during water and osmotic (mannitol) diuresis were determined, and compared

Transcription

1 J. Phy8iol. (1968), 197, pp With 8 textf gure Printed in Great Britain EFFECTS OF WATER DIURESIS AND OSMOTIC (MANNITOL) DIURESIS ON URINARY SOLUTE EXCRETION BY THE CONSCIOUS RAT BY J. C. ATHERTON, M. A. HAI AND S. THOMAS From the Department of Physiology, The University, Manchester, 13 (Received 26 February 1968) SUMMARY 1. The time course and extent of changes in urinary flow and in the outputs of urea, Na+, K+, and NH+ over a period of 71 hr in conscious rats during water and osmotic (mannitol) diuresis were determined, and compared with spontaneous changes in nondiuretic animals. 2. In nondiuretic rats, a morning rise and subsequent decline in urinary osmolal, sodium, potassium and ammonium outputs occurred, possibly attributable to circadian rhythms. 3. Water diuresis was accompanied by (i) a rapid increase in urea excretion during the phase of increasing urine flow, followed by a fall in later periods to values similar to those in nondiuresis, (ii) a slower increase in sodium output, continuing after the establishment of the constant water load, (iii) unchanged potassium excretion, but slightly increased ammonium outputs. 4. Mannitol diuresis was accompanied by (i) a rapid increase in urea outputs which subsequently fell but remained significantly higher, (ii) a steep rise in sodium and potassium outputs to values which remained far higher than those in nondiuretic and water diuretic animals. 5. The changes in mannitol diuresis are considered to result mainly from decreased tubular reabsorption, due to the lowered intraluminal sodium, potassium and urea concentrations and increased intratubular fluid flow. Some of the acute increase in urea excretion may be due to washout of medullary urea into the tubular fluid. 6. In water diuresis, some of the changes in solute excretion may similarly result from altered tubular reabsorption, perhaps influenced by suppression of antidiuretic hormone (A.D.H.). In addition, the slower changes in sodium output may be related to several consequences of change in body fluid volume.

2 396 J. C. ATHERTON AND OTHERS INTRODUCTION There are many reports in the literature that changes in urine flow may be accompanied by changes in urinary solute outputs, particularly sodium and urea. Thus, the diuresis caused by excretion of an osmotically active, metabolically inert solute such as mannitol has been reported, consistently, to be accompanied by increased output of sodium in man (e.g. Rapoport, West & Brodsky, 1949), dog (e.g. Wesson & Anslow, 1948) and rat (Windhager & Giebisch, 1961). Urea excretion is also increased in man (e.g. Chasis & Smith, 1938), dog (e.g. Shannon, 1938) and rat (Ullrich, SchmidtNielsen, O'Dell, Pehling, Gottschalk, Lassiter & Mylle, 1963). Changes in potassium and ammonium outputs are more variable in magnitude and even in direction. In water diuresis, the increase in urea clearance during rising urine flow and the higher urea clearance during sustained high flows are well known (e.g. SchmidtNielsen, 1958). However, reports of the effects of water diuresis in man and dog on sodium, potassium and ammonium excretion are less consistent and few observations have been made on the rat. For both water and mannitol diuresis, some of the inconsistency may be attributable to differences in experimental conditions. Secondly, little distinction has been made between changes in solute outputs during the transitional phase of increasing urine flow, which might involve modification in the operation of countercurrent mechanisms, and those evident during prolonged, sustained diuresis. Thirdly, simultaneous determinations of the major urinary solutes have been made in but few of these investigations in man or dog and no such investigation appears to have been reported in the rat. This report presents such data to compare the outputs of osmoles, sodium, potassium, ammonium and urea in nondiuretic rats with those during induction and prolonged maintenance of water and osmotic (mannitol) diuresis. Many of the data were accumulated in the course of investigations into the effects of water and mannitol diuresis on kidney composition. The results of such tissue analyses will be presented in accompanying reports (Atherton, Hai & Thomas, 1968a, b). METHODS Normal male albino rats (Wistar strain) weighing g, and fed on a standard rat cake (North Eastern Agricultural Society, Aberdeen, containing 217 % protein), were used. For 24 hr before each experiment, they were restrained in a finemeshed wire cage, placed over a funnel; this allowed collection under toluene of an overnight specimen of urine uncontaminated with faeces. Four groups of rats were then treated as follows: Group 1. In eighteen animals, plasma was obtained in the morning. Other than a qualitative check for normality of colour, urine analyses were not performed on this group.

