The effects of reducing dietary nitrogen and of increasing sodium chloride intake on urea excretion and reabsorption and on urine osmolality in sheep.

Renal responses to reducing dietary nitrogen were studied in four ewes during intravenous infusion of arginine vasopressin. The fall in urea excretion and in plasma urea concentration was accompanied by significant reduction in GFR and in urine osmolality. The fraction of filtered urea reabsorbed increased despite reduction in the urea U/P concentration ratio and this increase was sustained when the urea U/P ratio was further reduced at higher urine flows observed when the drinking water was replaced with saline. This procedure also sustained the RPF which in the absence of additional salt was significantly reduced on the low protein diet. It is suggested that the fall in GFR and the increase in the fraction of filtered urea reabsorbsed may contribute to nitrogen economy and that the increase in fractional reabsorption and the reduction in urine osmolality on the low protein diet provided evidence of active reabsorption of urea by renal tubules.     

Studies on renal excretion of potassium in the dik-dik antelope

1. In a study on the renal handling of potassium by the dik-dik antelope, plasma and urine samples were analysed for potassium, sodium and creatinine concentrations and osmolality during dehydration and intra-ruminal loading of potassium solutions. 2. The fractional excretion of potassium was 0.64 during the control period and rose up to as high as 2.3 during potassium loading. Urinary osmolality and potassium concentration decreased as the urine volume increased but the total amounts of potassium excreted were independent of urine volume. 3. Potassium loading led to a steady increase in its urinary excretion but a decrease in plasma potassium concentration was observed. This observation casts doubt on the hypothesis that alterations in potassium intake produce parallel alterations in plasma potassium concentration (which supposedly stimulates or depresses potassium excretion) and thereby maintain potassium homeostasis. 4. A possible alternative signal for increased potassium excretion following increased intake is discussed.     

Renal function in suckling and fasting pups of the northern elephant seal.

Elephant seals fast for prolonged periods without access to water. This is made possible, in part, by reductions in urine production. However, the mechanisms involved in reducing urine production are not understood. In this study, glomerular filtration rate (GFR) was measured in five northern elephant seal pups (Mirounga angustirostris) via the inulin clearance technique. Measurements were made during day 9 and day 18-22 of nursing and the second and eighth week of the postweaning fast. Plasma aldosterone and cortisol concentrations, quantified by radioimmunoassay, were measured in eight other weanlings during the second and eighth week of the fast. Mean GFR was 79.3+/-29.3 ml/min during the early suckling period and 78.2+/-17.1, 89.8+/-52.7, and 80.4+/-12.2 ml/min during the late suckling, early fasting and late fasting periods, respectively. Differences between nursing and fasting were insignificant, possibly because reduced protein oxidation during suckling and rapid recruitment of protein for tissue synthesis obviated the need for postprandial hyperfiltration. Alternatively, maintenance of GFR during fasting may facilitate urea concentration by compensating for reductions in the fractional excretion of urea. It is further hypothesized that aldosterone is primarily responsible for mediating renal water reabsorption in this system.     

Osmoregulating reactions following serotonin metabolism inhibition or activation

Effect of p-chlorphenilalanine and 5-hydroxytriptophan on the urine flow (V), glomerular filtration rate (GFR), free water reabsorption (TCH2O), and sodium fraction excretion (ENa.F%) in Wistar rats loaded with water or 2% sodium chloride solution, was studied. It was found that treatment of rats with inhibitor of serotonin biosynthesis, p-chlorphenilalanine (300 mg/kg, 48 hrs before the experiment) had no effect on the kidney response to the water loading in the experimental rats as compared to the control ones: changes in V, GFR, TCH2O and ENa.F% were the same. Treatment of rats with precursor of serotonin, 5-hydroxytriptophan which is known to increase the serotonin level in the brain (50 mg/kg) simultaneously with the water loading prevented the development of the diuretic reaction because of the high level of TCH2O reflected in the blood vasopressin concentration. Injection of 5-hydroxytriptophan at the maximum level of water diuresis resulted in the sharp increase in TCH2O and drop of the V. 5-hydroxytriptophan had no significant effect on the kidney response to the loading with the 2% sodium chloride solution. Under these conditions, increase in V was produced by suppression of the distal tubular sodium reabsorption, the TCH2O remaining at the high level. It is suggested that the brain serotonin manifested a significant stimulating effect on the vasopressin release from the neurohypophysis, but it is not involved in the mechanisms of suppression of its release into the blood. Serotonin seems not to interact with brain mechanisms regulating natriuretic function of the kidney.     

