Glomerular hemodynamics in rats with chronic sodium depletion. Effect of saralasin

Prerenal acute kidney injury—still a relevant term in modern clinical practice?

The traditional taxonomy of acute kidney injury (AKI) has remained pervasive in clinical nephrology. While the terms ‘prerenal’, ‘intrarenal’ and ‘postrenal’ highlight the diverse pathophysiology underlying AKI, they also imply discrete disease pathways and de-emphasize the nature of AKI as an evolving clinical syndrome with multiple, often simultaneous and overlapping, causes. In a similar vein, prerenal AKI comprises a diverse spectrum of kidney disorders, albeit one that is often managed by using a standardized clinical algorithm. We contend that the term ‘prerenal’ is too vague to adequately convey our current understanding of hypoperfusion-related AKI and that it should thus be avoided in the clinical setting. Practice patterns among nephrologists indicate that AKI-related terminology plays a significant role in the approaches that clinicians take to patients that have this complex disease. Thus, it appears that precise terminology does impact the treatment that patients receive. We will outline differences in the diagnosis and management of clinical conditions lying on the so-called prerenal disease spectrum to advocate caution when administering intravenous fluids to these clinically decompensated patients. An understanding of the underlying pathophysiology may, thus, avert clinical missteps such as fluid and vasopressor mismanagement in tenuous or critically ill patients.

Benign hyperfiltration after living kidney donation

Almost one-third of transplanted kidneys come from living donors, who sacrifice approximately 30% of their pre-donation glomerular filtration rate (GFR) after they experience compensatory hypertrophy and hyperfiltration in their remaining kidney. Although hyperfiltration can cause glomerular injury, many studies have suggested that donor nephrectomy itself does not cause long-term loss of GFR at a higher rate than what is seen in the normal aging population. However, when post-donation kidney diseases occur in an unfortunate few, recent studies suggest that GFR loss at donor nephrectomy increases the risk of eventual end-stage renal disease (ESRD). In this issue of the JCI, Lenihan and colleagues evaluated glomerular dynamics in a cohort of kidney donors prior to, within 1 year of, and several years after kidney donation. Their results suggest that adaptive hyperfiltration in the remaining kidney occurs without glomerular hypertension, furthering our understanding of the relatively benign renal outcomes for most living kidney donors.

Longitudinal study of living kidney donor glomerular dynamics after nephrectomy

BACKGROUND

Over 5,000 living kidney donor nephrectomies are performed annually in the US. While the physiological changes that occur early after nephrectomy are well documented, less is known about the long-term glomerular dynamics in living donors.

METHODS

We enrolled 21 adult living kidney donors to undergo detailed long-term clinical, physiological, and radiological evaluation pre-, early post- (median, 0.8 years), and late post- (median, 6.3 years) donation. A morphometric analysis of glomeruli obtained during nephrectomy was performed in 19 subjects.

RESULTS

Donors showed parallel increases in single-kidney renal plasma flow (RPF), renocortical volume, and glomerular filtration rate (GFR) early after the procedure, and these changes were sustained through to the late post-donation period. We used mathematical modeling to estimate the glomerular ultrafiltration coefficient (Kf), which also increased early and then remained constant through the late post-donation study. Assuming that the filtration surface area (and hence, Kf) increased in proportion to renocortical volume after donation, we calculated that the 40% elevation in the single-kidney GFR observed after donation could be attributed exclusively to an increase in the Kf. The prevalence of hypertension in donors increased from 14% in the early post-donation period to 57% in the late post-donation period. No subjects exhibited elevated levels of albuminuria.

CONCLUSIONS

Adaptive hyperfiltration after donor nephrectomy is attributable to hyperperfusion and hypertrophy of the remaining glomeruli. Our findings point away from the development of glomerular hypertension following kidney donation.

TRIAL REGISTRATION

Not applicable. FUNDING. NIH (R01DK064697 and K23DK087937); Astellas Pharma US; the John M. Sobrato Foundation; the Satellite Extramural Grant Foundation; and the American Society of Nephrology.

Glomerular and tubular function in the diabetic kidney

Diabetes mellitus with its attendant complications is a significant cause of morbidity and mortality with diabetic nephropathy being the leading cause of end stage renal disease in the Western world. Characteristic structural and functional changes in the kidney early in the course of diabetes have been shown to have enduring effects on the progression of disease. A better understanding of the mechanisms underlying these changes is imperative to the development of new therapeutic strategies. Renal hypertrophy and hyperfiltration along with proximal tubular hyperreabsorption are among the distinctive features of early diabetic nephropathy. Additionally, there are particular alterations in the sensitivity of the glomerular and tubular function to dietary salt intake in early diabetes. Herein, we focus on these early physiologic changes and discuss some of the primary and secondary mechanisms discovered in recent years which lead to these alterations in kidney function.

ADP-ribosyl cyclase and ryanodine receptor activity contribute to basal renal vasomotor tone and agonist-induced renal vasoconstriction in vivo

An important role for the enzyme ADP-ribosyl cyclase (ADPR cyclase) and its downstream targets, the ryanodine receptors (RyR), is emerging for a variety of vascular systems. We hypothesized that the ADPR cyclase/RyR pathway contributes to regulation of renal vasomotor tone in vivo. To test this, we continuously measured renal blood flow (RBF) in anesthetized Sprague-Dawley rats. Infusion of the ADPR cyclase inhibitor nicotinamide intrarenally at low doses inhibits angiotensin II (ANG II)- and norepinephrine (NE)-induced vasoconstriction by 72 and 67%, respectively ( P < 0.001). RBF studies in rats were extended to mice lacking the predominant form of ADPR cyclase (CD38). Acute renal vasoconstrictor responses to ANG II and NE are impaired by 59 and 52%, respectively, in anesthetized CD38−/− mice compared with wild-type controls ( P < 0.05). Intrarenal injection of the RyR activator FK506 decreases RBF by 22% ( P > 0.03). Furthermore, RyR inhibition with ruthenium red attenuates ANG II and NE responses by 50 and 59%, respectively ( P ≤ 0.01). Given at higher doses, nicotinamide increases basal RBF by 22% ( P > 0.001). Non-receptor-mediated renal vasoconstriction by L-type voltage-gated Ca2+channels is also dependent on ADPR cyclase and RyRs. Nicotinamide and ruthenium red inhibit constriction by the L-type channel agonist BAY K 8644 by 59% ( P > 0.02) and 63% ( P > 0.001). We conclude that 1) ADPR cyclase activity contributes to regulation of renal vasomotor tone under resting conditions, 2) renal vasoconstriction induced by G protein-coupled receptor agonists ANG II and NE is mediated in part by ADPR cyclase and RyRs, and 3) ADPR cyclase and RyRs participate in L-type channel-mediated renal vasoconstriction in vivo.