3 SOLUTE OUTPUTS IN DIURESIS 397 Group 2. Nondiuretic controls. In fifteen conscious rats, no infusions or injections were given. Urine samples were obtained after 1J hr restraint in a Perspex cage and subsequently every 2 hr for a further 6 hr. After the final urine sample, blood samples were obtained in five of these animals. Group 3. Water diuresis was induced in twenty animals, restrained in the Perspex cage, by continuous intravenous infusion of hypotonic dextrose (25 g/100 ml.) at a rate of 0033 ml./min/100 g body wt. via a fine nylon catheter previously inserted under light ether anaesthesia into a lateral tail vein, until a water load of 4% body weight was established over a period of 2 hr. This technique has been found (ZainulAbedin, 1967) to produce a diuresis with glucosefree urine of low osmolality. This positive water load was then sustained by infusing 25 % dextrose equal to the volume of urine voided in the preceding collection period. Urine samples were collected 1 hr from the start of infusion and subsequently every 30 min for a further 6i hr. After the final urine sample the kidneys of all twenty animals were removed under light ether anaesthesia for analysis as described in an accompanying paper (Atherton et al. 1968a). Blood samples were obtained at the end of the experiment from fifteen of these rats. Group 4. In fifteen conscious rats restrained in the Perspex cage osmotic diuresis was induced by a priming dose of 1 ml. mannitol (15 g/100 ml.), administered intravenously via a catheter as in Group 1 rats, and subsequently sustained by continuous intravenous infusion of 15% mannitol at 0 1 ml./min for 7j hr. Urine samples were collected every 30 min. After the final urine sample blood samples were obtained from fourteen of the rats and the kidneys of ten of these were removed for chemical analysis as described in another paper (Atherton et al. 1968a). Urine collections in Groups 2, 3 and 4, and the infusions in Groups 3 and 4 were made while the animals were firmly, but comfortably, restrained in a Perspex cylindrical cage, allowing collection of urine without contamination by water or faeces. Voiding of urine could be induced with ease by a variety of means, including a whiff of ether and gentle sensory stimulation. Steadiness of urine flow and osmolality in successive samples collected by such means and by spontaneous voiding during water diuresis indicated that these methods did not stimulate endogenous secretion of antidiuretic hormone. Plasma samples were obtained by centrifugation in heparinized tubes of blood collected by cardiac puncture from animals lightly anaesthetized with ether after the collection of the final urine sample. Analy8es. Urine and plasma sodium and potassium were determined by flame photometry (EEL); urine and plasma urea and urine ammonia by Conway's (1957) microdiffusion method; urine and plasma osmolality were determined by freezingpoint depression (Advanced osmometer, Model 31 LAS). RESULTS Plasma concentrations (mean+ s.e.) in nondiuretic rats sacrificed in the morning (Group 1) and in nondiuretic (Group 2), water diuretic (Group 3) and osmotic diuretic rats (Group 4) sacrificed after 71 hr restraint are given in Table 1. The morning values for the nondiuretic animals are similar to many reports in the literature for rats on normal protein intake (e.g. Ullrich et al. 1963; Clapp, 1966), as are the mean overnight urinary solute outputs included in Table 2. The urinary data represent the pooled results of all Groups, since analysis of variance showed no statistically significant differences. The number of collection periods during diuresis was greater than in