Renal haemodynamics and natriuretic responses to intravenous administration of diadenosine tetraphosphate (Ap4A) and nicotinamide adenine dinucleotide (NAD) in rat.

Effects of Ap4A and NAD--precursor of adenosine, on renal plasma flow (RPF), glomerular filtration rate (GFR) and urine excretion were determined in the anaesthetised rats. Infusion of Ap4A or NAD (i.v., bolus--1 micromol/kg followed by 10 nmol/min/kg) decreased RPF and GFR (by 30 and 40%, respectively). In spite of GFR reduction during Ap4A infusion, the significant increase in sodium excretion and urine flow was noticed: fractional sodium (FENa) and urine excretion (FEurine) rose 15-fold and 2.5-fold in comparison with the control value, respectively. In contrast to Ap4A, NAD-induced decrease in GFR was associated with parallel decrease in sodium and urine excretion, thus the FENa and FEurine did not significantly change. Pretreatment with adenosine deaminase (adenosine degrading enzyme, 2 U/min/kg) or theophylline (P1-receptors antagonist, 0.2 mmol/min/kg) ceased responses to NAD, whereas Ap4A-induced changes were not affected. Pre-treatment with suramin (P2-receptors antagonist, (i.v., bolus--12 mg/kg followed by 1.2 mg/min/kg) completely abolished the renal effects of Ap4A. We conclude that Ap4A may exert specific action on renal function. It acts different from NAD that modified renal function through its hydrolysis product--adenosine. Ap4A might reduce glomerular filtration rate and evoke natriuresis and diuresis, and its effects are probably mediated through stimulation of P2-receptors.     

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.     

Volume expansion during NOS substrate donation with L-arginine: regulatory offsetting of renal response?

The responses to infusion of nitric oxide synthase substrate (L-arginine 3 mg.kg(-1).min(-1)) and to slow volume expansion (saline 35 ml/kg for 90 min) alone and in combination were investigated in separate experiments. L-Arginine left blood pressure and plasma ANG II unaffected but decreased heart rate (6 +/- 2 beats/min) and urine osmolality, increased glomerular filtration rate (GFR) transiently, and caused sustained increases in sodium excretion (fourfold) and urine flow (0.2 +/- 0.0 to 0.7 +/- 0.1 ml/min). Volume expansion increased arterial blood pressure (102 +/- 3 to 114 +/- 3 mmHg), elevated GFR persistently by 24%, and enhanced sodium excretion to a peak of 251 +/- 31 micromol/min, together with marked increases in urine flow, osmolar and free water clearances, whereas plasma ANG II decreased (8.1 +/- 1.7 to 1.6 +/- 0.3 pg/ml). Combined volume expansion and L-arginine infusion tended to increase arterial blood pressure and increased GFR by 31%, whereas peak sodium excretion was enhanced to 335 +/- 23 micromol/min at plasma ANG II levels of 3.0 +/- 1.1 pg/ml; urine flow and osmolar clearance were increased at constant free water clearance. In conclusion, L-arginine 1) increases sodium excretion, 2) decreases basal urine osmolality, 3) exaggerates the natriuretic response to volume expansion by an average of 50% without persistent changes in GFR, and 4) abolishes the increase in free water clearance normally occurring during volume expansion. Thus L-arginine is a natriuretic substance compatible with a role of nitric oxide in sodium homeostasis, possibly by offsetting/shifting the renal response to sodium excess.     

