can result in root hair curling when three conditions are simultaneously fulfilled: 1) rhizobial growth inhibition is strong enough to prevent removal out of the tip growth range: 2) root hair surface growth between the attached Rhizobium and the root hair top is inhibited; 3) rhizobial growth inhibition is limited to one side of the root hair.
The results predict that root hair curling by stimulation of tip growth is improbable. This study accentuates the need for information about the growth processes contributing to tip growth in leguminous root hairs.