Disappearing cardiac masses--the importance of intraoperative transesophageal echocardiography

A 25-year-old man presented with sudden onset right-sided hemiplegia. Transthoracic and transesophageal echocardiograms demonstrated a mobile mass attached to the anterior mitral leaflet. The mass was, however, not found at operation.

Decrease in ambient [Cl-] stimulates nitric oxide release from cultured rat mesangial cells

It has been hypothesized that fluctuations of the ionic composition in the interstitium of juxtaglomerular apparatus (JGA) modulate the function of extraglomerular mesangial cells (MC), thereby participating in tubuloglomerular feedback (TGF) signal transmission. We examined the effects of isosmotic reductions in ambient sodium concentration ([Na+]) and [Cl-] on cytosolic calcium concentration ([Ca2+]i) in cultured rat MC. Rapid reduction of [Na+] or [Cl-] in the bath induced a concentration-dependent rise in [Ca2+]i. MC are much more sensitive to decreases in ambient [Cl-] than to [Na+]; a decrease in [Cl-] as small as 14 mM was sufficient to elicit a detectable [Ca2]i response. These observations suggest that MC can be readily stimulated by modest perturbations of extracellular [Cl-]. Next, we examined whether activation of MC by lowered ambient [Cl-] influences cellular nitric oxide (NO) production. Using an amperometric NO sensor, we found that a 13 mM decrease in ambient [Cl-] caused a rapid, Ca2+/calmodulin-dependent rise in NO release from MC. This response was not inhibitable by dexamethasone, indicating the involvement of the constitutive rather than the inducible type of NO synthase in MC. In addition, the NO release was blunted by indomethacin pretreatment, suggesting that a metabolite(s) of cyclooxygenase regulates the activation of NO synthase in MC. Our findings that small perturbations in external [Cl-] stimulate MC to release NO, a highly diffusible and rapidly acting vasodilator, provide a possible mechanism to explain the transmission of the signal for the TGF response within the JGA.

Numerical simulation of propagating concentration profiles in renal tubules

Method-dependent mechanisms that may affect dynamic numerical solutions of a hyperbolic partial differential equation that models concentration profiles in renal tubules are described. Some numerical methods that have been applied to the equation are summarized, and ways by which the methods may misrepresent true solutions are analysed. Comparison of these methods demonstrates the need for thoughtful application of computational mathematics when simulating complicated time-dependent phenomena.

Ascending myogenic autoregulation: interactions between tubuloglomerular feedback and myogenic mechanisms

A mathematical model of the renal vascular and tubular systems was used to examine the possibility that synergistic interactions might occur between the tubuloglomerular feedback (TGF) and myogenic autoregulatory mechanisms in the kidney. To simulate the myogenic mechanism, the renal vasculature was modelled with a resistance network where the total preglomerular resistance varies with intravascular pressure. In addition, a steady-state model of glomerular filtration, proximal and Henle's loop reabsorption, and TGF-modulation of afferent arteriolar resistance was derived. The results show that, if TGF acts on the distal portion of the preglomerular vasculature, then any TGF-induced vasoconstriction should raise upstream intravascular pressure and, thereby, trigger a myogenic (AMYO) response. The model further predicts that the magnitude of the AMYO response can be similar in magnitude to the TGF-induced increment in afferent resistance. Hence, the effects of TGF excitation on whole kidney hemodynamics may be much greater than predicted from measurements in single nephrons. Moreover, a significant fraction of the intrinsic myogenic autoregulatory response to increased renal perfusion pressure may result from a synergistic interaction between the TGF and myogenic mechanisms.