Renal function in the freshwater rainbow trout after dietary cadmium acclimation and waterborne cadmium challenge

Renal function was examined in adult rainbow trout (Oncorhynchus mykiss) after chronic exposure to a sublethal level of dietary Cd (500 mg/kg diet) for 52 d and during a subsequent challenge to waterborne Cd (10 microg/L) for 72 h. Dietary Cd had no major effects on UFR (urine flow rate) and GFR (glomerular filtration rate) but caused increased renal excretion of glucose, protein, and major ions (Mg(2+), Zn(2+), K(+), Na(+), Cl(-) but Ca(2+)). However, dietary Cd did not affect any plasma ions except Na(+) which was significantly elevated in the Cd-acclimated trout. Plasma glucose and ammonia levels fell by 25% and 36% respectively, but neither plasma nor urine urea was affected in Cd-acclimated fish. Dietary Cd exposure resulted in a remarkable increase of Cd load in the plasma (48-fold, approximately 22 ng/mL) and urine (60-fold, 8.9 ng/mL), but Cd excretion via the kidney was negligible on a mass-balance basis. Clearance ratio analysis indicates that all ions, Cd, and metabolites were reabsorbed strongly (58-100%) in both na?ve and dietary Cd exposed fish, except ammonia which was secreted in both groups. Mg(2+), Na(+), Cl(-) and K(+) reabsorption decreased significantly (3-15%) in the Cd-exposed fish relative to the control. Following waterborne Cd challenge, GFR and UFR were affected transiently, and only Mg(2+) and protein excretion remained elevated with no recovery with time in Cd-acclimated trout. Urinary Ca(2+) and Zn(2+) excretion rates dropped with an indication of renal compensation towards plasma declines of both ions. Cadmium challenge did not cause any notable effects on urinary excretion rates of metabolites. However, a significant decrease in Mg(2+) reabsorption but an increase in total ammonia secretion was observed in the Cd-acclimated fish. The study suggests that dietary Cd acclimation involves physiological costs in terms of renal dysfunction and elevated urinary losses.     

The effects of hypoxemia with progressive acidemia on fetal renal function in sheep

In order to determine the effects of fetal hypoxemia on renal blood flow, glomerular filtration rate (GFR) and urethral and urachal urine output, we examined the effects of 3 h maternally-induced (9% O2, 3% CO2, 88% N2) fetal hypoxaemia on 10 chronically-instrumented fetal sheep between 127-135 days of gestation. Fetal arterial pH fell significantly during the second and third hours of hypoxia and this coincided with a significant increase in fetal arterial blood pressure (P less than 0.05). During the second hour of hypoxia, with mild acidaemia, fetal GFR decreased significantly and then, during the third hour, fetal GFR, urethral and total urine output were significantly elevated. During the 2-h recovery period urachal and total, but not urethral urine output, were significantly elevated (P less than 0.05). The data suggested that the increase in GFR and urine output measured during the third hour of hypoxia and the recovery period may reflect a pressure diuresis.     

A preliminary study on T-786C endothelial nitric oxide synthase gene and renal hemodynamic and blood pressure responses to dietary sodium

The purpose of the present study was to examine the role of the T-786C endothelial nitric oxide synthase (eNOS) gene polymorphism on changes in renal hemodynamics and blood pressure due to Na(+) loading. Twenty-eight older (63+/-1 years), moderately obese (39+/-2 % fat) hypertensives had their glomerular filtration rate (GFR), renal plasma flow (RPF), blood pressure (BP) and plasma nitric oxide (NO(x)) levels determined after eight days of low (20 mEq) and high (200 mEq) Na(+) diets. The two Na(+) diets were separated by a 1-week washout period. Subjects were genotyped for the eNOS-786 site and were grouped on whether they were homozygous or heterozygous for the C allele (TC+CC, n=13) or only homozygous for the T allele (TT, n=15). The TC+CC genotype group had a significantly greater increase in diastolic (P=0.021) and mean arterial (P=0.018) BP and a significant decline in both RPF (P=0.007) and GFR (P=0.029) compared to the TT genotype group with Na(+) loading. Furthermore, Na(+) loading resulted in a significant (P=0.036) increase in plasma NO(x) in the TT, but not in the TC+CC genotype group as well as a trend (P=0.051) for an increase in urine NO(x) in TC+CC, but not in the TT genotype group. The increase in BP during Na(+) loading in older hypertensives was associated with the eNOS genotype and may be related to changes in renal hemodynamics due to changes in NO metabolism.     

