PubMed · 4163692
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A F Bak. 1967. Testing metal micro-electrodes.. https://doi.org/10.1016/0013-4694(67)90162-9
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Most higher plants develop severe toxicity symptoms when grown on ammonium (NH(4)(+)) as the sole nitrogen source. Recently, NH(4)(+) toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying NH(4)(+) toxicity have been extensively sought, the primary events conferring it at the cellular level are not understood. Using a high-precision positron tracing technique, we here present a cell-physiological characterization of NH(4)(+) acquisition in two major cereals, barley (Hordeum vulgare), known to be susceptible to toxicity, and rice (Oryza sativa), known for its exceptional tolerance to even high levels of NH(4)(+). We show that, at high external NH(4)(+) concentration ([NH(4)(+)](o)), barley root cells experience a breakdown in the regulation of NH(4)(+) influx, leading to the accumulation of excessive amounts of NH(4)(+) in the cytosol. Measurements of NH(4)(+) efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for NH(4)(+), reveal that, at elevated [NH(4)(+)](o), barley cells engage a high-capacity NH(4)(+)-efflux system that supports outward NH(4)(+) fluxes against a sizable gradient. Ammonium efflux is shown to constitute as much as 80% of primary influx, resulting in a never-before-documented futile cycling of nitrogen across the plasma membrane of root cells. This futile cycling carries a high energetic cost (we record a 40% increase in root respiration) that is independent of N metabolism and is accompanied by a decline in growth. In rice, by contrast, a cellular defense strategy has evolved that is characterized by an energetically neutral, near-Nernstian, equilibration of NH(4)(+) at high [NH(4)(+)](o). Thus our study has characterized the primary events in NH(4)(+) nutrition at the cellular level that may constitute the fundamental cause of NH(4)(+) toxicity in plants.
Chronic perineal pain, too easily considered in the past as psychogenic, has undergone significant reshapings in recent years because of a better understanding of pathophysiology. Progress in functional investigations, especially electrophysiological tests, has allowed to define new clinical entities such as pudendal neuralgia or pain linked to pelvic floor disorders. Treatments have been improved, but remain empirical outside these well defined entities. In practice, clinical examination is the main diagnostic step, which allows to exclude common anorectal or pelviperineal lesions, and to consider additional investigations. However diagnosis may remain entirely clinical in some typical presentations.
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