Direct visualization of renin-cell distribution in preglomerular vascular trees dissected from rat kidney

Three methods to visualize directly the distribution of granulated renin-positive cells in vascular trees microdissected from rat kidney were developed. Kidneys were removed from anesthesized rats, hemisectioned, macerated in HCl, and soaked in distilled water for 24-48 h. Cortical preglomerular vascular trees consisting of arcuate and cortical radial arteries and afferent arterioles were microdissected with the aid of a stereomicroscope. Granulated cells can be visualized in three ways. First, under transmitted or incident light observation, granulated cells are readily distinguished from the surrounding smooth muscle cells, because of marked differences in the refractive properties of these two cell types. Second, quinacrine, a fluorescent, intravital stain selective for dense-core granules, can be administered (2 mg/kg iv) to the rat 1 h before nephrectomy. When illuminated with 440-nm light, granulated cells fluorescence strongly at 510 nm. Third, specific immunostaining for renin can be obtained with a polyclonal anti-rat renin antibody and avidin-biotin immunoperoxidase staining in vascular trees subjected to cell permeabilization with Triton. These new techniques permit the direct visualization of the distribution of granulated renin-positive cells in preglomerular vessels under conditions in which the vascular architecture is largely preserved.

Effect of CsA on the expression of renin and angiotensin type 1 receptor genes in the rat kidney

To determine whether Cyclosporine A (CsA) alters the intrarenal expression of the renin and type 1 angiotensin II receptor genes, male adult Sprague-Dawley rats were given 25 mg/kg/day CsA s.c. for three weeks (CsA, N = 20) and were compared to pair-fed vehicle treated rats (Con, N = 20). The intrarenal distribution of renin and its mRNA was assessed by immunocytochemistry and in situ hybridization. In addition, kidney renin and type 1 angiotensin II (AT1) receptor mRNA levels were determined by Northern blot analysis. The percentage of juxtaglomerular apparatuses containing renin was higher in the CsA (84 +/- 5.5%) than in the Con (61 +/- 6.7%) group, (P < 0.05). The length of renin immunostaining along afferent arterioles was higher in the CsA (74 +/- 4.5 microns) than in the Con (37 +/- 5.1 microns) group, (P < 0.05). In contrast, neither renin mRNA levels nor its intrarenal distribution were altered by chronic CsA administration. Kidney AT1 receptor mRNA levels were lower in the CsA group than in the Con group. We conclude that chronic CsA: (1) induces recruitment of renin containing cells along the afferent arteriole, (2) causes no changes in intrarenal renin mRNA levels or distribution, suggesting that post-transcriptional events may be responsible for the persistence and/or uptake of renin by the preglomerular vasculature, (3) promotes a downregulation of AT1 receptor gene in the kidney, suggesting that local angiotensin II may control AT1 receptor gene expression by a negative feedback.

Autoregulation of intravascular pressure in preglomerular juxtamedullary vessels

To quantify the functional significance of autoregulatory responses in preglomerular juxtamedullary (JM) vessels, intravascular pressures (P chi) were measured with a servonull instrument in blood-perfused arcuate (ArcA) and interlobular arteries (ILA), afferent arterioles (AA), and glomerular capillaries (GC) in vitro. P chi was determined at perfusion pressures (Pp) between approximately 60 and 150 mmHg, and the slope of the relationship between Px and Pp was estimated by linear regression. From the regression, Px was 98 +/- 1, 96 +/- 2, 64 +/- 9, and 48 +/- 2 mmHg for ArcA, ILA, AA, and GC, respectively, at the reference perfusion pressure of 100 mmHg. The results show good autoregulation of GC pressure (Pg), with a response slope of 0.10 +/- 0.07 mmHg per mmHg change in Pp, corresponding to an autoregulation index of 0.20 +/- 0.15. The slopes of the Px vs. Pp relationships in ArcA, ILA, and AA were 0.96 +/- 0.02, 0.79 +/- 0.08, and 0.32 +/- 0.11, respectively. To determine whether these observed relationships in preglomerular vessels reflect significant upstream resistance changes, we derived a new autoregulation index for Px measured at an arbitrary preglomerular location. This analysis takes into account the extent to which outflow pressure, Pg, is autoregulated by the ensemble action of the entire preglomerular vasculature. The analysis indicates that 20% autoregulatory compensation occurs upstream from the late ILA, 65% upstream from the late AA, and 80% for the entire preglomerular vascular tree. Hence, in the JM preglomerular circulation, the AA is the major site of autoregulatory resistances adjustment, with a smaller but significant contribution by the ILA.

