Mechanisms of distal-nephron Li(+) reabsorption during dietary K(+) restriction in rats.

The mechanism by which dietary K(+) restriction induces distal-nephron Li(+) reabsorption was investigated by administration of bendroflumethiazide (BFTZ) or vehicle in conscious Wistar rats. Changes in fractional excretion of Li(+) following administration of amiloride (DeltaFE(Li)) were used as an index of distal tubular Li(+) reabsorption. The results revealed an absence of distal tubular Li(+) reabsorption in K(+)-replete rats (DeltaFE(Li) = 3. 6+/-2.4%), in contrast to K(+) restriction in which DeltaFE(Li) was 24.0+/-2.7%. The distal tubular Li(+) reabsorption in K(+)-depleted rats was significantly reduced by preadministration of BFTZ (DeltaFE(Li) = 9.2+/-0.9%). The fractions of Li(+) and Na(+) reabsorbed in the amiloride-sensitive segment were different in K(+)-replete rats (9+/-6 vs. 60+/-6%), but similar in K(+)-depleted rats (61+/-5 vs. 73+/-4%). BFTZ administration to K(+)-depleted rats resulted in a proportional decrease in these fractions, suggesting competition between Na(+) and Li(+) for reabsorption in the distal-nephron segment during K(+) depletion. These results are compatible with the hypothesis that during K(+) depletion the reabsorption of Li(+ )in the distal-nephron segment is competitively inhibited by Na(+).

N-substituted adenosines as novel neuroprotective A(1) agonists with diminished hypotensive effects.

The synthesis and pharmacological profile of a series of neuroprotective adenosine agonists are described. Novel A(1) agonists with potent central nervous system effects and diminished influence on the cardiovascular system are reported and compared to selected reference adenosine agonists. The novel compounds featured are derived structurally from two key lead structures: 2-chloro-N-(1-phenoxy-2-propyl)adenosine (NNC 21-0041, 9) and 2-chloro-N-(1-piperidinyl)adenosine (NNC 90-1515, 4). The agonists are characterized in terms of their in vitro profiles, both binding and functional, and in vivo activity in relevant animal models. Neuroprotective properties assessed after postischemic dosing in a Mongolian gerbil severe temporary forebrain ischemia paradigm, using hippocampal CA1 damage endpoints, and the efficacy of these agonists in an A(1) functional assay show similarities to some reference adenosine agonists. However, the new compounds we describe exhibit diminished cardiovascular effects in both anesthetized and awake rats when compared to reference A(1) agonists such as (R)-phenylisopropyladenosine (R-PIA, 5), N-cyclopentyladenosine (CPA, 2), 4, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR 79236, 26), N-cyclohexyl-2'-O-methyladenosine (SDZ WAG 994, 27), and N-[(2-methylphenyl)methyl]adenosine (Metrifudil, 28). In mouse permanent middle cerebral artery occlusion focal ischemia, 2-chloro-N-[(R)-[(2-benzothiazolyl)thio]-2-propyl]adenosine (NNC 21-0136, 12) exhibited significant neuroprotection at the remarkably low total intraperitoneal dose of 0.1 mg/kg, a dose at which no cardiovascular effects are observed in conscious rats. The novel agonists described inhibit 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate-induced seizures, and in mouse locomotor activity higher doses are required to reach ED(50) values than for reference A(1) agonists. We conclude that two of the novel adenosine derivatives revealed herein, 12 and 5'-deoxy-5'-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine (NNC 21-0147, 13), representatives of a new series of P(1) ligands, reinforce the fact that novel selective adenosine A(1) agonists have potential in the treatment of cerebral ischemia in humans.

Norethindrone acetate enhances the antiatherogenic effect of 17beta-estradiol: a secondary prevention study of aortic atherosclerosis in ovariectomized cholesterol-fed rabbits.

