Renal vasodilation and uncoupling of blood flow and filtration rate autoregulation

What's Old Is New Again: Harnessing the Power of Original Experiments to Learn Renal Physiology

Although medical schools across the United States have updated their curricula to incorporate active learning techniques, there has been little discussion on the nature of the content presented to students. Here, we share detailed examples of our experience in using original experiments to lay the groundwork for foundational concepts in renal physiology and pathophysiology. We believe that this approach offers distinct advantages over standard case-based teaching by (1) starting with simple concepts, (2) analyzing memorable visuals, (3) increasing graphical literacy, (4) translating observations to "rules," (5) encouraging critical thinking, and (6) providing historical perspective to the study of medicine. Although we developed this content for medical students, we have found that many of these lessons are also appropriate as foundational concepts for residents and fellows and serve as an excellent springboard for increasingly complex discussions of clinical applications of physiology. The use of original experiments for teaching and learning in renal physiology harnesses skills in critical thinking and provides a solid foundation that will help learners with subsequent case-based learning in the preclerkship curriculum and in the clinical arena.

Prostaglandin E2 sequentially activates E-prostanoid receptor-3 and thromboxane prostanoid receptor to evoke contraction and increase in resistance of the mouse renal vasculature

Although recognized to have an in vivo vasodepressor effect blunted by the vasoconstrictor effect of E-prostanoid receptor-3 (EP3), prostaglandin E₂ (PGE₂ ) evokes contractions of many vascular beds that are sensitive to antagonizing the thromboxane prostanoid receptor (TP). This study aimed to determine the direct effect of PGE₂ on renal arteries and/or the whole renal vasculature and how each of these two receptors is involved in the responses. Experiments were performed on isolated vessels and perfused kidneys of wild-type mice and/or mice with deficiency in TP (TP^-/- ), EP3 (EP3^-/- ), or both TP and EP3 (TP^-/- /EP3^-/- ). Here we show that PGE₂ (0.001-30 μM) evoked not only contraction of main renal arteries, but also a decrease of flow in perfused kidneys. EP3^-/- diminished the response to 0.001-0.3 μM PGE₂ , while TP^-/- reduced that to the prostanoid of higher concentrations. In TP^-/- /EP3^-/- vessels and perfused kidneys, PGE₂ did not evoke contraction but instead resulted in vasodilator responses. These results demonstrate that PGE₂ functions as an overall direct vasoconstrictor of the mouse renal vasculature with an effect reflecting the vasoconstrictor activities outweighing that of dilation. Also, our results suggest that EP3 dominates the vasoconstrictor effect of PGE₂ of low concentrations (≤0.001-0.3 μM), but its effect is further added by that of TP, which has a higher efficacy, although activated by higher concentrations (from 0.01 μM) of the same prostanoid PGE₂ .

Flow resistance along the rat renal tubule

The Reynolds number in the renal tubule is extremely low, consistent with laminar flow. Consequently, luminal flow can be described by the Hagen-Poiseuille laminar flow equation. This equation calculates the volumetric flow rate from the axial pressure gradient and flow resistance, which is dependent on the length and diameter of each renal tubule segment. Our goal was to calculate the pressure drop along each segment of the renal tubule and to determine the points of highest resistance. When the Hagen-Poiseuille equation was used for rat superficial nephrons based on known tubule flow rates, lengths, and diameters, it was found that the maximum pressure drop occurred in two segments: the thin descending limbs of Henle and the inner medullary collecting ducts. The high resistance in the thin descending limbs is due to their small diameters. The steep pressure drop observed in the inner medullary collecting ducts is due to the convergent structure of the tubules, which channels flow into fewer and fewer tubules toward the papillary tip. For short-looped nephrons, the calculated glomerular capsular pressure matched measured values, even with the high collecting duct flow rates seen in water diuresis, provided that tubule compliance was taken into account. In long-looped nephrons, the greater length of thin limb segments is likely compensated for by a larger luminal diameter. Simulation of the effect of proximal diuretics, namely acetazolamide or type 2 sodium-glucose transporter inhibitors, predicts a substantial back pressure in Bowman's capsule, which may contribute to observed decreases in glomerular filtration rate.

