ANF secretion and renal responses to volume expansion with equilibrated blood

To evaluate the role of atrial natriuretic factor (ANF) in the renal response to acute blood volume expansion without hemodilution, a reservoir syringe filled with donor rat blood was connected to the femoral artery and vein of anesthetized Sprague-Dawley rats to allow rapid equilibration of the reservoir with the intravascular blood. Volume expansion with blood from the reservoir in two steps (of 1 and 1.5% body wt, separated by 1 h, n = 5 rats) produced a mean peak increase in plasma immunoreactive ANF from 99 +/- 21 to 1,310 +/- 230 pg/ml (P less than 0.001); plasma ANF levels throughout these experiments correlated significantly with simultaneously measured urine flow (r = 0.74, P less than 0.005) and sodium excretion (r = 0.65, P less than 0.005). Another group (n = 7) underwent the same two-step procedure; after the second volume expansion, high-dose atriopeptin III infusion (0.4 microgram.kg-1.min-1 did not further increase fractional excretion of sodium (3.17 +/- 0.27 to 2.50 + 0.39%, P = NS). In another group (n = 9 rats), the same dose of atriopeptin III was started before any blood volume expansion. After the resulting hypotension was corrected by restoration of blood volume, an additional 1.5% body weight blood volume expansion did not further augment sodium excretion. We conclude that the diuresis and natriuresis, which occur in response to volume expansion without hemodilution, rise and fall in parallel with immunoreactive ANF in the plasma, and that ANF and acute blood volume expansion act on the kidney through a similar, saturable mechanism.

Angiotensin influences on tubuloglomerular feedback mechanism in hypertensive rats

The tubuloglomerular feedback (TGF) mechanism was evaluated in the nonclipped kidney of Goldblatt hypertensive rats from both stop flow pressure (SFP) and single nephron glomerular filtration rate (SNGFR) responses to step increases in late proximal perfusion rate from 0 to 40 nl/min. During control conditions, increases in late proximal perfusion rate produced flow dependent decreases in SFP and SNGFR with maximal values of 10.2 +/- 1.0 mm Hg and 12.9 +/- 2.5 nl/min, values similar to those obtained in normal rats. During ACE inhibition (MK 422; 0.6 mg/kg/hr), arterial pressure decreased from 168 +/- 8 to 137 +/- 7 mm Hg and there was a marked attenuation in the magnitude of SFP feedback responses (delta = 2.5 +/- 0.3 mm Hg). SNGFR feedback responses, however, were not significantly impaired. Direct decreases in renal arterial pressure reduced control SFP but SFP feedback responses were maintained, indicating that the attenuated SFP feedback responses during ACE inhibition were not due to decreased arterial pressure. Superimposed infusion of angiotensin II during ACE inhibition partially restored SFP feedback responses. In contrast, norepinephrine infusion did not result in a similar restoration of SFP feedback sensitivity. These results indicate that the nonclipped kidney of Goldblatt hypertensive rats has an intact TGF mechanism as assessed from SFP and SNGFR feedback responses. Furthermore, ACE inhibition attenuates SFP but not SNGFR feedback responses, and systemic angiotensin II infusions can restore SFP feedback responsiveness towards normal.

Enhanced tubuloglomerular feedback during peritubular infusions of angiotensins I and II

Experiments were performed in pentobarbital sodium-anesthetized rats to determine whether increases in intrarenal generation of angiotensin II (ANG II) can enhance the sensitivity of the tubuloglomerular feedback mechanism. Stop-flow pressure (SFP) feedback responses to step increases in late proximal perfusion rate were obtained during control conditions and during simultaneous peritubular capillary infusion of either angiotensin I (ANG I) or ANG II. Infusion of either 10(-7) M ANG II or 10(-5) M ANG I, at rates (18.3 +/- 0.9 and 14.8 +/- 1.5 nl/min, respectively) that did not affect resting SFP, enhanced the magnitude of SFP feedback responses both at a low proximal perfusion rate of 10 nl/min (2.9 +/- 0.9 vs. 0.3 +/- 0.2 and 4.5 +/- 1.0 vs. 0.1 +/- 0.1 mmHg, respectively) and at proximal perfusion rates (greater than 30 nl/min) that elicited a maximal feedback response (13.1 +/- 1.0 vs. 10.1 +/- 0.7 and 13.5 +/- 1.6 vs. 9.8 +/- 0.8 mmHg, respectively). With a higher ANG I infusion rate (20 nl/min), control SFP measured in the absence of distal volume delivery decreased from 39.2 +/- 0.6 to 12.0 +/- 2.8 mmHg (n = 18). These effects were blocked when the ANG II receptor antagonist, saralasin (10(-5) M, Sar), was added to the infusate. In addition, the magnitude of the maximal SFP feedback response was not altered during infusion of Sar alone or ANG I + Sar. These findings indicate that ANG II, either added or formed de novo beyond the glomerular circulation, can enhance the sensitivity of the tubuloglomerular feedback mechanism.

Tubuloglomerular feedback-dependent influence of angiotensin II on the kidney in rats

To examine the modulatory role of angiotensin II on the tubuloglomerular feedback (TGF) mechanism, TGF responses were assessed during control conditions, converting enzyme inhibition (CEI; MK 422, 0.6 mg/kg.hr) and during continued CEI with the replacement of angiotensin II. TGF responses were assessed from stop flow pressure (SFP) feedback responses obtained during step increases in the late proximal perfusion rate from 0-40 nl/min. SFP values in the absence of perfusion were used to estimate glomerular pressure (GP) under conditions where the influence of the TGF mechanism should be at a minimum. During CEI, the arterial pressure decreased from 124 +/- 3 to 106 +/- 3 mmHg and the estimated GP decreased from 53 +/- 1.4 to 49 +/- 0.8 mmHg. There was a marked attenuation in the magnitude of SFP feedback responses from 11.0 +/- 1.3 to 2.7 +/- 0.6 mmHg. TGF feedback responses, however, were restored towards normal during superimposed angiotensin II infusion (7.7 +/- 0.9 mmHg). These results indicate that converting enzyme inhibition decreases the effects of angiotensin II on the kidney through TGF dependent mechanism.

Evaluation of prostaglandins as mediators of tubuloglomerular feedback

Orthograde and retrograde microperfusion experiments were conducted in Sprague-Dawley rats to evaluate the participation of vasoconstrictive eicosanoids as mediators of tubuloglomerular feedback (TGF) signals. Retrograde perfusion with 160 microM arachidonic acid (AA) added to a hypotonic solution enhanced the stop-flow pressure (SFP) feedback responses compared with those obtained with the control hypotonic solution (delta SFP, 1.6 +/- 0.4 vs. 10.1 +/- 0.7 mmHg with AA). Blockade of thromboxane A2 (TxA2) with the receptor blocker EP 092 or the synthesis inhibitor UK 38485 did not alter the magnitude of the SFP feedback responses obtained with an isotonic solution. Similarly, nordihydroguaiaretic acid, a lipoxygenase inhibitor, did not alter maximal SFP feedback responses. Although indomethacin (5 mM) did induce attenuated SFP feedback responses (delta SFP, 9.5 +/- 0.7 vs. 0.5 +/- 0.4 mmHg with indomethacin), normal feedback responses were restored within 15-90 s after cessation of indomethacin perfusion. Additionally, SFP feedback responses were not inhibited with 5 mM piroxicam, a different cyclooxygenase inhibitor. These data fail to support a role for either TxA2 or lipoxygenase end products as mediators of TGF signals. The rapid restoration of feedback responses after indomethacin exposure and the lack of blockade with piroxicam suggest that transmission of feedback signals is not dependent on cyclooxygenase products.

