Extracellular ATP increases cytosolic calcium in cultured rat renal arterial smooth muscle cells

1. Experiments were conducted on cultured renal arterial smooth muscle cells to determine the ability of extracellular ATP to alter cytosolic calcium concentration and to determine the mechanisms by which this effect occurs. 2. ATP (100 mumol/L) caused the fluorescence ratio of fura-2 to increase from a control value of 1.06 +/- 0.05 to 2.06 +/- 0.13 (P < 0.01) before stabilizing at a sustained level of 1.35 +/- 0.04 (n = 8; P < 0.05). 3. Removal of extracellular calcium from the bathing medium resulted in an attenuation of the initial response to 100 mumol/L ATP with cell fluorescence increasing from 1.16 +/- 0.18 to 1.44 +/- 0.18 ratio units (n = 5). Furthermore, the initial increase in fluorescence ratio rapidly declined to 1.02 +/- 0.06, indicating that an influx of extracellular calcium is required to sustain the increase in fura-2 fluorescence. 4. Depletion of intracellular calcium pools with thapsigargin prevented the increase in fura-2 fluorescence evoked by ATP. 5. These data suggest that ATP-mediated increases in cytosolic calcium in cultured renal arterial smooth muscle cells involve calcium release from the thapsigargin-sensitive, intracellular pool in conjunction with calcium influx from the extracellular medium.

Medullary blood flow responses to changes in arterial pressure in canine kidney

Although it is well recognized that whole kidney and cortical blood flow exhibit efficient autoregulation in response to alterations in renal arterial pressure (RAP), the autoregulatory behavior of medullary blood flow (MBF) has remained uncertain. We have evaluated MBF responses to stepwise reductions in RAP for both short-term (2 min, n = 6) and longer periods (15 min, n = 7) using single-fiber laser-Doppler flowmetry with needle probes inserted into the mid-medullary region in denervated kidneys of 13 anesthetized dogs. The changes in cortical blood flow (CBF) were assessed with either a surface probe or a needle probe inserted into the cortex. Control total renal blood flow (RBF), assessed by electromagnetic flow probe in these dogs, was 5.2 +/- 0.3 ml.min-1.g-1, and glomerular filtration rate was 0.97 +/- 0.05 ml.min-1.g-1 (n = 7). RBF, MBF, and CBF all exhibited efficient autoregulatory behavior during changes in RAP from 150 to 75 mmHg. The slopes of RAP vs. RBF, CBF, as well as MBF, were not significantly different from zero within this range of RAP. Below RAP of 75 mmHg, all indexes of blood flow showed linear decreases with reductions in pressure. The data indicate that blood flow in the renal medulla of dogs exhibits efficient autoregulatory behavior, similar to that in the cortex.

Paracrine regulation of the renal microcirculation

There has been an explosive growth of interest in the multiple interacting paracrine systems that influence renal microvascular function. This review first discusses the membrane activation mechanisms for renal vascular control. Evidence is provided that there are differential activating mechanisms regulating pre- and postglomerular arteriolar vascular smooth muscle cells. The next section deals with the critical role of the endothelium in the control of renal vascular function and covers the recent findings related to the role of nitric oxide and other endothelial-derived factors. This section is followed by an analysis of the roles of vasoactive paracrine systems that have their origin from adjoining tubular structures. The interplay of signals between the epithelial cells and the vascular network to provide feedback regulation of renal hemodynamics is developed. Because of their well-recognized contributions to the regulation of renal microvascular function, three major paracrine systems are discussed in separate sections. Recent findings related to the role of intrarenally formed angiotensin II and the prominence of the AT1 receptors are described. The possible contribution of purinergic compounds is then discussed. Recognition of the emerging role of extracellular ATP operating via P2 receptors as well as the more recognized functions of the P1 receptors provides fertile ground for further studies. In the next section, the family of vasoactive arachidonic acid metabolites is described. Possibilities for a myriad of interacting functions operating both directly on vascular smooth muscle cells and indirectly via influences on endothelial and epithelial cells are discussed. Particular attention is given to the more recent developments related to hemodynamic actions of the cytochrome P-450 metabolites. The final section discusses unique mechanisms that may be responsible for differential regulation of medullary blood flow by locally formed paracrine agents. Several sections provide perspectives on the complex interactions among the multiple mechanisms responsible for paracrine regulation of the renal microcirculation. This plurality of regulatory interactions highlights the need for experimental strategies that include integrative approaches that allow manifestation of indirect as well as direct influences of these paracrine systems on renal microvascular function.