Digestive efficiency in two small, wild ruminants: the dik-dik and suni antelopes

A comparative study, using six dik-dik and eight suni antelope, was undertaken to identify similarities and differences that may exist in the digestive process of these two small, East African ruminant browsers. The suni antelope was the more select feeder, preferring the native, Grewia sinilis leaves over that of lucerne hay. Daily forage consumption rate, per unity body weight, was greater in the dik-dik (40.4 g/kg) than for the suni (30.6 g/kg), while daily fluid intake was considerably less (i.e. dik-dik, 68 ml/kg versus suni, 106 ml/kg body weight). Rumen fermentation studies suggested that the suni antelope attained the more rapid rumen and caecal fermentation activities, when compared to the dik-dik antelope. The difference in ruminal and caecal digestive process of the two antelope is suggested to be partly the result of the dik-dik's arid-adaptation strategies of less fluid intake and a more diverse (less selective) browse consumption, relative to the suni antelope.     

Control of renal function in larvalAmbystoma tigrinum

Summary Renal clearance methods were used to examine several factors which may be involved in control of urine flow in larvalAmbystoma tigrinum. Arginine vasotocin (10 ng/g) reduced both urine flow and GFR (–30%); this was reversed with mesotocin (5 ng/g). Adaptation to isosmotic media reduced GFR from 0.156 ml/10 g·h to 0.057. Neither hypophysectomy nor treatment with mesotocin altered this response. The adrenergic blocking agents propranolol and phentolamine did not reverse the antidiuresis in isosmotic medium-adapted larvac. Ten percent volume expansion with 50% and 100% Ringer's solution increased GFR in isosmotic medium-adapted animals. Hypophysectomy prevented this increase in GFR. Ten percent volume expansion with 200% Ringer's solution decreased GFR in tapwater adapted larvae; however, 20% volume expansion with 200% Ringer's increased GFR. Both volume and concentration of extracellular fluid appear to be important in control of GFR. Both arginine vasotocin and mesotocin may be involved in these responses, however other factors must also play a role.Abbreviations AVT arginine vasotocin - ECF extracellular fluid - GFR glomerular filtration rate - MT mesotocin     

Renal excretion of urea in response to changes in nitrogen intake in desert (black Bedouin) and non-desert (Swiss Saanen) goats

• 1.1. Renal responses were compared between desert (black Bedouin) and non-desert (Swiss Saanen) goats fed (a) Lucerne hay ad. lib., (b) Lucerne hay restricted to 65% of the ad. lib. consumption and (c) wheat straw.
• 2.2. Reduced feeding in both breeds resulted in proportional reduction in glomerular filtration rate (GFR) and the amount of urea excreted with no change in the percentage of urea reabsorbed.
• 3.3. In both breeds appreciably less urea was filtered at the glomerulus when a low-nitrogen diet was fed compared with a high nitrogen diet and the percentage of filtered urea reabsorbed increased from about 55 to about 90%.
• 4.4. In both breeds the reduction in GFR is quantitatively more important than the increase in the percentage of filtered urea reabsorbed in regard to renal urea conservation.
• 5.5. The results indicate no qualitative differences between desert and non-desert breeds of goat. The advantages of the Bedouin over the Saanen goats is related to the lower GFR values in the Bedouin goat and mainly their ability to reduce its GFR much more than the Saanen goats in response to lower nitrogen intake.

     

Metabolism and evaporative heat loss in the dik-dik antelope (Rhynchotragus kirki)

1. Under controlled conditions, the rate of oxygen consumption (VO2) respiratory frequency, evaporative water loss, heat balance, rectal (Trec) and surface temperatures were determined in the dik-dik antelopes at ambient temperatures (Ta) ranging from 1 to 44 degrees C. 2. The thermal neutral zone was found to be between 24 and 35 degrees C. 3. Respiratory frequency ranged between 27 and 630 breaths/min. 4. At a Ta of 44 degrees C, 95% of the heat produced by the dik-dik was lost via respiratory evaporation. Despite an increase in Trec, cutaneous evaporation did not increase. 5. During panting, VO2 increased in accordance with the expected Q10 effect, contrary to earlier findings. 6. Measurements of circadian rhythm [LD 12:12 (7-19) CT26 degrees C] in VO2 showed that the minimum VO2 (0.42 ml O2/g/hr) occurred at midnight while the maximum (0.78 ml O2/g/hr) occurred at midday. The 24 hr mean VO2 was 0.61 ml O2/g/hr. 7. These measurements suggest that in nature, determinants other than light may be responsible for triggering the variations observed in VO2.     