An unexpected role for angiotensin II in the link between dietary salt and proximal reabsorption

We set out to confirm the long-held, but untested, assumption that dietary salt affects proximal reabsorption through reciprocal effects on the renin-angiotensin system in a way that facilitates salt homeostasis. Wistar rats were fed standard or high-salt diets for 7 days and then subjected to renal micropuncture for determination of single-nephron GFR (SNGFR) and proximal reabsorption. The tubuloglomerular feedback (TGF) system was used as a tool to manipulate SNGFR in order to distinguish primary changes in net proximal reabsorption (Jprox) from changes due to glomerulotubular balance. The influence of Ang II over Jprox was determined by the sensitivity of Jprox to the AT1 receptor antagonist, losartan. Plasma, whole kidneys, and fluid from midproximal tubules were assayed for Ang II content by radioimmunoassay. In rats on the standard diet, losartan reduced Jprox by 25% and reduced the maximum range of the TGF response by 50%. The high-salt diet suppressed plasma and whole-kidney Ang II levels. But the high-salt diet failed to reduce the impact of losartan on Jprox or the TGF response and actually caused tubular fluid Ang II content to increase. The persistent effect of Ang II on Jprox prevented a major rise in late proximal flow rate in response to the high-salt diet. These observations challenge the traditional model and indicate that the role of proximal tubular Ang II in salt-replete rats is to stabilize nephron function rather than to contribute to salt homeostasis.

Role of oxidative stress in angiotensin-induced hypertension

Infusion of ANG II at a rate not sufficient to evoke an immediate vasoconstrictor response, produces a slow increase in blood pressure. Circulating levels of ANG II may be within ranges found in normotensive individuals, although inappropriately high with respect to sodium intake. When ANG II levels are dissociated from sodium levels, oxidative stress (OXST) occurs, which can increase blood pressure by several mechanisms. These include inadequate production or reduction of bioavailability of nitric oxide, alterations in metabolism of arachidonic acid, resulting in an increase in vasoconstrictors and decrease in vasodilators, and upregulation of endothelin. This cascade of events appears to be linked, because ANG II hypertension can be blocked by inhibition of any factor located distally, blockade of ANG II, OXST, or endothelin. Such characteristics are shared by other models of hypertension, such as essential hypertension, hypertension induced by reduction in renal mass, and renovascular hypertension. Thus these findings are clinically important because they reveal 1) uncoupling between ANG II and sodium, which can trigger pathological conditions; 2) the various OXST mechanisms that may be involved in hypertension; and 3) therapeutic interventions for hypertension developed with the knowledge of the cascade involving OXST.

Endogenous angiotensin II modulates rat proximal tubule transport with acute changes in extracellular volume

In the present study, we examined whether the effect of endogenously produced angiotensin II on proximal tubule transport in the male Sprague-Dawley rat is regulated by acute changes in extracellular volume. We measured the magnitude of endogenous angiotensin II-mediated stimulation of transport by sequentially perfusing proximal tubules in vivo, first with an ultrafiltrate-like solution, then by reperfusion of the same tubule with an ultrafiltrate-like solution containing 10(-8) M losartan (angiotensin II receptor antagonist). During volume contraction, 10(-8) M losartan decreased volume reabsorption from 4.20 +/- 0.50 to 1.70 +/- 0.30 nl . mm-1 . min-1 (P < 0.05), a decrease of 58.0 +/- 7.0%. In contrast, after acute volume expansion, 10(-8) M losartan decreased volume reabsorption from 1.84 +/- 0.20 to 1.31 +/- 0.20 nl . mm-1 . min-1 (P < 0.05), a decrease of 29.6 +/- 9.0%. In hydropenic rats, addition of exogenous luminal angiotensin II had no effect on transport. However, in volume-expanded rats, addition of 10(-8) M angiotensin II increased volume reabsorption from 2.10 +/- 0.34 to 4. 38 +/- 0.59 nl . mm-1 . min-1 (P < 0.005). These data are consistent with endogenously produced angiotensin II augmenting proximal tubule transport to a greater degree during volume contraction than after volume expansion.

Effects of daily sodium intake and ANG II on cortical and medullary renal blood flow in conscious rats

Implanted optical fibers and laser-Doppler flow measurement techniques were used for the sequential measurement of regional renal blood flow in conscious rats to determine the effects of an increase of daily NaCl intake on the renal cortical blood flow and blood flow to the outer and inner medulla. Cortical blood flow was increased significantly (32%) by the second day when NaCl intake was increased from 1 to 7 meq/day and was increased further (50%) on the second day after a further elevation of NaCl intake to 13 meq/day. Blood flow to the outer and inner medulla was not changed as NaCl intake was elevated. The increase in renal cortical flow was closely associated with significant reductions in circulating concentrations of ANG II from 31 to 16 pg/ml. Rats given a continuous infusion of nonpressor does of ANG II (5.0 ng.kg(-1).min-1) to maintain constant plasma concentrations of ANG II as sodium intake was increased exhibited no increase of cortical flow. We conclude that reductions of plasma ANG II associated with incremental increases of daily sodium intake result in a rise of renal cortical flow. The elevated blood flow to the renal cortex may enhance sodium excretion and contribute to long-term sodium homeostasis.

Angiotensinogen gene null-mutant mice lack homeostatic regulation of glomerular filtration and tubular reabsorption

Chronic volume depletion by dietary salt restriction causes marked decrease in glomerular filtration rate (GFR) with little increase in urine osmolality in angiotensinogen gene null mutant (Agt-/-) mice. Moreover, urine osmolality is insensitive to both water and vasopressin challenge. In contrast, in normal wild-type (Agt+/+) mice, GFR remains remarkably constant and urine osmolality is adjusted promptly. Changes in volume status also cause striking divergence in renal structure between Agt-/- and Agt+/+ mice. Thus, in contrast to the remarkably stable glomerular size of Agt+/+ mice, glomeruli of Agt-/- mice are atrophied during a low salt and hypertrophied during a high salt diet. Moreover, the renal papilla, a structure unique to mammals and essential for urine diluting and concentrating mechanisms, is hypoplastic in Agt-/- mice. Thus, angiotensin is essential for the two fundamental homeostatic functions of the mammalian kidney, namely stable GFR and high urine diluting and concentrating capacity during alteration in extracellular fluid (ECF) volume. This is not only accompanied by angiotensin's tonic effects on renal vasomotor tone and tubule transporters, but also accomplished through its capacity to affect the structure of both the glomerulus and the papilla directly or indirectly.