The influence of progestogens in combination with 17beta-estradiol (E2) on cardiovascular disease remains controversial. This study investigated the effect of norethindrone acetate (NETA) combined with E2 on aortic atherosclerosis. Eighty mature female rabbits were ovariectomized, then fed a cholesterol-rich diet (240 mg/d) for 14 weeks to induce aortic atherosclerosis. They were randomized to four equally large groups for the following 38-week intervention period. One group received placebo, another group oral E2 4 mg daily (E2), and the last two groups oral E2 4 mg daily combined with either NETA 1 mg (E2NETA1) or NETA 3 mg (E2NETA3). The cholesterol intake was reduced to a "maintenance" level of 80 mg/d during the intervention period. Total serum cholesterol and ultracentrifuged lipoproteins were analyzed enzymatically throughout the study. The cholesterol content in the aortic wall was 2.76+/-0.44 micromol/cm2 (mean+/-SEM) in the E2NETA1 group, 1.77+/-0.37 micromol/cm2 in the E2NETA3 group, 5.46+/-0.77 micromol/cm2 in the E2 group, and 7.20+/-0.94 micromol/cm2 in the placebo group (ANOVA P<0.0001). The difference (in the aortic cholesterol accumulation) between the E2 and each of the combined E2/NETA groups was statistically significant (P<0.01) but could only partly be explained by the differences in serum lipids and lipoproteins. In conclusion, NETA enhances the antiatherogenic effect of E2 in cholesterol-fed rabbits. This effect is only partially mediated through changes in serum lipids and lipoproteins.

Model explaining the relation between distal nephron Li+ reabsorption and urinary Na+ excretion in rats.

Li+ may be reabsorbed via an amiloride-sensitive mechanism in the collecting ducts of rats administered a low-Na+ diet. This was investigated by measuring the increase in fractional urinary excretion of Li+ (FELi) in response to amiloride in conscious rats at two different levels of plasma Li+ concentration and after administration of bendroflumethiazide (BFTZ), angiotensin III (ANG III), and aldosterone (Aldo). The results confirmed that amiloride increased (FELi) in rats on a low-Na+ diet (20 +/- 1 to 35 +/- 1%, means +/- SE), whereas no increase was observed in rats on a normal Na+ diet (37 +/- 1 to 38 +/- 1%). The lithiuretic effect of amiloride was 1) abolished by preadministration of BFTZ (32 +/- 1 to 33 +/- 2%) to Na(+)-deprived rats and 2) increased by ANG III (27 +/- 3 to 33 +/- 2%) and Aldo (25 +/- 2 to 37 +/- 2%) in Na(+)-replete rats. Amiloride-induced changes in FELi were independent of plasma Li+ concentration but inversely related to the fractional excretion of Na+ and the amiloride-sensitive excretion of K+. These results are compatible with the hypothesis that a low tubular Na+ concentration reduces end-tubular Na+ reabsorption and results in hyperpolarization of the apical membrane, thus favoring Li+ uptake into the cells.

A partial estrogen receptor agonist with strong antiatherogenic properties without noticeable effect on reproductive tissue in cholesterol-fed female and male rabbits.

Estrogen replacement therapy retards the development of cardiovascular disease and osteoporosis in postmenopausal women. However, long-term unopposed use increases the risk of cancer in endometrium and possibly in breast. The racemic compound ormeloxifene, widely used in India as an antifertility agent, is a partial estrogen receptor agonist with antiosteoporotic properties. The present study was undertaken to investigate the effect of the L-enantiomer (levormeloxifene) and the d-enantiomer (d-ormeloxifene) on the development of atherosclerosis. In a short-term experiment (6 weeks), 4 x 10 ovariectomized female rabbits were fed a 0.25% cholesterol-enriched diet and the effect on plasma cholesterol levels was studied. In a long-term experiment (13 weeks), 4 x 15 ovariectomized female and 4 x 15 shamoperated male rabbits were maintained at a similar plasma cholesterol level of 25 mmol/L and the effect on undamaged and balloon-injured arterial wall was studied. In both experiments, the rabbits were treated with levormeloxifene, d-ormeloxifene, 17 beta-estradiol, or placebo, respectively. In the short-term experiment, levormeloxifene, in contrast to d-ormeloxifene, significantly reduced plasma cholesterol by 30% compared with the placebo group. In the long-term experiment, levormeloxifene, in contrast to d-ormeloxifene, significantly reduced atherosclerosis by 50% in the undamaged arterial wall of both female and male rabbits. Because these rabbits were cholesterol-clamped, the antiatherogenic effect was not mediated via plasma cholesterol lowering. Like estrogen, levormeloxifene did not inhibit atherosclerosis in the endothelium-denuded site of aorta. The antiatherogenic effects of levormeloxifene were thus similar to those of estrogen, but produced in the absence of any noticeable estrogenic effect on uterine or testicular tissue.

