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Where do the experiments on rat kidneys exposed to puromycin leave us in attempting to evaluate further the pathophysiology of oedema in human nephrotic syndrome? They cannot be considered conclusive. The arguments favouring an intrinsic abnormality in the kidney as a major factor in sodium retention, rather than this being a secondary response to humoral and neural influences stimulated by changes in actual or perceived plasma volume, would be strengthened by experiments using another animal model of the nephrotic syndrome. But taken together with the longstanding clinical observations demonstrating the absence of any correlation between plasma oncotic pressure and diuresis or sodium retention, the work particularly of Dorhout-Mees and his colleagues on plasma volume, the frequent failure of infusions of salt-free albumin to induce natriuresis, and the profound resistance to intensive diuretic therapy in many nephrotics, the evidence for an important, probably predominant, role of intrinsic renal retention of sodium is strong. This is not to negate the importance of Starling forces in determining the distribution of the retained salt and water, nor to suggest that, on occasion at least, hypovolaemia, relative or absolute, may contribute to sodium retention. Certainly hypotensive shock due to hypovolemia in nephrotic patients untreated by diuretics has been observed often enough, particularly in children, and the risk of over-enthusiastic diuretic treatment resulting in tubular necrosis in the course of management of minimal change disease, is well recognised. An important role for leakage of plasma from the vascular compartment in the initiation of the oedema of the nephrotic state is certainly likely, but to consider this the major continuing mechanism is really not tenable.(ABSTRACT TRUNCATED AT 250 WORDS)

1. The kidney taken from a rat rendered nephrotic by exposure to puromycin aminonucleoside retains sodium abnormally when perfused in isolation and has an abnormally low vascular resistance (J. D. Firth et al., Clin. Sci. 1989; 76, 387-95). In this study the relation of oxygen consumption to sodium reabsorption has been examined in the isolated nephrotic organ, which has also been exposed to a variety of natriuretic agents and to the effect of inhibition of metabolism by cooling, in an attempt to discern the transport process, or processes, responsible for abnormal tubular handling of sodium. In addition, the effects of three endogenous vasoconstrictors, noradrenaline, angiotensin II and endothelin, on the function of the isolated nephrotic kidney have been examined. 2. The ratio of mol of sodium reabsorbed by the tubules of the isolated nephrotic kidney to mol of oxygen consumed was reduced in comparison with the control kidney (means +/- SEM): 9.22 +/- 0.97 versus 15.43 +/- 1.55 (P less than 0.002). 3. In the presence of ouabain (1 mmol/l), acetazolamide (1 mmol/l), frusemide (200 mumol/l), the combination of these three agents together, hydroflumethiazide (100 mumol/l), benzamil (100 nmol/l) or atrial natriuretic peptide (1000 pmol/l), a lesser increment in sodium excretion was induced in the isolated nephrotic kidney than in the control kidney and the nephrotic organ continued to excrete less sodium in both absolute and fractional terms. 4. This suggests that enhanced tubular sodium reabsorption in the isolated nephrotic kidney does not depend upon abnormally increased activity of the Na+/K(+)-adenosine triphosphatase, bicarbonate-dependent sodium transport, Na+/K+/2Cl- co-transport, electrically neutral proportionate reabsorption of sodium and chloride (distal tubule), epithelial sodium channel (distal tubule) or atrial natriuretic peptide-sensitive sodium transport processes. 5. When isolated nephrotic kidneys and normal kidneys were cooled to 8-10 degrees C the handling of sodium became virtually identical in the two groups. On re-warming to 37 degrees C, the original differences in sodium handling between nephrotic and control kidneys were restored. This implies that the mechanism responsible for the abnormal tendency to retain sodium is temperature-sensitive; as yet it remains otherwise undefined. 6. The sensitivity of the renal vessels to noradrenaline, angiotension II and endothelin, as judged by the percentage reduction in perfusate flow rate produced by a given concentration of any of these agents, was not substantially altered in the nephrotic kidney compared with the control kidney. Increase in vascular tone was not associated with amelioration of the tendency of the isolated nephrotic organ to retain sodium. Increasing concentrations of angiotensin II caused the filtration rate to increase in the nephrotic kidney. This effect was unexpected: in the control preparation, as anticipated, angiotensin II caused the filtration rate to decrease.

1. In cirrhosis the kidney tends to retain salt and water abnormally. Two theories have been proposed to account for this: the 'underfilling' theory, in which sodium retention is thought to occur secondary to perceived underfilling of the circulation, and the 'overflow' theory, in which sodium retention is considered to be due to a primary renal defect. 2. Using the model of cirrhosis produced by carbon tetrachloride administration in the rat, the ability of the kidney to excrete sodium has been examined in vivo and during isolated perfusion. 3. Cirrhotic animals demonstrated a reduced ability to excrete an acute sodium load: 6 h after 2 mmol of sodium was given by gavage, 27.5 +/- 10.5% had been excreted by the cirrhotic rats and 62.5 +/- 7.0% by control rats (P less than 0.025). 4. In contrast, during isolated perfusion, kidneys from cirrhotic animals excreted the same amount of sodium as control animals over a range of perfusion pressures from 90 to 150 mmHg (12 to 20 kPa). 5. The data are consistent with the view that in cirrhosis the kidney retains sodium in response to immediate external factors.