Effect of cyclosporine on endothelial albumin leakage in rats

Short-term treatment of rats with cyclosporine (cyclosporine A [CsA]; Sandimmune) results in a marked reduction in intravascular plasma volume, a factor that might contribute to the renal dysfunction associated with this potent immunosuppressant. To examine the role of plasma extravasation in CsA-induced hypovolemia, intravascular plasma volumes (PV), blood volumes, [125I]albumin disappearance, and changes in hematocrit (Hct) were measured in Inactin-anesthetized rats subjected to minimal surgery. The rats were treated for 3 wk with either 25 mg/kg/day of CsA s.c. or vehicle. Plasma creatinine and urea were significantly elevated, and magnesium was reduced in the CsA group (N = 6) as compared with controls (CON) (N = 6). CsA treatment had no effect on urinary protein and albumin excretion. Blood volume was significantly lower in CsA than in CON (8.4 +/- 0.5 versus 10.6 +/- 0.3 mL/100 g body wt) as was PV (4.3 +/- 0.2 versus 5.5 +/- 0.2 mL/100 g body wt). Two hours after injection, plasma [125I]albumin concentration had fallen by 41 +/- 4% in CsA versus 23 +/- 5% in CON. Because Hct, and, hence PV, was unchanged in both groups during these 2 h, these data indicate enhanced endothelial albumin leakage in the CsA group. In two additional groups of six rats each, acute volume expansion with fresh whole blood (2 mL/100 g body wt) resulted in extravasation of plasma. Hct rose by 8.0 +/- 0.2% in CsA versus 3.8 +/- 0.2% in CON after 150 min, corresponding to 27 +/- 3 and 15 +/- 2% decreases in total PV, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Bifurcation analysis of TGF-mediated oscillations in SNGFR

Recent micropuncture studies in rats have demonstrated the existence of oscillatory states in nephron filtration mediated by tubuloglomerular feedback (TGF). We develop a minimal mathematical model of the TGF system, consisting of a first-order hyperbolic partial differential equation describing thick ascending limb (TAL) NaCl reabsorption and an empirical feedback relation. An analytic bifurcation analysis of this model provides fundamental insight into how oscillatory states depend on the physiological parameters of the model. In the special case of no solute backleak in the TAL, the emergence of oscillations explicitly depends on two nondimensional parameters. The first corresponds to the delay time of the TGF response across the juxtaglomerular apparatus, and the second corresponds to the product of the slope of the TGF response curve at the steady-state operating point and the space derivative of the steady-state NaCl concentration profile in the TAL at the macula densa. Numerical calculations for the case without TAL backleak are consistent with this result. Numerical simulation of the more general case with TAL backleak shows that the bifurcation analysis still provides useful predictions concerning nephron dynamics. With typical parameter values, the analysis predicts that the TGF system will be in oscillatory state. However, the system is near enough to the boundary of the nonoscillatory region so that small changes in parameter values could result in nonoscillatory behavior.

