An ion''s view of the potassium channel. The structure of the permeation pathway as sensed by a variety of blocking ions

We have studied the block of potassium channels in voltage-clamped squid giant axons by nine organic and alkali cations, in order to learn how the channel selects among entering ions. When added to the internal solution, all of the ions blocked the channels, with inside-positive voltages enhancing the block. Cesium blocked the channels from the outside as well, with inside-negative voltages favoring block. We compared the depths to which different ions entered the channel by estimating the "apparent electrical distance" to the blocking site. Simulations with a three-barrier, double-occupancy model showed that the "apparent electrical distance," expressed as a fraction of the total transmembrane voltage, appears to be less than the actual value if the blocking ion can pass completely through the channel. These calculations strengthen our conclusion that sodium and cesium block at sites further into the channel than those occupied by lithium and the organic blockers. Our results, considered together with earlier work, demonstrate that the depth to which an ion can readily penetrate into the potassium channel depends both on its size and on the specific chemical groups on its molecular surface. The addition of hydroxyl groups to alkyl chains on a quaternary ammonium ion can both decrease the strength of binding and allow deeper penetration into the channel. For alkali cations, the degree of hydration is probably crucial in determining how far an ion penetrates. Lithium, the most strongly hydrated, appeared not to penetrate as far as sodium and cesium. Our data suggest that there are, minimally, four ion binding sites in the permeation pathway of the potassium channel, with simultaneous occupancy of at least two.     

Allometries of the durations of torpid and euthermic intervals during mammalian hibernation: A test of the theory of metabolic control of the timing of changes in body temperature

Summary The durations of the intervals of torpor and euthermia during mammalian hibernation were found to be dependent on body mass. These relationships support the concept that the timing of body temperature changes is controlled by some metabolic process. Data were obtained from species spanning nearly three orders of magnitude in size, that were able to hibernate for over six months without food at 5°C. The timing of body temperature changes was determined from the records of copper-constantan thermocouples placed directly underneath each animal. Because all species underwent seasonal changes in their patterns of hibernation, animals were compared in midwinter when the duration of euthermic intervals was short and relatively constant and when the duration of torpid intervals was at its longest. Large hibernators remained euthermic longer than small hibernators (Fig. 2). This was true among and within species. The duration of euthermic intervals increased with mass at the same rate (mass^0.38) that mass-specific rates of euthermic metabolism decrease, suggesting that hibernators remain at high body temperatures until a fixed amount of metabolism has been completed. These data are consistent with the theory that each interval of euthermia is necessary to restore some metabolic imbalance that developed during the previous bout of torpor. In addition, small species remained torpid for longer intervals, than large species (Fig. 3). The absolute differences between different-sized species were large, but, on a proportional basis, they were comparatively slight. Mass-specific rates of metabolism during torpor also appear to be much less dependent on body mass than those during euthermia, but the precision of these metabolic measurements is insufficient for them to provide a conclusive test of the metabolic theory. Finally, small species with high mass-specific rates of euthermic metabolism are under tighter energetic constraints during dormancy than large species. The data presented here show that, in midwinter, small species compensate both by spending less time at high body temperatures following each arousal episode and by arousing less frequently, although the former is far more important energetically than the latter.     

A comparison between Northern and Southern Hemisphere tundras and related ecosystems