Renal accumulation of circulating angiotensin II in angiotensin II-infused rats

Previous studies have demonstrated that low-dose angiotensin II (Ang II) infusion for 14 days mimics two-kidney, one clip Goldblatt hypertension and increases intrarenal Ang II levels. The objective of the present study was to determine whether the augmented intrarenal Ang II is due to intrarenal accumulation of the infused Ang II and/or to an increase in intrarenal formation of endogenous Ang II. Male Sprague-Dawley rats were uninephrectomized and divided into three groups: control (N=6), those infused with [Ile5]Ang II (endogenous form) (N=6), and those infused with [Val5]Ang II (n=8). [Ile5]Ang II or [Val5]Ang II was infused at 40 ng/min via an osmotic minipump implanted subcutaneously. By day 12, systolic blood pressure increased significantly in both [Val5]Ang II-infused rats (197 +/- 7 mm Hg) and [Ile5]Ang II-infused rats (173 +/- 3 mm Hg). Blood and kidney samples were harvested, subjected to high-performance liquid chromatography to separate [Val5]Ang II from [Ile5]Ang II, and then measured by radioimmunoassay. Plasma renin activity was markedly suppressed in both [Ile5]Ang II- and [Val5]Ang II-infused rats. Plasma Ang II levels were elevated in rats infused with both [Ile5]Ang II (121 +/- 24 fmol/mL) and [Val5]Ang II (119 +/- 14 fmol/mL) compared with controls (69 +/- 15 fmol/mL). Both [Ile5]Ang II- and [Val5]Ang II-infused rats exhibited an enhancement of total intrarenal Ang II. Only [Ile5]Ang II (358 +/- 53 fmol/g) was detected in the kidneys of rats infused with -Ile5-Ang II. In [Val5]Ang II-infused rats, a significant portion of total renal Ang II (371 +/- 57 fmol/g) was in the form of [Val5]Ang II (256 +/- 44 fmol/g). Renal [Ile5]Ang II levels were maintained in the [Val5]Ang II-infused rats (116 +/- 15 fmol/g) compared with control rats (116 +/- 11 fmol/g) despite marked suppression of renin release. These results support the hypothesis that infused circulating ANG II is bound to receptor or taken up intrarenally in a manner that protects against degradation.

Renal responses to intra-arterial administration of adrenomedullin in dogs

The present study was undertaken to evaluate the renal responses to intra-arterial infusion (12.5 pmol.kg-1.min-1) for 30 min of adrenomedullin (ADM) in denervated kidneys of anesthetized dogs (n = 8). Total renal blood flow (RBF) was measured with an electromagnetic flow probe, and two single-fiber laser-Doppler needle flow probes were used to measure relative changes in cortical (CBF) and medullary blood flow (MBF). ADM administration resulted in decreases in renal vascular resistance (32.3 +/- 4.9 to 22.3 +/- 3.0 mmHg.ml-1.min.g) and increases in RBF (4.5 +/- 0.5 to 6.0 +/- 0.6 ml.min-1.g-1), CBF (33 +/- 1.3%, n = 41, and MBF (32 +/- 5.7%, n = 5) without changes in glomerular filtration rate. There were also substantial increases in urine flow (11.3 +/- 1.9 to 28 +/- 4.8 microliters.min-1.g-1), as well as in sodium excretion (1.8 +/- 0.2 to 4.2 +/- 0.6 microliter.min-1.g-1). After cessation of ADM infusion, partial recovery of these responses was observed within a 30-min period. Responses to ADM were attenuated during inhibition of nitric oxide (NO) synthesis by nitro-L-arginine (50 micrograms.kg-1.min-1). These findings indicate that renal vasodilator, diuretic, and natriuretic responses to ADM may be mediated by the release of endogenous NO and suggest that ADM could play a role in regulating water and electrolyte excretion by the kidney.