Syncytial organization of cultured rat mesangial cells

To investigate communication competence of cultured rat mesangial cells, Lucifer yellow transfer was studied using microinjection and scrape-loading techniques. Both methods yielded results indicating considerable gap junctional communication between cultured mesangial cells. Gap junctional communication between mesangial cells was upregulated by adenosine 3',5'-cyclic monophosphate (cAMP). Conversely, cell-to-cell communication was attenuated by exposure to the tumor promoter phorbol myristate acetate, the Ca ionophore ionomycin, reduced oxygen intermediates, and cell acidification. Expression of voltage gated calcium channels by mesangial cells was studied microspectrofluorimetrically using fura-2 fluorescence. KCl-induced depolarization, BAY-K 8644, and readdition of calcium to Ca-free depolarizing medium all produced a nifedipine-inhibitable increase in cytosolic calcium concentration. The existence of voltage-gated calcium channels in communication-competent cells suggests the possibility of propagation of depolarizing signals across the syncytium. This was studied by microapplication of KCl to the microenvironment of a single cell and monitoring fura-2 fluorescence in remote cells. This maneuver resulted in propagating calcium waves in communication-competent monolayers; calcium waves could not be evoked in monolayers exposed to an alkanol-type gap junction uncoupler, octanol. It is concluded that cultured rat mesangial cells form a syncytium capable of propagating calcium transients from a single depolarized cell to its coupled neighbors.

Arteriolar renin and vascular effects of angiotensin II in juxtamedullary nephrons

The SJMNs made accessible in the in vitro juxtamedullary nephron preparation possess an arteriolar renin and Ang II distribution similar to other nephron populations; renin-rich cells prevail in the JAA. Physiological studies have documented that exogenous Ang II constricts both pre- and postglomerular vessels of SJMNs. However, Ang II predominantly increases afferent resistance along the JAA. A functional interaction between Ang II and prostaglandins is likely to occur in preglomerular vessels and deserves further assessment. An additive, functional interaction was demonstrated between Ang II and autoregulation in afferent arterioles. Calcium channel blockers interfere with Ang II-induced constriction only in preglomerular vessels of SJMNs.

Tubuloglomerular feedback dependence of autoregulation in rat juxtamedullary afferent arterioles

Experiments were performed in blood-perfused juxtamedullary nephrons in vitro to evaluate the tubuloglomerular feedback (TGF) dependence of autoregulatory vasoconstriction in mid-to-late (mAA) and juxtaglomerular (jAA) afferent arterioles. Videometric measurements were made of perfusion pressure (PP) dependent changes in lumen diameter of superficial vessels before and after acute inhibition of the TGF mechanism by direct microinfusion of 0.1 mM furosemide solution into the macula densa (MD) segment. When PP was raised from 60 to 123 +/- 7 mm Hg in seven vessels, jAA diameter decreased by 29 +/- 3% (SEM, N = 7). During furosemide infusion with the same change in PP, jAA diameter decreased only 7 +/- 2%. After calcium channel blockade with 1 micromolar nimodipine, jAA lumen diameter increased by 21 +/- 7%. A similar pattern of responses was observed in eight jAA where TGF was inhibited with an oil block at the MD. mAA autoregulatory responses were also blunted by TGF inhibition. Raising PP from 60 to 120 mm Hg resulted in 15 +/- 2% and 7 +/- 2% decreases in mAA luminal diameter before and after TGF inhibition. These results demonstrate that the autoregulatory responses in mid- and juxtaglomerular afferent arteriolar segments are mediated by both TGF and a TGF-independent myogenic mechanism.

Indirect assessment of renal dysfunction in patients taking cyclosporin A for autoimmune diseases

The reliability of increases in serum creatinine and serum urea above initial values in detecting decreases in glomerular filtration rate below initial values was assessed in patients taking cyclosporin A (CyA) for autoimmune diseases. Both serum creatinine and serum urea provided reasonable estimates of the decline in glomerular filtration rate when measured simultaneously and when compared to patients' own baseline values. Combination of both serum creatinine and serum urea improved the assessment of renal dysfunction, partly by reducing the influence of any analytical errors. Frequent measurements, precise baseline evaluation and standardization of procedures can be expected to increase further the precision of assessing renal dysfunction from serum measurements. Hence, careful evaluation of changes in serum creatinine, with the option to include changes in serum urea, can provide a simple and reliable way of monitoring changes in renal function in patients taking CyA for autoimmune diseases.