Summary Climatic, edaphic, and vegetation parameters are compared among three Subantarctic islands and one Maritime Antarctic island, and Northern Hemisphere tundras and tundra-like ecosystems (ranging from higharctic to cool-temperate oceanic and temperate alpine), mainly by Principal Component Analyses (PCA). All analyses of abiotic variables emphasized the extreme oceanicity of the Southern Hemisphere sites and showed the strong influence of seaspray on soil nutrient ratios in the southern sites. Analyses of climate classified all the southern sites as Cold Oceanic within a series of very oceanic climates that were distinct from the continental/subalpine climates more typical of the northern sites. Four factorial groups were derived from the vectors in PCA of soils and climate combined. Each was a grouping of several variables that was essentially independent of the other groups. They were: general climatic severity, soil base status, supply of labile nutrients, and a combined index of oceanicity with organic/mineral balance and drainage. The ecological significance of these is discussed. The range of soil nutrient levels in the southern sites is approximately equal to that of the northern sites; however, in the southern sites soil nutrient levels are inversely related to climatic severity while in the north the reverse is true. The two most important causes of these opposed trends are the relative ages of the sites and the much greater effects of sea-based vertebrates (e.g. seals and seabirds) in the southern islands. Cluster analyses of component scores derived from climatic and soil data linked the southern sites with northern sites at higher latitudes, indicating the effect of the Antarctic continent on the Southern Ocean (hence on the overall climate of the southern islands) and of wind-chill on the aerial and soil climates of the southern sites. Vegetation patterns (derived from PCA of life form data) were more complex because of the serial replacement of one lifeform by onother along continuous environmental gradients. Wind exposure was an important element in the first two vectors derived from the PCA of the botanical data. The Southern Hemisphere sites exhibited almost the full range of vegetation types found in the Northern Hemisphere, despite their floristic poverty. Variation within individual islands was comparable with that found at considerably higher (up to 30° more) northern latitudes and reflects the overriding importance of wind exposure in the southern islands. Subarctic ecosystems are generally less severe forms of Arctic ones, and decreasing latitude leads to increasingly milder environments with no great changes overall in continentality. In contrast, the Subantarctic combines elements from the extremes of the range of northen tundras (i.e. high-Arctic and cool-temperate oceanic) with its own peculiar features (e.g. animal influences) to produce ecosystems that are qualitatively different both from Subarctic systems and from the continental Antarctic regions to which they are geographically closest.     

13C- and 31P-NMR studies of the conformation of carcinogen-modified nucleic acid dimers

The effect of the carcinogen acetylaminofluorene (AAF) on nucleic acid structure was examined using ¹³C- and ³¹P-NMR spectroscopies. Conformational effects were compared in two AAF-modified dinucleoside monophosphates (ApG and GpA) and two AAF-modified deoxydinucleotides (dpApG and dpGpA). Changes in adenine ¹³C chemical shifts on formation of the AAF-adduct and as a function of temperature provided evidence of base stacking. Differences in fluorene ¹³C chemical shifts between the AAF-modified dimer and AAF-modified monomer provided evidence of fluorene stacking. The effect of forming the adduct on the phosphate backbone was examined using ³¹P-NMR. A correlation was demonstrated between the degree of adenine-fluorene stacking on one hand and the change in conformation of the backbone conformation on the other.     

The effects of microtubule dissociating agents on the physiology and cytology of the sensory neuron in the femoral tactile spine of the cockroach, periplaneta americana L

Microtubules are prominent cellular components of the mechanosensory and chemosensory sensilla associated with the insect cuticle, and a range of hypotheses have been proposed to account for their role in sensory transduction. Chemical agents such as colchicine and vinblastine, which dissociate microtubules, also interfere with transduction in these sensilla, and this has been attributed to their anti-microtubule activity. We have now examined the dynamic properties of sensory transduction in the mechanosensitive neuron of the cockroach femoral tactile spine, after the application of colchicine, vinblastine and lumicolchicine. Concurrently we have examined the ultrastructure of the same sensory ending by transmission electron microscopy. All of the drugs reduced the mechanical sensitivity o the receptor. Colchicine and vinblastine achieved this reduction without altering the dynamic properties of the receptor but lumicolchicine changed the dynamic response, and increased the relative sensitivity to rapid movements. Conduction velocity, another measure of neuronal function, which relies upon ionic currents flowing through the membrane, was reduced by all three drugs. The effects of the drugs upon the ultrastructure of the sensory ending were also disparate. In the case of colchicine there was complete dissociation of microtubules in the tubular body and distal dendrite before a total loss of mechanical sensitivity. Vinblastine was less effective in dissociating microtubules, although more effective in the reduction of mechanical sensitivity. With lumicolchicine the dominant morphological effect was a severe disruption of the dendritic membrane. We conclude from these experiments that microtubules are not essential in the transduction of mechanical stimuli by cuticular receptors and that the effects of these drugs upon mechanosensitivity are not directly related to their dissociation of the microtubules in the tubular body, but are more likely to arise from actions upon the cell membrane. These actions could include effects upon tubulin in the membrane or upon other membrane components.     

Photosynthesis in sugar beet depends on root growth

Summary Sugar-beet plants, germinated in growth cabinets at 20°C and transplanted into the field after 3 weeks, developed much larger roots than plants grown from seed drilled directly into the soil. At the end of the season, the roots of transplants were 39% greater than from drilled seed—an increase of 14 m tons per hectare. The increased yield was mainly due to a sustained increase in photosynthesis because of the larger sink for carbohydrates provided by plants from the growth cabinets.