Hemodynamic effects of furosemide on renal perfusion as evaluated by ASL-MRI

RATIONALE AND OBJECTIVES

The aim of this study was to investigate the short-term effects of furosemide on renal perfusion by using arterial spin labeling (ASL) magnetic resonance imaging.

MATERIALS AND METHODS

Eleven healthy human subjects were enrolled in the study. The measurement of renal blood flow (RBF) was performed by applying an ASL technique with flow-sensitive alternating inversion recovery spin preparation and a single-shot fast spin-echo imaging strategy on a 3.0-T magnetic resonance scanner. For all subjects, the ASL magnetic resonance images were obtained before agent injection as a baseline scan. Then 20 mg of furosemide was injected intravenously. Postfurosemide ASL images were acquired following administration to evaluate the renal hemodynamic response.

RESULTS

Postinjection scans showed that cortical RBF decreased from 366.59 ± 41.19 mL/100 g/min at baseline to 314.33 ± 48.83 mL/100 g/min at 10 minutes after the administration of furosemide (paired t test, P = .04 vs baseline), and medullary RBF decreased from 118.59 ± 24.69 mL/100 g/min at baseline to 97.38 ± 18.40 mL/100 g/min at 10 minutes after the administration of furosemide (paired t test, P = .01 vs baseline). There was a negative correlation between the furosemide-induced diuretic effect and the reduction of RBF (Spearman's r = -0.61).

CONCLUSIONS

The dominant hemodynamic effect of furosemide on the kidney is associated with a decrease in both cortical and medullary blood perfusion. Furthermore, the quantitative ASL technique may provide an alternative way to noninvasively monitor the change in renal function due to furosemide administration.

Mechanisms of pressure natriuresis

A central component of the feedback system for long-term control of arterial pressure is the pressure-natriuresis mechanism, whereby increases in renal perfusion pressure lead to decreases in sodium reabsorption and increases in sodium excretion. The specific intrarenal mechanism for the decrease in tubular reabsorption in response to increases in renal perfusion pressure appears to be related to increases in hemodynamic factors such as medullary blood flow and renal interstitial hydrostatic pressure (RIHP), and renal autocoids such as nitric oxide, prostaglandins, kinins, and angiotensin II. Increases in renal perfusion pressure are associated with significant increases in RIHP, nitric oxide, prostaglandin E2, and kinins, and decreases in angiotensin II. The mechanism whereby RIHP increases in the absence of discernible changes in whole kidney renal blood flow and peritubular capillary hydrostatic and/or oncotic pressures may be related to increases in renal medullary flow as a result of nitric oxide-induced reductions in renal medullary vascular resistance. Several lines of investigation support an important quantitative role for RIHP in mediating pressure natriuresis. Preventing RIHP from increasing in response to increases in renal perfusion pressure markedly attenuates pressure natriuresis. Furthermore, direct increases in RIHP, comparable to increases measured in response to increases in renal perfusion pressure, have been shown to significantly decrease tubular reabsorption of sodium in the proximal tubule and increase sodium excretion. The exact mechanism whereby RIHP influences tubular reabsorption is unknown, but may be related to alterations in tight junctional permeability to sodium in proximal tubules, redistribution of apical sodium transporters, and/or release of renal autacoids such as prostaglandin E2.

Diurnal variations of glomerular filtration rate and albuminuria in diabetic nephropathy

BACKGROUND

The aim of our study was to evaluate the diurnal variation in glomerular filtration rate (GFR), and the potential mechanisms responsible for such variations in GFR and albuminuria in diabetic nephropathy.

METHODS

In three 24-hour urine samples, divided into a night- and daytime portion, diurnal variation in albuminuria (ELISA) was assessed. Furthermore, during recumbency changes in albuminuria, GFR (51Cr-EDTA plasma clearance) and arterial blood pressure (TM2420) from nighttime (00:00 to 05:00 hours) to subsequent daytime (08:00 to 13:00 hours) were examined in 20 type 1 diabetic patients with diabetic nephropathy.