Effect of luminal angiotensin II on rabbit proximal convoluted tubule bicarbonate absorption

The present in vitro microperfusion study examined the effect of luminal angiotensin II on proximal convoluted tubule (PCT) volume absorption and bicarbonate transport. Neither 10(-11) M, 10(-10) M, nor 2 x 10(-8) M luminal angiotensin II significantly affected PCT transport. When tubules were first perfused with enalaprilat to inhibit endogenous angiotensin II production, addition of 10(-10) M luminal angiotensin II increased volume absorption (0.72 +/- 0.08 vs. 0.86 +/- 0.07 nl x mm(-1) xmin(-1), P < 0.01) and bicarbonate transport (52.3 +/- 3.7 vs. 67.9 +/- 4.2 pmol x mm(-1) min(-1), P < 0.01). Addition of 10(-6) M losartan, an AT1 inhibitor, to the luminal perfusate inhibited volume absorption (0.95 +/- 0.14 vs. 0.72 +/- 0.11 nl x mm(-1) x min(-1), P < 0.05) and bicarbonate transport (65.0 +/- 7.3 vs. 54.7 +/- 9.2 pmol x mm(-1) x min(-1), P < 0.05). Addition of 10(-4) M luminal PD-123319, an AT2 inhibitor, was without effect. In tubules perfused with 10(-4) M luminal enalaprilat and 10(-4) M luminal PD-123319, addition of 10(-10) M luminal angiotensin II in the experimental period resulted in a stimulation in volume absorption (0.61 +/- 0.08 vs. 0.81 +/- 0.10 nl x mm(-1) x min(-1), P < 0.01) and bicarbonate transport (49.9 +/- 6.3 vs. 77.4 +/- 14.3 pmol x mm(-1) x min(-1), P < 0.01). In tubules perfused with 10(-6) M losartan and 10(-4) M enalaprilat, addition of luminal 10(-10) M angiotensin II resulted in no change in transport. These data are consistent with endogenous angiotensin II affecting PCT bicarbonate transport in vitro via luminal AT1 receptors.

A novel in vivo mechanism for angiotensin type 1 receptor regulation

This study examined whether a regulatory mechanism exists for the angiotensin II receptor that is compatible with in vivo homeostatic need. Experiments were conducted under two different experimental stresses, (1) deletion of receptor protein and (2) chronic extracellular fluid (ECF) volume depletion. To circumvent potentially dampening intermediary feedback signals in vivo, any feedback gain was completely averted through genetic engineering. The coding exon of angiotensin type 1A (AT1A) receptor gene (Agtr1a) was targeting-replaced with a reporter gene, lacZ, so that the transcription of lacZ, instead of Agtr1a, is driven by the native Agtr1a promoter. ECF volume depletion by dietary sodium restriction enhanced Agtr1a gene expression in the adrenal gland of wild-type mice. However, although blood pressure fell in the homozygous targeted mice, Agtr1a gene expression remained unchanged in the adrenal, indicating that adrenal Agtr1a gene expression is regulated entirely through angiotensin receptor-ligand interactions. In the kidney, AT1A mRNA assessed by Northern blotting also did not change in AT1A null-mutated mice with or without sodium restriction. However, tissue examinations for lacZ mRNA and activities indicated that sodium restriction and receptor protein depletion result in dramatic up-regulation of Agtr1a gene expression within the renal arterioles, which can be nullified by an experimental normalization of blood pressure. No such change was observed in wild-type mice. This study demonstrates a presence within the resistance vessel of a blood pressure-sensitive mechanism for AT1 receptor regulation that opposes a down-regulatory influence of the ligand during ECF volume depletion.

Angiotensin-independent mechanism for aldosterone synthesis during chronic extracellular fluid volume depletion

Wild-type (Agt+/+) and homozygous angiotensinogen deletion mutant (Agt-/-) littermates were placed on normal (NS) or low Na diet (LS) for 2 weeks. Plasma aldosterone levels (P(aldo)) were comparable during NS, and similarly elevated during LS in Agt+/+ and Agt-/-. Moreover, in both, the elevation in P(aldo) was accompanied by marked increase in adrenal zona glomerulosa cells and adrenal P450aldo mRNA. Agt-/- mice were distinguished from Agt+/+ mice by their higher plasma K level, by approximately 1.5 and approximately 3.8 mEq/liter during NS and LS, respectively. Within the Agt-/- group, P(aldo) was directly proportional to plasma K. The importance of K for the hyperaldosteronism during dietary Na restriction was verified by the observation that superimposition of K restriction led to hypotension in Agt+/+ and uniform death in Agt-/- mice along with a reduction in P(aldo) by 75 and 90%, respectively. Thus, suppression of potassium, but not angiotensin, led to a marked attenuation of hyperaldosteronism during dietary Na restriction. Therefore, (a) a powerful angiotensin-independent mechanism exists for the hyperaldosteronism during LS; (b) high K is a central component of this mechanism; (c) contrary to current belief, the tonic effect of high K on aldosterone synthesis and release does not require an intact renin-angiotensin system; and (d) normally, intermediary feedback signals for hyperaldosteronism, i.e., both hypotension and high K, are effectively masked by aldosterone actions.

Cellular expression of angiotensin type-1 receptor mRNA in the kidney

Angiotensin II has multiple renal effects that are important in the regulation of renal hemodynamics and electrolyte secretion, and binding sites for angiotensin II have been demonstrated in different cells of the kidney. In the present study the cellular localization of mRNA for the angiotensin type 1 (AT1) subtype of the angiotensin II receptor was studied in adult rat kidney using a cRNA probe and in situ hybridization. Strong labeling was demonstrated in tubule cells of the inner and outer stripe of the outer medulla. In emulsion-dipped sections, counter-stained with hematoxylin-eosin, labeling was identified in segment S3 of proximal tubules and in the thick ascending limb of loop of Henle (mTAL). The results suggest expression of AT1-receptor mRNA with a distinct compartmentalization within the nephron.

