Osmoregulatory responses of glaucous-winged gulls (Larus glaucescens) to dehydration and hemorrhage

The effects of dehydration and hemorrhage on plasma ionic, osmotic, and antidiuretic hormone (arginine vasotocin) concentrations and of hemorrhage on salt gland secretion and glomerular filtration rate were evaluated in glaucous-winged gulls, Larus glaucescens. Dehydration for 24 h did not affect plasma ionic, osmotic or arginine vasotocin concentrations; 72 h dehydration significantly elevated plasma osmolality, plasma sodium and chloride concentrations, and plasma arginine vasotocin concentration, but did not affect plasma potassium concentration. Constant infusion of 0.8 mol·l^-1 NaCl increased plasma arginine vasotocin concentration and produced salt gland secretion in seven gulls; four secreted well, while three secreted less well. Removal of 20% blood volume during saline infusion immediately reduced (P<0.001) salt gland secretion rate in all gulls. After bleeding, good secretors maintained glomerular filtration rate and urine flow rate; the poorer secretors increased glomerular filtration rate and became diuretic. Blood replacement returned salt gland secretion rate to the prebleeding level (P<0.05) without affecting salt gland secretions sodium concentration in gulls which secreted well, but did not restimulate salt gland secretion in gulls which secreted poorly. Reinfusion of blood had no effect on glomerular filtration rate. Bleeding and blood replacement did not affect plasma arginine vasotocin concentration.Abbreviations AVT arginine vasotocin - ECF extracellular fluid - ECFV extracellular fluid volume - EDTA ethylenediaminetetra-acetate - EWL evaporative water loss - GFR glomerular filtration rate - Hct hematocrit - LB large blood sample - [Na⁺]_pl plasma sodium concentration - Osm_pl plasma osmolality - PEG polyethylene glycol - RH relative humidity - RIA radioimmunoassay - SB small blood sample - SGS salt gland secretion - T _a ambient temperature - TFA trifluoroacetic acid - UFR urine flow rate     

Renal compensation for cecal loss in Gambel's quail (Callipepla gambelii)

Some studies have implicated the avian digestive cecae as important sites of water and solute reclamation working in concert with the lower intestine and the kidneys as part of an integrated osmoregulatory system. In Gambel's quail (Callipepla gambelii), we studied compensatory adjustments in renal function on days 6–7 and 16–17 following ligation of cecae. Plasma osmolality (P_osm) varied significantly between groups with sham-operatd birds (Cs), with an average (P_osm) of 348 mOsm/kg H₂O and quail with ligated cecae (Cx) having a (P_osm) of 355 mOsm/kg H₂O. We detected no change in the rate of glomerular filtration (GFR) between experimental and control groups either shortly after cecectomy or after 16–17 d following surgery. Regression analysis of GFR and urine flow rate (V) showed that Cx birds had a significantly lower V at a given GFR than did controls, evidence that Cx quail absorbed more fluid in their renal tubules. Increased fluid reabsorption was apparently driven by an enhanced reabsorption of sodium. Indeed, sodium excretion was lower in Cx quail as compared to sham-operated birds. On days 6–7, Cx quail drank more water than Cs birds, but by days 16–17 drinking rates were similar. At the end of the experiments, Cx quail showed a proliferation of microvilli along the apical membrane of the rectum, an adjustment consistent with the idea that the rectum alters its absorption capacity to adjust for the loss of cecal function.     

Renal function and organic anion and cation transport during dehydration and/or food restriction in chickens

The effect of dehydration in the presence or absence of continued food intake on renal function was evaluated in chickens. In addition, renal transport of organic anions and cations under these conditions was assessed in vitro by uptake of ¹⁴C-para-aminohippuric acid and ¹⁴C-tetraethylammonium bromide by renal slices. Water restriction with continued food intake resulted in increases in serum osmolality and serum concentrations of sodium, uric acid, calcium and total protein. If food was restricted in addition to water, only serum osmolality and sodium concentration were significantly increased after 48 hours. Dehydration with continued access to food resulted in marked decreases in extracellular fluid volume, glomerular filtration rate and effective renal plasma flow. If food was restricted during dehydration, the decrease in effective renal plasma flow was attenuated despite reductions in glomerular filtration rate and extracellular fluid comparable to that seen in dehydrated birds allowed free access to food. Transport of organic anions was significantly increased after 24 and 48 hours of water restriction, regardless of whether food was withheld. Enhanced transport of organic anions in the presence of decreased glomerular filtration rate and effective renal plasma flow during dehydration may promote precipitation of urates and nephrosis in chickens.Abbreviations cpm counts per minute - dpm disintegrations per minute - ECF extracellular fluid - ERPF effective renal plasma flow - GFR glomerular filtration rate - PAH para-aminohippuric acid - SEM standard error of the mean - TEA tetraethylammonium bromide     