Direct evaluation of the microvascular actions of ANP in juxtamedullary nephrons

The renal vascular actions of atrial natriuretic peptide (ANP) remain incompletely understood. The purpose of this study is to evaluate the effects of ANP on microvascular structures of the normal kidney. The in vitro blood-perfused juxtamedullary nephron technique was utilized to allow visualization of arcuate arteries and afferent and efferent arterioles. Donor rats were pretreated with captopril to eliminate possible interactions between angiotensin II and atriopeptin III (AP III). The effects of topical administration of 3 nM AP III were determined by videometric analysis of vessel inside diameters. Under control conditions, arcuate arterial diameter averaged 83 +/- 14 microns (n = 7), afferent arteriolar diameter was 20 +/- 4 microns (n = 7), and efferent arteriolar diameter was 16 +/- 2 microns (n = 7). During superfusion with AP III, arcuate arteries and afferent arterioles dilated 73 +/- 9 and 23 +/- 5%, respectively. Both returned to their control values when AP III was removed from the superfusate. Further experiments on arcuate arteries (n = 5) revealed that 0.3 nM AP III also vasodilated these vessels (26 +/- 9%); however, no significant effect was elicited by 0.03 nM AP III. In contrast to the vasodilator influence of AP III on preglomerular vessels, efferent arteriolar diameter was not altered by AP III exposure. These observations reveal that AP III can induce selective preglomerular vasodilation involving arcuate arteries as well as afferent arterioles, while efferent arteriolar diameter is not perceptibly influenced.

Renal autoregulation and pressure natriuresis during ANF-induced diuresis

We examined the autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) in the anesthetized dog during selective renal arterial infusion of two different synthetic atrial natriuretic factor (ANF) analogues. Rat atriopeptin II (5 X 10(-8) M in renal arterial blood) caused increases in sodium and water excretion but left RBF and GFR unchanged. A similar response was seen with rat 8-33 atrial natriuretic peptide (ANP) (10(-9) M), but a twofold higher dose of this peptide produced a transient increase in RBF and a sustained 16% increase in GFR. The normal pattern of RBF autoregulation in response to decreases in renal perfusion pressure was not altered by either peptide. GFR was also efficiently autoregulated during ANF infusion; however, there was a threefold increase in the slope of the relationship between sodium excretion and arterial pressure (pressure natriuresis) during 8-33 ANP infusion (control 1.11 +/- 0.39, 8-33 ANP 4.00 +/- 0.86 mu eq/mmHg, P less than 0.01). We conclude that ANF-induced diuresis can be sustained without detectable changes in either the autoregulation-responsive or autoregulation-independent components of renal vascular resistance. Factors other than GFR, which are highly responsive to renal perfusion pressure, are important in modulating the natriuresis caused by ANF. The augmentation of pressure natriuresis within the GFR autoregulatory range suggests an influence of ANF on the magnitude of arterial pressure-induced changes in tubular sodium reabsorption.

De novo intrarenal formation of angiotensin II during control and enhanced renin secretion

In previous studies it has not been possible to determine net intrarenal formation of angiotensin II (ANG II) from arteriovenous ANG II concentrations because of the high intrarenal ANG II degradation rates (DR). This study was designed to determine ANG II-DR and to estimate net intrarenal ANG II formation during normal and enhanced renin secretion rate (RSR). In anesthetized dogs, plasma renin activity and ANG II were measured in arterial and renal venous blood by radioimmunoassay during four periods: control, renal arterial constriction (RAC), angiotensin converting enzyme (ACE) inhibition (MK 422), and MK 422 plus systemic arterial ANG II infusion. ANG II-DR was determined in each dog from the arterial-renal venous ANG II concentration difference during the period of ANG II infusion in the presence of ACE inhibition; this value was used to estimate net ANG II formation by predicting the amount of arterially delivered ANG II that escaped degradation. The average percent ANG II-DR calculated during ANG II infusion (range of 0.05 to 0.20 microgram/min) was 89 +/- 2%. In response to RAC, RSR increased from 11 +/- 3 to 24 +/- 5 ng ANG I X h-1 X min-1 X g-1. Arterial ANG II (67 +/- 11 pg/ml) and renal venous ANG II (29 +/- 6 pg/ml) increased to 133 +/- 18 and 61 +/- 10 pg/ml, respectively. Net intrarenal ANG II formation increased from 44 +/- 11 to 83 +/- 13 pg X min-1 X g-1 after renal arterial constriction. There was a significant relationship between the change in RSR and the change in ANG II formation rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Superficial nephron responses to peritubular capillary infusions of angiotensins I and II

Proximal tubular reabsorption, stop-flow pressure (SFP), and single nephron glomerular filtration rate (SNGFR) were measured in the absence of and during infusion of an isotonic saline solution containing either angiotensin I (ANG I; 10(-6) to 10(-5) M) or angiotensin II (ANG II; 10(-9) to 10(-7) M) into an adjacent peritubular capillary at a rate of 20 nl/min. Dilution of the infused ANG I and ANG II occurred in the peritubular capillary blood and as the peptides diffused into the interstitium. Infusion of either 10(-7) M ANG II or 10(-5) M ANG I increased proximal fractional fluid reabsorption (FRH2O) and decreased both SFP and SNGFR. There were no significant changes in FRH2O or SNGFR during infusion of 10(-5) M ANG I when the converting enzyme inhibitor enalaprilat (MK 422, 10(-3) M) was added to the infusate. Similarly, peritubular infusion at lower concentrations of either ANG II (10(-9) or 10(-8) M) or ANG I (10(-6) M) did not alter FRH2O, SFP, or SNGFR. These data indicate that conversion of ANG I to ANG II can occur in the peritubular capillary or interstitial environment and that increases above the normal endogenous levels in the postglomerular interstitial ANG II concentration can enhance proximal tubular reabsorption and increase preglomerular resistance and thereby reduce SNGFR.