RESULTS

The 24-hour urine collections showed an average rise in albuminuria from night- to daytime of 51% (95% CI; 16 to 95; P < 0.01). During recumbency a non-significant rise was recorded from night- to daytime in albuminuria (22%, -8 to 61, P=0.15), simultaneously with an increase in GFR of 9.0% (3.4 to 14.5, P < 0.005) and mean arterial blood pressure (MABP) of 8.0% (4.3 to 11.7, P < 0.0001). No diurnal variation in fractional clearance of albumin was found. Significant associations between MABP and albuminuria were demonstrated during night- (R2=0.50; P < 0.001) and daytime (R2=0.48; P < 0.005). A linear regression analysis between diurnal variations in MABP and GFR showed that an increase in MABP (of 10%) from night- to daytime was associated with a significant increase in GFR (of 8.0%, 0.2 to 4.1, P < 0.02).

CONCLUSIONS

Our study revealed diurnal variations in GFR, albuminuria and MABP in diabetic nephropathy, with lowest values during sleep at night. The observed diurnal variation in albuminuria seems to be explained partly by mechanisms related to orthostasis, and partly by the diurnal variation in GFR and serum albumin concentration. The diurnal variation of blood pressure seems to play a role for the diurnal changes in GFR and albuminuria.

Contribution of endothelin to renal vascular tone and autoregulation in the conscious dog

Exogenous endothelin-1 (ET-1) is a strong vasoconstrictor in the canine kidney and causes a decrease in renal blood flow (RBF) by stimulating the ETA receptor subtype. The aim of the present study was to investigate the role of endogenously generated ET-1 in renal hemodynamics under physiological conditions. In six conscious foxhounds, the time course of the effects of the selective ETA receptor antagonist LU-135252 (10 mg/kg iv) on mean arterial blood pressure (MAP), heart rate (HR), RBF, and glomerular filtration rate (GFR), as well as its effects on renal autoregulation, were examined. LU-135252 increased RBF by 20% (from 270 +/- 21 to 323 +/- 41 ml/min, P < 0.05) and HR from 76 +/- 5 to 97 +/- 8 beats/min (P < 0. 05), but did not alter MAP, GFR, or autoregulation of RBF and GFR. Since a number of interactions between ET-1 and the renin-angiotensin system have been reported previously, experiments were repeated during angiotensin converting enzyme (ACE) inhibition by trandolaprilat (2 mg/kg iv). When ETA receptor blockade was combined with ACE inhibition, which by itself had no effects on renal hemodynamics, marked changes were observed: MAP decreased from 91 +/- 4 to 80 +/- 5 mmHg (P < 0.05), HR increased from 85 +/- 5 to 102 +/- 11 beats/min (P < 0.05), and RBF increased from 278 +/- 23 to 412 +/- 45 ml/min (P < 0.05). Despite a pronounced decrease in renal vascular resistance over the entire pressure range investigated (40-100 mmHg), the capacity of the kidneys to autoregulate RBF was not impaired. The GFR remained completely unaffected at all pressure levels. These results demonstrate that endogenously generated ET-1 contributes significantly to renal vascular tone but does not interfere with the mechanisms of renal autoregulation. If ETA receptors are blocked, then the vasoconstrictor effects of ET-1 in the kidney are compensated for to a large extent by an augmented influence of ANG II. Thus ET-1 and ANG II appear to constitute a major interrelated vasoconstrictor system in the control of RBF.

Heterogeneous activation mechanisms in the renal microvasculature

Vascular smooth muscle cells in different renal microvascular segments utilize different activation mechanisms to respond to mechanical and vasoactive stimuli. L-type Ca2+ channel blockers vasodilate primarily the preglomerular vascular resistance component responsible for autoregulation. Local interstitial infiltration of Ca2+ channel blockers increases glomerular pressure and markedly reduces vascular responsiveness of the tubuloglomerular feedback mechanism. Ca2+ channel blockers selectively attenuate the afferent vasoconstrictor responses to increases in perfusion pressure. Although both afferent and efferent arterioles constrict in response to angiotensin II (Ang II), afferent but not efferent constriction requires Ca2+ influx through L-type Ca2+ channels. Sensitivity of the preglomerular arterioles to Ang II is also heterogeneous with the greatest sensitivity in glomerulus-near, terminal segments. Adenosine triphosphate (ATP) is a vasoconstrictor agonist that selectively activates Ca2+ entry pathways in afferent arterioles but has no effect on efferent arterioles. In isolated preglomerular smooth muscle cells, increasing extracellular [KCl] increases intracellular Ca2+ by stimulating voltage-dependent Ca2+ influx. Ang II, norepinephrine, and ATP also elicit similar increases in intracellular Ca2+. Mechanical and agonist-induced voltage-dependent Ca2+ influx is thus a primary pathway in the control of cytosolic Ca2+ in afferent arterioles. Efferent arterioles, however, rely primarily on intracellular Ca2+ mobilization and other Ca2+ influx pathways.

