Long-term antiorthostatic hypokinesia stimulates storage of osmotically inactive sodium in the human body

Changes in the water and sodium balances and in the states of the fluid compartments of the human body observed in experiments performed with healthy subjects exposed to long-term (120 days) antiorthostatic hypokinesia (ANOH) were analyzed. A hypothesis was suggested that the normal dietary consumption of sodium could be associated with the accumulation of osmotically inactive sodium in the body of a healthy person (independently of changes in the total water content). The results agree with the assumption that considerable amounts of osmotically inactive sodium may be stored in the human body. This hypothesis was confirmed by the inversion of the correlation between the cumulative sodium balance and the total water content of the body found both in the group-averaged data and in individual data. This nonsmotic sodium accumulation may take place not only during deviations from its normal consumption, but also during its regular dietary supply. Accumulation of sodium in these stores and its depletion are not associated with any significant changes in the volumes of body fluids. Infradian rhythmic changes in the sodium balance observed in some subjects exposed to the long-term ANOH, which were not caused by any periodic external influences, indicated the existence of a specific mechanism regulating the sodium content of the body. This mechanism must be significantly more inert and less precise than the fast regulation of the volume, osmolality, and ionic composition of extracellular fluids.     

Are large amounts of sodium stored in an osmotically inactive form during sodium retention? Balance studies in freely moving dogs

Alterations in total body sodium (TBSodium) that covered the range from moderate deficit to large surplus were induced by 10 experimental protocols in 66 dogs to study whether large amounts of Na+ are stored in an osmotically inactive form during Na+ retention. Changes in TBSodium, total body potassium (TBPotassium), and total body water (TBWater) were determined by 4-day balance studies. A rather close correlation was found between individual changes in TBSodium and those in TBWater (r2 = 0.83). Changes in TBSodium were often accompanied by changes in TBPotassium. Taking changes of both TBSodium and TBPotassium into account, the correlation with TBWater changes became very close (r2 = 0.93). The sum of changes in TBSodium and TBPotassium was accompanied by osmotically adequate TBWater changes, and plasma osmolality remained unchanged. Calculations reveal that even moderate TBSodium changes often included substantial Na+/K+ exchanges between extracellular and cellular space. The results support the theory that osmocontrol effectively adjusts TBWater to the body's present content of the major cations, Na+ and K+, and do not support the notion that, during Na+ retention, large portions of Na+ are stored in an osmotically inactive form. Furthermore, the finding that TBSodium changes are often accompanied by TBPotassium changes and also include Na+/K+ redistributions between fluid compartments suggests that cells may serve as readily available Na+ store. This Na+ storage, however, is osmotically active, since osmotical equilibration is achieved by opposite redistribution of K+.     

Feedback models allowing estimation of thresholds for self-promoting body weight gain

Objective: Most people maintain almost constant body weight over long time with varying physical activity and food intake. This indicates the existence of a regulation that works well for most individuals. Yet some people develop obesity, indicating that this regulation sometimes fails. The difference between the two situations is typically an energy imbalance of about 1% over a long period of time.Theory: Weight gain increases basal metabolic rate. Weight gain is often associated with a decrease in physical activity, although not to such an extent that it prevents an increase in total energy expenditure and energy intake. Dependent on the precise balance between these effects of weight gain, they may make the body weight unstable and tend to further promote weight gain. With the aim of identifying the thresholds beyond which such self-promoting weight gain may take place, we develop a simple mathematical model of the body as an energy-consuming machine in which the changes in physical activity and food intake are described as feedback effects in addition to the effect of the weight gain on basal metabolic rate. The feedback parameters of the model may differ between individuals and only in some cases do they take values that make weight gain self-promoting.Results: We determine the quantitative conditions under which body weight gain becomes self-promoting. We find that these conditions can easily be met, and that they are so small that they are not observable with currently available techniques. This phenomenon encourages emphasis on even minor changes in food intake and physical activity to abate or stop weight gain.     

Excessive Weight Gain during the First Year of Peritoneal Dialysis Is Associated with Inflammation,Diabetes Mellitus,and a Rapid Decrease in Residual Renal Function

Objectives

Significant weight gain is a potential problem in most patients starting peritoneal dialysis (PD); however, few studies have explored the clinical effects of increased body weight (BW) in these patients. We evaluated the effect of excess weight gain during the first year after PD on residual renal function (RRF).