Interactions between arterial pressure and sodium excretion

Recent developments have led to renewed interest in the mechanisms that mediate the changes in sodium excretion in response to changes in arterial pressure, a phenomenon termed pressure-natriuresis. Pressure-natriuresis occurs in the absence of changes in filtered load and thus, the changes in sodium excretion are caused by changes in net tubular sodium reabsorption. Both proximal and distal nephron segments have been implicated as the sites of altered reabsorption. The specific mechanism responsible for pressure-natriuresis remains unresolved. One proposal suggests that changes in renal interstitial fluid pressure which may be due, in part, to a less efficient autoregulation of the renal medullary circulation than of the cortex, alter sodium reabsorption. Alternatively, recent studies indicate that increases in arterial pressure increase endothelial nitric oxide formation which inhibits sodium reabsorption via direct effects on the tubules as well as hemodynamically mediated effects.

Modulation of tubuloglomerular feedback by angiotensin II type 1 receptors during the development of Goldblatt hypertension

It has been suggested that the increased levels of angiotensin II (Ang II) in the contralateral kidney of two-kidney, one clip (2K1C) Goldblatt hypertensive rats act to enhance tubuloglomerular feedback responsiveness and proximal tubular reabsorption and thereby exert a substantial sodium-retaining influence on the nonclipped kidney. The current study investigated the Ang II dependency of tubuloglomerular feedback responsiveness in the nonclipped kidney during the early stages of development of 2K1C hypertension. Stop-flow pressure feedback responses were assessed in the nonclipped kidney of 2K1C rats during control conditions and after systemic administration of the Ang II type 1 receptor antagonist losartan (10 mg/kg). In 1-week clipped and sham-operated rats, losartan administration decreased mean arterial pressure (from 143 +/- 6 to 123 +/- 2 mm Hg, P < .01, and from 129 +/- 2 to 106 +/- 5 mm Hg, P < .01, respectively) and attenuated the magnitude of the maximal feedback responses (from -12.9 +/- 1.2 to -3.0 +/- 0.3 mm Hg, P < .01, and from -13.2 +/- 1.5 to -3.6 +/- 1.1 mm Hg, P < .01, respectively). The decreases in mean arterial pressure were not significantly different in sham-operated and 1-week clipped rats. In 3-week clipped rats, mean arterial pressure was further elevated (163 +/- 6 mm Hg) compared with sham-operated rats (134 +/- 4 mm Hg, P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium activation mechanisms in the renal microvascular response to extracellular ATP

Previous studies have suggested a paracrine role for extracellular ATP in the regulation of afferent arteriolar tone. The current study was conducted to determine the dependence of this response on calcium entry mechanisms. Experiments were performed in vitro using the blood-perfused juxtamedullary nephron technique combined with video microscopy. The afferent arteriolar response to alpha, beta-methylene ATP was determined before and after treatment with the calcium channel blockers, diltiazem or felodipine. alpha, beta-Methylene ATP was used to obviate concerns over responses being elicited by ATP or by ATP hydrolysis products such as adenosine. Previous studies have shown that afferent arteriolar responses to alpha, beta-methylene ATP are comparable to those elicited by ATP. alpha, beta-Methylene ATP (1.0 microM) induced a rapid initial afferent vasoconstriction of 72.5 +/- 10.6%, which partially recovered to a stable diameter 11.3 +/- 1.7% smaller than control (P < 0.01 vs control). Afferent diameter returned to control diameter on removal of ATP from the bath. Diltiazem or felodipine treatment significantly increased afferent diameter by 5.6 +/- 2.3 and 16.4 +/- 4.6%, respectively (P < 0.05). In the presence of either diltiazem or felodipine, the initial vasoconstriction to alpha, beta-methylene ATP was attenuated, and the sustained vasoconstriction was completely blocked. Removal of calcium from the extracellular medium completely abolished both the initial and sustained vasoconstrictor response.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired renal autoregulatory ability in dogs with reduced renal mass