Nephroprotective effect of antihypertensive drugs in essential hypertension

BACKGROUND

Effective antihypertensive treatment has prevented target-organ involvement in hypertension, markedly reducing morbidity and mortality from strokes, coronary heart disease, cardiac failure, and hypertensive emergencies. However, the incidence of hypertension-related end-stage renal disease continues to increase, suggesting that therapeutic reduction in arterial pressure by itself is not sufficient to prevent the development of hypertensive renal failure.

OBJECTIVE

To examine experimental and clinical data concerning the protective effect of reduction of arterial pressure on the progression of hypertension-related renal disease, and the evidence indicating that some antihypertensive agents may afford more nephroprotection, over and above that attributable to reduction of arterial pressure.

RESULTS

Results of numerous studies clearly indicate that adequate control of arterial pressure, irrespective of the antihypertensive agent used, slowed the progression of renal disease. Results of some studies suggest that lowering arterial pressure below the level that is usually considered adequate has an additional beneficial effect by slowing the progression of renal injury.

CONCLUSION

Results of a number of studies evaluating nephroprotective effects of various drugs and regimens have indicated that certain agents, most notably angiotensin converting enzyme inhibitors and their combination with calcium antagonists, afford more protection than do others at similar levels of reduction of arterial pressure. Results of still other studies suggest that certain agents that exert greater nephroprotection are more efficient at controlling arterial pressure. Therefore, further data are needed before any final conclusion can be drawn. However, it is clear that, in order to establish nephroprotection in patients with essential hypertension, the problem should not be further complicated by additional comorbid diseases such as diabetes mellitus.

Increased glomerular filtration rate after withdrawal of long-term antihypertensive treatment in diabetic nephropathy

Initiation of antihypertensive treatment (AHT) in hypertensive insulin-dependent diabetic (IDDM) patients with diabetic nephropathy (DN) induces a faster initial (0 to 6 months) and a slower subsequent (6 months to end of observation) decline in GFR [delta GFR (ml/min/month) approximately 1.5 vs. 0.35]. Whether this initial phenomenon is reversible (hemodynamic) or irreversible (structural damage) after prolonged AHT is not known. To elucidate these mechanisms we investigated 42 hypertensive IDDM patients (16F/26M, age 40 +/- 7 years, mean +/- SD) with DN receiving AHT (angiotensin converting enzyme inhibition, N = 30) for 6 (2 to 15) years [median (range)]. GFR (ml/min/1.73 m2), arterial blood pressure (BP, mm Hg) and albuminuria (mg/24 hr) were measured the last day on AHT and one month after withdrawal of AHT. The measured variables were all significantly elevated after withdrawal of AHT: GFR [mean(SEM)] from 76(4) to 81(4) (P < 0.0001), BP [mean(SEM)] from 140/82 (2/1) to 151/89 (2/1) (P < 0.0005) and albuminuria [geometric mean (antilog SEM)] from 704 (1.2) to 1122 (1.2) (P < 0.0001). A correlation between relative rise in systolic blood pressure (delta Sys%) and relative change in GFR (delta GFR%) was found (r = 0.44, P < 0.005). Our results render some support of the hypothesis that the faster initial decline in GFR is due to a functional (hemodynamic) effect of AHT, which does not attenuate over time, while the subsequent slower decline reflects the beneficial effect on progression of diabetic nephropathy.

Renal autoregulation in midterm and late-pregnant rats

OBJECTIVES

The normal kidney autoregulates (that is, maintains) renal blood flow over a wide range of arterial blood pressures. During normal pregnancy blood pressure falls and the kidney vasodilates. The purpose of this study was to investigate whether renal autoregulatory ability was influenced during pregnancy in the normal rat.