Significant reduction of the antiatherogenic effect of estrogen by long-term inhibition of nitric oxide synthesis in cholesterol-clamped rabbits.

The purpose of this study was to investigate whether endothelium-derived nitric oxide (NO) is involved in the plasma lipid-independent antiatherogenic effect of estrogen and levormeloxifene, a partial estrogen receptor agonist. 85 rabbits were ovariectomized and balloon-injured in the middle thoracic aorta. The rabbits were fed a cholesterol-enriched diet supplemented with 17beta-estradiol, levormeloxifene, or placebo, either alone, or together with 160 microg/ml NG-nitro- -arginine methyl ester (-NAME), an NO synthase inhibitor, in their drinking water for 12 wk. Plasma cholesterol was maintained at 25-30 mmol/liter by individualized cholesterol feeding. In the undamaged aorta, the extent of atherosclerosis in the estrogen group was only one-third that in the placebo group. Simultaneous administration of -NAME, however, significantly reduced the antiatherogenic effect of estrogen (P < 0.01). There was no significant difference between the placebo group given -NAME and the group treated with placebo alone. At the previously endothelium-denuded site, estrogen had no effect on atherosclerosis development, whereas -NAME combined with estrogen significantly increased atherogenesis (P < 0.05). The effects of levormeloxifene were almost similar to those of estrogen. Active vascular concentrations of -NAME were demonstrated in an additional study, in which maximal aortic/coronary endothelium-dependent relaxation was significantly inhibited in rabbits given -NAME. Thus, in this study a considerable part of the plasma lipid-independent antiatherogenic effect of estrogen was mediated through its effect on endothelial NO in cholesterol-fed rabbits. The results for levormeloxifene suggest a common mechanism of action for estrogen and partial estrogen receptor agonists on atherogenesis.

Antihypertensive action of non-natriuretic doses of furosemide in Dahl salt-sensitive rats.

BACKGROUND: That non-natriuretic doses of loop diuretics exert an antihypertensive action has been suggested, but not confirmed, by simultaneous measurements of the arterial pressure and sodium balance during therapy. OBJECTIVE: To examine the relationship between changes in arterial pressure and changes in sodium balance during furosemide treatment. DESIGN: Twenty hypertensive Dahl salt-sensitive rats fed a 4% NaCl diet were allocated to four groups and administered the following treatments: placebo once a day intraperitoneally, continuous infusion of 4 mg/day furosemide intraperitoneally, 4 mg furosemide once a day intraperitoneally and 12 mg furosemide once every third day intraperitoneally. METHODS: The mean arterial pressure (MAP) was measured continuously with radiotelemetry and the sodium balance was measured with the rats in metabolic cages. RESULTS: Administration of furosemide as a bolus injection once a day (P < 0.01) or once every third day (P < 0.05) lowered the MAP significantly compared with placebo, whereas continuous infusion of furosemide had no significant effect on the MAP (P < 0.07). Fast Fourier transformation analysis detected an acute antihypertensive action related to the temporary diuretic and natriuretic responses during the period 0-6 h after intraperitoneal bolus injections of 4 and 12 mg furosemide. None of the treatment regimens produced 24 h sodium or potassium losses. At the end of the study, the total body water, extracellular fluid volume, total body sodium and potassium were similar for rats in all groups. CONCLUSIONS: Furosemide has an acute antihypertensive action in Dahl salt-sensitive rats fed a 4% NaCl diet that is related to renal sodium and volume losses whereas the long-term antihypertensive effect is independent of changes in extracellular fluid volume, total body water, sodium and potassium.

Effects of renal papillary-medullary lesion on the antihypertensive effect of furosemide and development of salt-sensitive hypertension in Dahl-S rats.