In normal dogs, the renal autoregulatory mechanism limits the effect of changes in renal arterial pressure (RAP) on RBF, renal outer cortical blood flow (OCBF), and GFR by adjusting the vascular tone of the preglomerular arterioles. To determine the extent to which autoregulatory ability was impaired in remnant renal tissue in dogs, the effects of variations in RAP on RBF, OCBF, and GFR were studied after sham-operation (Group 1; N = 5), 3/4 nephrectomy (Group 2; N = 5), or 7/8 nephrectomy (Group 3; N = 5). In Group 1, the RBF, OCBF, and GFR were not significantly affected by variations in RAP between 75 and 125 mm Hg, indicating intact renal autoregulatory ability. In contrast, both groups of partially nephrectomized dogs exhibited a loss of renal autoregulatory ability below 100 mm Hg (P < 0.05). As RAP rose above 100 mm Hg, dogs with 7/8 nephrectomy exhibited a greater increase than control dogs in RBF (0.586 +/- 0.211 versus -0.080 +/- 0.030 percent change in RBF/mm Hg change in RAP; P < 0.05), OCBF (0.408 +/- 0.157 versus -0.059 +/- 0.054 percent change in RBF/mm Hg change in RAP; P < 0.05), and GFR (0.784 +/- 0.230 versus 0.134 +/- 0.049 percent change in RBF/mm Hg change in RAP; P < 0.05). The ability of the renal vasculature to maintain renal function stable above 100 mm Hg was intermediate in Group 2 and not significantly different from corresponding values for Group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation between pressure natriuresis and urinary excretion of nitrate/nitrite in anesthetized dogs

Alterations in intrarenal nitric oxide (NO) formation during changes in renal arterial pressure (RAP) have been suggested as a mechanism mediating pressure natriuresis. To test this hypothesis further, we examined the relation between RAP and the urinary excretion rate of nitrate/nitrite (NO3-/NO2-; NO metabolites) in anesthetized sodium-replete dogs before (n = 9) and during (n = 6) intrarenal infusion of the NO synthesis inhibitor nitro-L-arginine (NLA; 50 micrograms.kg-1.min-1). Urinary NO3-/NO2- concentrations were measured with the Griess reaction and spectrophotometry methods after enzymatic reduction of NO3- to NO2- in the samples. During control conditions, there were decreases in the urinary NO3-/NO2- excretion rate in response to reductions in RAP (150 to 75 mm Hg; slope, 0.04 +/- 0.01 nmol.min-1.g-1.mm Hg-1) in association with decreases in urinary sodium excretion (UNaV). There was a positive correlation between changes in NO3-/NO2- excretion rate and changes in RAP (r = .48; P < .005) or UNaV (r = .59; P < .001). NLA infusion resulted in decreases in NO3-/NO2- excretion rate (4.8 +/- 1.4 to 1.0 +/- 0.3 nmol.min-1.g-1) in association with reductions in UNaV (4.3 +/- 0.3 to 0.7 +/- 0.2 microL.min-1.g-1), fractional excretion of sodium (2.9 +/- 0.2% to 0.5 +/- 0.1%), and renal blood flow (4.8 +/- 0.3 to 3.3 +/- 0.2 mL.min-1.g-1), without changes in glomerular filtration rate. Furthermore, there was a marked attenuation of the NO3-/NO2- and sodium excretory responses to alterations in RAP during NO synthesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoregulation of afferent arteriolar blood flow in juxtamedullary nephrons

Utilizing the in vitro blood-perfused juxtamedullary nephron preparation, we examined the effects of alterations in renal arterial pressure on afferent arteriolar blood flow. With video microscopy and cross-correlation techniques, arteriolar inside diameters and centerline erythrocyte velocity were measured to estimate single afferent arteriolar blood flow. In response to random changes in perfusion pressure, afferent arteriolar diameter (n = 8) varied inversely (-0.53 +/- 0.02%/mmHg), and erythrocyte velocity was directly related (1.4 +/- 0.1%/mmHg). Above 95 mmHg, the slope of the relationship between perfusion pressure and afferent arteriolar blood flow did not differ from zero (0.081 +/- 0.053%/mmHg), suggesting efficient autoregulation. When the tubuloglomerular feedback pathway was interrupted by the addition of furosemide (n = 9) or papillectomy (n = 7), there was attenuation of pressure-induced afferent arteriolar constriction, with impairment in blood flow autoregulation (0.60 +/- 0.05%/mmHg). Superfusion with diltiazem abolished autoregulatory responses in afferent arteriolar diameter and blood flow (1.5 +/- 0.2%/mmHg). These data demonstrate the autoregulation of blood flow of individual afferent arterioles in juxtamedullary nephrons and suggest that both tubuloglomerular feedback-dependent and -independent mechanisms are required for autoregulatory responses.