Methods

A total of 148 incident PD patients were analyzed in a longitudinal observational study. The mean duration of follow-up was 23.8 months. RRF was measured at baseline (within 1 month of starting PD) and thereafter at 6-month intervals for 2–3 years or until loss of RRF. BW was measured at the time of RRF measurement, and excess weight gain was defined as a BW increase over the median value (3.0%).

Results

The median 1-year increase in BW was 2.3kg (IQR, 1.01–4.58) or 3.0% (IQR, 1.13–5.31). The mean slope of RRF decline was –0.068 ± 0.053 mL/min/month/1.73m², and RRF loss developed in 48 patients at a mean follow-up time of 19.4 ± 6.8 months. Patients with BW increases > 3.0% showed significantly increased RRF decline rate compared to those without excess weight gain (p<0.001), and the BW increase (%/year) correlated significantly with higher hs-CRP levels and RRF decline rate. High systolic blood pressure, diabetes, large amount of proteinuria and excess BW gain significantly influenced the RRF decline rate. Also, it increased the risk of RRF loss by 4.17-fold (95% confidence intervals, 1.87–9.28; p<0.001).

Conclusions

Excess weight gain during the first year of PD was closely linked to systemic inflammation, diabetes and rapid decline in RRF.     

Water relations of polyol accumulation by Zygosaccharomyces rouxii in continuous culture

Summary Glycerol and arabitol were the main polyols accumulated by Zygosaccharomyces rouxii in continuous culture but the intracellular and extracellular concentrations of the polyols varied with the dilution rate and osmoticum used to adjust the water activity (a_w) to 0.960. When the a_w was adjusted with NaCl, glycerol was the main polyol accumulated intracellularly whereas glycerol and arabitol were accumulated when polyethylene glycol (PEG) 400 was used. The extracellular glycerol and arabitol concentrations at 0.960 a_w (NaCl or PEG 400) were similar or decreased relative to cultures at 0.998 a_w. Compared to steady-state cultivation at 0.998 a_w, the yeast retained at 0.960 a_w (NaCl or PEG 400) a greater proportion of the total glycerol intracellularly against an increased concentration ratio without significantly greater production of glycerol. Arabitol was only significant in osmoregulation when cultivated at 0.960 a_w (PEG 400). The intracellular glycerol concentration was insufficient to balance the a_w across the membrane, but an equilibrium could be achieved under certain conditions if arabitol was also osmotically active. Offprint requests to: P. J. van Zyl     

Short-term osmotic responses of cells and tissues of the sea anemone,Condylactis gigantea

• 1.1. Cells of tentacles and body wall of the sea anemone Condylactis gigantea behaved as simple osmometers during 5hr exposure to 50, 67, 83, 100 and 125% sea-water.
• 2.2. All intracellular water appeared to be osmotically active.
• 3.3. Cell sodium, chloride and total osmolyte content remained invariable, with taurine decreasing and potassium increasing as sea-water concentration was reduced.
• 4.4. Tissues, as a whole, exhibited a pseudoregulatory response to changes in salinity as the large and osmotically inert extracellular space buffered volume changes to a considerable extent.

     

Osmotic Properties of Mitochondria

The osmotic behavior of rat liver mitochondria has been studied in a sucrose medium. The mitochondria behave like a two compartment system. One compartment is permeable to sucrose and has a volume of 1.22 µl/(mg mitochondrial dry weight) in a 272 milliosmol sucrose medium; the second, inaccessible to sucrose, has a volume of 0.555 µl/mg dry weight) under the same conditions. Part of the water in the sucrose inaccessible space is apparently not free to participate in osmotic phenomena. This volume is 0.272 µl/(mg dry weight) under the same conditions. It is suggested that the osmotically inactive water corresponds to the water of hydration of the mitochondrial macromolecules. The volume of the remainder of the water in the sucrose inaccessible space depends inversely on the osmolality of the medium, as is to be expected. The volume of water in the sucrose accessible space is constant, independent of the osmolality of the medium, as is the volume of the mitochondrial framework plus the nonvolatile solutes.     