Intrarenal mechanisms mediating pressure natriuresis: role of angiotensin and prostaglandins

The ability of the kidney to increase sodium and water excretion in response to increases in perfusion pressure has been recognized for more than 50 years. Because glomerular filtration rate is tightly autoregulated, pressure natriuresis occurs as the result of decreased tubular sodium reabsorption rather than increased filtered load. Micropuncture and microperfusion data support the contention that acute changes in arterial pressure can alter proximal tubule reabsorption; however, studies have failed to show a consistent association between changes in sodium excretion and peritubular, interstitial, or tubular pressures. Thus, the specific intrarenal mechanism for the change in tubular reabsorption in response to an acute change in arterial pressure does not appear to be related to the peritubular physical factors at the level of outer cortical nephrons. The possible roles of angiotensin and prostaglandins as humoral mediators of pressure natriuresis are considered in this report. Although angiotensin II is a powerful modulator of the slope of the pressure natriuresis relationship, the responsiveness of sodium excretion to arterial pressure is actually enhanced by angiotensin-converting enzyme inhibitors. These data suggest that angiotensin does not mediate the basic phenomenon. Recent experiments indicate that intrarenal prostaglandins also modulate the magnitude of the pressure natriuresis relationship, but these hormones do not appear to be essential for its basic manifestation.

Angiotensin II effects on microvascular diameters of in vitro blood-perfused juxtamedullary nephrons

The purpose of this study was to determine the specific renal microvascular segments that are functionally responsive to angiotensin II (ANG II) and other vasoactive hormones. Experiments were performed on juxtamedullary tissue from captopril-treated rats during perfusion with blood at a constant pressure of 110 mmHg. Epifluorescence videomicroscopy was utilized to measure diameters of arcuate and interlobular arteries (ART), mid- (MA) and late- (LA) afferent arterioles, and efferent arterioles (EA). Norepinephrine (700 nM) significantly decreased, and sodium nitroprusside (380 nM) increased, inside diameters of all segments. Topical application of ANG II (0.01 to 1 nM) induced significant reductions in diameters of all vessel segments: ART, 17.5 +/- 2.0%; MA, 19.6 +/- 2.5%; LA, 13.5 +/- 1.5%; and EA, 16.9 +/- 2.7%. The preglomerular response to ANG II was blocked by saralasin (10 microM) and, in most cases, was dose dependent; however, an initial hypersensitivity to low ANG II doses (30% decrease in diameter) was exhibited by 38% of the preglomerular vessels studied. Under these experimental conditions, single-nephron glomerular filtration rate decreased significantly in response to 0.01 nM ANG II exposure. These observations demonstrate that physiological concentrations of ANG II can elicit receptor-dependent and reversible vasoconstriction of the juxtamedullary nephron microvasculature at both pre- and postglomerular sites.

Effects of calcium channel blockade on renal vascular resistance responses to changes in perfusion pressure and angiotensin-converting enzyme inhibition in dogs

We conducted these experiments to evaluate the selectivity of calcium channel blockade on the renal autoregulatory mechanism and on angiotensin II-mediated renal vasoconstriction. Experiments were performed in anesthetized dogs in which renal arterial pressure, renal blood flow, and glomerular filtration rate were measured at normal and reduced renal arterial pressure. At control arterial pressures, renal arterial infusions of verapamil increased renal blood flow and glomerular filtration rate significantly. The decreases in renal vascular resistance elicited with verapamil (n = 13) and nifedipine (n = 4) occurred only at renal arterial pressure levels within the normal autoregulatory range. Renal blood flow autoregulatory efficiency was markedly attenuated, and the pressure-flow relationship obtained during calcium channel blockade approached that of a passive system. Systemic infusions of an angiotensin-converting enzyme inhibitor (captopril) during continued verapamil infusion caused further vasodilation at all renal arterial pressure values, as evidenced by an increase in slope of 27% of the pressure-blood flow relationship. This response was reversed by angiotensin II infusions. This shift indicates a reduction in minimal vascular resistance elicited by captopril, not obtainable with verapamil alone, and sensitive to angiotensin II. The effects of verapamil and nifedipine on renal blood flow autoregulation suggest a specific effect at preglomerular sites of potential operated membrane calcium channels in the autoregulatory phenomenon. The additional vasodilation elicited with captopril and reversed by angiotensin II indicates the presence of an angiotensin-sensitive postglomerular resistance component which is not influenced by calcium entry blockers.

Arterial pressure and renal function in two-kidney, one clip Goldblatt hypertensive rats maintained on a high-salt intake

Arterial blood pressure and renal function of both clipped and non-clipped kidneys of benign two-kidney, one clip (2K1C) Goldblatt hypertension were evaluated in order to determine whether high-salt intake alters the course of the development and magnitude of hypertension or influences renal function. The administration of 0.9% sodium chloride as a drinking solution for 3 weeks suppressed plasma renin activity (PRA) and kidney renin content of the clipped kidney to normal values. Despite suppression of PRA and kidney renin content, the saline-drinking clipped rats still developed hypertension of the same magnitude as the water-drinking clipped rats. However, the onset of hypertension was delayed by 4 days. Urine flow, glomerular filtration rate (GFR) and sodium excretion rate from the clipped kidneys of the saline-drinking clipped rats were higher than the corresponding values in the water-drinking rats, and approached those observed in control animals. Thus, the high-salt intake which was associated with suppression of the activity of the renin-angiotensin system delayed the onset of, but not the final magnitude of, the hypertension. In addition, kidney function in the clipped kidneys of saline-drinking clipped rats was enhanced compared with that observed in the water-drinking clipped rats.

Effects of locally formed angiotensin II on renal hemodynamics

The kidney produces angiotensin II (AngII) by conversion of both locally formed and systemically delivered angiotensin I (AngI). The latter may be physiologically significant because the kidney can convert 20-25% of systemically delivered AngI. To determine possible differences between the effects of circulating and locally converted AngII, we compared the renal responses to renal arterial infusions of AngI and AngII in equiconstrictor doses. Both reduced the renal blood flow and increased the filtration fraction; it is important that the AngI infusions consistently reduced glomerular filtration rates (GFR), which indicates effects proximal to or at the glomerulus. Micropuncture experiments revealed that AngI infusions reduced proximal tubular and peritubular capillary pressures and the single-nephron GFR; glomerular capillary pressure was not altered significantly. AngI infusions increased both pre- and postglomerular resistances and reduced the glomerular filtration coefficient. In other studies designed to estimate net intrarenal AngII generation, it was determined that the kidney degrades about 90% of arterially delivered AngII. Thus, most of the AngII in renal venous blood was formed intrarenally. Local production of AngII was enhanced, in association with increased renin release, after reductions in renal arterial pressure. Such increases in intrarenal AngII production may contribute to the AngII-dependent changes in renal vascular resistance that occur in conditions where the renin-angiotensin system is stimulated.