4 398 J. C. ATHERTON AND OTHERS nondiuretic experiments; therefore, the effects of diuresis on solute outputs have been assessed by comparing outputs in the nondiuretic series with the outputs over corresponding times in the diuretic experiments, calculated by pooling several periods. The levels of statistical significance are included in Figs. 2, 3, 4, 7 and 8. TABLE 1. Group 1. Nondiuretic (morning) (18) 2. Nondiuretic (afternioon) (5) 3. Water diuresis (15) 4. Osmotic diuresis (15) Plasma concentrations (mean+ s.x.) in nondiuretic rats, and after 7l hr water or osmotic diuresis Sodium (mequiv/l.) Potassium (mequiv/l.) Urea (mmole/l.) Osmoles (mosmole/l.) ( )* * Sum of chemically determined solutes, i.e. (Urea + 2(Na+ + K+ + NH')). Number of experiments in each series in parentheses. TABLE 2. Mean (+ S.E.) overnight urinary concentrations and outputs in fifty rats subsequently used in fifteen nondiuretic (Group 2), twenty water diuretic (Group 3), and fifteen osmotic diuretic (Group 4) experiments Output Concentration Osmolal Urea (mosmole/l.) (mmole/l.) Osmoles Urea Sodium Potassium Ammonium (/umolel/ (,umole/ (,uequiv/ (,uequiv/ (/tequiv/ min) min) min) min) min) Nondiuresis. Comparison with values obtained in the animals sacrificed in the morning indicated that there were no statistically significant changes in plasma sodium and osmolal concentrations. Plasma urea concentration, however, fell (Table 1). In noninfused rats (and in the water diuretic series) urinary solute outputs were usually lower in the first collection period (Figs. 24) than in the overnight samples (Table 2). This is presumed to be related to the restraint imposed by immobilization in the cage. Subsequently, solute outputs tended to rise to reach maximal values in, usually, the 315 hr sample. These changes were usually small compared with those observed in the diuretic series, but were fairly consistent. Thus, by analysis of variance, the changes in osmolal, sodium, potassium and ammonium outputs were highly significant (P < 0.005). Since water intake was allowed ad libiturm, the significance (statistical or otherwise) of a similar pattern of change in urine flow, and therefore of inverse changes in solute concentrations, is not certain. Water diuresis. The drop in plasma urea, sodium and osmolal concentrations below the values seen in nondiuretic animals (Table 1) corresponds,

5 SOLUTE OUTPUTS IN DIURESIS 399 approximately, to that expected from diluting body fluids by the 4 % body weight water load. Plasma urea concentration fell to a mean value similar to that observed in the nondiuretic controls (Table 1) restrained for the same period. 400 [ 200 _ E a 800 0_ t t 02 En_ Time (hr) Fig. 1. Mean concentrations of urea, sodium and osmoles and urinary flows during water diuresis (Group 3; n = 20). In this and subsequent figures, vertical bars represent+s±.e. of mean. The patterns of change in urine flow and in individual solute concentrations in water diuresis are shown in Fig. 1. The changes in the outputs of osmoles (Fig. 2), sodium and potassium (Fig. 3) and urea and ammonium (Fig. 4) are plotted with the corresponding data for the Group 2 control series for comparison. Increased flow and decreased total osmolal and individual solute concentrations were usually evident in the first collection period; maximal flows and minimal osmolality were achieved at about 3 hr (Fig. 1) but the steepness and extent of the fall in concentration differed between individual solutes. The changes in urinary ammonium and potassium concentrations were similar, qualitatively, to those in total osmolality and are not presented separately. The changes in mean urinary sodium concentration differed from those in the other solutes in that after 26 Phy. I97

6 400 J. C. ATHERTON AND OTHERS reaching minimal values at 1J72 hr, there occurred an increase, variable in magnitude between individual experiments, which continued long after the time of establishment of the constant water load (Fig. 1). Accordingly, increases in urinary sodium outputs, highly variable between individual experiments but significantly greater than the spontaneous changes observed in the nondiuretic controls in the later collection periods, also continued for many hours (Fig. 3) _ <005 <0 001 <005 < _ A._ 0 5 I Time (hr) Fig. 2. Mean (± s.e.) urinary osmolal outputs and flows during nondiuresis ( Group 2; n = 15), and water diuresis ( Group 3; n = 20). P values represent the statistical significance of differences between Group 2 results and the pooled corresponding periods in Group 3. Conversely, urinary urea concentration decreased less steeply than other solutes (Fig. 1) reaching minimal values at about 5 hr; urinary urea outputs (Fig. 4), therefore, were considerably and significantly increased during the developing diuresis, reaching peak values much earlier than maximal sodium outputs, but then decreased progressively to low values at 71 hr. Differences from the nondiuretic values over the last 4 hr were small and nonsignificant. The changes in potassium excretion were similar to those observed in the