Renin expression in renal proximal tubule

Angiotensinogen, angiotensin-converting enzyme, and renin constitute the components of the renin-angiotensin system. The mammalian renal proximal tubule contains angiotensinogen, angiotensin-converting enzyme, and angiotensin receptors. Previous immunohistochemical studies describing the presence of renin in the proximal tubule could not distinguish synthesized renin from renin trapped from the glomerular filtrate. In the present study, we examined the presence of renin activity and mRNA in rabbit proximal tubule cells in primary culture and renin mRNA in microdissected proximal tubules. Renin activity was present in lysates of proximal tubule cells in primary culture. Cellular renin content in cultured proximal tubule cells was increased by incubation with 10(-5) M isoproterenol and 10(-5) M forskolin by 150 and 110%, respectively. In addition, renin transcripts were detected in poly(A)+ RNA from cultured proximal tubule cells by RNA blots under high stringency conditions. In microdissected tubules from normal rats, renin mRNA was not detectable with reverse transcription and polymerase chain reaction. However, in tubules from rats administered the angiotensinogen-converting-enzyme inhibitor, enalapril, renin was easily detected in the S2 segment of the proximal tubule. We postulate the existence of a local renin-angiotensin system that enables the proximal tubule to generate angiotensin II, thereby providing an autocrine system that could locally modulate NaHCO3 and NaCl absorption.

Effect of acute changes in glomerular filtration rate on Na+/H+ exchange in rat renal cortex

Studies were undertaken in Munich-Wistar rats to assess the influence of changes in filtered bicarbonate (FLHCO3), induced by changes in GFR, on Na+/H+ exchange activity in renal brush border membrane vesicles (BBMV). Whole-kidney and micropuncture measurements of GFR, FLHCO3, and whole-kidney and proximal tubule HCO3 reabsorption (APRHCO3) were coupled with BBMV measurements of H+ gradient-driven 22Na+ uptake in each animal studied. 22Na+ uptake was measured at three Na+ concentration gradients to allow calculation of Vmax and Km for Na+/H+ exchange. GFR was varied by studying animals under conditions of hydropenia, plasma repletion, and acute plasma expansion. The increase in GFR, FLHCO3, and APRHCO3 induced by plasma administration correlated directly with an increase in the Vmax for Na+/H+ exchange in BBMV. The Km for sodium was unaffected. In the plasma-expanded rats, the Vmax for Na+/H+ exchange was 22% greater than in the hydropenic rats (P less than 0.025) whereas APRHCO3 was 86% greater (P less than 0.001). These results indicate that increases in FLHCO3, induced by acute increases in GFR, stimulate Na+/H+ exchange activity in proximal tubular epithelium. This stimulation is a mechanism which can, in part, account for the delivery dependence of proximal bicarbonate reabsorption.

Renal functional reserve: its significance in normal and salt depletion conditions

The aim of this study was the evaluation, in healthy subjects, of the renal functional reserve (RFR), that is, the GFR increase induced by a combined infusion of amino acids (AA) and dopamine (D), in conditions of extracellular volume depletion caused by diuretic administration. In particular, this study was undertaken: a) to evaluate whether and to which extent, AA + D can reverse the functional GFR impairment induced by salt depletion, without volume restoration; b) to study whether any relationship may be found between the GFR in normal condition (the so-called "resting" GFR), and/or the renal functional reserve and the GFR impairment induced by salt depletion, in order to understand the role of both "resting" GFR and RFR in the degree of renal dysfunction induced by salt depletion. In control conditions the i.v. infusion of AA + D significantly increased RPF (+ 41% vs. baseline period) with a mean absolute increase of 211 ml/min. A similar pattern was observed in GFR behavior (+31.5% with 34 ml/min of mean absolute increase). A significant inverse exponential relationship was observed between GFR before AA + D i.v. infusion ("resting" GFR) and renal functional reserve (P less than 0.05), suggesting that, in normal conditions, these inversely related parameters may widely vary according to the tone of the glomerular arterioles. Following salt depletion, we observed a variable degree of GFR impairment. Both GFR and RPF were significantly decreased (-25.9%, P less than 0.05 and -29%, P less than 0.05, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Proximal nephron and renal effects of DuP 753, a nonpeptide angiotensin II receptor antagonist

The purpose of these studies was to quantitatively assess the role of endogenous angiotensin II activity in controlling transport in the proximal convoluted tubule (PCT) and whole nephron. We used the nonpeptide angiotensin II receptor antagonist DuP 753, which lacks the agonist and kinin/prostaglandin-inducing properties of saralasin and captopril, respectively. During in vivo microperfusion in the Munich-Wistar rat, we found that DuP 753 had a powerful inhibitory effect on bicarbonate (370 +/- 3 to 200 +/- 9 pEq/mm.min, P less than 0.001), chloride (214 +/- 3 to 105 +/- 9 pEq/mm.min, P less than 0.001), and water (5.2 +/- 0.1 to 2.8 +/- 0.2 nl/mm.min, P less than 0.001) absorption in the S1 subsegment of the PCT. At maximally effective doses, DuP 753 (10 mg/kg i.v.) was significantly more effective than was captopril (3 mg/kg i.v.) in inhibiting sodium chloride transport in the S1 PCT. DuP 753 is the most potent diuretic ever described in this segment. Consistent with the axial decline of angiotensin II receptor density in the PCT, DuP 753 was a less effective transport inhibitor in the S2 subsegment of the PCT, similar to captopril. Though downstream reabsorptive elements partially compensate for the action in the earliest segment of the nephron, we also showed using free-flow micropuncture and clearance techniques that DuP 753 induces a substantial diuresis, natriuresis and chloruresis. In conclusion, the marked decrease in S1 PCT fluid and electrolyte absorption induced by DuP 753 indicates that endogenous angiotensin II exerts significant tonic support of proximal transport in vivo.

Renal hemodynamics and segmental tubular reabsorption in early type 1 diabetes

To investigate mechanisms underlying GFR control in diabetes mellitus, renal hemodynamics and segmental tubular handling of sodium, using lithium clearance, were assessed in 41 insulin-dependent diabetics (IDD) treated by insulin for 11 +/- 8 days, and in 19 normal controls. Average GFR and effective renal plasma flow (ERPF) were slightly but not significantly higher (136 +/- 22 vs. 123 +/- 16 ml/min.1.73 m2) in IDD than in normal subjects. GFR and ERPF were positively and strongly correlated in controls (r = 0.61, P less than 0.001) and in diabetics (r = 0.72, P less than 0.0001) indicating the marked flow dependency of GFR in both populations. After adjustment for ERPF, GFR was significantly higher in diabetics, suggesting a role of increased glomerular capillary pressure and ultrafiltration coefficient in the subset of "hyperfiltering" patients. Both fractional (FPRNa) and absolute (APRNa) proximal sodium reabsorption were significantly higher in IDD and significantly correlated with GFR. The ensuing decrease in sodium distal delivery could deactivate the tubuloglomerular feedback response and thus favor sustained vasodilation and high GFR in some diabetics. The renal effects of acute administration of drugs acting predominantly at either the pre- or the postglomerular resistance using nicardipine (N = 16) or captopril (N = 25) were further evaluated in IDD. The renal response to captopril or nicardipine was different in IDD. Whereas both drugs induced a marked decrease in renal vascular resistance, GFR was slightly decreased by captopril and was unchanged after nicardipine; these results are similar to those observed in normotensive non-diabetic subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of mesangial cell contraction in adaptation of the glomerular tuft to changes in extracellular volume