STUDY DESIGN

Experiments were conducted on three groups of anesthetized rats, studied in the virgin state, at midterm, and late in pregnancy (gestation lasts 22 days in the rat). Renal blood flow was measured by electromagnetic flow probe at normal renal perfusion pressures, during elevation in renal perfusion pressure with bilateral carotid occlusion, and at subnormal renal perfusion pressures after application of an aortic snare.

RESULTS

Blood pressure was similar in midterm and virgin rats but lower in late pregnancy in the control state. Control renal blood flow was similar between virgin and late-pregnant rats but significantly elevated at midterm. During elevation in blood pressure with bilateral carotid occlusion, autoregulation was maintained in all three groups. During graded aortic occlusion, renal autoregulatory ability was lost. The autoregulatory threshold was between 90 and 100 mm Hg for midterm pregnant and virgin rats and was slightly lower at about 88 mm Hg for late-pregnant rats.

CONCLUSIONS

Midterm pregnant rats are able to autoregulate renal blood flow as well as virgins are in spite of underlying gestational renal vasodilation. The slight shift in renal autoregulatory threshold seen in late-pregnant rats may prevent the kidney from hypoperfusion during late-gestation hypotension.

Effects of bradykinin and papaverine on renal autoregulation and renin release in the anaesthetized dog

The present study on six anaesthetized dogs investigates the influences of two different vasodilators, bradykinin and papaverine, on the relationship between autoregulation of renal blood flow and glomerular filtration rate, sodium excretion and renin release. At control conditions renal blood flow and glomerular filtration rate was autoregulated to the same levels of renal arterial pressure, 55 +/- 3 and 58 +/- 3 mmHg, respectively. Renin release increased from 0.3 +/- 0.1 to 22 +/- 4 micrograms AI min-1, and sodium excretion decreased from 99 +/- 29 to 4.6 +/- 3.3 mumol min-1 when renal arterial pressure was reduced from 122 +/- 6 to 44 +/- 2 mmHg. Infusion of bradykinin (50 ng kg-1 min-1) increased renal blood flow by 50% at control blood pressure without changing glomerular filtration rate, and both renal blood flow and glomerular filtration rate autoregulated to the same pressure levels as during control. Sodium excretion increased threefold at control renal arterial pressure, but was unchanged at low renal arterial pressure. Bradykinin did not change renin release neither at control nor low renal arterial pressure. Papaverine infusion at a rate of 4 mg min-1 increased renal blood flow 50% without changing glomerular filtration rate. The lower pressure limits of renal blood flow and glomerular filtration rate autoregulation were increased to 94 +/- 6 and 93 +/- 6 mmHg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of renal actions of atrial natriuretic peptide. Lack of effect of atrial natriuretic peptide on pressure-induced vasoconstriction

We have previously demonstrated that atrial natriuretic peptide (ANP) completely reverses norepinephrine-induced afferent arteriolar (AA) vasoconstriction. In the present study we characterized the effects of ANP on pressure-induced vasoconstriction of AA. Chronic unilateral hydronephrosis was induced to facilitate direct visualization of the renal microcirculation. Hydronephrotic kidneys were perfused in vitro, and AA diameters were measured during stepwise alterations in renal arterial pressure. Increasing renal arterial pressure from 80 to 180 mm Hg decreased AA diameter by 22 +/- 2% (from 18.5 +/- 1.0 to 14.4 +/- 1.0 microns, p less than 0.005). In the presence of 100 nM ANP [human ANP-(4-28)], AA vasoconstricted by 23 +/- 4%, indicating that ANP failed to modify the pressure-induced AA vasoconstriction. Furthermore, both nitroprusside (10 microM) and 8-bromoguanosine 3':5'-cyclic monophosphate (30 microM) only partially inhibited pressure-induced AA vasoconstriction (31 +/- 5% and 47 +/- 7%, respectively), whereas these vasodilators completely abolished norepinephrine-induced AA vasoconstriction. In contrast, nifedipine completely inhibited pressure-induced AA vasoconstriction. In summary, pressure-induced AA vasoconstriction is insensitive to the action of ANP, is relatively refractory to cyclic GMP-mediated vasorelaxation, but is completely inhibited by calcium channel blockade. Furthermore, since ANP completely abolishes norepinephrine-induced vasoconstriction but fails to affect pressure-induced vasoconstriction, it is apparent that the type of underlying vasoconstrictor stimuli constitutes a major determinant of the renal microvascular response to ANP.