Renal autoregulatory efficiency during angiotensin-converting enzyme inhibition in dogs on a low sodium diet

Autoregulatory efficiency of renal blood flow (RBF) and glomerular filtration rate (GFR) was evaluated in 12 anesthetized dogs that had been maintained on low-sodium diet during control conditions and following infusion of an angiotensin-converting enzyme inhibitor (captopril). Converting enzyme inhibition (CEI) decreased systemic blood pressure by 15.5 +/- 3.5%, increased RBF by 36.3 +/- 6.5%, and increased GFR by 25.9 +/- 10.7%. In response to reductions in renal arterial pressure, RBF was efficiently autoregulated and did not change significantly until the 89- to 75-mm Hg range during the control period and the 74- to 54-mm Hg range during CEI. Overall GFR autoregulatory efficiency was generally well maintained during CEI; however, evaluation of the coupled autoregulatory efficiency of RBF and GFR indicated that during angiotensin blockade, there was a greater incidence of a dissociation between RBF and GFR autoregulatory efficiency. Six of the 12 dogs showed reduced GFR autoregulatory efficiency at renal arterial pressures where RBF was still well maintained. Thus, while the data indicate that blockade of the renin-angiotensin system does not abolish the basic capability of the kidney to autoregulate either RBF or GFR efficiently, more subtle influences on the coupling of RBF and GFR autoregulatory efficiency were observed at the lower level of the autoregulatory range.

Microvascular reactivity of in vitro blood perfused juxtamedullary nephrons from rats

The effects of angiotensin II (AII), epinephrine, and changes in perfusion pressure on glomerular capillary and afferent arteriolar pressures were assessed using the in vitro blood perfused juxtamedullary nephron (JMN) preparation. At a perfusion pressure of 102 +/- 1 mm Hg, glomerular capillary pressure (GCP) averaged 55 +/- 1 mm Hg. Afferent arteriolar pressure (AAP), measured at early-to-mid afferent locations, was 88 +/- 2 mm Hg and decreased to 71 +/- 7 mm Hg at the most terminal segments, 50 to 80 micron from the glomerulus. In some nephrons, readjustments of GCP occurred in response to step changes in perfusion pressure within the range of 90 to 165 mm Hg. In 37 nephrons, bolus injections of AII into the blood caused dose-dependent and reversible decreases in GCP, ranging from -4 +/- 1 mm Hg (12 to 25 pg) to -26 +/- 4 mm Hg (20 ng). Similar decreases in GCP ranging from -9 +/- 3 to -22 +/- 3 mm Hg were observed in response to epinephrine (1.25 to 20 ng). Epinephrine also consistently reduced AAP by 37 +/- 10% (N = 8). In contrast, AII typically increased pressure in the early and mid segments of the afferent arteriole, but caused variable responses in the late afferent arteriole. The responses to vasoconstrictor agents were not mimicked by increases in perfusion pressure per se. These results indicate that the preglomerular vasculature of in vitro JMN can exhibit autoregulatory behavior and is responsive to humoral vasoconstrictors. The response to epinephrine was generalized, occurring along the entire preglomerular vasculature, while the predominant effects of AII were localized to terminal afferent structures, which may include intraglomerular constrictor elements.

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.

Tubular transport responses to angiotensin

Angiotensin II (ANG II) is a powerful effector agent in the regulation of extracellular volume and exerts an important influence on renal sodium excretion. In addition to its effects on aldosterone secretion, ANG II acts directly on the kidney causing retention of sodium at low (physiological) doses and enhanced sodium excretion at high doses. The mechanism for these responses involves vasoconstrictor actions of ANG II on the renal vasculature and a direct action of the peptide on tubular reabsorption. Micropuncture and microperfusion studies have demonstrated that proximal tubular sodium and water transport are stimulated by physiological concentrations (10(-12) to 10(-10) M) of ANG II on the peritubular side, whereas higher doses (10(-7) M) cause inhibition. A luminal site of action in the proximal tubule has also been reported and additional more distal sites are indicated. [125I]ANG II binding sites on the brush border and basolateral membranes of proximal tubule cells have high affinity (Kd in the nanomolar range) for ANG II and lower affinity for ANG III. The biphasic action of ANG II is exerted directly on the epithelial cells and appears to be electroneutral. The data indicate that ANG II binds to receptors on the basolateral cell membrane and alters the rate of entry of sodium through the luminal membrane to increase or decrease, depending on the concentration of peptide. Several possible cellular mechanisms that could mediate these responses are discussed.

Prostaglandins in the sodium excretory response to altered renal arterial pressure in dogs

Acute variations in renal arterial pressure are associated with corresponding alterations in absolute and fractional sodium excretion even under conditions of highly efficient autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR). Since prostaglandins recently have been implicated in the regulation of sodium excretion, we investigated the hypothesis that the renal prostaglandin system participates in "pressure natriuresis." Anesthetized sodium-replete dogs were subjected to partial carotid artery constriction to elevate systemic arterial pressure. Under these control conditions, sodium excretion was 103 +/- 18 mueq/min (n = 17) and urinary prostaglandin E2 excretion averaged 4.6 +/- 1.5 ng/min (n = 8). Decreases in renal arterial pressure within the auto-regulatory range reduced sodium excretion (2.1%/mmHg) and prostaglandin E2 excretion (1.7%/mmHg), whereas GFR and RBF were not affected. There was a significant correlation between the changes in sodium and prostaglandin E2 excretion rates (r = 0.932, P less than 0.01). In nine dogs treated with indomethacin, sodium excretion was reduced by 70% while GFR and autoregulatory capability were unaffected. There was a marked attenuation of the effect of changes in arterial pressure on sodium excretion, with this parameter exhibiting changes averaging 0.6%/mmHg (P less than 0.001). These observations suggest that the renal prostaglandin system may exert an important influence on the pressure-natriuresis mechanism.

Tubuloglomerular feedback-mediated decreases in glomerular pressure in Munich-Wistar rats

These experiments were performed to evaluate directly measured glomerular capillary pressure and single nephron glomerular filtration rate (SNGFR) tubuloglomerular feedback responses in Munich-Wistar rats during increased distal flow rate achieved by infusing an isotonic electrolyte solution into unblocked late proximal tubules. Arterial pressure averaged 114 +/- 2 mmHg and proximal tubule pressure was 14 +/- 1 mmHg. In eight tubules, control SNGFR based on distal tubular fluid collections averaged 22 +/- 3 nl/min, decreased to 15 +/- 2.3 nl/min when 10-12 nl/min of perfusate was infused into the late proximal tubule, and further decreased to 9 +/- 1.7 nl/min at an infusion of 20-24 nl/min. In 22 tubules, control glomerular capillary pressure was 55 +/- 1.6 mmHg, decreased to 43 +/- 2.5 mmHg with addition of perfusate into a late proximal tubule at a rate of 24 nl/min, and returned to 53 +/- 3.1 mmHg when perfusion was stopped. In eight nephrons, glomerular capillary pressure was shown to be responsive to smaller increments in the late proximal infusion rate and was reduced by 4 +/- 0.5 and 7 +/- 1.1 mmHg at the intermediate rates of 10 and 15 nl/min, respectively. These results demonstrate that glomerular pressure decreases during increased distal delivery even when the tubule is not blocked. They are consistent with the hypothesis that increases in afferent arteriolar resistance are primarily responsible for feedback-mediated reductions in glomerular filtration rate.