Different chronic states of mesangial cell contraction were induced by variation of extracellular volume in Munich-Wistar rats for 6 days to study the influence of mesangial cells on the geometry of the glomerular tuft. Stereological analysis of superficial glomeruli in volume-expanded rats (VE, treated with enalapril) and volume-reduced rats (VR, treated with indomethacin) revealed a glomerular tuft volume 28.7% smaller, and a capillary luminal volume 32% smaller in VR than in VE rats. The filtration area [defined as glomerular basement membrane (GBM) area facing fenestrated endothelium] was greatly reduced in VR rats (97 +/- 16 X 10(3) micron 2 vs 137 +/- 13 x 10(3) micron 2). The surface density (Sv) of the GBM was higher by approximately 10% in VR rats primarily due to the considerable increase in Sv of the perimesangial GBM subdivision (0.189 +/- 0.01 micron 2/micron 3 vs 0.153 +/- 0.01 micron 2/micron 3), indicating a higher degree of mesangial cell contraction in these animals. Our results suggest (1) that mesangial cell contraction plays a major role in the adaptation of the glomerular tuft to variations in extracellular volume; (2) that the relevance of mesangial cell contraction for the regulation of glomerular haemodynamics appears to be small; and (3) that the reduction in filtration area, although prominent, cannot fully account for the considerable decreases in the ultrafiltration coefficient observed by others in acute and chronic studies.

Intrarenal mechanisms that regulate sodium excretion in relationship to changes in blood pressure

Because pressure-related natriuresis may be central to the regulatory role of the kidney on blood pressure, it is important to understand the relationship of humoral systems involved in the control of renal hemodynamics and tubular function. The preglomerular endothelial synthesis of prostaglandin I2 and endothelium-derived relaxing factor seem to modulate autoregulatory control by the afferent arterioles and the release of renin by the juxtaglomerular apparatus. The release of renin is followed by an increase in angiotensin II in the renal interstitium, which is responsible for adjusting the vascular tone of the efferent arterioles and vasa recta and for stimulating proximal tubular reabsorption of sodium. Variations in medullary circulation induced by angiotensin II could alter medullary interstitial pressure and the medullary production of prostaglandins E2 and I2 and, ultimately, could modulate sodium reabsorption in the medullary thick ascending limbs and the collecting ducts.

Comparison of the effects of calcium antagonists and converting enzyme inhibitors on renal function under normal and hypertensive conditions

Calcium antagonists decrease the ability of the kidney to autoregulate renal blood flow (RBF) and glomerular filtration rate (GFR). Therefore, when afferent renovascular resistance is elevated, as in essential hypertension, there is a resultant increase in RBF and GFR with the administration of calcium antagonists. These agents also induce a marked natriuresis because of direct tubular action through unknown mechanisms. The natriuresis can be dissociated from renal and systemic hemodynamic actions, indicating that the decreased sodium reabsorption could override other compensatory mechanisms explaining the absence of sodium retention during the treatment. The renal effects of converting enzyme inhibitors (CEIs) can be explained by the reduction of intrarenal formation in angiotensin II. Because the activation of the renin-angiotensin system is mainly responsible for inducing sodium retention during a decrease in systemic blood pressure, CEIs could have a protecting effect without disturbing other homeostatic mechanisms. CEIs decrease efferent glomerular resistance, reducing capillary pressure and thereby reducing GFR. This effect is not translated in sodium retention because the reduction of GFR is mild during captopril administration in kidneys with normal or increased renal perfusion pressure. At low renal perfusion pressure, the reduced glomerular afferent vasoconstriction can compromise GFR, leading to renal insufficiency. Although these situations are not likely to be encountered during the treatment of uncomplicated essential hypertension, in severe hypertension with hypertrophy of pre-glomerular vessels, glomerular perfusion may decrease. Combination therapy of calcium antagonists and CEIs has been reported to be an effective treatment of severe hypertension. Currently, little information is available on the manner in which renal function is affected by simultaneous administration of both drugs.

Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

It has long been known that increments in renal perfusion pressure can induce an elevation of urine sodium excretion without changing renal blood flow or glomerular filtration rate. The mechanism underlying this pressure-related natriuresis remains undefined, although the interest in its elucidation has been stimulated by the notion that it may constitute the central phenomenon through which the kidney regulates blood volume and, thereby, blood pressure. Recently, the use of novel experimental techniques has disclosed some important clues about changes in renal hemodynamics that, along with changes in renal humoral regulators, allow us to visualize a possible sequence of events responsible for pressure-related natriuresis. According to this hypothesis, the autoregulatory responses responsible for maintaining glomerular filtration rate are elicited in preglomerular vasculature by changes in renal perfusion pressure. These myogenic responses are coupled through Ca2+ entry in juxtaglomerular cells with inversely related changes in the release of renin and, consequently, with the amount of angiotensin II generated in renal interstitium. The release of renin from juxtaglomerular cells is modulated by the synthesis of prostaglandin I2 from the adjacent endothelial cells. Interstitial angiotensin II could influence sodium tubular reabsorption directly by stimulating sodium transport in proximal renal tubules and indirectly by altering medullary blood flow and, thereby, medullary interstitial pressure. In the renal medulla, the effects of interstitial pressure on sodium reabsorption can be amplified by the release of prostaglandin E2 from interstitial cells. A deficient regulation of this relationship could result in a shift of the pressure-natriuresis curve, leading to hypertension.