Tubular fluid concentrations and kidney contents of angiotensins I and II in anesthetized rats

Previous micropuncture studies have reported nanomolar concentrations of angiotensin II in proximal tubular fluid and have indicated that angiotensin II or a precursor may be secreted into the tubular lumen. Further experiments were performed to determine if proximal tubular fluid angiotensin I concentrations are also greater than plasma and kidney levels and to estimate the degree of intrarenal compartmentalization of the angiotensin peptides. Free-flow proximal tubular fluid samples were collected in micropipets and were pooled for each animal. At the end of each experiment, a blood sample was collected and the micropunctured left kidney was harvested and homogenized in methanol. The angiotensin I concentration in proximal tubular fluid samples averaged 6.1 +/- 1.2 pmol/mL, whereas the angiotensin II concentration averaged 8.1 +/- 1.6 pmol/mL (N = 13). HPLC analysis of a separate sample pooled from collections in five rats indicated that the immunoreactive angiotensin I and angiotensin II primarily represented authentic angiotensin I and II. Plasma concentrations of angiotensin I and angiotensin II averaged 0.39 +/- 0.09 and 0.15 +/- 0.03 pmol/mL, respectively. The kidney contents of angiotensin I and angiotensin II were 1.28 +/- 0.24 and 0.97 +/- 0.17 pmol/g of kidney, respectively. These findings indicate that proximal tubular fluid contains nanomolar concentrations of angiotensin I as well as angiotensin II. These high tubular fluid concentrations, which greatly exceed the plasma and kidney levels, likely reflect net secretion of the angiotensin peptides by proximal tubule cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of distal nephron sodium transport attenuates pressure natriuresis in dogs

The sodium excretory responses (UNaV) to acute changes in renal arterial pressure (RAP) during blockade of distal nephron sodium transport were evaluated in seven sodium-replete anesthetized dogs. The major distal sodium entry pathways were blocked by intrarenal infusion of amiloride (AM, 10(-5) mol/L) and bendroflumethiazide (BZ, 10(-6) mol/L). Infusion of AM plus BZ caused slight increases in renal blood flow (RBF, 4.1 +/- 0.5 to 4.6 +/- 0.4 mL.min-1.g-1; P < .001) but no changes in glomerular filtration rate (GFR, 0.96 +/- 0.05 to 1.01 +/- 0.07 mL.min-1.g-1; P = NS) or autoregulatory efficiency of RBF and GFR. There were significant increases in UNaV (2.7 +/- 0.7 to 5.2 +/- 0.6 mumol.min-1.g-1) and fractional excretion of sodium (FENa, 1.8 +/- 0.4% to 3.5 +/- 0.3%) and decreases in potassium excretion (0.59 +/- 0.10 to 0.35 +/- 0.06 mumol.min-1.g-1) during AM plus BZ infusion. During the control period and during repeat measurements in time control studies, decreases in RAP (150 to 100 mm Hg) elicited the usual decreases in UNaV (slope, 0.022 +/- 0.007 mumol.min-1.g-1.mm Hg-1; P < .01). After administration of AM plus BZ, there was a marked attenuation of the pressure-natriuretic responses, and the slopes of the RAP versus UNaV and RAP versus FENa relations at RAP levels above 100 mm Hg were not significantly different from zero. However, the pressure-natriuresis response was maintained at arterial pressure between 75 and 100 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

[The regulatory mechanisms of glomerular filtration: the tubuloglomerular feedback system, physiological aspects and their participation in the physiopathology of kidney diseases]

The tubuloglomerular feedback system (TGF), the main component of the autoregulation mechanism, maintains constant glomerular blood flow and filtration rate. The system is based in the morpho-functional association among the macula densa, the afferent and efferent arterioles, the glomerulus and the extraglomerular mesangial cells. The macula densa cells sense the changes in solute concentrations and osmolarity in the tubular fluid and send vasoconstrictor or vasodilatory signals to the arterioles that modify blood flow and glomerular filtration rate. However, the mechanism by which the macula densa sends signals to the arterioles remains unknown. A chemical mediator such as prostaglandins or adenosine has been postulated. Our studies discarded the participation of prostaglandins as mediators. Further studies from our group with adenosine analogs indicate that this nucleotide participates in the activation of the TGF responses, and suggest alternate activation pathways not previously described. To evaluate the role of TGF feedback in the glomerular hemodynamic changes associated with progression of renal damage, we studied TGF responses in rats with Goldblatt hypertension and partial ablation of the contralateral kidney. Our results suggest that the sensitivity of the system is decreased. This alteration may contribute to glomerular hypertension and progression to renal damage.