Effects of renal arterial angiotensin I infusion on glomerular dynamics in sodium replete dogs

During intrarenal infusion of angiotensin I (AI), conversion to angiotensin II (AII) within the kidney has been shown to occur early enough to decrease glomerular filtration rate (GFR). To evaluate further the mechanism by which AI decreases GFR, micropuncture studies were conducted in sodium replete dogs. Feedback-mediated alterations in glomerular function were minimized by reducing renal arterial pressure to 90 mm Hg. During infusion of AI (0.82 +/- 0.01 micrograms min-1), renal blood flow (3.91 +/- 0.25 ml min-1 g-1) and GFR (0.63 +/- 0.04 ml min-1 g-1) decreased by 36.7 +/- 6.1% and 18.9 +/- 6.1%, respectively. Similarly, single nephron GFR decreased from 66.4 +/- 3.8 to 40.0 +/- 3.2 nl min-1 and estimated glomerular plasma flow (280 +/- 49 nl min-1) decreased by 55 +/- 6%. Stop-flow pressure (40.5 +/- 3.6 mm Hg) did not change significantly, while proximal tubular (21.8 +/- 1.4 mm Hg) and peritubular capillary pressures (13.2 +/- 1.8 mm Hg) decreased by 25.5 +/- 2.8% and 49.4% +/- 5.1%, respectively. Glomerular capillary and effective filtration pressures were not altered significantly. There were increases in both preglomerular (168%) and efferent (203%) arteriolar resistances, along with a decrease in the glomerular filtration coefficient (Kf) from 4.6 +/- 0.6 to 2.5 +/- 0.5 nl mm Hg-1 min-1. These data indicate that augmented intrarenal conversion of circulating AI reduces GFR as a consequence of decreases in Kf as well as in glomerular plasma flow, the latter being due to concomitant increases in preglomerular and efferent arteriolar resistances.

Evidence for angiotensin-stimulated proximal tubular fluid reabsorption in normotensive and hypertensive rats: effect of acute administration of captopril

The effects of captopril on mean arterial blood pressure and proximal tubular fluid reabsorption (JV) were examined in anaesthetized normotensive rats and in the non-clipped kidneys of two-kidney, one-clip Goldblatt hypertensive rats. In the normotensive animals, captopril reduced arterial blood pressure from 121 +/- SD 9 to 106 +/- 10 mmHg and JV decreased from 3.78 +/- 0.45 to 2.57 +/- 0.58 X 10(-4) mm3 mm-2 s-1. Captopril had a greater effect on blood pressure in the hypertensive animals (172 +/- 17 reduced to 133 +/- 23 mmHg) although the decrease in JV from 3.62 +/- 0.12 to 2.40 +/- 0.40 was similar to that observed in normotensive animals. These results provide evidence that, in the anaesthetized rat, angiotensin II contributes to the maintenance of the rate of proximal fluid reabsorption. The magnitude of the angiotensin-stimulated component of proximal fluid absorption is similar in normotensive and two-kidney, one-clip Goldblatt hypertensive rats.

In vitro perfusion of juxtamedullary nephrons in rats

Anatomical studies of rat kidneys revealed the presence of a unique population of juxtamedullary glomeruli (JMG) located at the inside cortical surface apposed to the pelvic lining and arcuate veins. These "superficial" JMG were exposed by longitudinally bisecting the kidney, reflecting the papilla, removing the pelvic mucosa, and transecting the venous walls. A microperfusion system was developed to allow blood perfusion of these nephrons via arcuate arteries. At a perfusion pressure of 100 mmHg, with most of the preglomerular pressure drop being localized to the terminal afferent arteriolar segment. In subsequent studies, blood hematocrit was reduced to approximately 30% with physiological solutions devoid of or containing albumin, and rats were treated with a converting enzyme inhibitor. In these conditions, single nephron glomerular filtration rate averaged 34 +/- 4 nl/min (low plasma colloid osmotic pressure, PCOP) and 23.3 nl/min (maintained PCOP). Proximal tubule reabsorption ranged from 17 to 29%. In conclusion, the integrity of nephron function is maintained in this model, which may provide further insights into the dynamics of filtration and reabsorption processes of juxtamedullary nephrons.

Intrarenal angiotensin I conversion at normal and reduced renal blood flow in the dog

Intrarenal conversion of angiotensin I (ANG I) to angiotensin II (ANG II) under conditions of normal and reduced renal blood flow (RBF) elicited by constriction of the renal artery was examined in pentobarbital-anesthetized dogs. In eight animals, tracer doses of 125I-ANG I (5-12 pmol) were injected into the renal artery and 125I-ANG I, 125I-ANG II, and 125I-labeled metabolites were measured in renal venous effluent by high-voltage paper electrophoresis. The mean conversion of ANG I to ANG II during a single passage through the kidney was 21.8 +/- 2.1% at control RBF. When RBF was decreased by 25 and 53%, percent ANG I conversion was not altered significantly. In six dogs percent conversion of 125I-[Sar1, Ile5]ANG I, an ANG I analogue refractory to hydrolysis by aminopeptidases, was 18.1 +/- 1.7% at control RBF and did not change significantly when the RBF was reduced by 55%. Although there were severalfold increases in renal renin secretion rate and net ANG I generation rate during reduced RBF, net renal ANG II formation rate did not change significantly. These data indicate that there is substantial conversion of ANG I in a single passage through the dog kidney and that intrarenal ANG I conversion is independent of RBF even under conditions in which renin secretion rate and ANG I generation rate are increased severalfold.

Effects on renal hemodynamics of intra-arterial infusions of angiotensins I and II

Experiments were conducted in anesthetized dogs to evaluate the differences between the effects of intrarenal conversion of angiotensin I (ANG I) to angiotensin II (ANG II) and those of circulating ANG II on renal blood flow (RBF), glomerular filtration rate (GFR), peritubular capillary pressure (PCP), proximal tubular free-flow pressure (PTP), and stop-flow pressure (SFP). Equiconstrictor doses of ANG I and ANG II were infused into the renal arteries of dogs kept on normal and high sodium diets. In clearance experiments, RBF decreased by 23% (low dose) and 33% (high dose) during the infusion of either ANG I or ANG II; GFR was significantly reduced only during the ANG I infusion. In micropuncture experiments, in which the GFR responses were similar, there were significant reductions in PTP (23 +/- 3%) and PCP (33 +/- 3%) during the intrarenal ANG I infusion; SFP was not altered significantly. Afferent and efferent arteriolar resistances increased significantly during ANG I infusion as well as during infusion of ANG II. These results indicate that during intra-arterial infusion of ANG I, the conversion to ANG II within the kidney occurs early enough to decrease glomerular filtration rate through an apparent increase in preglomerular resistance.