Salt intake and renal hemodynamics in immature and mature Dahl salt-sensitive (DS/JR) and salt-resistant (DR/JR) rats

To determine if abnormalities in the maturation of renal function in Dahl salt-sensitive rats are associated with the development of hypertension, studies were performed in anesthetized 3 week old salt-sensitive (DS/JR) and salt-resistant (DR/JR) rats whose mothers were maintained on 0.15% (low-salt) during gestation and either 0.15% or 2.0% (high-salt) NaC1 diets after parturition. Mature DS/JR and DR/JR rats were maintained on either 0.15% or 2.0% NaC1 diets after weaning and studied at 8 to 9 weeks of age. High-salt diet raised blood pressure (BP) and reduced glomerular filtration rate (GFR) and renal blood flow (RBF) in mature DS/JR rats, but had no effect on BP, GFR and RBF in mature DR/JR rats. In immature DS/JR and DR/JR animals, high-salt intake resulted in poor growth with reductions in GFR and RBF in the DS/JR group. The response to acute volume expansion, (5% body weight physiologic saline infusion) differed among the groups. Mature rats all vasodilated while immature high-salt DS/JR did not, and immature low-salt DS/JR vasoconstricted. These studies demonstrated that both mature and immature DS/JR rats evidence abnormal responses to acute and chronic salt loading. Early exposure to high-salt intake affects the maturation of renal function in the DS/JR group. An enhanced vascular sensitivity to sodium is present at critical periods of postnatal development in DS/JR rats.

Autoregulation and tubuloglomerular feedback in juxtamedullary glomerular arterioles

Videometric measurements of changes in vessel lumen diameters were made to investigate autoregulatory and tubuloglomerular feedback (TGF) responses of early efferent arterioles (EA), mid-to-late afferent arterioles (MAA), and terminal, juxtaglomerular afferent arterioles (JAA) in rat juxtamedullary nephrons in vitro. High-contrast shadow-cast images of blood-perfused arterioles at the glomerular vascular pole were obtained with incident illumination and long-working-distance objectives fitted to a compound microscope. In response to an increase in blood perfusion pressure from 60 to 140 mmHg, strong autoregulatory vasoconstriction was observed in the MAA and JAA, with respective reductions in mean luminal diameter of 23 +/- 4 and 40 +/- 4% (mean +/- SE); EA diameter was unchanged. In response to TGF excitation by direct microinjection of Ringer solution into the cortical thick ascending limb segment near the macula densa, JAA luminal diameter decreased by 34 +/- 5%. The TGF responses were completely inhibited by the addition of 0.1 mM furosemide to the tubular injectate. Calcium channel blockade achieved by adding 1 microM nimodipine to the superfusate had no effect on early EA diameter but produced a blood pressure-dependent JAA and MAA vasodilation and complete inhibition of autoregulatory responses. These results provide direct evidence that the distal afferent arteriole in juxtamedullary nephrons is a major effector site for both renal autoregulation and tubuloglomerular feedback.

Transport-coupling hypothesis of tubuloglomerular feedback signal transmission

A mathematical model was used to explore the transport-coupling hypothesis of tubuloglomerular feedback (TGF) signal transmission from the macula densa (MD) to the extraglomerular mesangium (EGM) within the juxtaglomerular apparatus (JGA). The transport-coupling hypothesis supposes that changes in MD transport alter the ionic composition of the EGM interstitium, thereby stimulating the Goormaghtigh cells. This hypothesis is based on the avascularity of the EGM and the presence of a narrow cleft (JGA cleft) between the MD and Goormaghtigh cells. The model describes NaCl and water transport by MD cells and mass conservation in the JGA cleft. It calculates cleft water flow and NaCl concentration [( NaCl]). If the cleft is narrow, the model predicts that cleft [NaCl] will vary directly with luminal [NaCl] and net MD NaCl transport. With strong active NaCl transport, the MD cell plaque may act as an ionic amplifier, in that small changes in luminal [NaCl] might elicit much larger concentration changes within the cleft. Even without active NaCl transport, cleft [NaCl] could remain coupled to luminal [NaCl] if the MD cells passively secrete NaCl. With high hydraulic conductivity, cleft [NaCl] also varies with luminal osmolarity. With a wide cleft, a low diffusional resistance of the EGM interstitium, or swollen MD lateral intercellular spaces, the transport coupling between cleft and luminal [NaCl] is markedly attenuated. The predictions of our model of large changes in JGA interstitial composition agree well with published measurements made in Amphiuma. However, the low Na+-K+-ATPase content and high water permeability of mammalian MD suggest that the transport-coupling effects in mammals may be significantly less pronounced than in Amphiuma.