Renal blood flow and flow velocity in children and adolescents: duplex Doppler evaluation

The renal arteries of 62 children and adolescents aged 1-16 years without renal or renovascular disease were examined by computer Doppler duplex sonography (DDS) to measure absolute renal blood flow velocities. Maximum systolic velocity (Vmax) and time average velocity (TAV) were not age-dependent. In addition, absolute values of renal artery and renal blood flow were measured. Renal blood flow was 4.1 +/- 1.2 ml/min per gram kidney (two standard deviations), independent of age and comparable to commonly accepted physiological values. The coefficient of variation of blood flow calculations was 6%-15% depending on vessel diameter.

Pressure-induced vasoconstriction of renal microvessels in normotensive and hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

The capacity of small arteries to respond to increased intravascular pressure may be altered in hypertension. In the kidney, hypertension is associated with a compensatory shift in the autoregulatory response to pressure. To directly determine the effects of established hypertension on the renal microvascular response to changes of perfusion pressure, we evaluated pressure-induced vasoconstriction in hydronephrotic kidneys isolated from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Vessel diameters of interlobular arteries (ILAs) and afferent and efferent arterioles were determined by computer-assisted videomicroscopy during alterations in renal arterial pressure (RAP) from 80 to 180 mm Hg. Increased RAP induced a pressure-dependent vasoconstriction in preglomerular vessels (afferent arterioles and ILAs), but not in postglomerular vessels (efferent arterioles). The calcium antagonist nifedipine prevented pressure-induced afferent arteriolar vasoconstriction with a similar half-maximal inhibitory concentration (IC50) (WKY, 63 +/- 27 vs. SHR, 60 +/- 32 nM). The pressure-activation curves for ILAs in SHR and WKY were similar. In contrast, the pressure-activation curve for afferent arterioles in SHR kidneys exhibited a rightward shift, which was observed at every segment of the afferent arteriole (i.e., near ILA, at midportion, and near glomerulus). These findings demonstrate that the ILA and the afferent arteriole both possess the ability to constrict in response to increased pressure, whereas this property is lacking in the efferent arteriole. Hypertension was associated with a compensatory shift in the pressure response of the afferent arteriole, such that higher RAPs were required to elicit vasoconstriction in this vessel.

Acute prostaglandin reduction with indomethacin and chronic prostaglandin reduction with an essential fatty acid deficient diet both decrease plasma flow to the renal papilla in the rat

Renal distribution of prostaglandin synthetase is mainly medullary, whereas the major degrading enzyme, prostaglandin dehydrogenase is primarily cortical. This suggests that prostaglandins (PG) released from the renal medulla could affect the medullary blood vessels. In two different experiments we studied the role of PG in the regulation of renal papillary plasma flow in the rat. First study: PG synthesis were stimulated in 34 adult Sprague-Dawley rats by bleeding from the femoral artery 1% of the body weight over a period of 10 minutes. Following this, indomethacin (a PG inhibitor, 10 mg/kg i.v.) was given slowly and then renal papillary plasma flow was measured 25 minutes after the end of infusion. In 17 indomethacin rats the renal papillary plasma flow averaged 18.8 ml/100 g/minute, whereas it averaged 23.0 in 17 non-indomethacin rats given diluent, an 18% reduction (p less than .025). Second study: Male Sprague-Dawley rats were made prostaglandin deficient by fasting rats for one week, followed by 10% dextrose fluid for one week and subsequent institution of an essential fatty acid (EFA) deficient diet for two weeks. With urinary PG excretion in prostaglandin deficient rats 28 ng/24 hours compared to 149 ng in control rats, they could be considered as prostaglandin deficient. When renal papillary plasma flow was measured, the 16 prostaglandin deficient rats had a 16% lower papillary plasma flow than 16 control rats, 21.6 vs 25.6 (p less than .005). These results clearly demonstrate that PG inhibition in rats decreases plasma flow to the papilla, strongly suggesting that PG are vasodilators for the vessels supplying the renal papilla.