Extracellular ATP in the regulation of renal microvascular function

Considerable attention has been focused on the purine nucleoside, adenosine, in the control of renal blood flow, epithelial transport, and renin secretion; however, surprisingly little attention has been directed toward the renal effects of purine nucleotides such as adenosine triphosphate (ATP). Recent studies utilizing in vivo micropuncture and in vitro techniques have demonstrated that renal vascular, epithelial, and mesangial cells respond to extracellular ATP via mechanisms distinct from those elicited by adenosine. ATP vasoconstricts afferent but not efferent arterioles whereas adenosine vasoconstricts both vascular segments. Adenosine-mediated afferent arteriolar vasoconstriction is abolished by adenosine receptor antagonists, whereas the response to ATP is enhanced. ATP-mediated vasoconstriction reaches a maximum within seconds of exposure while the vasoconstriction induced by adenosine develops more slowly. L-type calcium channel antagonists such as diltiazem or felodipine prevent the sustained afferent vasoconstriction produced by ATP. Data from micropuncture experiments indicate that peritubular capillary infusion of ATP reduces glomerular pressure and results in marked attenuation of the tubuloglomerular feedback mechanism, which transmits signals from the macula densa to the afferent arteriole. These data support the existence of ATP-sensitive P2 purinoceptors in the preglomerular microvasculature that contribute to the control of renal vascular function via activation of calcium channels.

Augmentation of intrarenal angiotensin II levels by chronic angiotensin II infusion

The objective of this study was to investigate the singular role of elevated angiotensin II (ANG II) levels in the development of two-kidney, one-clip (2K1C) Goldblatt hypertension in the rat and specifically in the altered intrarenal ANG II levels that occur in the nonclipped kidney. As a substitute for the clipped kidney, chronic delivery of ANG II (40 ng/min) via an osmotic minipump implanted subcutaneously was used to mimic plasma ANG II levels observed in 2K1C rats during the developmental phase of hypertension. Arterial pressure increased gradually over a period of 14 days, and a pressure profile similar in magnitude and temporal pattern to that of the 2K1C rats was observed. Systemic ANG II was elevated to similar levels in the 2K1C (60 +/- 13 fmol/ml) and ANG II-infused rats (72 +/- 15 fmol/ml) compared with intact two-kidney control animals (31 +/- 6 fmol/ml; P < 0.05) or uninephrectomized rats (13 +/- 1 fmol/ml; P < 0.05). Although renin content was markedly suppressed (80%), intrarenal ANG II content of the contralateral kidneys of the 2K1C groups (86 +/- 12 fmol/g) and the ANG II-infused group (150 +/- 17 fmol/g) was greater than that of the two-kidney control (53 +/- 7 fmol/g; P < 0.05) and uninephrectomized control animals (42 +/- 5 fmol/g; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of renin-angiotensin and kallikrein gene expression in experimental hypertension

Previous studies have shown that chronic low-dose administration of 40 ng/min angiotensin II by osmotic minipump to uninephrectomized rats mimics the temporal hypertensive response and the circulating angiotensin II levels observed in two-kidney, one clip Goldblatt rats. Furthermore, renal tissue angiotensin II contents were higher than the circulating angiotensin II levels, suggesting that circulating angiotensin II induces endogenous intrarenal angiotensin II production. The present study examined the molecular mechanisms by which intrarenal angiotensin II production is modulated in angiotensin II-induced and two-kidney Goldblatt hypertension. Two weeks after clipping, intrarenal renin mRNA levels were elevated threefold in the clipped kidney of Goldblatt rats but were markedly suppressed in the nonclipped kidneys of Goldblatt rats (28% of control values) and in the remaining kidney of uninephrectomized angiotensin II-infused rats (7% of control values). In contrast, there were sustained levels of angiotensinogen mRNA in the kidneys and livers of Goldblatt and angiotensin II-infused rats, indicating differential regulation of the genes of the renin-angiotensin system. Renal kallikrein gene expression was not altered in either of the hypertensive groups 14 days after the induction of hypertension, suggesting the absence of an enhanced counteracting kinin influence.