Contributions of the kidneys and intestines to water conservation,and plasma levels of antidiuretic hormone,during dehydration in house sparrows (Passer domesticus)

Summary The contributions of the kidneys, the small intestine and the lower intestine (rectum plus cloaca) to water conservation during dehydration in unanaesthetized, unrestrained house sparrows (Passer domesticus) were assessed. Thirty hours of acute dehydration resulted in a 12% loss in body mass and a significant increase in plasma osmolality. Glomerular filtration rate declined by 55%, from 7.7 to 3.5 ml/h, and urine flow rate delined by more than 80%, from 0.2 to 0.03 ml/h. These changes are likely attributable to a large increase in plasma levels of arginine vasotocin during dehydration, from <26 pg/ml in hydrated birds to greater than 200 pg/ml after 30 h dehydration. Flow of water from the ileum to the lower intestine was reduced during dehydration, primarily because of a reduced flow of dry matter (with no significant reduction in water content). The rate of water loss in the excreta declined from 0.2 ml/h in hydrated birds to 0.04 ml/h in dehydrated birds. The rate of water reabsorption in the lower intestine (equal to the rate of water loss in the excreta minus the combined rates of inflow into the lower intestine from the urine and the ileal contents) slightly exceeded the rate of water flow from the ileum in both hydrated and dehydrated birds. We suggest that much of the water reabsorbed in the lower intestine of hydrated birds derives from the urine, but that primarily water from ileal contents is reabsorbed in dehydrated birds. That is, urine undergoes significant post-renal modification in hydrated but not dehydrated house sparrows.     

The effect of maternal fluid intake on the volume and composition of fetal urine

The effects on fetal renal function of restricting maternal water intake to 1 l/day for 6 days was investigated in 7 chronically-catheterized fetuses (gestation age 118-131 days). Restriction of water intake caused a significant decrease in maternal urine flow rate and significant increases in maternal plasma and urinary osmolality. Fetal renal function was investigated on the third and sixth days of the period of restricted maternal intake of water. Urine flow rate from the fetus was depressed significantly, and urinary osmolality increased significantly. The glomerular filtration rate remained unchanged, and free water clearance was decreased. These changes indicate increased water reabsorption in the distal parts of the nephron, probably consequent upon increased circulating levels of antidiuretic hormone. In 3 fetuses whose mothers subsequently had free access to water, these changes in urine flow rate and free water clearance that occurred during water restriction were reversed. There was an inverse correlation between maternal plasma osmolality and fetal free water clearance corrected for glomerular filtration rate. It is concluded that when water intake by a pregnant animal is restricted, the availability of water to the fetus is reduced and fetal sheep respond by producing a concentrated urine.     

Micromanipulation of male and female gametes ofNicotiana tabacum: II. Preliminary attempts for in vitro fertilization and egg cell culture

This research is part of an attempt to establish an in vitro fertilization system in tobacco to aid in understanding mechanisms of fertilization. Fusions of isolated male and female gametes were induced in a polyethylene glycol solution. Fusion appears similar to that in maize. One nuclear division of both an unfertilized egg cell and a synergid was induced in KM8p medium with 1 mg/l 2,4-dichlorophenoxyacetic acid in a microchamber culture; one cellular division of the egg cell was also induced in the same medium in solid-drop culture. The osmolality of suspension culture feeder cells was critical for the development of these cells. These results indicate that in vitro fertilization is possible in tobacco, which would be the first such system in dicots.Abbreviations 2,4-D 2,4-Dichlorophenoxyacetic acid - PEG Polyethylene glycol     

Renal function and plasma arginine vasotocin during an acute salt load in feral chickens