Effects of interrupting the renin-angiotensin system on sodium excretion in man

1. Twelve normal volunteers were studied on 2 separate days, having taken a range of diets providing 50-300 mmol sodium per day for 3 days and having been dehydrated overnight. Each volunteer received a background intravenous infusion of arginine vasopressin (5 x 10(-7) i.u. kg-1 min-1) on both days, and also received 6 mg captopril orally on one day and a placebo tablet on the other. The ensuing changes in arterial pressure, and in urinary solute and solute-free water excretion were recorded. 2. Captopril did not significantly alter arterial pressure. It increased the rate of excretion of sodium but not of potassium, and it did not significantly change urinary osmolality or creatinine clearance. 3. Captopril increased the rate of solute-free water reabsorption and did so in direct proportion to its effect of increasing sodium excretion. 4. A further twelve normal, dehydrated volunteers on free diets were studied on each of 2 days, after taking 500 mg lithium carbonate on the previous evening. On each day, they also received a loading dose and maintenance infusion of inulin. On one day they received 50 mg captopril orally, and, on the other, they received a placebo tablet. The arterial pressure, urinary excretion of electrolytes, and inulin clearance were recorded. 5. Captopril increased the rates of excretion of sodium, lithium and potassium, but there were no significant changes in inulin clearance or arterial pressure. 6. The natriuretic effect of captopril in either group of twelve volunteers was not significantly less in those volunteers who were already excreting more sodium, at least over the range of dietary sodium loading to which the volunteers were subjected. 7. Six normal volunteers were studied on a further 2 days, having taken a diet supplying 30 mmol sodium per day for 3 days and being dehydrated overnight. Each volunteer received a background intravenous infusion of arginine vasopressin (5 x 10(-7) i.u. kg-1 min-1) on both days and also received an intravenous infusion of saralasin acetate (50 ng kg-1 min-1) plus carrier on one day and carrier alone on the other. The ensuing changes in arterial pressure, and in urinary solute and solute-free water excretion were recorded. 8. There was a small but significant fall in systolic arterial pressure during the infusion of saralasin.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin II: a potent regulator of acidification in the rat early proximal convoluted tubule

The early proximal convoluted tubule (PCT) is the site of 50% of bicarbonate reabsorption in the nephron, but its control by angiotensin II has not been previously studied. In vivo microperfusion was used in both the early and late PCT in Munich-Wistar rats. Systemic angiotensin II administration (20 ng/kg X min) or inhibition of endogenous angiotensin II activity with saralasin (1 microgram/kg X min) caused profound changes in bicarbonate absorption in the early PCT (169 +/- 25 and -187 +/- 15 peq/mm X min, respectively). Because the bicarbonate absorptive capacity of the early PCT under free-flow conditions is 500 peq/mm X min, angiotensin II administration or inhibition affected greater than 60% of proton secretion in this segment. Both agents less markedly affected bicarbonate absorption in the late PCT (+/- 28 peq/mm X min) or chloride absorption (+/- 68-99 peq/mm X min) in both the early and late PCT. Because of its potential for controlling the majority of bicarbonate absorption in the early PCT (hence greater than or equal to 30% of bicarbonate absorption in the entire nephron), angiotensin II may be a powerful physiologic regulator of renal acidification.

Effects of angiotensins II and III on glomerulotubular balance in rats

1. The role of angiotensin as a modulator of proximal glomerulotubular (GT) balance was investigated in anaesthetized rats by examining the relationship between glomerular filtration rate (GFR) and absolute proximal reabsorption (APR) during removal of endogenous angiotensin II (AII) and III (AIII) with enalaprilat (CEI) and then during their subsequent replacement by intravenous infusions. 2. Enalaprilat lowered mean arterial blood pressure (MABP) and increased renal blood flow (RBF), GFR, urine flow rate and sodium excretion. Filtration fraction (FF) was not altered. Absolute proximal reabsorption, derived from fractional lithium clearance, increased by only 48% of the change expected for 'perfect' GT balance. 3. Angiotensin II replacement corrected MABP, GFR and plasma renin level, but reduced RBF and increased FF; APR was decreased and GT balance was restored. Urine flow and sodium excretion remained above control values with AII. 4. Replacement with AIII did not correct the hypotension but completely reversed the renal and renin responses to enalaprilat and restored GT balance without affecting FF. 5. It was concluded that the relation between proximal reabsorption and GFR is considerably modified by the intrarenal angiotensin concentration. The findings are best explained by a direct stimulation of proximal tubular sodium transport by angiotensin at the concentrations existing in anaesthetized rats.

Role for angiotensin II in an overt functional proteinuria

A partial renal vein constriction (RVC) was induced acutely in Munich-Wistar rats. RVC caused a marked reduction in glomerular plasma flow rate, and rises in glomerular transcapillary hydraulic pressure difference and efferent arteriolar resistance. These changes were associated with a marked increase in urinary protein excretion, on average from a baseline level of 8 to approximately 120 mg/24 hrs per kidney. Infusion of saralasin, an angiotensin II (AII) antagonist, largely normalized these indices, including urinary protein excretion (to approximately 35 mg/24 hrs per kidney), despite continued RVC. In separate rats, fractional clearances of neutral [125I]dextrans (molecular radii = 18-60 A) (CDEX/CIN) were measured. RVC caused a significant increase in CDEX/CIN for large dextrans (greater than or equal to 44A), but not small dextrans (less than or equal to 42A). Saralasin infusion led to a partial return toward baseline values of CDEX/CIN for the large dextrans. On the basis of the heteroporous membrane theory for glomerular filtration, the glomerular sieving defect during RVC was attributed to an increase in the relative fluid flux through a group of large non-selective pores. A marked alteration in glomerular microcirculatory pattern induced by enhanced action of endogenous AII in turn seemed to account largely, although not entirely, for the impairment of glomerular size-selectivity during RVC.

An analysis of glomerular-tubular balance in the rat proximal tubule

An analysis of glomerulo-tubular balance in the rat proximal tubule. Flow dependence of absolute proximal reabsorption (APR) or glomerulo-tubular balance (GTB) has been observed with spontaneous alterations in flow and attributed to both intraluminal and extraluminal factors. Flow dependent alterations in APR were demonstrated when 1. nephron filtration rate (SNGFR) was decreased by tubulo-glomerular feedback mechanisms by increasing late proximal tubular microperfusion rates, and 2. when SNGFR was increased by addition of [Sar1, Ala8] angiotensin II to the adjacent peritubular capillary flow. Selective reduction in early proximal tubular flow rate by pump aspiration also resulted in flow dependent reductions in APR. However, selective additions of perfusion fluids of various native and artificial constituency to the early proximal tubule did not result in flow dependent increase in APR. Conclusions. 1. GTB with both increases and decreases in SNGFR can be demonstrated at the level of the single nephron, 2. selective reductions in luminal flow rate produces parallel reductions in APR; however, 3. increases in flow rate with either artificial or native fluids of different ionic concentrations did not result in increases in APR. This lack GTB may be due to lack of parallel changes in peritubular physical factors or that APR in the S2 segment is less sensitive to increase in flow rate.