7 SOLUTE OUTPUTS IN DIURESIS 401 nondiuretic controls (Fig. 3) but ammonium outputs were higher (Fig. 4). The significantly higher osmolal outputs seen in water diuresis (Fig. 2) were largely attributable to the increased sodium and urea outputs. Osmotic diuresis. Plasma concentrations at 71 hr are given in Table 1 A. The difference between the plasma osmolality determined directly (373 m osmoles/l.) and that calculated by summing the chemically determined solutes (300 mosmoles/l.) is presumed to represent mannitol. <0 05 <0 05 < Tim Time (hr) Fig. 3. Mean (+s.e.) urinary outputs of sodium and potassium in nondiuresis ( Group 2; n = 15), and water diuresis ( Group 3; n = 20). P values were calculated as in Fig. 2. Changes in urine flow, in osmolal concentration and outputs and in individual solute concentrations and outputs are presented in Figs. 58. In Figs. 5 and 6 calculated osmolality refers to the value (Urea + 2(Na+ + K+ + NH)+) for concentration and output, that is, to chemically determined solute which, in nondiuretic and water diuretic urines, corresponds closely with directly determined total osmolal concentration and output. The differences between total and calculated values, therefore, can be taken to approximate mannitol concentration and output. On this basis, it is evident that after about 1 hr mannitol output remained essentially constant and equal to the rate of mannitol infusion. It is also evident (Fig. 5) that the changes in urine flow are more closely related to changes in calculated osmolality than to those in mannitol concentration. 262

8 402 J. C. ATHERTON AND OTHERS Urinary flow increased in the first halfhour period reaching maximal values of about 0x2 ml./min (similar to those observed in water diuresis) at 12 hr; the flow then declined, slowly, to mean values of less than 015 ml./min in the later periods. Conversely, urinary urea, ammonium and osmolal concentrations fell sharply in the first period, but, with the decreasing flow in the later periods, mean urinary osmolality rose from minimal values of 555 mosmole/l. at i1 hr to 680 mosmole/l. These <0o01 < <001 <0001 <0001 <0001 ZL ~ ~ 4 a 1 1 r I I I Sz; Time (hr) Fig. 4. Mean (+s.e.) urinary outputs of urea and ammonium in nondiuresis ( Group 2; n = 15), and water diuresis ( Group 3; n = 20). P values were calculated as for Fig. 2. changes in urinary flow and osmolality may be related to the progressive negative water balance which increased from mean values of 6 ml. at 2 hr to 15 ml. at 4 hr and to over 20 ml. at about 6 hr. After the initial decrease in urinary solute concentrations, there were marked differences between subsequent changes in the concentrations and, therefore, outputs of the various solutes. Urinary urea concentration fell relatively much less than the further increase in flow, so that mean urinary urea output rose sharply to reach maximal values at 12 hr (Fig. 8). Although urea outputs declined in subsequent periods they remained significantly higher than those observed in the control nondiuretic experiments.

9 SOLUTE OUTPUTS IN DIURESIS 403 After the initial fall urinary sodium and potassium concentrations (Fig. 5) actually rose despite increasing flow, sodium steeply to values approaching the prediuretic samples, potassium more slowly. Accordingly, both sodium and potassium outputs increased considerably (Fig. 7) to values far higher than those observed in water diuresis and in nondiuretic 100_ Urea (mmole/l.) 1 ' 0 40 Na (mequiv/l.) Osmolal (mosmole/l.) 400 k Flow (ml./min) 02 L o L I I Time (hr) Fig. 5. Mean (± s.e.) concentrations of urea, sodium and osmoles (total and calculated) and urinary flows during mannitol diuresis (Group 4; n = 15). The calculated osmolal concentration was determined as the sum (urea + 2(Na + K + NH4)); the difference between this value and total osmolal concentration is an approximation of mannitol concentration. controls. After these peak values the outputs ofboth sodium and potassium declined progressively, but were still far higher than the nondiuretic values (P < 0.001) over the final periods. Ammonium outputs increased to values higher than those in the nondiuretic animals in the early periods. Subsequent differences were small (Fig. 8). DISCUSSION The initial fall in urinary solute outputs in Series 2, 3 and 4 is presumably related to the restraint imposed by immobilization in the rat cage. The subsequent increase and then decrease in sodium and potassium outputs in the nondiuretic rats (Series 2) are similar to human circadian rhythms