Summary Renal clearance experiments were conducted on feral chickens descended from birds collected from a coral island off the coast of Queensland, Australia. Following a control period when 0.13 M NaCl was used as a vehicle for the renal function markers, a salt load was imposed by infusion of 1 M NaCl. The hypertonic NaCl infusion resulted in increases in glomerular filtration rate (GFR), effective renal blood flow (ERBF), and urine flow rate (V), whereas filtration fraction decreased. Haematocrit was reduced and plasma osmolality, sodium, chloride and potassium concentrations increased. Plasma arginine vasotocin (P_AVT) levels increased from 31.4±2.3 pg·ml^-1 during the control infusion to 56.0±1.7 pg·ml^-1 following a salt load of 12 mmol Nacl·kg^-1 The sensitivity of release of AVT was 0.69±0.11 pg·ml^-1 per mosmol·kg^-1. The concentrations of Na and Cl in urine increased, whereas urine osmolality and potassium concentration decreased. The expansion of the extracellular fluid volume induced by the salt loading would tend to suppress the release of AVT, whereas the osmotic stimulus would provide a stimulus for the release of AVT. In this study, GFR, ERBF and ERPF increased at the same time as P_AVT increased.Abbreviations AVP arginine vasopressin - AVT arginine vasotocin - ERBF effective renal blood flow - ERPF effective renal plasma flow - GFR glomerular filtration rate - P_avt plasma arginine vasotocin concentration - PAH paraaminohippuric acid - SEM standard error of mean - SNGER single nephron glomerular filtration rate - U/P urine to plasma ratio - V urine flow rate     

Fractional reabsorption of calcium,magnesium and phosphate in the kidneys of freshwater North American eels (Anguilla rostrata LeSueur) following removal of the corpuscles of Stannius

Renal function was observed in freshwater North American eels (Anguilla rostrata LeSueur) 2 weeks after the removal of the corpuscles of Stannius. There was a positive linear correlation between glomerular filtration rates and urine flow rates in both sham-operated and stanniectomized eels but there was no difference in slope or elevation between the two groups nor did urine flow rates ever exceed glomerular filtration rates. Osmolar clearance and free-water clearance were unchanged following stanniectomy. Plasma Ca²⁺ and K⁺ concentrations increased and plasma Mg²⁺, phosphate, Na⁺ and Cl^- concentrations decreased following stanniectomy. Plasma ultrafilterable Ca increased and ultrafilterable Mg decreased after stanniectomy but neither changed relative to its total plasma concentration. Stanniectomy was followed by a decreased renal tubular reabsorption of Mg²⁺ relative to the amount filtered (C _Mg/C _In); the same applies to C _Na/C _In. Even though the filtered load of Ca increased in conjuction with the predictable hypercalcemia, there was no change in the fraction of filtered Ca reabsorbed. Net tubular secretion of phosphate was observed in both sham-operated and stanniectomized cels together with a slight increase in C _phos/C _In following stanniectomy. Some or all of these changes in plasma electrolytes and/or the modified renal transport of Na⁺, Mg²⁺ and possibly phosphate may be caused by the changes in cardiovascular function that were recently shown to follow stanniectomy.Abbreviations CSX Stanniectomized/stanniectomy - ER endoplasmatic reticulum - FW fresh water - GFR glomerular filtration rate - Pi inorganic phosphate - RAS renin-angiotensin system - SHM sham-operated - SW sea water - UFR urine flow rate - U/P ratio urine/plasma ratio     

Fasting endurance and cold resistance without hypothermia in a small predatory bird: the metabolic strategy of Tengmalm's owl,Aegolius funereus