Glucose response to an oral glucose tolerance test predicts weight change in non-diabetic subjects

Objective: Glucose exerts a dual action in the regulation of energy balance, consisting of inhibition of energy intake and stimulation of energy expenditure. Whether blood glucose affects long‐term regulation of body weight in humans remains to be established. We sought to test the hypothesis that the post‐challenge glucose response is a predictor of weight change. Research Methods and Procedures: We performed a prospective analysis of the impact of glucose response to an oral glucose tolerance test (OGTT) and a mixed‐meal test (MT) on subsequent changes in body weight (BW) on 253 Pima Indians (166 men and 87 women) with normal glucose regulation at baseline and follow‐up (follow‐up: 7 ± 4 years). Main outcome measures included BW change (total, percent, and annual), plasma glucose and insulin concentrations during OGTT and MT [total and incremental areas under the curve (AUCs)], resting metabolic rate (RMR; indirect calorimetry), and insulin action (euglycemic‐hyperinsulinemic clamp). Results: Total and incremental glucose AUCs during the OGTT (but not the MT) were negatively associated with BW change (total, percent, and annual), both before and after adjusting for sex, age, initial BW, follow‐up time, insulin action, RMR, fasting plasma glucose and insulin concentrations, and insulin response. Total and incremental glucose AUCs during the OGTT were independent determinants of final BW with age, initial BW, follow‐up time, fasting plasma insulin concentrations, and RMR. Discussion: Higher post‐challenge glucose response protects against BW gain in subjects with normal glucose regulation. We propose that this action may be because of the effect of glucose on food intake and/or thermogenesis.     

Evolutionary Advantages of Participation of Vasopressin instead of Vasotocin in Regulation of Water–Salt Balance in Mammals

In experiments on non-anesthetized Wistar white rats there was studied reaction of kidney to an intramuscular injection of arginine vasotocin or arginine vasopressin at doses from 0.001 to 0.05 µg/100 g body mass on the background of a water load. Water (5 ml/100 g body mass) was administered through a catheter into stomach to suppress secretion of endogenous antidiuretic hormone (ADH). In experiments with water administration, diuresis increased due to a decrease of osmotic permeability of renal tubules and to excretion of osmotically free water, with the constant clearance of sodium ions. Injection of 0.05 µg arginine vasopressin led to a marked decrease of diuresis due to a rise of reabsorption of osmotically free water without elevation of excretion of osmotically active substances. Injection of the same dose of arginine vasotocin resulted in no increase of diuresis; however, reabsorption of osmotically free water and excretion of osmotically active substances including sodium ions were more pronounced. Hence, both vasotocin and vasopressin increased osmotic permeability of the tubular epithelium, but vasotocin, unlike vasopressin, promoted reduction of reabsorption of sodium ions and their loss with urine. A suggestion is made that one of the reasons for replacement in mammals of the molecular ADH forms (vasotocin by vasopressin) was the absence of the pronounced natriuretic effect in arginine vasopressin. This was of crucial significance to preserve sodium ions in the organism, to maintain water–salt balance in animals adapted to the terrestrial life, and to provide not only osmo-, but also volumoregulation.     

Volume Control by Muscle Fibers of the Blue Crab : Volume readjustment in hypotonic salines

Single isolated muscle fibers from the walking legs of the blue crab, Callinectes sapidus act as Boyle-van't Hoff osmometers with an osmotically inactive volume of 33 %. Fibers in hypotonic salines undergo a spontaneous volume readjustment toward the initial volumes of the cells found in isotonic salines. The volume readjustment is initiated by the increase in cell volume in hypotonic salines and appears to be dependent on the duration of exposure of the fiber to external sodium, the sodium concentration, and the pH of the external medium. The volume-readjusted cells continue to behave as osmometers, but with an increased relative osmotically inactive volume and a decreased internal resistivity. The decreases in cell volumes appear to be, in large part, due to losses of osmotically active nonelectrolytes from the cells.     