Inhibition of myogenic autoregulation in cyclosporine nephrotoxicity in the rat

The mechanisms responsible for the impairment of renal blood flow (RBF) autoregulation in cyclosporine nephrotoxicity were investigated with clearance and micropuncture studies in anesthetized rats. Early chronic cyclosporine nephrotoxicity (CCN) was induced in male rats by daily intramuscular injection of 10 mg/kg/day cyclosporine-A in olive oil for 7 days; control (CON) rats received vehicle injections. Glomerular filtration rate and RBF were both reduced by 33% in CCN when compared to CON rats. RBF autoregulation was also significantly impaired in CCN, with an autoregulation index (AI) of 0.53 +/- 0.03 vs. 0.16 +/- 0.01 in CON rats. Micropuncture studies showed that the tubuloglomerular feedback (TGF) system is not impaired in CCN. Rather, in CCN there was a slight resetting such that the maximum TGF response was greater and the onset occurred at lower rates of perfusion than in CON. In contrast, further micropuncture studies demonstrated that TGF-independent autoregulation of glomerular capillary pressure was significantly impaired in CCN, with an AI of 0.86 +/- 0.09 vs. 0.57 +/- 0.06 in CON. These results indicate that the loss of autoregulatory ability in rats with CCN results from substantial impairment of the myogenic autoregulatory mechanism that is an intrinsic property of the preglomerular vasculature of the kidney.

Cyclosporine nephrotoxicity: blood volume, sodium conservation, and renal hemodynamics

Glomerular filtration rate (GFR) and renal blood flow (RBF) are depressed by chronic cyclosporine treatment. We examined the hypothesis that depletion of extracellular or intravascular fluid volume contributes to the renal vasoconstriction of early cyclosporine nephrotoxicity (CCN). Control and CCN rats were given 10 mg/kg cyclosporine A or vehicle intramuscularly daily for 7 days. The effects of extracellular volume expansion, both acute (AVE, 10% body wt saline) and chronic (CVE, 10% body wt/day saline ip, 10 days including cyclosporine A treatment period), on renal hemodynamics were measured. In CCN, AVE completely normalized GFR and RBF, whereas CVE partially prevented the development of CCN. Renal autoregulatory ability was depressed in CCN but was largely restored by AVE. Intravascular volumes were measured with Evans blue and 51Cr-labeled red cells. Plasma and red cell volumes were reduced by 24% in CCN, indicating circulatory hypovolemia. Acute repletion of the deficit in blood volume by acute administration of an isoncotic solution (1.8 ml/100 g body wt of 5% albumin in isotonic saline) restored GFR and RBF to levels similar to those in control rats. Extracellular fluid volume, estimated as inulin space, was similar in both CCN and control groups. A metabolic study (7 day) showed stool Na loss in CCN to be twice that in controls but both groups remained in sodium balance. We conclude that the renal vasoconstriction produced in the rat by short-term cyclosporine treatment is, at least in part, prerenal in origin and related to the development of circulatory hypovolemia.