To test the hypothesis that the long-term antihypertensive action of furosemide is mediated by a renomedullary vasodepressor substance, we measured mean arterial pressure (MAP) by radiotelemetry in Dahl-S rats with either intact or bromoethylamine-induced (BEA, 100 mg/kg i.p.) lesion of the renal papilla and medulla. Seven days of recovery after BEA administration, the rats diet was changed from 1 to 4% NaCl, and during days 8 to 31, rats were randomized to daily treatment with placebo or furosemide (50 mg/kg p.o.). Then furosemide treatment was stopped and the rat food was changed to 1% NaCl diet. After a 10-day wash-out period, renal function was measured. BEA produced a rapid (within min) and sustained increase in MAP which was accelerated during 4% NaCl diet. Furosemide prevented 4% NaCl-induced hypertension in both rats with intact kidneys and in rats with BEA-induced renal papillary-medullary lesion. A significant decrease in renal plasma flow (-34%) and glomerular filtration rate (-40%) was observed in all BEA-treated rats independent of previous furosemide treatment. In response to an i.v. load of isotonic saline (10% body weight), rats with renal papillary-medullary lesion had an impaired ability to excrete sodium. Histological examination showed that BEA-treated rats had severe lesions of the renal papilla and medulla, with light-to-moderate changes in the renal cortex. It is concluded that the antihypertensive effect of furosemide is not mediated by a renomedullary vasodepressor substance. The accelerated NaCI-sensitive hypertension in rats with BEA-induced renal papillary-medullary lesion is related to an impaired ability to excrete excess NaCl.

Effects of perindopril and hydrochlorothiazide on the long-term progression of lithium-induced chronic renal failure in rats.

Administration of lithium in the diet to new-born rats induces chronic renal failure associated with hypertension, proteinuria and irreversible tubulo-interstitial morphological changes. In the present study we induced chronic renal failure by administration of lithium for 16 weeks to new-born rats, and examined the spontaneous course of this nephropathy and the effects of antihypertensive treatment with either perindopril (12 mg/kg diet) or hydrochlorothiazide (500-1000 mg/kg diet) during a 24 weeks follow up period without lithium. In the placebo group, progression to terminal uraemia occurred in all rats with severe renal failure (initial Purea > 15 mM) (10 of 18). Rats with mild-moderate renal failure (Purea 9-15 mM) showed no deterioration in renal function despite persistent systolic hypertension and irreversible structural renal changes. Perindopril normalized the blood pressure in all rats but did not prevent the progression to terminal uraemia (8 to 18). Hydrochlorothiazide partially controlled the hypertension and accellerated the progression of uraemia without increasing the mortality (7 of 17). Irrespective of treatments, the predominant quantitative structural changes (e.g. decreased volume of proximal tubular cells) showed significant correlations with the degree of renal dysfunction, but not with systolic blood pressure in the surviving rats. It is concluded that progression of lithium-induced nephropathy to terminal uraemia occurs when the nephrotoxic insult results in a more than 50% reduction of the glomerular filtration rate, judged from Purea levels. The failure of effective antihypertensive treatment with an angiotension-converting enzyme inhibitor to modify the progression suggests that in this model, systemic or glomerular hypertension may not be an important pathophysiological factor. The structural and functional deterioration observed in Li-uraemic rats during treatment with hydrochlorothiazide remains unexplained.

Effect of adrenalectomy on distal nephron lithium reabsorption induced by potassium depletion.

In potassium-depleted rats lithium is reabsorbed by an amiloride-sensitive transport mechanism in the distal nephron segments, and the urinary fractional excretion of lithium (FE[Li]) is reduced by almost 50% compared to that of potassium-replete rats. We have used renal clearance techniques to study the effects of adrenalectomy (Adx) or sham operation on amiloride-sensitive lithium reabsorption in conscious potassium-deprived (K5) and control (K200) rats. In the sham-operated rats, administration of a low potassium diet led to a significant reduction in FE(Li) from 27.0 to 16.6% (p < 0.01), whereas in the Adx rats the reduction in FE(Li) was smaller (from 27.0 to 22.6) and not statistically significant. Urinary sodium excretion was similar (1,100 nmol/min/100 g body weight) in all groups. During subsequent amiloride infusion in order to block the distal nephron reabsorption of lithium, urinary sodium excretion increased nearly twofold in the sham-operated groups whereas no change was evident in the Adx rats. Similarly, amiloride led to an increase in FE(Li) in the sham-K5 group but failed to increase FE(Li) in the Adx-K5 group. The results suggest that amiloride-sensitive lithium transport seen during potassium depletion is influenced by the presence of the adrenal glands, most likely due to their production of aldosterone.