Pathophysiology of hemorrhagic shock. A model for studying the effects of acute blood loss in the rat

A model of acute blood loss in rats with a reproducible mortality rate over a wide range of body weights was developed by withdrawing various amounts of fixed blood volume per 100 g body weight via the left common carotid artery and observing the survival of the animals. Younger (lighter) animals survived the bleeding longer than older (heavier) animals. As early as 70 min following the shock episode there was evidence of acute tubular necrosis in kidney proximal tubules and focal centrilobular necrosis in the liver. The 84%, 50%, or 16% body weight - mortality line was: V84 = -0.17 BW + 3.25; V50 = -0.18 BW + 3.16; or V16 = -0.18 BW + 3.01 in animals ranging from 250 to 400 g body weight (where V = bleeding volume ml/100 g body weight, BW = body weight in grams X 10(-2). To produce the same mortality rate, the bleeding volume per unit body weight decreased with increased body weight. On the other hand, the bleeding volume per total blood volume or per unit body surface area increased with increased body weight. The body weight-mortality line is a useful method to calculate the bleeding volume to produce predetermined mortality rate. This method can be easily applied to various pathophysiological and metabolic studies on the nature of acute blood loss as well as in the treatment of acute blood loss.     

Volume regulation during dehydration of desert beetles

In arid areas in East Africa, dietary water is available only during the rainy seasons. Since the rainy seasons are separated by dry seasons, which may last for many months and in extreme cases for more than a year, the beetles may lose more than 80% of their body water. The water loss takes place mainly at the expense of the extracellular fluid, i.e. as the haemolymph volume drops to zero, the cell volume is only moderately reduced. The protection of cell volume at the expense of the haemolymph requires that solutes are removed from the haemolymph. The solutes are either excreted from the body or sequestered within the body in an osmotically inactive state. In predatory beetles of the family Carabidae, where Na is the dominating extracellular solute, Na is excreted, but it can easily be replaced from the diet. In most herbivorous beetles, such as the Tenebrionidae, which feed on a low Na diet, and which have low extracellular Na levels, Na is usually, but not always, deposited within the body. Free amino acids are moved from haemolymph to cells, but some seem to be made osmotically inactive by polymerization to peptides. As beetles become rehydrated, the peptides are rapidly depolymerized and the amino acids released to the haemolymph. Another factor, which may be important in the stabilisation of cell volume, is the colloid osmotic contribution of intracellular proteins, which may have a steep increase in their osmotic activity with increasing concentration.     

Effects of saline infusion and acute metabolic acidosis and alkalosis on water and electrolyte transport in the human colon.

Both the kidney and colon secrete bicarbonate and transport water and electrolytes. The respective contributions of these two organs to acid-base and electrolyte balance in normal man has thus been studied in eight healthy male volunteers who underwent simultaneous renal clearance studies, and colonic perfusion with a 0.9% saline or 7.2% mannitol solution, during metabolic alkalosis and acidosis, extracellular volume expansion, and control conditions. There was no influence of these acid-base conditions on electrolyte transport in the colon. In the urine, preferential loss of chloride over sodium averaged 81, 143 (P less than 0.001), and 141 (P less than 0.05) muequiv./min, during control, metabolic acidosis, and extracellular volume expansion conditions, respectively. During alkalosis more sodium than chloride was lost (146 muequiv./min) (P less than 0.001). Colonic pH averaged 7.41 during saline and 6.75 (P less than 0.005) during mannitol perfusion. Titratable acid was not produced in the colon during saline perfusion, and averaged 18 muequiv./min during mannitol perfusion. Urinary titratable acid increased from 19 to 25 muequiv./min (P less than 0.01) during volume expansion. With saline perfusion, bicarbonate secretion rate in the colon rose from 249 muequiv./min during control conditions to 289 muequiv./min during metabolic alkalosis (P less than 0.05). More bicarbonate was excreted in the urine during alkalosis when mannitol was introduced in the colon (243 muequiv./min) than when saline was perfused (152 muequiv./min) (P less than 0.05). This study indicates that the response of the human colon is trivial compared with that of the kidney during acute changes in acid-base balance.     