In vivo measurement of tubular fluid ferrocyanide with carbon-fiber microelectrodes

Techniques to construct carbon-fiber microelectrodes and to measure ferrocyanide ion concentration in single nephrons are described. The measurement involves polarizing an inert carbon-fiber microelectrode 500 mV positive with respect to a Ag-AgCl reference, while measuring the faradic current produced by the oxidation of ferrocyanide. A carbon fiber (5-7 micron diam) is heat sealed into a glass micropipette that is then sharpened, silanized, and electrochemically pretreated to minimize electrode degradation by protein. Circuit diagrams for an inexpensive voltage clamp-current monitor and a data sampling device are presented. The electrodes show a linear response to changes in ferrocyanide concentration in large and very small (20 nl) volumes in vitro. The electrodes were used in an electrochemical microassay to determine tubular fluid-to-plasma ferrocyanide concentration ratios and nephron filtration rates with proximal micropuncture samples. The results show excellent agreement with paired determinations using [3H]inulin. In vivo proximal tubule perfusion experiments show a rapid linear response to changes in tubular fluid ferrocyanide concentration. These electrodes permit rapid quantitative measurements of ferrocyanide concentration and water transport in the proximal tubule and may be useful in other biological systems.

Cyclosporine nephrotoxicity: sodium excretion, autoregulation, and angiotensin II

Cyclosporine-induced nephrotoxicity (CIN) was studied in rats treated for 7 days with cyclosporine (10 mg x kg-1 x day-1 im) or vehicle (CON). CIN rats displayed characteristic reductions in glomerular filtration (GFR) and renal blood blood flow (RBF), and electron microscopy showed injury to proximal cells. Metabolic studies (7 day) showed significantly lower renal sodium excretion in conscious CIN rats compared with CON. In anesthetized rats at similar blood pressures, nephron GFR (SNGFR) was lower in CIN than CON, but fractional Na reabsorption was similar. In CIN, SNGFR, measured proximally to block flow to the sensing site of tubuloglomerular feedback (TGF) at the macula densa, was not significantly different than distal SNGFR. The rate of distal fluid delivery was significantly lower in CIN than in CON. Inhibition of the renin-angiotensin system (RAS) with captopril (CAP, 10 mg/kg iv), or saralasin (SAR, 0.3 mg x kg-1 x h-1 iv) caused marked arterial hypotension in CIN and a fall in renal vascular resistance (RVR). With arterial pressure controlled, CAP or SAR increased GFR and RBF, and reduced RVR in CIN, but did not reverse the renal deficits compared with similarly treated CON. RBF autoregulation in CIN was impaired between 90 and 140 mmHg but was partially restored by CAP. We conclude that both the filtered load and excretion rate of sodium in CIN are significantly reduced compared with controls, that SNGFR in CIN is not depressed by TGF in response to elevated distal fluid delivery, and that the RAS is not a primarily mediator of the renal vasoconstriction in CIN.

Tubuloglomerular feedback control of distal fluid delivery: effect of extracellular volume

A closed-feedback loop micropuncture method was used to examine tubuloglomerular feedback (TGF) and the regulation of distal fluid delivery in hydropenic rats (CON), moderately hemorrhaged rats (HEM), and rats given desoxycorticosterone (DOC) and 0.6% saline to drink. Distal delivery and TGF response curves were measured with four samples per nephron: a spontaneous early distal collection, two distal collections during moderate (7.5 nl/min) and saturating (30 nl/min) perturbations in nephron fluid load, and a proximal collection to measure single-nephron glomerular filtration rate (SNGFR) during TGF inhibition. Arterial pressure, predistal volume reabsorption, SNGFR, and early distal flow were significantly higher in DOC than in HEM; the CON group exhibited intermediate values. Except for a greater maximum TGF response in HEM, the normalized TGF responses were similar in all three groups, as was the regulation of distal fluid delivery. However, the TGF onset threshold and the TGF operating point, defined by the spontaneous rates of early distal flow and SNGFR, were reset such that distal fluid delivery and SNGFR were higher in DOC than in HEM, as was renal sodium excretion. The results show that the level around which TGF stabilizes distal fluid delivery is reset when extracellular fluid volume is altered.