The effect of glucagon on glomerular filtration rate in dogs during reduction of renal blood flow.

Glucagon in small intravenous (i.v.) doses markedly increases glomerular filtration rate (GFR) in normal anesthetized dogs. In this study, the effects of glucagon 5 mug/min (i.v.) on renal hemodynamics was tested in four canine models of acute pre-renal failure (hemorrhage, barbiturate overdose; renal arterial clamping and renal arterial infusions of noradrenaline) and in a model of unilateral acute tubular necrosis at 4 h and 6-7 days following completion of the ischemic insult. Following hemorrhage and barbiturate excess, with arterial blood pressure maintained at 65-70 mm Hg, whole-kidney GFR and clearance rate of p-aminohippurate decreased by 50-70%. During this reduction of perfusion pressure, the subsequent infusion of glucagon increased GFR by 90-130%. In models where arterial pressure was normal during the period of ischemia (clamping and noradrenaline infusion), not only did glucagon significantly increase renal perfusion, but the ischemic kidney proved to be far more sensitive to the hemodynamic effects of glucagon (delta GFR - 120-160%) than the contralateral control (deltaGFR = 30-40%). In three dogs completely anuric following renal arterial clamping, glucagon was able to improve blood flow and restart urine formation. Glucagon, but not dopamine, was able to simulate the beneficial effects of hypertonic mannitol on renal function in dogs with hemorrhagic hypotension. Glucagon was without effect in established acute tubular necrosis. This study, therefore, indicates that, during renal ischemia, glucagon may be quite effective in preserving urine output and perfusion of the kidneys.     

Body water handling in response to hypertonic-saline induced diuresis in fasting northern elephant seal pups (Mirounga angustirostris)

During natural fasting conditions in postweaned northern elephant seal (NES) (Mirounga angustirostris) pups, urinary water loss is minimized and percent total body water (TBW) is maintained constant. However, following infusion of hypertonic saline, glomerular filtration rate (GFR) and urine output increased in fasting pups. Therefore, we quantified the magnitude of the hypernatremia-induced diuresis relative to the animal's total body water (TBW) pool and the percentage of filtered water reabsorbed. Following a 24 h control period, naturally fasting NES pups (n=7) were infused (4 ml min(-1)) with hypertonic saline (16.7%) at a dose of 3 mmol NaCl kg(-1) body mass. Total body water was estimated prior to infusion by tritium dilution, GFR was estimated by standard creatinine clearance, and urine output (V) was measured for 24 h during the control and post infusion periods. Percentage of filtered water reabsorbed was calculated as (1-(V/GFR))x100. Twenty-four hours following the infusion, GFR (control: 69+/-12 ml min(-1) and post-infusion: 118+/-19 ml min(-1); mean+/-S.E.) increased 77+/-28% above control and the percentage of filtered water reabsorbed was decreased 0.4+/-0.1%. The increase in urine output (control: 218+/-47 ml d(-1) and post-infusion: 883+/-92 ml d(-1)) accounted for 1.7+/-0.2% of the pups' TBW. The hypernatremia-induced diuresis was accompanied by the loss of body water indicating the lack of water retention. Although the 77% increase in GFR was only associated with a 0.4% decrease in the percentage of filtered water reabsorbed, this decrease was significant enough to result in a 4-fold increase in urine output. Despite the observed diuresis, fasting NES pups appear to possess an efficient water recycling mechanism requiring only a small percentage of body water to excrete an excess salt load. This water recycling mechanism may allow pups to avoid negative perturbations in body water as they initiate feeding in a marine environment following the fast.     

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     

Renal effects of dopamine and dopexamine in the newborn anesthetized rabbit.