1. In order to examine the handling of sodium by the nephrotic kidney when separated from the immediate influences of renal nerves and humoral factors, kidneys were taken from nephrotic rats (puromycin aminonucleoside) and studied over a range of perfusion pressures using the isolated perfused kidney technique. 2. When perfused with medium containing 6.7 g/dl albumin, the nephrotic kidneys performed differently from controls with a reduction in sodium excretion at all pressures [(mean +/- SEM) 1.14 +/- 0.43 vs 4.20 +/- 0.69 mumol/min at 105 mmHg (14 kPa); 6.32 +/- 1.56 vs 44.60 +/- 5.30 mumol/min at 150 mmHg (20 kPa)]. Renal vascular resistance, inulin clearance, fractional sodium excretion and fractional lithium excretion were also reduced. 3. When kidneys were perfused without oncotic agent these differences between nephrotic and control kidneys remained. Perfusion with medium containing 10 g/dl albumin, designed to prevent glomerular filtration, abolished the difference in vascular resistance between the two groups. Captopril had no effect on the sodium retention or vascular resistance of nephrotic kidneys. 4. It was concluded that (a) the isolated nephrotic kidney demonstrates increased avidity for sodium, (b) the abnormality of sodium handling is not dependent on the presence of altered oncotic forces, and (c) the alteration in vascular resistance is conditional upon glomerular filtration.

Studies were performed to determine the effects of elevation of renal venous pressure on sodium excretion by the isolated perfused rat kidney in the presence and absence of a specific V2-receptor agonist, 1-des-amino-8-D-arginine vasopressin (dDAVP), at a concentration (1 ng/ml) expected to have maximal antidiuretic activity but minor vasopressor action. In either the presence or absence of dDAVP, increments in venous pressure led to falls in perfusate flow rate and glomerular filtration rate, which became significant at an imposed pressure greater than or equal to 18.75 mmHg. In the absence of dDAVP, absolute sodium excretion fell as venous pressure increased, and there was a negative correlation between fractional sodium excretion (FENa) and renal venous pressure (RVP) within each experiment and when all data points were combined: FENa = 3.46-0.072RVP (r = -0.608, P less than 0.01). In contrast, in the presence of dDAVP, absolute sodium excretion was unchanged, and in four of five experiments FENa rose as venous pressure increased (in one it remained unchanged). Linear regression analysis of all data points showed a positive correlation between FENa and RVP: FENa = 1.27 + 0.127RVP (r = 0.392, P less than 0.05). The slopes of the two regression lines were significantly different (P less than 0.001). It is postulated that this effect of dDAVP may be mediated via changes in the distal tubular pressure response to elevation of RVP. Such an effect of vasopressin could explain the observation that the response to renal vein constriction in vivo is dependent on volume status.

Contemporary theories of oedema formation are often based on the idea that "effective" blood volume is reduced, and that sodium retention and oedema are a result of the kidney responding, as in haemorrhage, to a perception by receptors in the circulation that blood volume is inadequate. This idea has enhanced understanding of the pathophysiology of such conditions as cardiac failure and cirrhosis, but has obscured the fact that blood volume is almost always increased in oedematous states. Evidence is presented that an increase in renal venous pressure can cause sodium retention by a direct action on the kidney: a rise in venous pressure could thereby initiate a vicious circle by causing sodium retention, expansion of plasma volume, and further increase in venous pressure. This sequence of events may be crucial in the pathophysiology of cor pulmonale, and an exacerbating factor in other oedematous states.

1. Isolated kidneys taken from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were perfused over a range of perfusion pressures. 2. Lithium clearance was used as an index of proximal tubule sodium handling. 3. When the perfusate contained an oncotic agent (albumin, 6.7 g/dl) the SHR kidneys performed differently from the WKY kidneys with a reduction in inulin clearance, sodium excretion, fractional sodium excretion and fractional lithium excretion [at 105 mmHg (14 kPa) perfusion pressure, SHR 6.0 +/- 1.1% vs WKY 12.6 +/- 2.4% (mean +/- SEM); at 150 mmHg (20 kPa), SHR 17.1 +/- 1.6% vs WKY 27.0 +/- 2.3%]. Calculated indices of distal tubular function showed no major differences between SHR and WKY. 4. When kidneys were perfused without oncotic agent in the perfusate the differences between SHR and WKY in tubular handling of sodium and lithium were largely abolished. 5. These findings are consistent with the hypothesis that increased sodium reabsorption occurs in the proximal tubules of the kidneys of SHR and suggest that this is an intrinsic property of the kidney, not immediately dependent on neural or humoral factors. Increased sodium reabsorption in the proximal tubule may contribute significantly to the existence of hypertension in the SHR.