The energetic adaptations of non-breeding Tengmalm's owls (Aegolius funereus) to temperature and fasting were studied during the birds' autumnal irruptions in western Finland. Allometric analysis (including literature data and two larger owl species measured in this study) indicates that the basal metabolic rate of owls is below the mean level of non-passerine birds. However, the basal metabolic rate of the 130-g Tengmalm's owl (1.13 W) is higher than in other owls of similar size. This is probably related to its northern distribution and nomadic life history. Relative to its size, Tengmalm's owl has excellent cold resistance due to effective insulation (lower critical temperature +10°C, minimum conductance 0.19 mW·cm^-2·°C^-1). Radiotelemetric measurements of body temperature showed that the level of body temperature is lower than for birds in general (39.4°C at zero activity) and that the amplitude of the diurnal cycle is also low (0.2–0.6°C). In contrast to many other small birds, Tengmalm's owls do not enter hypothermia during a 5-day fast at thermoneutrality or in cold. Moreover, while the metabolic rate per bird shows the expected mass-dependent decrease, the mass-specific rate decreases only slightly during the fast. In line with this, there was no decrease in the plasma triiodothyronine concentration during the fast in the owl, whereas a dramtic drop was observed in the pigeon and Japanese quail that were used as a reference. Despite this, the owl has an excellent capacity for fasting because of its ability to accumulate extensive fat depots and its low overall metabolic rate. Fasting reduced evaporative water loss to 50% of that in the fed state. Calculations show that the oxygen consumption observed in fasting birds would involve a production of metabolic water barely sufficient to compensate for evaporative water loss. The threat of dehydration may thus set a limit to the decrease in metabolic rate in fasting owls (owls rely totally on water either ingested with food or produced metabolically). We conclude that the metabolic strategy in Tengmalm's owl is largely dictated by an evolutionary pressure for fasting endurance. With the restrictions set by small body size and water economy, this bird has apparently taken these adaptations to an extreme. The constraints that preclude hypothermia, which could increase the capacity for fasting even more, remain unknown.Abbreviations BM body mass - BMR basal metabolic rate - EWL vaporative water loss - MR metabolic rate - T₃ triiodothyronine - T _a ambient temperature - T _b body temperature - VO₂ oxygen consumption     

Renal function in red wattlebirds in response to varying fluid intake

Red wattlebirds (Anthochaera carunculata) are among the more nectarivorous of the Australian honeyeaters (family Meliphagidae). As such, they potentially ingest large and dilute fluid loads as food, and they produce copious dilute urine in the field. We examined in the laboratory the renal mechanisms by which such fluid loads are processed. Wattlebirds received one of three liquid diets [a mix of honey, water, and Complan (Boots) complete dietary supplement] of varying concentration (250, 1000, and 1750 mmol/kg, Na⁺/K⁺ concentrations of 4/4, 12/15, and 23/30 mmol/l, respectively). We measured renal function via infusion of a filtration marker (¹⁴C-polyethylene glycol) from osmotic minipumps implanted intraperitoneally. Wattlebirds consumed volumes of the three diets sufficient to provide nearly equal caloric intakes (approximately 200 kJ/day), and as a consequence had water turnover rates varying from 30 to 200 ml/day (approximately 50–335% of total body water per day). Renal function in the morning, before feeding, did not differ among diet groups (glomerular filtration rate =18 ml/h, urine flow rate =0.4 ml/h). In the afternoon, after feeding, urine flow did vary, from 3 ml/h in birds on the most concentrated diet to 6 ml/h on the most dilute. This was accomplished by varying the rate of tubular reabsorption of water (from a high of >90% on the most concentrated diet to a low of just over 70% on the most dilute), with little variation in the rate of glomerular filtration (mean ∼23 ml/h). Comparisons between dietary intakes and urinary outputs of water and electrolytes suggest that not all dietary water was absorbed from the gut, but that there was significant post-renal reabsorption of Na⁺. The reduced fractional water reabsorption on the dilute diet was accompanied by a decrease in the circulating concentration of arginine vasotocin (from >4 pg/ml in birds on the two more concentrated diets to <1 pg/ml on the most dilute diet). In contrast, concentrations of aldosterone (10–20 pg/ml) did not differ among diets. Perhaps in consequence, renal fractional absorption of Na⁺ also did not differ, and so birds on the dilute diet, with their higher urine flows, had higher rates of Na⁺ excretion and suffered a decreased concentration of Na⁺ in the plasma. Accepted: 14 January 1988     

Metabolism during starvation/dehydration stress in two species of galagos

Two species of galagos (G. senegalensis moholi andG. garnettii) were subjected to dehydration and starvation stress in order to determine whether, as is common in other animals, these hypometabolic prosimians would lower their metabolic rate even further. Dehydration was confirmed by losses in body mass, a decrease in fecal water content and a rise in urine osmolality. At the height of dehydration, 20 to 25% reduction in body mass, 30 to 40% reduction in fecal water content and urine osmolality ranging from 1.8 to 3.5 Osmol kg⁻¹ H₂O, were recorded in some of the animals. Basal metabolic rate of 0.536 ml O₂ (g·h)⁻¹ inG. s. moholi and 0.302 ml O₂ (g·h)⁻¹ inG. garnettii were recorded, representing 50 to 42% reduction in metabolic rate, respectively, compared with mass specific values. In none of the tested animals did we observe significant reduction in basal metabolism during dehydration/starvation stress compared with the rates observed during the control period. Basal metabolism in the bushbabies seems to have reached the lowest level and no further adjustment is apparently possible as a strategy for energy saving during starvation and/or dehydration stress.     