Glomerular filtration dynamics during renal vasodilation with acetylcholine in the dog

The reason for the failure of glomerular filtration rate (GFR) to exhibit plasma flow dependency during pharmacologic vasodilation remains unclear although it has been suggested on the basis of experiments in rats that vasodilators may lead to a reduction in the glomerular filtration coefficient (Kf). To evaluate the applicability of this hypothesis to the dog, the effects of vasodilation with acetylcholine on glomerular dynamics and Kf were evaluated in two groups of dogs. One group (n = 19) was studied at spontaneous arterial pressures to allow maximum vasodilation to occur. In the other group (n = 5), renal arterial pressure was reduced and maintained at approximately 89 mmHg. Glomerular filtration rate and single nephron glomerular filtration rate were not altered significantly during acetylcholine infusion in either of the two groups. Both whole kidney and superficial filtration fractions decreased significantly. At spontaneous arterial pressures, transglomerular hydrostatic pressure was not altered significantly because of equivalent increases in proximal tubule pressure and in glomerular pressure. In the dogs studied at reduced renal perfusion pressure, glomerular capillary pressure did not change, but proximal tubule pressure increased slightly. Average effective filtration pressures and Kf were not significantly altered during the infusion of acetylcholine either at spontaneous or reduced renal perfusion pressures. These observations indicate that Kf in the dog is not significantly decreased by acetylcholine and that GFR is not affected during infusion of this agent because the effective filtration pressure is not significantly altered.

Renal hemodynamic effects of captopril in anesthetized sodium-restricted dogs. Relative contributions of prostaglandin stimulation and suppressed angiotensin activity

The mechanism of captopril-induced alterations in arterial pressure (AP), glomerular filtration rate (GFR), renal blood flow (RBF), and renal vascular resistance (RVR) was studied in pentobarbital anesthetized sodium-restricted dogs. In 7 dogs, captopril caused decreases in AP (16 +/- 3%) and RVR (46 +/- 5%), as well as increases in RBF (62 +/- 12%) and sodium excretion (399 +/- 73%). These responses were reversed by angiotensin II infusion at a rate sufficient to restore RBF to control levels. The captopril-induced increase in GFR (29 +/- 8%) was partially reversed by the intravenous angiotensin II infusion to a level not significantly different from control. In 5 dogs, indomethacin increased AP (10 +/- 2%) and RVR (38 +/- 8%); the slight decreases in RBF and GFR were not statistically significant. Subsequent captopril treatment decreased AP (20 +/- 3%) and RVR (42 +/- 4%), while RBF and GFR increased by 45 +/- 8% and 32 +/- 10%, respectively. These observations suggest that the renal response to captopril in sodium-restricted dogs is not dependent upon alterations in prostaglandin synthesis but, instead, is primarily due to diminished angiotensin II activity.

Angiotensin-mediated alterations in nephron function in Goldblatt hypertensive rats

The present study was performed to evaluate superficial nephron responses of the nonclipped kidney to angiotensin I converting enzyme inhibitor (CEI) (SQ 20,881, 3 mg . kg-1 . h-1) in two-kidney, one-clip Goldblatt hypertensive (GH) rats. Late proximal and early distal tubule collections were obtained before and during CEI. Significant increases in glomerular filtration rate, urine flow, sodium excretion, proximal and distal tubule flow rates, and single nephron glomerular filtration rate (from 24.6 +/- 1.7 to 27.5 +/- 1.6 nl/min) occurred despite reductions in arterial blood pressure (from 160 +/- 5 to 137 +/- 6 mmHg) during CEI. Proximal tubule absolute and fractional reabsorption of fluid, chloride, and total solute decreased significantly. In the nephron segment between the two collection sites, there were increases in absolute but decreases in fractional reabsorption. At the distal tubule level, fractional reabsorption but not absolute reabsorption decreased significantly. Proximal and distal tubule hydrostatic pressures increased significantly while peritubular capillary pressure decreased slightly. Responses following inhibition of angiotensin II formation suggest that there exists an angiotensin II-mediated enhancement in tubular reabsorption in the nonclipped kidney of Goldblatt hypertensive rats.

Relationship between tubulo-glomerular feedback responses and perfusate hypotonicity

Previous studies have established that during orthograde perfusion from a late proximal tubule site, there is a direct relationship between the magnitude of the feedback response and the level of distal tubular fluid sodium chloride concentration. The present study was conducted in the rat to extend this observation by assessing stop flow pressure (SFP) feedback responses during retrograde perfusion into the early distal tubule with solutions varying in total solute concentration and in the anionic constituent. SFP was measured after blockade of the intermediate proximal and late distal tubular segments with wax. Retrograde perfusion was initiated from an early distal tubular site at 15 nl/min. All solutions contained a 38 mOsm/kg matrix base, and the total solute concentration was increased with either sodium chloride or sodium isethionate to achieve osmolalities of 68, 85, and 120 mOsm/kg. For comparison, feedback responses during perfusion with a 120 mOsm/kg choline chloride solution were evaluated. During perfusion with the 120 mOsm/kg solutions, SFP decreased by 13 +/- 1.3 mm Hg with the sodium chloride solution, 12 +/- 1.5 mm Hg with the sodium isethionate solution, and 12 +/- 1.3 mm Hg with the choline chloride solution. During perfusion with solutions having an osmolality of 85 mOsm/kg, SFP decreased by 8 +/- 1.3 mm Hg with sodium chloride and 8 +/- 0.8 mm Hg with sodium isethionate. The 68 mOsm/kg solutions elicited decreases in SFP of 4.4 +/- 0.4 mm Hg and 5 +/- 0.5 mm Hg. During perfusion with the 38 mOsm/kg matrix solution, SFP decreased by an average of 1.4 +/- 0.9 mm Hg. Linear regression analysis revealed a 1 mm Hg decrease in SFP for every 7.7 mOsm/kg decrease in perfusate osmolality below 120 mOsm/kg. These results confirm previous findings that the magnitude of the feedback response is associated closely with the concentration of the perfusate over a narrow range from 38 to 120 mOsm/kg. Since the responses with sodium isethionate solutions were similar to the responses obtained with sodium chloride containing solutions, these studies provide evidence that the magnitude of the feedback responses are not specifically dependent on alterations in chloride concentration.

Role of a macula densa feedback mechanism as a mediator of renal autoregulation

The phenomenon of renal autoregulation demonstrates the presence of a sensitive intrarenal mechanism capable of maintaining GFR stable even during extrinsic disturbances that would be expected to alter renal hemodynamics. Substantial evidence has accumulated indicating that autoregulatory capability is dependent on the integrity of normal distal tubule flow dynamics and an intact distal tubuloglomerular feedback mechanism. Several whole-kidney and micropuncture studies have shown that interruption of volume delivery to the distal nephron interferes with autoregulation of renal blood flow (RBF) and GFR. The autoregulatory adjustments are probably localized at the afferent arterioles because the pressure in the larger arterioles does not exhibit autoregulation in response to decreases in renal perfusion pressure. It remains uncertain if the distal tubuloglomerular feedback mechanism is entirely responsible for autoregulatory responses. Data obtained in dog experiments indicate that under conditions of interrupted delivery to the distal nephron, SNGFR responses to decreases in arterial pressure are approximately those expected of a passive system where proximal tubule pressure is allowed to adjust to new steady-state levels with regard to the rapidity with which signals are transmitted to the distal nephron. Whole-kidney experiments indicate that, under conditions of a mild osmotic diuresis, the changes in urine flow following an increase in arterial pressure occur within 1 sec of the initiation of autoregulatory adjustments in vascular resistance. These experiments are consistent with the view that the major fraction of renal autoregulatory adjustments in resistance is mediated by the distal tubuloglomerular feedback mechanism that responds to some component of distal tubular flow and transmits signals to the afferent arteriolar segment of the same nephron.