10 404 J. C. ATHERTON AND OTHERS (Mills, 1966), even though the animal is normally nocturnal. Similar rhythmic changes in urine flow (Konig & Meyer, 1967) and various urinary solutes (Kurcz, 1959) have been described in the rat. The changes characteristic of osmotic diuresisrising urine flow and osmolal output with urinary osmolality declining towards plasma osmolalitywere similar to those described previously in the rat (Corcoran, Del Greco & Masson, 1956; Koike & Kellogg, 1957) as in dog and man, and will not be discussed further ~.5 80 o4o40 t 0 E0.2 i01 I Pi Time (hr) Fig. 6. Mean (+ S.E.) urinary flows and outputs of total osmoles in nondiuresis ( Group 2; n = 15), and mannitol diuresis ( Group 4; n = 15) and of chemically determined solutes ( Calc) in Group 4. The calculated osmolal output was determined as the sum (urea+ 2(Na+K + NH4)); the difference between this value and total osmolal output is an approximation of mannitol output. The fact that excretion of osmotically active mannitol is accompanied by increased outputs of other solutes, particularly sodium, is also well known. However, in the present experiments the quantitative contribution of individual solutes to the increased total osmolal output changed with time, particularly in the periods of sustained diuresis. Various factors contributing to such changes in solute excretionfor example, glomerular filtration rate (G.F.R.), transtubular concentration gradients, membrane permeability, active transport mechanismsprobably also changed with time. Increased solute excretion during mannitol loading must result primarily

11 SOLUTE OUTPUTS IN DIURESIS 405 from altered tubular transport, since the filtered load of solutes, other than mannitol, decreases; thus, plasma solute concentrations, other than mannitol, fall during the early phase of loading with hypertonic mannitol (West & Rapoport, 1950; Atherton et al. 1968a), presumably owing to osmotically induced movement of water from cells into the extracellular 6.5.> * 2 v rr ' > L r r.j.. w I 'I Time (hr) Fig. 7. Mean (±s.e.) outputs of sodium and potassium in nondiuresis ( Group 2; n = 15), and mannitol diuresis ( calculated as for Fig. 2. Group 4; n = 15). P values were space (Stahl, 1965). Furthermore, mannitol loading appears to cause decreased G.F.R. in the rat (Windhager & Giebisch, 1961), possibly owing to efferent renal arteriolar dilatation (Nashat & Portal, 1967). The presence of poorly reabsorbable mannitol impedes isosmotic reabsorption of water in the proximal tubule; changes in intratubular sodium concentration depress active proximal and distal tubular reabsorption (Windhager & Giebisch, 1961), accounting for the prompt and sustained increased excretion of sodium observed in the rat (present experiments; Windhager & Giebisch, 1961) as in man and dog. Depressed proximal tubular reabsorption of potassium during mannitol loading in rat (Rector, Bloomer & Seldin, 1964) may similarly contribute

12 406 J. C. ATHERTON AND OTHERS to the consistently increased potassium excretion observed here, as some workers have reported in the dog (e.g. Wesson & Anslow, 1948). However, there are also reports of unchanged or variable responses in both dog and man. Changes in urea clearance with urine flow have been conventionally described as secondary to altered passive tubular reabsorption (Smith, 1951). Thus, the increased urea excretion during mannitol diuresis in man ~L 2 _<001 <0 01 < <0001 <005 <005 1 Sz; Time (hr) Fig. 8. Mean ( s+.e.) outputs of urea and amnmonium in nondiuresis ( Group 2; n = 15), and mannitol diuresis ( Group 4; n = 15). P values were calculated as for Fig. 2. (Chasis & Smith, 1938), dog (e.g. Shannon, 1938) and rat (Ullrich et al. 1963), as in the present experiments, probably results from a changed concentration gradient, less favourable for passive urea diffusion from the proximal tubule and collecting duct (Ullrich et al. 1963), due to mannitolobligated water retention in the nephron. There seems no reason to suggest the participation of active urea transport, either in the acute changes observed during the early stages or in the subsequent decrease, although it is claimed that active urea transport out of rat collecting duct is demonstrable during mannitol diuresis (Bray & Preston, 1961; Lassiter, Mylle & Gottschalk, 1966; Clapp, 1966). Previous reports of ammonium outputs during mannitol diuresis have