Kidney structure and function of obligate and facultative hibernators: the white-tailed prairie dog (Cynomys leucurus) and the black-tailed prairie dog (Cynomys ludovicianus)

The white-tailed prairie dog is an obligate hibernator that enters a heterothermic phase when maintained in the cold with low intensity light and ad libitum food and water. The black-tailed prairie dog (a facultative hibernator) will not hibernate under similar conditions. It has been suggested that the black tailed prairie dog remains active during the winter because it can conserve water more effectively due to a more efficient kidney. The present study revealed no significant differences between the species in renal morphology: relative medullary thickness, nephron heterogeneity, renal vasculature, or fornix dimensions, all of which are structures associated with the urinary concentrating mechanism. In addition, there was no difference in number of nephrons between the two species. The black-tailed prairie dog does produce a more concentrated urine when food and water deprived. However, this difference was not observed when the animals were salt loaded. The water-deprivation and salt-loading experiments suggest that the higher urine osmolality produced by the back-tailed prairie dog during fasting is a result of a higher urea load due to a greater protein catabolism and not because of a differential capacity to concentrate urine.Abbreviations C cortex - GFR glomerular filtration rate - H height - IS inner stripe - IZ inner zone of medulla - L length - OS outer stripe - PE polythylene - RMT relative medullary thickness - T _a ambient temperature - W width     

Renal,respiratory and ionic regulation in Atlantic salmon (Salmo salar L.) kelts following transfer from fresh water to seawater

Summary Atlantic salmon may return to the sea after spawning in fresh water. These fish, known as kelts, reportedly show a limited ability to hypoosmoregulate. However, this study shows that fresh-water-adapted kelts exposed to seawater demonstrate rapid adaptation (within 48 h) in osmoregulatory parameters to values characteristic of seawater-adapted salmonids. The urine flow rate falls from 1.2 to 0.2 ml·kg^-1·h^-1 within 24 h. Over the same period, urine osmolality increases from 48 mosmol·kg^-1 to become isosmotic with the plasma, and Mg²⁺ secretion by the kidney tubules elevates the urine concentration from 0.5 to 100 mmol·l^-1. As is characteristic for marine teleosts, kelts drink seawater and process the ingested water in the gut to replace body water lost by osmosis to the hyperosmotic medium. Seawater exposure causes a marked hypoxia, arterial oxygen tension falling by 43% within minutes and persisting for at least 4 days at this low level. This is associated with large changes in blood pH and acid-base balance. The physiological mechanisms involved in adaptation to a hyperosmotic external medium are discussed, and the osmoregulatory capacity of kelts is compared with that of salmon at other stages of the life cycle.Abbreviations FW fresh water - GFR glomerular filtration rate - Hb haemoglobin - Hct haematocrit - MCHC mean cell haemoglobin concentration - pH_a pH in arterial blood - P _aO₂ partial pressure of oxygen in arterial blood - SEM standard error of mean - SW seawater - UFR urine flow rate     

Renal response to saline infusion in chicks of Leach's storm petrel (Oceanodroma leucorhoa)