Comparison of the renal effects of six sedating agents in rats.

This study was designed to compare the renal effects of sedation with alphaxalone-alphadolone, etomidate, propofol, midazolam, fentanyl-fluanisone, and thiopental in rats. The sedative dose was defined as the highest dose that abolished the escape response without affecting the righting reflex. Female Wistar rats were chronically catheterized with a jugular vein catheter, and urine flow rate and renal clearances of inulin (glomerular filtration rate = GFR), sodium, and lithium (used as an index of proximal tubular function) were measured in the conscious, unrestrained state (n = 107 experiments). In a separate series (n = 70 experiments), the effect of sedative doses of each drug on the nociceptive threshold was tested with the tail-flick test. Responses in sedated animals were compared to responses in animals infused with the vehicle. Fentanyl-fluanisone and thiopental had hypoalgesic actions in sedating doses. Propofol, fentanyl-fluanisone, and thiopental reduced GFR by 20-30%. Urine flow rate was significantly decreased by propofol (-24%) and thiopental (-48%). Propofol and fentanyl-fluanisone reduced fractional lithium excretion by 9-13%. Only alphaxalone-alphadolone, etomidate, and midazolam produced sedation without affecting renal function in rats. Because midazolam produced the most consistent degree of sedation, we conclude that midazolam is the least confounding sedative agent for renal function studies in conscious rats.

Supra-additive natriuretic synergism between bendroflumethiazide and furosemide in rats.

To examine the diuretic and natriuretic synergism between bendroflumethiazide (BFTZ) and furosemide (FUR), the acute diuretic and natriuretic response to BFTZ was compared when given alone, during acute and during chronic FUR infusion. Responses to diuretics were assessed in conscious, chronically instrumented rats and, to avoid confounding influences of diuretic-induced volume contraction, extracellular fluid volume was kept constant by a computer-driven servo-control technique. Two groups were pretreated with osmotic minipumps chronically infusing either FUR (0.25 mg/hr; group CF; n = 9) or vehicle (8% ethanolamine; group CV; n = 8) i.p. for 7 days before the experiment. During the experiment, two other groups received either acute infusion with FUR (0.25 mg/hr; group AF; n = 16) or acute infusion with vehicle (150 mM glucose; group AV; n = 11). After a 60-min control period, BFTZ (0.083 mg; 0.25 mg/hr) was administered for 90 min to all four groups. Whereas the diuretic and natriuretic response to BFTZ was similar in groups AF, AV and CV, a supra-additive response was observed in the CF group (vs. group CV: delta urine flow rate: +78%; delta urinary Na excretion: +69%). BFTZ had no effect on the fractional excretion of Li, which suggests that the supra-additive effect was due to inhibition of an enhanced distal tubular Na reabsorption induced by chronic FUR administration. Because before BFTZ administration urinary Na excretion was 3-fold higher during acute than during chronic FUR infusion, an increased delivery of NaCl to the thiazide segment cannot by itself explain the exaggerated BFTZ response during chronic FUR treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the adrenal medulla in control of blood pressure and renal function during frusemide-induced volume depletion.

OBJECTIVE: To examine the role of the adrenal medulla in reflex control of mean arterial pressure (MAP), heart rate and renal function parameters during frusemide-induced volume depletion. DESIGN: Frusemide (6 mg/kg per h) was administered intravenously to two groups of chronically instrumented rats with either sham adrenal medullectomy (n = 15) or adrenal medullectomy (n = 11). RESULTS: Adrenal medullectomy reduced adrenal adrenaline concentration by 99% and plasma adrenaline concentration by 97%. Plasma corticosterone levels were similar in the two groups, which suggests that adrenal cortical function was not affected by adrenal medullectomy. Although frusemide did not affect arterial pressure in the sham-operated rats, MAP fell by 21 +/- 3 mmHg (mean +/- SEM) during frusemide diuresis in the rats with adrenal medullectomy. Heart rate was lower in the rats with adrenal medullectomy than in the sham-operated rats throughout the study. There were no major differences in renal haemodynamics or overall renal water and sodium handling between the two groups but, at the time of maximal activation of compensatory sodium reabsorption, fractional sodium excretion in the distal nephron segment was significantly higher in the rats with adrenal medullectomy than in the sham-operated rats. CONCLUSIONS: Adrenal medullary adrenaline release is essential in the control of MAP during frusemide-induced volume depletion. Circulating adrenaline might contribute, by modifying distal tubular sodium reabsorption, to compensatory sodium reabsorption during frusemide-induced volume depletion.