Renal hemodynamics and natriuresis induced by the dopamine-1 agonist, SKF 82526

The intrarenal infusion of dopamine (DA) during alpha- and beta-adrenergic blockade has been reported to increase renal blood flow (RBF) and sodium excretion by occupation of DA-1 receptors. In addition, DA may potentially influence renal function by occupation of DA-2 receptor subtypes. This study was designed to examine the hemodynamic and/or tubular mechanisms of the natriuretic effect of DA-1 in dogs anesthetized with pentobarbital. The intrarenal infusion of the DA-1 agonist, SKF 82526 (10(-9), 10(-8), 10(-7) M), resulted in dose related increases in RBF and absolute and fractional sodium excretion. These changes were not associated with alterations in urinary prostaglandin E2, F2 alpha, or kallikrein excretion. To determine the role of RBF in the natriuresis due to SKF 82526 infusion (10(-7) M), the renal artery was constricted to return RBF to control levels during continued SKF 82526 infusion. Although absolute and fractional sodium excretion decreased during this maneuver, they remained higher than control. These studies support both a hemodynamic and a tubular mechanism for the natriuretic effect of the DA-1 agonist, SKF 82526. These effects do not appear to be mediated by the renal prostaglandin or kallikrein systems.

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.

Histamine H1 receptor antagonists inhibit autoregulation of renal blood flow in the dog

Histamine H1 and H2 receptors are present in the canine renal circulation. We have examined the effects of H1 and H2 receptor antagonists on autoregulation of renal blood flow in the dog. Renal arterial pressure was reduced in a step-wise fashion to 80 mm Hg by means of an adjustable aortic clamp positioned above the left renal artery. Infusion of H2 antagonists, cimetidine or ranitidine, into the left renal artery at 10(-5) mol/min had no effect on autoregulation of renal blood flow or on the reactive hyperemia that occurred when the aortic constriction was released. By contrast, intrarenal infusion of 10(-5) mol/min chlorpheniramine, an H1 receptor antagonist, reversibly attenuated reactive hyperemia and the ability of the kidney to autoregulate renal blood flow. Similar results were obtained with other, chemically dissimilar H1 antagonists (terfenadine, diphenhydramine, and pyrilamine). The effects of chlorpheniramine on autoregulation of glomerular filtration rate also were evaluated. Before chlorpheniramine was infused (at 10(-5) mol/min), the reduction of renal arterial pressure to 90 mm Hg had no effect on the glomerular filtration rate, whereas, during infusion of the H1 antagonist, the glomerular filtration rate fell significantly when renal arterial pressure was reduced to 90 mm Hg. Infusion of histamine (1 microgram/kg per min) with increasing amounts of cimetidine, chlorpheniramine, diphenhydramine, or pyrilamine resulted in virtually identical dose-dependent decreases in histamine-induced renal vasodilation. However, even with 10(-5) mol/min cimetidine or 10(-5) mol/min chlorpheniramine, diphenhydramine, or pyrilamine, a significant histamine-induced renal vasodilation was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Total and local renal blood flow and filtration in the rat during reduced renal arterial blood pressure

Discrepancies concerning the lower pressure limit of renal blood flow (RBF) autoregulation, different autoregulatory adjustments in deep and superficial renal zones and dissociation of RBF and filtrate production stimulated the present study. Autoregulation of renal blood flow was investigated at reduced renal arterial blood pressure (RAP) in Sprague-Dawley rats using 4 different flow methods: uptake of 125I-iodoantipyrine (I-Ap) and 86Rb, local detection of hydrogen gas washout rate (H2) and, in the autoperfused kidney, electromagnetic flowmetry (FM). With I-Ap and 86Rb, RBF was maintained at a RAP as low as 80 mmHg, compared to contralateral RBF. However, with the other two methods where each kidney serves as its own control, a 15% RBF reduction was obtained at this RAP. This discrepancy (p less than 0.001) infers a contralateral renal vasoconstriction during ipsilateral renal hypotension and vasodilation. Arterial blood pressure increased during unilateral renal hypotension, suggesting that contralateral renal constriction was part of a general increase in total body vascular resistance. Following abrupt RAP reduction RBF was immediately readjusted (2-3 s) and maintained for up to 40 min. No significant change in intrarenal blood flow distribution was observed with I-Ap. Superficial and deep cortical single nephron glomerular filtration rates were equally reduced at lowered RAP as determined by the ferrocyanide technique. However, a dissociation between the autoregulation of RBF and glomerular filtration rate (GFR) in the direction of less well maintained GFR was observed. Renal arterial acetylcholine infusion increased RBF by about 40% and effectively abolished RBF autoregulation.