The renal response to infusion of three different saline solutions was studied in chicks of Leach's storm petrel (Oceanodroma leucorhoa). Each of the solutions (125 mmol·1^-1 NaCl at 5.3 ml·h^-1, 250 mmol·l^-1 NaCl at 2.6 ml·h^-1, and 550 mmol·l^-1 NaCl at 1.2 ml·h^-1) provided the same delivery of Nacl but in different volumes of water. Birds infused with 125 mmol·l^-1 NaCl had a glomerular filtration rate of 25.7 ml·h^-1, a urine flow rate of 4.4 ml·h^-1, and excreted 71% of the infused Na⁺ load in the urine. With infusion of 250 mmol·l^-1 NaCl, the glomerular filtration rate was unchanged (23.3 ml·h^-1), but urine flow rate was reduced to 0.93 ml·h^-1 and only 35% of the Na⁺ load was excreted in the urine. Infusion of 550 mmol·l^-1 NaCl induced a sharp decrease in glomerular filtration rate (to 3.8 ml·h^-1) and urine flow rate (to 0.08 ml·h^-1), and only 1.4% of the infused Na⁺ was excreted in the urine. The contribution of different nephron populations to filtration was assessed by the pattern of staining of glomeruli by alcian blue infused during the last 30 min of the saline infusion. The numbers of stained glomeruli did not differ between birds infused with 125 and 250 mmol·l^-1 NaCl (59000 and 55000 glomeruli per kidney, respectively), and the patterns of staining were similar for birds in these two groups. Birds infused with 550 mmol·l^-1 NaCl had lighter staining overall and fewer stained glomeruli (37000 per kidney). This absence of staining was predominant in the smaller size classes of glomeruli, suggesting a selective shutdown of smaller (reptilian-type) nephrons during times of osmotic challenge in these birds. This may be part of an overall suite of water-conserving strategies employed by these chicks during their long confinement with irregular feeding in the nesting burrow.Abbreviations ADH antidiuretic hormone - GFR glomerular filtration rate - MT mammalian-type - P plasma inulin concentration - RT reptilian-type - U urine inulin concentration - V urine flow rate     

Effects of caecal ligation and saline acclimation on plasma concentration and organ mass in male and female Pekin ducks,Anas platyrhynchos

Summary The intestinal caeca reabsorb urinary sodium chloride (NaCl) and water (Rice and Skadhauge 1982). Free water may be generated if the reabsorbed NaCl is secreted via salt gland secretion (Schmidt-Nielsen et al. 1958). Therefore ceacal ligation should (a) reduce hingut NaCl and water reabsorption, (b) enhance the increase in plasma osmolality during saline acclimation, and (c) affect drakes more than ducks. Twelve Pekin drakes and 13 Pekin ducks, Anas platyrhynchos, were caecally ligated or sham operated before acclimation to 450 mmol · 1 NaCl. Body mass, hematocrit, plasma osmolality, and inonic concentrations of plasma, cloacal fluid, and salt gland secretion were measured after each increase in drinking water salinity. Osmoregulatory organ masses were determined. Caecal ligation did not effect plasma osmolality or ion concentrations of plasma, cloacal fluid, or salt gland secretion, but reduced salt gland size in ducks. Drakes and ducks drinking fresh water had the same hematocrit, plasma osmolality, and plasma concentrations of Na⁺ and Cl^–. In both sexes exposure to 75 mmol · 1^-1 NaCl significantly decreased plasma [Na⁺] and doubled cloacal fluid [Na⁺]. Exposure to 450 mmol · 1^-1 NaCl decreased body mass and increased hematocrit, plasma [Na⁺], [Cl^–], and plasma osmolality (more in drakes than in ducks); cloacal fluid osmolality nearly doubled compared to freshwater-adapted ducks, due mainly to osmolytes other than Na⁺ and Cl^–. The [Cl^–] in salt gland secretion only slightly exceeded drinking water [Cl^–].Abbreviations AVT antiduretic hormone - CF cloacal fluid - ECFV extraoellular fluid volume - FW freshwater acclimated - Hct hematocrit - MDWE mean daily water flux - [Na ⁺]_cf cloacal fluid sodium concentration - [Na ⁺]_pl plasma sodium concentration - Osm _cf cloacal fluid osmolality - Osm _pl plasma osmolality - SGS salt gland secretion - TBW total body water     

Maximum Urine Concentrating Capability in a Mathematical Model of the Inner Medulla of the Rat Kidney