Relevance of the tubuloglomerular feedback mechanism in pathophysiology

The balance between a high filtration rate and high reabsorption rate in the kidney is critical in the maintenance of extracellular fluid volume. One of the mechanisms that maintain this balance is the tubuloglomerular feedback (TGF) mechanism, which operates at the level of the macula densa assessing the load and/or solute concentration coming out of the loop of Henle and controlling this load by adjusting the GFR. This review discusses the potential role of the TGF system with respect to volume homeostasis in various conditions where GFR is maintained, decreased, or increased. In most of the states discussed, the TGF system seems to act appropriately regarding volume control; however, trade-off effects occasionally occur. After acetazolamide administration, during extracellular fluid volume contraction or expansion or acute hyperkalemia, the TGF mechanism responds appropriately with regard to volume balance. After a large reduction of renal mass, the system adjusts to function at a higher level of GFR and distal delivery. In chloride-depletion metabolic alkalosis, glomerulonephritis, diabetes mellitus, and acute renal failure, the adaptation of the TGF system appears to be appropriate with regard to volume control; however, it may lead to trade-off effects, such as maintenance of metabolic alkalosis, glomerular hypertension and sclerosis, or depression of GFR, respectively. Because the TGF mechanism often contributes to compensatory adjustments to or development of disease, it can be appreciated that any in-depth evaluation of the mechanisms responsible for various pathophysiologic conditions should include an assessment of the potential role of the TGF mechanism.

Renin, angiotensinogen, and kallikrein gene expression in two-kidney Goldblatt hypertensive rats

An imbalance in the activity of the vasopressor renin-angiotensin and vasodepressor kallikrein-kinin systems may play an important role in the pathogenesis of hypertension after unilateral renal artery constriction. To test this hypothesis, we examined the expression of the renin, angiotensinogen (Ao), and tissue kallikrein genes 7 and 25 days after placement of a 0.25-mm clip on the left renal artery of rats. One week after clipping, renin mRNA levels were 4.6-fold higher in the clipped and 50% lower in the nonclipped kidneys compared with kidneys from sham-operated rats. At 25 days, renin mRNA levels in the clipped kidneys were not different from sham kidneys, but were suppressed to almost undetectable levels in the nonclipped kidneys. Steady-state Ao mRNA levels in the clipped kidneys were not different from those of nonclipped or sham kidneys at either 7 or 25 days. However, at 25 days, Ao mRNA levels were lower in the liver (70%), left ventricle (55%), and aorta (45%) of clipped than sham-operated rats. The expression of the renal kallikrein gene was unchanged at 7 days and was suppressed by 50% at 25 days. These results are consistent with the notion that activation of the intrarenal renin-angiotensin system occurs during the initial phase of the two-kidney, one-clip hypertension model. The renal kallikrein gene, in marked contrast to renin, becomes downregulated in the chronic phase. The differential regulation of renin-angiotensin and kallikrein genes may be an important pathogenetic factor in renovascular hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of angiotensin II to renal hemodynamic and excretory responses to nitric oxide synthesis inhibition in the rat

This study was performed to evaluate the contribution of angiotensin II to the effects of nitric oxide (NO) synthesis inhibition on renal hemodynamics and excretory function in rats. Intravenous infusion of N omega-nitro-L-arginine (NLA; 20 micrograms/100 g.min) increased renal arterial pressure (RAP) from 128 +/- 2 to 143 +/- 3 mm Hg (P < 0.05; N = 6) and decreased RBF by 64 +/- 3% (P < 0.01) and GFR by 41 +/- 5% (P < 0.05). In response to reduction of RAP to control levels (127 +/- 2 mm Hg) by means of an adjustable clamp (CL) placed on the suprarenal aorta, RBF and GFR exhibited efficient autoregulation and were not altered. In rats (N = 6) pretreated with the AT1 angiotensin II receptor antagonist losartan (10 mg/kg iv), the infusion of NLA increased RAP (from 114 +/- 1 to 135 +/- 2 mm Hg; P < 0.05) and decreased RBF by 42 +/- 3% (P < 0.05). However, NLA did not decrease GFR in the losartan-treated rats. As in the control rats, the reduction of RAP to 113 +/- 1 mm Hg elicited autoregulatory responses that maintained RBF and GFR. In the untreated rats, at similar RAP (128 +/- 2 (control) versus 127 +/- 2 mm Hg (NLA+CL)). NO synthesis inhibition decreased urine flow and sodium excretion (P < 0.05, in both cases). However, during blockade of AT1 receptors, NLA infusion failed to decrease urine flow and sodium excretion, even when RAP was controlled (114 +/- 1 (control) versus 113 +/- 1 mm Hg (NLA+CL)).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal responses to intra-arterial administration of nitric oxide donor in dogs