Improved analysis of picomole quantities of lithium, sodium, and potassium in biological fluids.

The analysis of picomolar lithium, sodium, and potassium by electrothermal atomic absorption spectrophotometry was studied using a Perkin-Elmer Zeeman 3030 spectrophotometer. With ordinary pyrolytically coated graphite tubes, a number of interference effects associated with the sample matrix were observed. In particular, the lithium and potassium absorbance signal was depressed by chloride, an effect shown to be dependent on the preatomization heating. When an in situ tantalum-coated atomization surface was used, matrix interferences observed in lithium and potassium analyses were abolished, and the linear range for the potassium assay was extended. Technical difficulties encountered during sodium analysis at the primary wavelength were effectively circumvented by analysis at a less-sensitive wavelength (303.3 nm), at which tantalum coating also prevented significant chloride interference. The improved microanalyses were employed to reevaluate the handling of lithium, sodium, and potassium along the proximal convoluted tubule (PCT) of the anesthetized rat. The average tubular fluid-to-plasma concentration ratios for lithium [(TF/P)Li] and sodium [(TF/P)Na] were 1.13 +/- 0.08, n = 26, and 0.99 +/- 0.07 (n = 26), respectively. The tubular fluid-to-plasma ultrafiltrate concentration ratio for potassium [(TF/UF)K] was 1.09 +/- 0.05 (n = 13). Ratios did not change significantly with puncture site along the PCT for any of the ions. (TF/P)Li and (TF/UF)K were significantly greater than (TF/P)Na, indicating that lithium and potassium reabsorption do not directly parallel sodium reabsorption in the PCT.

Renal adaptations to continuous administration of furosemide and bendroflumethiazide in rats.

During continuous treatment with diuretics, the kidney adapts to the initial Na loss by activating antinatriuretic mechanisms which serve to prevent further Na and volume losses. To study the renal sites of adaptations to constant diuretic treatment, bendroflumethiazide (4 mg daily), furosemide (8 mg daily) or vehicle (0.24 ml daily) was infused intraperitoneally to female Wistar rats by implanted osmotic minipumps. Half of the animals (groups vol.) were randomized to receive a balanced saline solution to drink in addition to water in order to replace Na, K and volume losses. On the 6th day of treatment, clearances of inulin, Na, and Li were determined during four consecutive 6 hr periods. Circadian changes in renal excretions occurred in all groups with highest excretions of Na, Li and water in the dark period (6 p.m. to 6 a.m.). Renal changes induced by continuous infusion of diuretics were most pronounced in the dark period and would probably not have been disclosed if the clearance experiments had been restricted to the daytime. The average 24-hour clearance for inulin (glomerular filtration rate) was not different among groups, except for a 20% decrease in the furosemide group. The 24-hour fractional Na excretion, being approximately 0.5% in the vehicle group, increased to approximately 0.8% in group (bendroflumethiazide+vol) and to approximately 2.8% in group (furosemide+vol) but was not different from the vehicle group in the diuretic groups without volume replacement.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel computer-driven, servo-controlled fluid replacement technique and its application to renal function studies in conscious rats.

1. A new rat model has been developed allowing body fluid status to be accurately controlled and maintained throughout experimentation by computer-driven, servo-controlled replacement of spontaneous urinary fluid losses. 2. Experiments in vitro were performed to test the accuracy of the servo system, and experiments in vivo were carried out to re-assess basic renal function in servo-controlled vasopressin-replete Long Evans and vasopressin-deficient Brattleboro rats. The model was further evaluated in water-diuretic Wistar rats with or without administration of a vasopressin V2-receptor agonist, 1-desamino-8-D-arginine vasopressin. 3. The data gained from the present study indicate the suitability of the servo-controlled replacement system for conscious renal function studies in three different rat strains. Haemodynamic and renal function variables measured were demonstrated to be stable throughout a 5 h experimental procedure and reproducible between repeated experimental occasions over a 14 day post-operative period. 4. Using the servo-control technique, the expected action of 1-desamino-8-D-arginine vasopressin on renal water handling was demonstrated, but the natriuretic effect reported by some workers was not evident. 5. Since the servo-controlled fluid replacement technique maintains many of the inherent differences between vasopressin-replete Long Evans and vasopressin-deficient Brattleboro rats and eliminates the changes in body fluid volume during transition from a diuretic to an antidiuretic state, the model confers an advantage over previously employed constant infusion protocols.