In a mathematical model of the urine concentrating mechanism of the inner medulla of the rat kidney, a nonlinear optimization technique was used to estimate parameter sets that maximize the urine-to-plasma osmolality ratio (U/P) while maintaining the urine flow rate within a plausible physiologic range. The model, which used a central core formulation, represented loops of Henle turning at all levels of the inner medulla and a composite collecting duct (CD). The parameters varied were: water flow and urea concentration in tubular fluid entering the descending thin limbs and the composite CD at the outer-inner medullary boundary; scaling factors for the number of loops of Henle and CDs as a function of medullary depth; location and increase rate of the urea permeability profile along the CD; and a scaling factor for the maximum rate of NaCl transport from the CD. The optimization algorithm sought to maximize a quantity E that equaled U/P minus a penalty function for insufficient urine flow. Maxima of E were sought by changing parameter values in the direction in parameter space in which E increased. The algorithm attained a maximum E that increased urine osmolality and inner medullary concentrating capability by 37.5% and 80.2%, respectively, above base-case values; the corresponding urine flow rate and the concentrations of NaCl and urea were all within or near reported experimental ranges. Our results predict that urine osmolality is particularly sensitive to three parameters: the urea concentration in tubular fluid entering the CD at the outer-inner medullary boundary, the location and increase rate of the urea permeability profile along the CD, and the rate of decrease of the CD population (and thus of CD surface area) along the cortico-medullary axis.     

The effect of intravenous hypertonic saline infusion on renal function and vasopressin excretion in sheep.

Conscious Merino ewes were given an intravenous hypertonic sodium chloride load of 4 mmol.min-1 for 100 min. This resulted in increases in urine flow, sodium and potassium excretion and plasma sodium concentration and osmolality. Urinary vasopressin output and solute-free water reabsorption increased and plasma renin activity declined. Renal plasma flow and glomerular filtration rate (GFR) rose, as did the solute clearance. The change in urinary osmolality was related to the initial urine osmolality such that when the initial urine osmolality was high the urine became more dilute, and vice versa. Tubular sodium reabsorption increased but the fractional reabsorption rate fell. It is suggested that the increase in GFR was at least partly due to the increase in AVP and that the electrolyte loss can be accounted for by the increase in GFR without necessarily involving AVP or other hormonal effects at the tubular level.     

Effect of water restriction on energy and water balance and osmoregulation of the fruit bat Rousettus aegyptiacus

The energy budget, water balance and osmoregulation of the fruit bat, Rousettus aegyptiacus, were studied during normal hydration and during water restriction (oven-dried apple diet). The water input and output were balanced during both normal hydration and water restriction. The kidney of the fruit bat is well adapted to handle the water load from its fruit diet by excreting large volumes (14% of the body mass per day) of dilute urine (113±25 mosmol·kg H₂P^-1) as well as reducing urine volume (-95%) and increasing urine osmotic concentration (555±280 mosmol·kg H₂O^-1) during water restriction. The haematocrit, plasma haemoglobin and total protein concentrations did not increase during water restriction and heat exposure, suggesting the conservation of plasma volume. Gross energy intake was not alfected by water restriction. However, digested energy intake and digestibility were significantly reduced. The effective regulation of energy and water budgets during water restriction suggests that the fruit bat can cope with seasonal climatic changes and with variable fruit supply during various seasons.Abbreviations BM body mass - DEI digested energy intake - EWL evaporative water loss - GEL gross energy intake - NH normal hydration - T _a ambient temperature - WR water restriction     

Renal excretion of urea in a small east African antelope, the dik-dik (Rhynchotragus kirkii)

1. A study on the renal handling of urea by the dik-dik antelope (Rhynchotragus kirkii) was conducted. 2. Plasma and urine samples were analysed for osmolality, urea and creatinine concentrations during dehydration and intra-ruminal loading of potassium and sodium solutions. 3. The glomerular filtration rate (GFR) of the dik-dik was found to be 182.6 +/- 11.7 ml/min/100 kg body mass. 4. Dehydration was observed to increase tubular urea reabsorption and increase plasma and urine osmolalities, but had no effect on the amount of urea filtered at the glomerulus. 5. Potassium loading increased both GFR and urine flow rate.     

Urinary concentrating defect in glutathione-depleted rats

The effect of an acute depletion of glutathione by diethyl maleate injection on renal concentrating function was examined in rats. The parameters tested were the concentration and dilution of urine, applying conventional clearance techniques. Tissue osmolality and Na+-K+ ATPase activity were also measured. Diethyl maleate treated rats showed a diminished renal glutathione concentration and an impairment in the glomerular filtration rate and in electrolyte and water excretion. Treated rats also showed a diminished urine-to-plasma osmolality ratio as compared with controls. The studies on free water formation revealed a marked difference between groups; these data were supported with a diminished medullary Na+-K+ ATPase and a diminished corticomedullary osmolality gradient in the treated rats. The studies suggest that one area of target cells of glutathione depletion is that of the ascending limb of Henle's loop.