13 SOLUTE OUTPUTS IN DIURESIS 407 been inconsistent: increased excretion in man (Beck, 1958; Steinmetz & Bank, 1963) and unchanged or decreased excretion in dog (West & Bayless, 1957). No data have been presented previously for the rat, although the results of micropuncture experiments (Hayes, Mayson, Owen & Robinson, 1964) suggest increased excretion as observed in the present experiments in the earlier periods. This may be due to a reduction in ph in plasma (Beck, 1958) and tubules (Hayes et al. 1964) during mannitol infusion; but no ph data are available in the present experiments. The causes of the reductions in outputs of sodium, potassium, ammonium and urea in the later periods are uncertain, but it seems likely that the increasing negative water balance would lead to a decreasing G.F.R. and filtered solute load. The decreasing plasma urea concentration would also contribute to a decreased filtered load. Similar considerations apply to any changes in solute outputs during water diuresis. The small, but statistically significant, increase in sodium excretion in the rat has not been reported previously. However, there was considerable variation in the magnitude of this increase in individual experiments. Such variability, lack of assessment of the contribution of spontaneous, circadian rhythmic variations, differences in preexisting diet and hydration, and differences in the mode of water loading may account for much of the inconsistency in reports of changes in both man and dog (e.g. Coxon & Ramsay, 1967). The latter authors emphasized the transient nature of the salt loss which occurred during the phase of rising flow. This may be related to the different means of hydration. The present finding of unchanged potassium excretion during the generation and maintenance of water diuresis in the rat has not been reported previously. As with sodium. there is inconsistency in the literature on man and dog (e.g. Coxon & Ramsay, 1967). The tubular factors directly involved in changes in solute outputs in osmotic diuresis have been mentioned above. In water diuresis, these tubular factors may be further influenced by changes in the production of endogenous antidiuretic hormone (A.D.H.) and by the sustained expansion of body water. On conventional theory diminished endogenous production of A.D.H. during the developing diuresis would not be expected to influence G.F.R. which, in the rat, has been reported to be independent of urine flow (Dicker & Heller, 1945; Friedman, 1947). However, sustained expansion of body fluid volume might lead to an increased G.F.R. during the water loading phase, as can occur in dog (Martino & Earley, 1967). Increasing G.F.R. may thus contribute to the small, but significant, increase in sodium output and to the more substantial increase in urea excretion. Since the time courses of the changes in sodium and urea excretion were different, however, changes in tubular transport must have been involved.

14 408 J. C. ATHERTON AND OTHERS The decreased endogenous production of A.D.H. is unlikely to have been responsible for such increased sodium excretion since exogenous A.D.H. caused increased sodium excretion in the rat (Atherton, Hai & Thomas, 1967). However, water loading can lead to depressed proximal tubular reabsorption ofsodium, by intrarenal mechanisms (Martino & Earley, 1967). Furthermore, expansion of extracellular fluid and plasma volumes would cause reduced aldosterone production (Bartter, Liddle, Duncan, Barber & Delea, 1956) and, therefore, reduced tubular reabsorption of sodium. As discussed already with osmotic diuresis, urea clearance is increased at higher urine flows in man (Chasis & Smith, 1938) and dog (Shannon, 1936), and the high values observed during sustained water diuresis in the present experiments may also be explained by diminished passive reabsorption from dilute intratubular fluid. Similarly, the diminution in urea output over the later periods may be associated with falling plasma urea concentrations to the low values observed at 71 hr. Ammonium outputs were increased in the present experiments, in agreement with previous suggestions in rat (Richterich, 1962). There is considerable variation in the literature, however, concerning the flowdependence of ammonium excretion, and the relation to urinary ph (see, e.g. Richterich, 1962). In the present experiments, the possible influence of urinary ph has not been assessed. The high medullary contents of solute present during antidiuresis might act as an additional source of urinary solute, at least during the phase ofincreasing urine flow. The phenomenon of' exaltation 'a transient rise in urea clearance after water administration to values higher than those found at similar, but steady, flowshas been explained as washout of the medullary urea into the urine (SchmidtNielsen, 1958). This may have contributed to the changes in urea excretion in the earlier periods of the present experiments, particularly in the mannitol series. Hypertonic mannitol would be expected to stimulate or sustain endogenous A.D.H. production (Windhager & Giebisch, 1961). In the presence of A.D.H., the collecting duct wall is permeable to urea (Jaenike, 1961; Thomas, 1964; Foulkes, 1966), allowing passive diffusion into the nephron during the generation of the diuresis if the collecting duct fluid urea concentration fell more rapidly than that of the medullary interstitium. If progressive suppression of endogenous A.D.H. leads to reduced collecting duct permeability to urea, this would be a transient phenomenon in water diuresis, despite the maintenance of medullary solute concentration gradients (Atherton et al. 1968b). Any contribution made by medullary solute to an increment in the output of individual solutes would depend on the relative time courses and magnitude of changes in medullary interstitial and tubular concentrations, and in tubular membrane permeability.

