The steady-state concept of Na(+) homeostasis, based on short-term investigations of responses to high salt intake, maintains that dietary Na(+) is rapidly eliminated into urine, thereby achieving constant total-body Na(+) and water content. We introduced the reverse experimental approach by fixing salt intake of men participating in space flight simulations at 12 g, 9 g, and 6 g/day for months and tested for the predicted constancy in urinary excretion and total-body Na(+) content. At constant salt intake, daily Na(+) excretion exhibited aldosterone-dependent, weekly (circaseptan) rhythms, resulting in periodic Na(+) storage. Changes in total-body Na(+) (±200-400 mmol) exhibited longer infradian rhythm periods (about monthly and longer period lengths) without parallel changes in body weight and extracellular water and were directly related to urinary aldosterone excretion and inversely to urinary cortisol, suggesting rhythmic hormonal control. Our findings define rhythmic Na(+) excretory and retention patterns independent of blood pressure or body water, which occur independent of salt intake.
Cell-to-cell fusion has great potential in membrane research, membrane reconstitution and genetic mapping [25,34,49]. Furthermore, both somatic hybridization and genetic engineering offer a way of modifying plant cells and, in turn, of improving crops [49,53]. If lymphocyte cells are fused with a permanent cell line so-called hybridoma cells are produced [32] which are capable of producing monoclonal antibodies of predetermined antigenic specificity. These hybridoma cells (antibody-producing hybrids) can be envisaged as important tools in future clinical diagnosis and therapy as well as in the purification and enrichment of compounds of cell-biological and medical interest [18,65, 72]. However, despite many promising results and considerable efforts, fusion still seems to be something of an art rather than a precise science.Phenomenologically, cell-to-cell fusion is achieved presently in vitro by chemicals or inactivated virus [2,30,31,33,34,48,64]. Fusion can usually only be achieved by using membrane-disrupting agents and procedures and/or unphysiological conditions (e.g., high Ca/+ concentrations, high or low pH values, hypotonic conditions, etc.)The field of chemically and virus-induced fusion has been excellently reviewed by several authors over the last years [17,34,45,47,48]. In view of the bewildering array of data on chemically and virusinduced fusion we are obliged to conclude that we are still far from solving the molecular mechanism underlying the fusion process.Any progress in this field and in the development of novel fusion techniques will have to be evaluated by comparison with the inherent limitations of the current fusion procedures, which are listed below:
Accurately collected 24-hour urine collections are presumed to be valid for estimating salt intake in individuals. We performed two independent ultra-long-term salt balance studies lasting 105 (4 men) and 205 (6 men) days in 10 men simulating a flight to Mars. We controlled dietary intake of all constituents for months at salt intakes of 12, 9, and 6 grams per day and collected all urine. The subjects’ daily menus consisted of 27,279 individual servings, out of which 83.0% were completely consumed, 16.5% completely rejected, and 0.5% incompletely consumed. Urinary recovery of dietary salt was 92% of recorded intake, indicating long-term steady state sodium balance in both studies. Even at fixed salt intake, 24-hour sodium excretion (UNaV) showed infradian rhythmicity. We defined a ±25 mmol deviation from the average difference between recorded sodium intake and UNaV as the prediction interval to accurately classify a 3-gram difference in salt intake. Due to the biological variability in UNaV, only every-other daily urine sample correctly classified a 3-gram difference in salt intake (49%). By increasing the observations to three consecutive 24-hour collections and sodium intakes, classification accuracy improved to 75%. Collecting seven 24-hour urines and sodium intake samples improved classification accuracy to 92%. We conclude that single 24-hour urine collections at intakes ranging from 6–12 grams salt per day were not suitable to detect a 3-gram difference in individual salt intake. Repeated measurements of 24-hour UNaV improve precision. This knowledge could be relevant to patient care and the conduct of intervention trials.
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