Apportioning protein requirements for maintenance v. growth for blue-breasted quail (Excalfactoria chinensis) from 7 to 21 days of age

The aim of this study was to investigate protein requirements for the maintenance and growth of blue-breasted quail (Excalfactoria chinensis) from 7 to 21 days of age. A total of 180 quails, 7 days old, were randomly assigned to 36 cages and for 2 weeks were fed diets with a metabolisable energy concentration of 12.13 MJ/kg and a dietary CP concentration of 125, 150, 175, 200, 225 or 250 g/kg. The average BW per cage and the feed intake per cage were recorded daily. The results showed that quails fed 125 g/kg CP could not maintain their BW and had negative feed efficiency. There were linear and quadratic relationships between CP level and response criteria, including BW, weight gain, feed intake, feed efficiency, final body nitrogen mass and body nitrogen accretion (P<0.05). The dietary CP requirements, as calculated using a one-slope quadratic broken-line model, were 211 and 202 g/kg according to weight gain and feed efficiency, respectively. The regression equations, on the basis of metabolic BW, of daily weight gain on daily protein intake according to the model were Y=0.137-2.128(0.113-X) if X<0.113 and Y=0.137 if X>or=0.113 (R2=0.96, P<0.001), which meant that the protein requirement for maintenance was 0.049 times the metabolic BW and that to gain 1 g weight quails needed to ingest an extra 0.47 g protein after the maintenance requirement was satisfied. The regression equations, on the basis of metabolic BW, of daily body nitrogen accretion on daily protein intake according to the model were Y=5.667-76.700(0.119-X) if X<0.119 and Y=5.667 if X>or=0.119 (R2=0.95, P<0.001), which meant that quails had to receive an amount of protein equal to their metabolic BW multiplied by 0.045 to satisfy the requirement for maintenance and then ingest an extra 13 g protein to accrete 1 g body nitrogen. In conclusion, growth or protein accretion rates should be regulated according to dietary CP for specific experimental purposes via apportioning protein requirements for maintenance v. growth.     

No effect of isolation on blood pressure and daily electrolyte excretion in rats

The effects of isolation stress on mean blood pressure (BP) and on body weight, water and food intake as well as on urine flow, urinary sodium and potassium excretion were studied in CFY and Long Evans rats. During a 7 day isolation period, food and water intake as well as urine flow, urinary sodium and potassium excretion, as expressed for 100 g body weight, were not changed in either group. Body weight increased similarly in isolated (38 +/- 2 g) and aggregated (41 +/- 5 g) CFY rats. Compared to group housed rats, BP in male CFY animals was not increased after a 7 day isolation (111 +/- 3 vs 111 +/- 3 mmHg, NS). In additional experiments high sodium intake by physiological saline drinking slightly elevated blood pressure but failed to induce arterial hypertension in isolated rats (118 +/- 2 vs 121 +/- 3 mmHg, NS). We conclude that, contrary to some reports from other laboratories, isolation stress has no detectable effect on BP and/or water and electrolyte balance.     

Partial desiccation induced by sub-zero temperatures as a component of the survival strategy of the Arctic collembolan Onychiurus arcticus (Tullberg)

The mechanism by which the freeze susceptible Arctic collembolan Onychiurus arcticus survives winter temperatures of -25 degrees C in the field is not fully understood but exposure to sub-zero temperatures (e.g. -2.5 degrees C) is known to induce dehydration and lower the supercooling point (SCP). In this study, changes in the water status and certain biochemical parameters (measured in individual Collembola) during a 3-week exposure to decreasing temperatures from 0 to -5.5 degrees C were studied. Osmotically active and inactive body water contents were measured by differential scanning calorimetry (DSC), water soluble carbohydrates by high performances liquid chromatography (HPLC) and glycogen by enzymatic assays. The activity of trehalase and trehalose 6-phosphate synthase were also measured. During the experiment, total water content decreased from 70 to 40% of fresh weight, mostly by the loss of osmotically active water with only a small reduction in the osmotically inactive component. The SCP decreased from -7 to -17 degrees C. Analysis of the results shows that if O. arcticus is exposed to -7 degrees C in the presence of ice, all osmotically active water would be lost due to the vapour pressure gradient between the animals supercooled body fluids and the ice. Under these conditions the estimated SCP would reach a minimum of c. -27 degrees C, but the Collembola may never freeze as all the osmotically active water has been lost, the animal becoming almost anhydrobiotic. Trehalose concentration increased from 0.9 to 94.7&mgr;g mg(-1)fw while glycogen reserves declined from 160 to 7.7 nmol glucose equivalents mg(-1) protein. Trehalase activity declined as the temperature was reduced, while trehalose 6-phosphate activity peaked at 0 degrees C. By adopting a strategy of near anhydrobiosis induced by sub-zero temperatures, O. arcticus, which was previously thought to be poorly adapted to survive severe winter temperatures, is able to colonise high Arctic habitats.     