Angiotensin II control of the renal microcirculation: effect of blockade by saralasin

The hydronephrotic rat kidney with intact circulation and innervation was split and spread out as a thin sheet in a tissue bath. The microvasculature was observed in vivo via television microscopy. We quantitated the effects of increasing concentrations (10(-9) to 10(-5) M) of saralasin (angiotensin II antagonist) applied locally in the tissue bath on microvascular diameters and on relative glomerular blood flow (measured using fluorescent labeled RBCs). Saralasin produced an increase in preglomerular diameters which was largest (37 +/- 11%) in the interlobular artery (there was no dilation in the afferent arteriole near the glomerulus), an increase in postglomerular diameters which was largest (17 +/- 4%) in the efferent arteriole near the glomerulus, and an increase in blood flow (19 +/- 4%). If these types of findings would hold for the normal kidney, it would suggest a role for angiotensin II in the control of total renal blood flow, in the regional distribution of flow, and in the control of filtration fraction. We also made control micropressure measurements using the servo-nulling approach. Pressures measured were: afferent arteriole, 65 +/- 5 mm Hg; intraglomerulus, 50 +/- 5 mm Hg; and efferent arteriole, 19 +/- 3 mm Hg. These data indicate that there is major vascular resistance near the glomerulus, especially in the efferent arteriole.

Failure of an angiotensin II antagonist to influence isoprenaline-induced antidiuresis in rats

The purpose of this study was to elucidate the role of angiotensin II in isoprenaline-induced antidiuresis by use of the competitive angiotensin II antagonist saralasin. Isoprenaline is known to enhance the formation of angiotensin II. Since angiotensin II has been shown to increase proximal tubular salt and volume reabsorption and to decrease renal fluid and salt excretion the renal effects of isoprenaline might, at least partly, be mediated by angiotensin II. Isoprenaline, infused at 0.1 microgram/kg . min i.v. into anaesthetized rats, led to a marked decrease of urine flow, sodium and potassium excretion and to an increase in urinary osmolality, confirming previous data. Effective renal plasma flow increased significantly. Immediately after onset of the isoprenaline infusion a transient small fall in GFR was observed. On continuation of the infusion, GFR recovered, but marked antidiuresis persisted. Changes in renal hemodynamics, therefore, can be excluded as being responsible for the antidiuretic effect. Saralasin, infused at 6 micrograms/kg . min i.v., did not affect isoprenaline-induced antidiuresis, antinatriuresis, antikaliuresis, and the increase in urinary osmolality, but prevented the rise in renal plasma flow observed during isoprenaline infusion, probably as a consequence of a weak angiotensin II-like effect of saralasin on renal vessels. It is concluded that angiotensin II does not play a role in isoprenaline-induced antidiuresis. The results are compatible with the view that beta-adrenoceptor stimulation may directly affect the tubular handling of water and salt.

Role of renal sympathetic nerves in mediating hypoperfusion of renal cortical microcirculation in experimental congestive heart failure and acute extracellular fluid volume depletion

To evaluate the pathophysiologic importance of renal nerves in regulating the renal vasomotor tone, we measured several parameters of renal cortical microcirculation before and after acute renal denervation (DNx) in the following three groups of anesthetized Munich-Wistar rats: (group 1) congestive heart failure after surgically induced myocardial infarction (n = 10), (group 2) acute extracellular fluid volume depletion after deprivation of drinking water for 48 h (n = 8), and (group 3) sham or nontreated controls (n = 6). In the myocardial-infarcted rats, DNx led to a uniform increase in glomerular plasma flow rate of, on average, 36%. Single nephron glomerular filtration rate of myocardial-infarcted rats also increased despite a reduction in glomerular capillary hydraulic pressure. These changes were associated with a fall in arteriolar resistances, particularly in the efferent arteriole. The glomerular capillary ultrafiltration coefficient rose in all but one myocardial-infarcted animal. A similar hemodynamic pattern was seen after DNx in water-deprived animals. In every water-deprived animal, glomerular plasma flow rate and single nephron GFR increased on average by 28 and 14%, respectively. Again, afferent and efferent arteriolar resistances decreased significantly. Furthermore, the ultrafiltration coefficient increased uniformly and substantially with DNx. To ascertain the potential importance of the interaction between the renal nerves and angiotensin II in these circumstances, we compared the renal cortical hemodynamics in additional groups of water-deprived rats (group 4) after DNx (n = 15), (group 5) during inhibition of angiotensin II with saralasin (n = 15), and (group 6) during treatment with both saralasin and DNx (n = 15). No appreciable difference was detected between group 4 vs. 6. In contrast, substantial differences were noted between group 5 vs. 6: on average, the glomerular plasma flow rate was 26% higher and the afferent and efferent arteriolar resistances 25% and 27% lower, respectively, in group 6. These observations provide direct evidence to indicate pathophysiologic importance of renal nerves in the profound intrarenal circulatory adjustments in prerenal circulatory impairment. The vasoconstrictive effects of renal nerves appear to be mediated in part by their stimulatory influence on angiotensin II release and their direct constrictor actions on pre- and post-glomerular vessels as well.

Nephron responses to converting enzyme inhibition in non-clipped kidney of Goldblatt hypertensive rat at normotensive pressures

To evaluate the effects of angiotensin converting enzyme inhibition (SQ 20881, CEI) on superficial nephron function of the non-clipped kidney in Goldblatt hypertensive rats in the absence of alterations in renal arterial pressure, control renal arterial pressure (RAP) was reduced first to the range generally obtained during CEI (124 +/- 4 mm Hg). RAP was maintained during the CEI period by adjustment of a suprarenal aortic clamp. At the reduced RAP, whole kidney and single nephron glomerular filtration rates (GFR) were reduced from the hypertensive levels and were lower than the measurements in normotensive control rats. During CEI, whole kidney GFR and single nephron GFR increased by 55 and 42%, respectively. There were decreases in absolute as well as fractional proximal reabsorption rates. In the intermediate nephron segment, fractional reabsorption was decreased, but absolute fluid reabsorption increased in proportion to the increased delivery rate. Proximal tubule and peritubular capillary hydrostatic pressures increased significantly during CEI also. These results indicate that an increased activity of the renin-angiotensin system occurring in Goldblatt hypertensive rats subjected to aortic constriction exerts effects to lower GFR and increase proximal reabsorption rate. The concomitant superficial nephron and whole kidney GFR responses to CEI when arterial pressure was maintained suggests that the pre-existing levels of angiotensin exerted similar influences on the total nephron population.