Effects of angiotensin-converting enzyme inhibition on renal adaptations to acute furosemide administration in conscious rats.

During administration of loop diuretics the initial volume depletion activates Na-conserving mechanisms, which reduces glomerular filtration rate (GFR) and stimulates renal tubular reabsorption of Na and water. By i.v. infusion of the angiotensin-converting enzyme inhibitor enalaprilat (100 micrograms bolus; 100 micrograms/h) we examined the role of angiotensin II for the compensatory renal responses occurring during furosemide administration in conscious rats. To evaluate the significance of hydration for the compensatory renal effects of angiotensin II, experiments were performed in groups of rats with or without i.v. replacement of urinary volume losses. Furosemide was administered i.v. (6 mg/kg/h) for 3 1/2 hr. Furosemide infusion produced a short-lasting increase in urine flow rate, Na, Li and K excretion after which the renal excretion rates returned toward pretreatment levels, along with significant reductions in effective renal plasma flow and GFR and increases in effective filtration fraction and effective renal vascular resistance. Sustained increases in urine flow and urinary excretion rates of Na, Li and K were observed in absence of changes in GFR in rats given furosemide with volume replacement. Enalaprilat did not alter the tubular response to furosemide during either euvolemia or volume depletion. However, enalaprilat attenuated the furosemide-induced increases in effective filtration fraction and effective renal vascular resistance. It is concluded that angiotensin II is not essential for the compensatory response of decreased GFR and increased tubular Na reabsorption. However, angiotensin II is an important mediator of renal vasoconstriction during furosemide infusion.

Effect of low dietary sodium and potassium on lithium clearance in rats.

The influence of dietary sodium and potassium intake on lithium clearance (CLi) was examined in conscious Wistar rats without surgical stress. CLi was not influenced by variations in the dietary content of Na and K in the range of 50-600 mmol/kg, but decreased at values of 25 mmol/kg and was lowered by about 50% at a value of 5 mmol/kg of either ion. The lowering of CLi produced by a reduction in one of the ions was independent of variations in the dietary content of the other. The reductions in CLi were additive, CLi being reduced to almost zero in rats given a diet low in both Na and K. The reductions in the fractional excretion of lithium produced by either a low Na or a low K diet were both fully reversible after administration of amiloride, which suggests that they were due to reabsorption of Li in the distal nephron segments. It is concluded that a low Na diet and a low K diet both lead to distal tubular reabsorption of Li and that the effect is additive. If CLi is to be used as an index of proximal tubular fluid output, the dietary contents of Na and K should both be at least 50 mmol/kg in unanesthetized Wistar rats without surgical stress.

Inhibition of tubular lithium reabsorption by amiloride in the conscious sodium-restricted rat.

We examined the effects of amiloride administration on the renal lithium clearance (CLi) in three series of conscious, unrestrained rats maintained on either a low (5 mmol/kg) or normal (200 mmol/kg) sodium diet. In series 1, six doses of amiloride (0.5-16 mg/kg) were administered s.c., and the renal electrolyte excretion was assessed over a 3-h clearance period. In series 2, the time profile of changes in renal electrolyte excretion following 4 mg/kg of amiloride s.c. was examined, and in series 3, the effect of amiloride infusion i.v. (1 mg/kg followed by 2 mg/kg per h) on renal function was investigated. In all series CLi was lower in sodium-restricted rats than in controls. Amiloride administered s.c. to sodium-restricted rats did not increase the 3-h CLi to the levels found in control rats. When amiloride was administered s.c. or i.v. and urine collected in 30-minute periods, CLi in rats fed a sodium-deficient diet increased to control levels. We conclude that amiloride-induced losses of sodium and water in CLi studies may lead to an erroneous interpretation of data. However, distal tubular lithium reabsorption may be recognized, if present, by the administration of amiloride i.v. or s.c. during collections of urine in short time intervals.