The renal effects of dopexamine, a new dopaminergic agonist with marked beta 2-adrenergic agonist properties, but no alpha-adrenergic effect, has been studied in 8 newborn New Zealand rabbits, whose renal functional characteristics show close similarities with those of premature infants. Six animals were used as controls. After a control period, dopexamine was infused intravenously at a rate of 4 micrograms/kg per min and after a wash-out period, at 10 micrograms/kg per min. The renal effects of dopamine were studied in similar conditions. Glomerular filtration rate (GFR) and renal plasma flow (RPF) were determined by inulin and para-aminohippuric acid clearances, respectively. Dopexamine, 4 micrograms/kg per min, did not induce changes in cardiovascular and renal hemodynamics or in renal functions. At 10 micrograms/kg per min, a significant increase in urine flow rate (25 +/- 5%; p less than 0.01), urine sodium excretion (77 +/- 17%; p less than 0.01) and fractional sodium excretion (69 +/- 25%; p less than 0.05) was observed. The GFR, RPF and renal vascular resistance (RVR) were not affected. Heart rate increased slightly but significantly (8 +/- 3%; p less than 0.05), without change in mean blood pressure (MBP). Dopamine, 4 micrograms/kg per min, decreased slightly albeit significantly MBP (3 +/- 1%; p less than 0.05). At 10 micrograms/kg per min the only renal effect was a significant increase in RVR (19 +/- 6%; p less than 0.02). The different actions of these two dopaminergic agonists in this immature model could be explained by their respective ability to activate electively the adrenergic and dopaminergic peripheral receptors. The natriuretic and diuretic effect of dopexamine in normal immature rabbits, in the absence of changes in RPF or GFR is probably mediated by a direct action of this agent on dopaminergic tubular receptors. Failure of these two drugs to increase RPF may be related to an immaturity of the dopaminergic vascular receptors.     

脑室内注射高张盐水抑制近曲小管对水和钠的重吸收

实验在麻醉大鼠上进行。用锂清除率为指标观察脑室内注射高张盐水对近曲小管重吸收水和钠的影响。在切断单侧肾神经的动物中,脑室内注射高张盐水后的锂清除率与肾小球滤过率比值在去神经侧肾脏从0.37±0.04增加至0.51±0.05(P<0.01);神经完好侧肾脏则从0.26±0.03增加至0.31±0.04(P<0.05);双侧肾脏的肾小球滤过率、尿量、尿钠和尿钾量均增加,且去肾神经肾脏的增加幅度高于肾神经完好肾脏。在肾小管微穿刺实验中,脑室内注射高张盐水后,近曲小管末段小管液流量从24.42±1.84nl/min增加至31.86±3.09nl/min(P<0.01),小管液的渗透压无显著变化。以上实验结果表明,脑室内注射高张盐水引起的利尿、尿钠增多反应与肾小球滤过率增加和近曲小管对水、钠重吸收减少有关,体液因素在该反应中可能起主要作用。     

Renal effects of fresh water-induced hypo-osmolality in a marine adapted seal

With few exceptions, marine mammals are not exposed to fresh water; however quantifying the endocrine and renal responses of a marine-adapted mammal to the infusion of fresh water could provide insight on the evolutionary adaptation of kidney function and on the renal capabilities of these mammals. Therefore, renal function and hormonal changes associated with fresh water-induced diuresis were examined in four, fasting northern elephant seal ( Mirounga angustirostris) (NES) pups. A series of plasma samples and 24-h urine voids were collected prior to (control) and after the infusion of water. Water infusion resulted in an osmotic diuresis associated with an increase in glomerular filtration rate (GFR), but not an increase in free water clearance. The increase in excreted urea accounted for 96% of the increase in osmotic excretion. Following infusion of fresh water, plasma osmolality and renin activity decreased, while plasma aldosterone increased. Although primary regulators of aldosterone release (Na(+), K(+) and angiotensin II) were not significantly altered in the appropriate directions to individually stimulate aldosterone secretion, increased aldosterone may have resulted from multiple, non-significant changes acting in concert. Aldosterone release may also be hypersensitive to slight reductions in plasma Na(+), which may be an adaptive mechanism in a species not known to drink seawater. Excreted aldosterone and urea were correlated suggesting aldosterone may regulate urea excretion during hypo-osmotic conditions in NES pups. Urea excretion appears to be a significant mechanism by which NES pups sustain electrolyte resorption during conditions that can negatively affect ionic homeostasis such as prolonged fasting.