Effects of saralasin infusion on bilateral renal function in two-kidney, one-clip Goldblatt hypertensive rats

1. Previous studies have shown that administration of converting enzyme inhibitor (CEI, SQ 20 881) to two-kidney, one-clip Goldblatt hypertensive (GH) rats clipped for 3-4 weeks resulted in marked increases in glomerular filtration rate (GFR), water and sodium excretion by the non-clipped kidneys. The clipped kidneys exhibited reduced function that was due, in part, to the reductions in arterial pressure. To evaluate further the hypothesis that the renal responses to CEI were due primarily to the inhibition of angiotensin II rather than other factors, we infused the angiotensin II competitive blocker, saralasin, into GH rats under sodium pentobarbital anaesthesia and examined renal haemodynamics and excretory function of each kidney before and during saralasin infusion and after cessation of saralasin infusion. 2. Saralasin reduced mean arterial blood pressure from 164 +/- 4 to 124 +/- 4 mmHg. Despite the profound fall of arterial pressure, significant increases in renal blood flow from 5.82 +/- 0.22 to 9.15 +/- 0.76 ml/min and glomerular filtration rate from 1.46 +/- 0.10 to 2.18 +/- 0.14 ml/min were observed in the non-clipped kidneys. Renal vascular resistance decreased from 2.34 (+/- 0.14) x 10(5) to 1.17 (+/- 0.19) x 10(5) kPa 1(-1) s [2.34 (+/- 0.14) x 10(6) to 1.17 (+/- 0.19) x 10(6) dyn s cm-5]. Also, concomitant diuresis and kaliuresis and a delayed natriuresis occurred. 3. The clipped kidneys exhibited reductions in renal blood flow, GFR and excretory function during saralasin infusion. 4. Normal rats receiving the identical dose of saralasin responded with a slight but significant decrease in arterial pressure. The increase in renal blood flow and GFR were less than those observed in the non-clipped kidneys of hypertensive rats. 5. These data provide further support to the hypothesis that an angiotensin II-mediated elevation in renal vascular resistance and impairment of renal function exist in the non-clipped kidneys of GH rats.

Cytoplasmic calcium in the mediation of macula densa tubulo-glomerular feedback responses

Within each nephron of the mammalian kidney, a feedback mechanism operating between the macula densa segment of the distal tubule and the afferent arteriole participates in the regulation of glomerular filtration rate. Retrograde microperfusion studies in rats were conducted to test the hypothesis that activation of macula densa cytoplasmic calcium is involved in the transmission of feedback signals to the vascular elements. Perfusion into distal tubules with a hypotonic solution (70 milliosmolar) elicited moderate decreases in glomerular pressure of 6 +/- 0.8 millimeters of mercury. With the addition of a calcium ionophore (A23187) glomerular pressure decreased by 16 +/- 1.1 millimeters of mercury. When a solution devoid of calcium but containing A23187 was used, the feedback response was inhibited. Thus, cytoplasmic calcium within the receptor cells may participate in the transmission of feedback signals to the contractile cells.

Attenuated pressure natriuresis in hypertensive rats

We studied the isolated blood-free perfused nonclipped kidneys from the 2-kidney Goldblatt hypertensive rat model (GHR) to evaluate intrinsic excretory responses to changes in perfusion pressure. We examined kidneys from 10 control rats (in vivo systolic BP 110 +/- 3.6 mm Hg), from 9 rats with nonmalignant hypertension (HBP) (in vivo systolic BP 158 +/- 6.5 mm Hg), and from 5 rats with malignant HBP (in vivo systolic BP 183 +/- 6.4 mm Hg). We found that at all levels of perfusion pressure, the renal vascular resistances were significantly higher and glomerular filtration rate (GFR) lower in kidneys from hypertensive rats than in kidneys from control rats. Kidneys from hypertensive rats had lower urinary excretion of sodium (UNaV) and urine flow than kidneys from control rats at all levels of pressure above 100 mm Hg. The most striking differences in all functional parameters were noted in kidneys from rats with malignant HBP. Kidneys from both hypertensive and control rats failed to show changes in vascular resistance in response to Saralasin. We conclude that the nonclipped kidney in GHR exhibits a blunted natriuresis in response to elevated perfusion pressure which occurs in the absence of angiotensin II (AII) and renin substrate. This diminished pressure natriuresis may be caused partly by the lower GFR and by reduced pressure transmission due to greater renal vascular resistance and thus may be partially responsible for the maintenance of the hypertensive state.

Influence of bradykinin and papaverine on renal and glomerular hemodynamics in dogs

Although it is recognized that vasodilator-induced increases in renal plasma flow are not associated with increases in glomerular filtration rate (GFR), the mechanism responsible for the failure of GFR to exhibit plasma flow dependency under these circumstances remains uncertain. To evaluate this problem further, the effects of intra-arterial infusion of two vasodilators, bradykinin (n = 7) and papaverine (n = 6), on renal hemodynamics and glomerular function of dogs were determined. Both agents increased renal blood flow; however, GFR and single nephron GFR (SNGFR) remained unchanged during bradykinin infusion and decreased significantly during papaverine infusion. Transglomerular hydrostatic pressure and mean effective filtration pressure were not altered during bradykinin infusion but were significantly reduced by papaverine. Neither agent altered the glomerular filtration coefficient. These observations indicate that, in the dog, the GFR responses during renal vasodilation are dependent primarily on the effective filtration pressure.

Influence of converting enzyme inhibition on renal hemodynamics and glomerular dynamics in sodium-restricted dogs

Clearance and micropuncture experiments were performed to evaluate the influence of converting enzyme inhibition (CEI) (SQ 14,225) on renal hemodynamics, glomerular filtration rate (GFR), segmental vascular resistances, and superficial nephron function in anesthetized sodium restricted dogs. In one series (n = 8), renal blood flow (RBF) and GFR exhibited a high degree of autoregulatory efficiency when renal arterial pressure (RAP) was reduced from 126 +/- 5 to 86 +/- 1 mm Hg. With RAP maintained at the reduced level, CEI elicited increases in RBF (3.9 +/- 0.3 to 5.8 +/- 0.5 ml/min per g kw) and GFR (0.81 +/- 0.03 to 0.94 +/- 0.04 ml/min per g kw). With return of RAP to spontaneous levels during continued CEI, RBF and GFR autoregulatory efficiency was maintained, and was similar to that observed in control dogs subjected to the same procedures (n = 5). In the micropuncture experiments (n = 12), RAP was maintained at the reduced level (87.5 +/- .9 mm Hg), and measurements were made before and during CEI. Proximal tubule pressure, peritubular capillary pressure, stop flow pressure, and single nephron GFR (SNGFR) increased significantly. Regression analysis suggested that the increases in SNGFR were associated with small increases in the filtration coefficient. CEI reduced preglomerular resistance by 29% to 35% and efferent arteriolar resistance by 24% to 32%. These results indicate that the increased activity of the renin-angiotensin system that occurs during salt restriction exerts approximately equivalent vasoconstrictor influences on both preglomerular and postglomerular vascular resistance elements.