Effects of wire-bottom caging on heart rate, activity and body temperature in telemetry-implanted rats

Some experimental procedures are associated with placement of animals in wire-bottom cages. The goal of this study was to evaluate stress-related physiological parameters (heart rate [HR], body temperature [BT], locomotor activity [LA], body weight [BW] and food consumption) in rats under two housing conditions, namely in wire-bottom cages and in bedding-bottom cages. Telemetry devices were surgically implanted in male Sprague-Dawley rats. HR, BT and LA were recorded at 5 min intervals. Analysis under each housing condition was performed from 16:00 to 08:00 h of the following day (4 h light, 12 h dark). During almost all of the light phase, the HR of rats housed in wire-bottom cages remained high (371 ± 35 bpm; mean ± SD; n = 6) and was significantly different from that of rats housed in bedding-bottom cages (340 ± 29 bpm; n = 6; P < 0.001; Student's t-test). In general, BT was similar under the two housing conditions. However, when rats were in wire-bottom cages, BT tended to fluctuate more widely during the dark phase. LA decreased when animals were housed in wire-bottom cages, in particular during the dark phase. Moreover, there was a significant difference with respect to the gain in BW: BW of rats housed in bedding-bottom cages increased 12 ± 2 g, whereas that of rats in wire-bottom cages decreased by 2 ± 3 g (P < 0.001). Our results demonstrate that housing rats in wire-bottom cages overnight leads to immediate alterations of HR, BW and LA, which might be related to a stress response.     

Polymorphisms in the 5' regulatory region of myostatin gene are associated with early growth traits in Yorkshire pigs

Myostatin is a negative regulator of skeletal muscle mass. The present study cloned the 5' regulatory region of porcine myostatin gene, screened its polymorphisms and analyzed their associations with early growth traits in Yorkshire pigs. The results indicated that a fragment length polymorphism and a polymorphism concerning two nucleotide changes exist in the 5' regulatory region of porcine myostatin gene. At sites 435 and 447, allele A and allele B have the haplotypes of A-G and G-A, respectively. The allelic frequency of B is 0.475 in Yorkshire pigs. No homozygous BB genotype was detected in 9 Laiwu Black pigs. Allele B was found to have positive effect on body weight on day 21 (BW21) (P＜0.01), body weight on day 28 (BW28) (P＜0.05), body weight on day 70 (BW70) (P＜0.05), average daily gain from birth to 21 d (ADG1) (P＜0.01), average daily gain from birth to 28 d (ADG2) (P＜0.05) and average daily gain from 21 d to 70 d (ADG3) (P＜0.01), respectively. The additive effect of allele B on BW21, BW28, BW70, ADG1, ADG2 and ADG3 was 0.596±0.205 kg (P=0.0041), 0.498±0.200 kg (P=0.0136), 1.409±0.551 kg (P=0.0112), 28.39±9.74 g P=0.0041), 17.78±7.15 g (P=0.0136) and 37.00±16.92 g (P=0.0304), respectively, whereas its effect on average daily gain from 28 d to 70 d (ADG4) was not significant (P＞0.1), although BB individuals are superior in average daily gain to AA and AB.     