Mechanism of preservation of glomerular perfusion and filtration during acute extracellular fluid volume depletion. Importance of intrarenal vasopressin-prostaglandin interaction for protecting kidneys from constrictor action of vasopressin

Glomerular circulatory dynamics were assessed in 60 adult anesthetized rats, which were either deprived or not deprived of water for 24-48 h. Water-deprived rats (n = 21) were characterized by a depressed level of single nephron glomerular filtration rate (SNGFR) when compared with nonwater-deprived controls (n = 8) (23.2 +/- 1.3 vs. 44.8 +/- 4.1 nl/min). This was primarily due to decreased glomerular plasma flow rate (71 +/- 5 vs. 169 +/- 23 nl/min) and glomerular capillary ultrafiltration coefficient (0.028 +/- 0.003 vs. 0.087 +/- 0.011 nl/[s . mmHg]). Infusion of saralasin to these water-deprived rats resulted in significant increases in plasma flow rate and ultrafiltration coefficient, and decline in arteriolar resistances. Consequently, SNGFR increased by approximately 50% from pre-saralasin levels. When water-deprived saralasin-treated rats were given a specific antagonist to the vascular action of arginine vasopressin (AVP), d(CH2)5Tyr(Me)AVP, a fall in systemic blood pressure occurred, on average from 102 +/- 5 to 80 +/- 5 mmHg, unaccompanied by dilation of renal arterioles, so that both plasma flow rate (129 +/- 8 vs. 85 +/- 13 nl/min) and SNGFR (31.0 +/- 2.9 vs. 18.2 +/- 4.4 nl/min) decreased. This more selective extrarenal constrictor action of AVP was further documented in additional studies in which cardiac output and whole kidney blood flow rate were simultaneously measured. In water-diuretic rats, administration of a moderately pressor dose of AVP (4 mU/kg per min) resulted in a significant rise in kidney blood flow rate (from 8.8 +/- 1.2 to 9.6 +/- 1.3 ml/min). The higher kidney blood flow rate occurred despite a fall in cardiac output (from 111 +/- 7 to 98 +/- 9 ml/min), and was associated with a significant increase in the ratio of systemic vascular to renal vascular resistance (on average from 0.083 +/- 0.014 to 0.106 +/- 0.019). Furthermore, infusion of d(CH2)5Tyr(Me)AVP to water-deprived animals (n = 6) to antagonize endogenous AVP resulted in systemic but not renal vasodilation, so that kidney blood flow rate fell (by approximately 30%), as did systemic-to-renal resistance ratio (by approximately 30%). When the above two experiments were repeated in indomethacin-treated animals, exogenous AVP administration in water-diuretic rats (n = 6) and antagonism of endogenous AVP in water-deprived rats (n = 7) caused, respectively, parallel constriction and dilation in systemic and renal vasculatures. The net effect was unaltered systemic to renal vascular resistance ratio in both cases. These results indicate that (1) unlike angiotensin II, AVP maintains glomerular perfusion and filtration in acute extracellular fluid volume depletion by a more selective constriction of the extrarenal vasculature. (2) The relative renal insensitivity to the vasoconstrictor action of AVP appears to be due to an AVP-induced release of a potent renal vasodilator, sensitive to indomethacin, presumably prostaglandins.

Renal adaptation to sodium deprivation. Effect of captopril in the rat

Dietary sodium restriction is associated with a rapid decrease in urinary sodium excretion and achievement of a new sodium balance within three to five days. In addition, renal vasoconstriction and progressive activation of intrarenal systems with vasoconstrictor (renin-angiotensin) or vasodilating (kallikrein-kinin and prostaglandins) properties are observed. The relationship between sodium homeostasis and the renin-angiotensin system was assessed through the use of captopril in the rat. Treatment with captopril, before and during a six-day period after suppression of dietary sodium, was associated with sodium wasting (urinary sodium always exceeded sodium intake during the observation period); in addition, the normal increase in urinary aldosterone was blunted by about 80 percent. When captopril treatment was given for six days to rats maintained on long-term sodium restriction (at least four weeks) urinary sodium increased, although transiently; at the end of the study, renal vasodilatation together with a redistribution of glomerular blood flow to nonsuperficial glomeruli was observed. These studies indicate that captopril administration markedly blunts the renal and systemic adaptations to a reduced sodium intake in the rat. They suggest that the renin-angiotensin system is probably indispensable in preventing sodium loss when dietary sodium is suppressed.

Glomerular actions of angiotensin II

An abundance of evidence suggests that glomerular ultrafiltration is significantly influenced by alterations in circulating or local concentrations of a variety of vasoactive substances. Angiotensin II regulates whole kidney glomerular filtration rate by influencing one or more of the determinants of single nephron glomerular filtration rate, particularly the ultrafiltration coefficient and glomerular plasma flow rate. The glomerular mesangial cell may be an important target for angiotensin II by virtue of its ability to contract and thereby reduce glomerular capillary surface area and ultrafiltration coefficient. These control mechanisms are not only important under physiologic conditions but they may also be activated by a variety of insults and thereby contribute to the decrease in glomerular filtration rate observed in a broad spectrum of renal disease.

Angiotensin II directly stimulates sodium transport in rabbit proximal convoluted tubules

Numerous previous studies have proposed a role for angiotensin II (AII) in the renal regulation of salt balance. At least one nephron site, the proximal convoluted segment, has been implicated in this role. We used in vitro microperfusion of rabbit proximal convoluted tubules to further examine this question. To insure use of appropriate in vivo concentrations as well as potency of the hormone in vitro, we measured plasma AII levels by radioimmunoassay in normal, sodium-depleted, and adrenalectomized rabbits, and measured AII activity by bioassay after incubation in various microperfusion baths. Plasma levels ranged from approximately 2 X 10(-11) to 5 X 10(-11) M. AII activity was stable in Ringer's solution plus albumin, but not in rabbit serum or Ringer's solution plus fetal calf serum. In Ringer's solution plus albumin, physiologic concentrations of AII stimulated volume reabsorption (Jv). 10(-11) M AII increased Jv by 16% (P less than 0.01). 10(-10) M AII produced a lesser increase, 7.5% (P less than 0.05). At a frequently studied, but probably pharmacologic dose, 10(-7) M AII inhibited Jv by 24% (P less than 0.001). AII at 10(-11) M did not stimulate Jv in the presence of 10(-7) M saralasin. Though previous studies have suggested agonistic effects of saralasin alone in epithelia, we found no significant effect of 10(-7) M saralasin on Jv. None of the AII doses measurably changed transepithelial voltage. We conclude that AII in physiologic doses directly stimulates Jv in proximal convoluted tubules and this effect is probably receptor mediated and, within the limits of detection, electroneutral.