Hemodynamic and single nephron function during the maintenance phase of ischemic acute renal failure in the dog

We studied ischemic acute renal failure in 28 dogs by micropuncture, microsphere, morphologic, and whole kidney hemodynamic techniques, 18 to 24 hours after the renal artery was clamped (clamping time, 60 to 90 min). Before the artery was clamped, renal blood flow (RBF) averaged 3.49 +/- (SEM) 0.23 ml/min x g and was not significantly different (3.70 +/- 0.34 ml/min x g) 18 hours after the ischemic episode. RBF autoregulatory capability was, however, significantly reduced. Fractional outer cortical blood flow decreased slightly from 41 +/- 2 to 36 +/- 3% (P less than 0.05) postischemia. Single nephron glomerular filtration rate (SNGFR) was highly variable from one animal to the next and ranged from 0 to 87 nl/min (mean, 36 +/- [SEM] 7 nl/min) in a manner similar to whole kidney inulin clearance, which ranged from 0 to 0.56 ml/min x g (mean, 0.30 +/- 0.05 ml+min x g). The correlation coefficient between SNGFR and inulin clearance was highly significant, indicating an association between SNGFR and whole kidney GFR. Proximal tubule pressure (PTP) averaged 20 +/- (SEM) 1 mm Hg. In 6 dogs, the glomerular filtration coefficient (Kf) was determined by measurements of stop-flow pressure, colloid osmotic pressure, SNGFR, PTP, and single nephron filtration fraction, Kf was below that obtained for control animals. Scanning electron microscopy (SEM) studies indicated that the endothelial fenestrations were reduced in number and size. These studies suggest that one major characteristic of ischemic nephropathy in the dog is a derangement in the filtration process. The maintenance of RBF in the postischemic phase may occur by utilization of the autoregulatory reserve of the renal vasculature.

Dissociation of tubuloglomerular feedback responses from distal tubular chloride concentration in the rat

Previous studies have demonstrated that stop-flow pressure (SFP) feedback responses can occur during orthograde perfusion with solutions having low amounts of sodium or chloride. However, retrograde perfusion studies have suggested a specific role for chloride concentration in mediating feedback responses. These studies were conducted to compare SFP feedback responses during orthograde and retrograde perfusion with an artificial tubular fluid solution (ATF) (Cl- = 135 meq/liter) and a Na+ isethionate solution (Cl- = 6 meq/liter). With ATF, increases in perfusion rate from 10 to 35 nl/min led to decreases in SFP of 11 +/- 1.4 mmHg, increases in distal tubular fluid Cl- of 46 +/- 4.9 meq/liter, and osmolality of 58 +/- 10 mosmol/kg. There were significant inverse relationships between SFP and changes in Cl- and osmolality. With Na+ isethionate, SFP decreased by 8.4 +/- 1.0 mmHg, osmolality increased by 43 +/- 8 mosmol/kg, and Cl- did not change. There was a significant relationship between SFP and osmolality, but not with Cl-. During retrograde perfusion at 15 nl/min, SFP decreased by 12 +/- 1.2 mmHg with ATF and by 12 +/- 1.2 mmHg with Na+ isethionate. These results demonstrate that feedback-mediated decreases in SFP can occur in the absence of concomitant increases in distal Cl- and suggest that the receptor system does not have a unique and specific requirement for chloride.

Role of feedback mechanism in renal autoregulation and sensing step in feedback pathway

The unique morphology of the juxtaglomerular complex has resulted in many investigations evaluating its potential physiological roles. One hypothesis that has stimulated considerable interest is that feedback signals originating from distal tubule cells, presumably at the macula densa segment, participate in the phenomenon of renal autoregulation. Data obtained from dog experiments indicate tha autoregulation of single nephron glomerular filtration rate (SNGFR) is most consistently observed when fluid delivery to the early distal tubule is not interrupted. In contrast, techniques that interfere with normal orthograde fluid delivery to the distal nephron have usually resulted in increases in SNGFR and an inability to exhibit appropriate autoregulatory responses to decreases in arterial pressure. There is considerable uncertainty concerning the nature of the intraluminal component of the early distal tubule fluid responsible for initiating feedback responses. The studies reported in this paper have indicated that feedback mediated decreases in stop-flow pressure and SNGFR in response to increases in distal microperfusion rate can occur with a variety of artificial perfusion solutions, including solutions that contain low concentrations of chloride, sodium, or total electrolytes. Microperfusion studies in the rat have demonstrated that feedback mediated decreases in stop-flow pressure can occur in the absence of associated increases in distal tubule fluid chloride concentration. These results are consistent with the concept that some function of distal tubule fluid osmolality or distal tubule solute delivery may participate in the initiation of feedback signals.

Impaired renal blood flow and cortical pressure autoregulation in contralateral kidneys of Goldblatt hypertensive rats

Experiments were conducted on two-kidney, one clip renal vascular hypertensive rats to assess the ability of the kidney contralateral to renal vascular stenosis to autoregulate renal blood flow (RBF), glomerular filtration rate (GFR), and hydrostatic pressures in cortical structures during conditions of acutely reduced renal arterial blood pressure (BP). When observed at their respective, spontaneous BPs, RBF and GFR were not different in the contralateral kidneys of the hypertensive rats (n = 11) compared to normal animals (n = 7). However, the contralateral kidneys exhibited a significantly higher renal vascular resistance (RVR), 28.9 +/- 2.8 mm Hg . min/ml than the control animals, 23.1 +/- 1.5 mm Hg . min/ml. At spontaneous BP (169 +/- 5 mm Hg), urine flow, absolute and fractional sodium excretion, and absolute and fractional potassium excretion were all significantly greater in the contralateral kidneys of hypertensive rats than in kidneys of normal rats. Hydrostatic pressures in cortical structures were similar in the two groups. When BP was reduced acutely, the kidney contralateral to the renal artery stenosis achieved only small decreases in RVR that failed to allow RBF, GFR, or cortical pressures to be maintained. In contrast, normal rats efficiently autoregulated RBF and GFR. In addition, hydrostatic pressures in proximal tubules, distal tubules, and first order peritubular capillaries were maintained during reductions in BP to as low as 100 mm Hg. Urine flow and electrolyte excretion decreased to a greater extent in the hypertensive kidneys, such that at comparable BP these indices of excretory function were not different in the two groups. These observations indicate that the capacity of the contralateral kidney to maintain hemodynamic and glomerular function at reduced BP is compromised severely and suggest the possibility that the impaired autoregulatory capability may contribute to the maintenance of hypertension observed in this model.