Production and composition of Iberian sow's milk and use of milk nutrients by the suckling Iberian piglet

Sixteen purebred Iberian (IB) sows were used in two consecutive trials to determine the efficiency of conversion of sow's milk into piglet body weight (BW) gain and the relationship between milk protein and body protein retention and between milk energy yield and body energy retention in the nursing IB piglet. In each trial, four sows were selected in order to evaluate their milk production, litter growth and nutrient balance measurements, together with four additional sows for milk sampling. Litter size was equalized to six piglets. Daily milk yield (MY) was determined weekly by the weigh-suckle-weigh technique over a 34-day lactation period. Piglets were weighed individually at birth and then weekly from day 5 of lactation. Milk samples were collected on days 5, 12, 19, 26 and 34 post partum. The comparative slaughter procedure was used to determine piglet nutrient and energy retention. One piglet from each litter was slaughtered at birth and four on the morning of day 35. Total MY was on average 5.175 ± 0.157 kg/day. The average chemical composition (g/kg) of the milk was 179 ± 4 dry matter, 53.4 ± 1.0 CP, 58.5 ± 3.8 fat, 10.4 ± 0.3 ash and 56.9 ± 2.3 lactose. Milk gross energy (GE) was 4.626 ± 0.145 MJ/kg. Milk intake per piglet tended to increase in trial 2 (832 v. 893 g/day; P = 0.066). Piglet BW gain contained (g/kg) 172.1 ± 1.3 protein, 151.5 ± 3.5 fat, 41.4 ± 0.6 ash and 635 ± 3 water and 10.127 ± 0.126 MJ GE/kg. Throughout the 34-day nursing period, the piglets grew at an average rate of 168 ± 3 g/day. The ratio of daily piglet BW gain to daily MY was 0.195 ± 0.002 g/g and the gain per MJ milk GE intake was 41.9 ± 0.5 g/MJ. The overall efficiency of protein accretion (g CP gain/g CP milk intake) was low and declined in trial 2 (0.619 v. 0.571; P = 0.016). Nutrient and energy deposition between birth and weaning were 27.4 ± 0.5 g/day protein, 24.2 ± 0.8 g/day fat and 1615 ± 40 kJ/day energy. Piglet energy requirements for maintenance were 404 kJ metabolizable energy (ME)/kg BW0.75. ME was used for growth with a net efficiency of 0.584. These results suggest that poor efficiency in the use of sow's milk nutrients rather than a shortage in milk nutrient supply might explain the low growth rate of the suckling IB piglet.     

Are extracellular osmolality and sodium concentration determined by Donnan effects of intracellular protein charges and of pumped sodium?

Although we are used to attribute almost identical extracellular fluid (ECF) sodium concentrations in birds, amphibians, reptiles, and mammals to the composition of the primordial oceans in which, presumably, all life originated, this interpretation is not supported by geological data suggesting that the ocean salinity was never much lower than the present-day values, still four times higher than our plasma sodium.Here presented interpretation is that the similar ECF salt concentrations are dictated by the opposed Donnan effects on the cell membrane. The only way for the cell to reach the osmotic equilibrium is to alter cell volume, until concentration of nondiffusible intracellular ions (mainly charges on intracellular proteins) is equal to the ECF restricted ions (mainly Na⁺ ions, restricted by pumping out of cells).The achievement of electroneutrality requires that the sum of all anions equals concentration of positive ions in the cell (mainly K⁺). Negative charges on cytoplasmic proteins are the most stable component among ionized particles and other ions have to adapt to their concentration. Positive and negative soluble intracellular ions are all osmotically active and to achieve balance of osmotic forces on the cell membrane, the sum of their intracellular concentrations must equal the concentration of osmotically active extracellular particles. Since almost half the osmotically active ECF particles are sodium ions, the ECF sodium concentration seems related to concentration of charges on cytoplasmic proteins and concentration of intracellular phosphates.Our ancestors could not leave the salty ocean and move to brackish, or even fresh waters, without adequate regulation of their ECF sodium concentration and osmolality. Concentration of charges on cytoplasmic proteins or of intracellular phosphate buffers could not be altered, since this would compromise cell functioning. The remaining solution was to maintain the lowest ECF Na⁺ concentration effective in counteracting the average Donnan effect of charges on cytoplasmic proteins. When the optimal ECF sodium concentration had once become the reference point for osmoreceptors (controlling thirst and ADH secretion) and other regulatory mechanisms (secretion of renin/angiotensin/aldosterone, natriuretic factors), it made an important survival advantage that allowed spreading of animal life in fresh water and conquering of earth. The actual common value had to be a compromise that reduces the average osmotic burden on body cells to zero. Individual cells can reduce eventual residual osmotic forces on their membrane through altering cell volume by chloride shift, and by modulating the Na⁺K⁺-ATPase function.