Inhibition of nitric oxide synthesis by intra-arterial administration of nitro-L-arginine (NLA) leads to attenuation of the slope of the relation between renal arterial pressure (RAP) and sodium excretion without an alteration in renal autoregulatory efficiency. In the present study, we examined whether only the presence of nitric oxide or, alternatively, changes in nitric oxide production during changes in RAP are required for pressure natriuresis to occur. Anesthetized sodium-replete dogs (n = 8) were treated with NLA (50 micrograms.kg-1 x min-1) to inhibit endogenous nitric oxide formation, and S-nitroso-n-acetylpenicillamine (SNAP) was infused intra-arterially at a constant rate (2 micrograms.kg-1 x min-1) to replenish intrarenal nitric oxide levels. Renal responses to reductions in RAP within the autoregulatory range were assessed before and during NLA infusion followed by SNAP+NLA infusion. As reported previously, NLA infusion alone increased renal vascular resistance and decreased renal blood flow, urine flow, sodium excretion, and fractional excretion of sodium, with no change in glomerular filtration rate. Autoregulatory efficiency remained intact, whereas the pressure-induced natriuretic responses were attenuated. During SNAP+NLA infusion, renal blood flow increased from 2.8 +/- 0.3 to 3.5 +/- 0.3 mL.min-1 x g-1 (P < .001), without significant changes in glomerular filtration rate (0.75 +/- 0.07 to 0.81 +/- 0.05 mL.min-1 x g-1); the autoregulatory efficiency of renal blood flow and glomerular filtration rate remained intact. SNAP increased urine flow (4.8 +/- 1.8 to 10.0 +/- 2.5 microL.min-1 x g-1), sodium excretion (0.63 +/- 0.26 to 1.70 +/- 0.37 mumol.min-1 x g-1), and fractional excretion of sodium (0.55 +/- 0.20% to 1.38 +/- 0.27%).(ABSTRACT TRUNCATED AT 250 WORDS)

Proximal tubular secretion of angiotensin II in rats

It is now established that all of the components necessary for the local formation of angiotensin II (ANG II) coexist in the kidney and can alter local ANG II production rate. However, data on ANG II concentrations in different compartments within the kidney are limited. Recently, proximal tubule fluid ANG II concentrations in the nanomolar range were reported. Using an ANG II radioimmunoassay procedure with enhanced sensitivity, we performed experiments to explore proximal tubular fluid ANG II levels further and to determine the source of the ANG II. Total free-flow proximal tubular fluid samples (n = 11) had an average ANG II concentration of 13 +/- 2 nM. These concentrations were similar (10 +/- 2 nM) in samples collected into pipettes containing the inhibitors enalaprilat and EDTA (n = 17). Fluid collected from blocked proximal tubules that were perfused with artificial tubular fluid showed similar ANG II concentrations both in the presence (22 +/- 3 nM) and absence (22 +/- 4 nM) of the angiotensin-converting-enzyme inhibitor, enalaprilat, in the perfusate. Plasma ANG II concentrations were much lower and averaged 155 +/- 26 pM. Isotonic saline expansion lowered plasma ANG II levels to 30 +/- 5 pM (P < 0.01) but did not significantly decrease intraluminal ANG II (8 +/- 1 nM). These data provide further evidence that intratubular ANG II concentrations are in the nanomolar range and are regulated independently of the plasma ANG II levels. The data obtained from perfused tubules indicate that the proximal tubule adds substantial amounts of ANG II or a precursor into the tubular lumen.