15 SOLUTE OUTPUTS IN DIURESIS 409 In summary, the changes in solute excretion in the rat are, in general, similar to changes previously reported in other species. But we wish to emphasize differences in the time course of these changes between the various solutes. In particular, it is evident that even with fairly consistent urine flow and osmolality, considerable changes in the concentrations of individual solutes may occur. Consequently, it is possible that changes in renal tissue composition may occur despite apparent stability of urinary flow and osmolality. We wish to thank Mr K. Fletcher for technical assistance, and Professor J. N. Mills for advice in the presentation. REFERENCES ATHERTON, J. C., HAT, M. A. & THOMAS, S. (1967). Transient saluresis due to lysinevasopressin administration in the conscious water diuretic rat. J. Phy8iol. 190, 3031P. ATHERTON, J. C., HAi, M. A. & THOMAS, S. (1968a). The time course of changes in renal tissue composition during mannitol diuresis in the rat. J. Phy8iol. 197, ATHERTON, J. C., HAI, M. A. & THOMAS, S. (1968b). The time course of changes in renal tissue composition during water diuresis in the rat. J. Physiol. 197, BARTTER, F. C., LIDDLE, G. W., DUNCAN, L. E. JNR., BARBER, JEAN K. & DELEA, CATHERINE. (1956). The regulation of aldosterone secretion in man: the role of fluid volume. J. clin. Invest. 35, BECK, R. N. (1958). Osmotic diuresis and the base sparing function of the kidney. Clin. Sci. 17, BRAY, G. A. & PRESTON, A. S. (1961). Effect of urea on urine concentration in the rat. J. clin. Invest. 40, CHASIS, H. & SMITH, H. W. (1938). The excretion of urea in normal man and in subjects with glomerulonephritis. J. clin. Invest. 17, CLAPP, J. R. (1966). Renal tubular reabsorption of urea in normal and proteindepleted rats. Am. J. Physiol. 210, CONWAY, E. J. (1957). Microdiffusion Analysis and Volumetric Error, 4th edn. London: Crosby, Lockwood and Son. CORCORAN, A. C., DEL GRECO, F. & MASSON, G. M. C. (1956). Osmotic (mannitol) diuresis in the anesthetized rat. Effectiveness of water conserving mechanisms. Am. J. Physiol. 187, CoxoN, R. V. & RAMSAY, D. J. (1967). The effect of water diuresis on electrolyte excretion in unanaesthetized dogs. J. Physiol. 191, DICKER, S. E. & HELLER, H. (1945). The mechanism of water diuresis in normal rats and rabbits as analysed by inulin and diodone clearances. J. Physiol. 103, FOULKES, E. C. (1966). The action of pitressin on solute permeability of the rabbit nephron in vivo. J. gen. Physiol. 50, 18. FRIEDMAN, M. (1947). The creatinine, inulin and hippurate clearance in the rat. Am. J. Physiol. 148, HAYES, C. P. JNR., MAYSON, J. S., OWEN, E. E. & ROBINSON, R. R. (1964). A micropuncture evaluation of renal ammonia excretion in the rat. Am. J. Physiol. 207, JAENIKE, J. R. (1961). The influence of vasopressin on the permeability of the mammalian collecting duct to urea. J. clin. Invest. 40, KOIKE, T. I. & KELLOGG, R. H. (1957). Osmotic diuresis in the unanesthetized hydropenic rat. Am. J. Physiol. 191, KONIG, A. & MEYER, A. (1967). Tagesperiodische schwankungen der urinausscheidung und des adiuretingehaltes im hypophysenhinterlappen mannlicher wistarratten. Acta endocr., Copenh. 54, KURCZ, M. (1959). Diurnal variation in the volume and composition of the urine of albino rat. Acta biol. hung. suppl. 3, 43.

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