Influence of uranyl nitrate upon tubular reabsorption and glomerular filtration in blood perfused isolated dog kidneys

The early changes in tubular reabsorption, glomerular filtration, blood flow and sodium excretion brought about by uranyl nitrate were investigated in isolated, blood-perfused dog kidneys during water diuresis. No significant changes in urine volume were observed; the decrease in fluid reabsorption was counterbalanced quantitatively by a reduction in glomerular filtration rate; only a small diminution of renal blood flow was found. The balance between reabsorption and filtration was observed as well when angiotensin action or prostaglandin synthesis were inhibited. The intrarenal venous pressure rose, suggesting that an increase in proximal intratubular hydrostatic pressure caused the decrease in filtration. Tubular back-leak of fluid, or back-diffusion, induced by the toxin, were excluded. The presence of natriuretic compounds in the urine was confirmed.

Vitamin E inhibits arachidonic acid induced mortality and renal IgM deposition in rabbits

Prior administration of DL-alpha tocopherol inhibits arachidonic acid induced mortality in rabbits. Glomerular deposition of fibrin and IgM due to arachidonic acid infusion is also inhibited by prior administration of DL-alpha tocopherol.

Impaired renal blood flow autoregulation in ischemic acute renal failure

We used a model of ischemic acute renal failure featuring normal renal blood flow (RBF) to evaluate the autoregulatory capability in a preparation having a marked reduction of inulin clearance (GFR). In 10 dogs, we clamped the renal artery for 90 min (experimental); 6 dogs, 1 min only (sham). Approximately 18 hours later, we determined the autoregulatory ability from RBF responses to renal arterial constriction. GFR of the experimental dogs was 10 +/- 4 ml/min, significantly lower than GFR in the sham dogs (43 /+- 9 ml/min). RBF in the experimental dogs (189 +/- 17 ml/min) was not significantly different from that in the sham dogs (206 +/- 32). An autoregulation index, ranging from 0.49 to 1.09 (mean 0.690), was significantly larger than was that of sham dogs, which ranged from zero to 0.23 (mean 0.060). At control arterial pressures, vascular resistance was comparable in both groups; however, at reduced arterial pressures below the normal autoregulatory range, average resistance of the experimental dogs (0.62 +/- 0.12 mm Hg/[ml/min]) was significantly greater than was that of the sham dogs (0.38 +/- 0.06 mm Hg/[ml/min]). These studies indicate that a substantial loss of renal hemodynamic responsiveness follows ischemic injury to the dog even when RBF is maintained within the normal range. The loss of autoregulatory capacity associated with a severely attenuated GFR is consistent with a role for tubular flow in the normal mechanism of autoregulation.

Influence of raising albumin concentration on renin release in isolated perfused rat kidneys

1. Experiments were conducted in isolated perfused rat kidneys to determine the effect of raising perfusate albumin concentration on renin release.2. Raising albumin concentration in the perfusion fluid from 20 g/l. to 60 g/l. (high albumin concentration) increased renin release and renal perfusate flow rate. The effect was reversible.3. Ureteral occlusion did not prevent the rise in renin release and renal perfusate flow induced by high albumin concentration.4. Propranolol (0.28 mM) did not block the renin release stimulated by high albumin concentration, but it inhibited the release stimulated by isoprenaline (2.43 muM).5. Clonidine (10 muM) and oxymetazoline (10 muM) constricted the renal vasculature and stimulated renin release during high perfusate albumin concentration providing perfusion pressure was kept constant.6. Low renal perfusion pressure (50 mmHg) and isoprenaline (2.43 muM) stimulated renin release in perfusion experiments with both 20 and 60 g/l., but the rate of renin release was substantially greater with 60 g/l.7. On the other hand, perfusion fluid deprived of calcium induced a greater increase in renin release in kidneys perfused with 20 g/l. than in those with 60 g/l.8. We conclude that high albumin concentration stimulates renin release in isolated perfused rat kidneys by a mechanism which does not involve the renal nerve, direct renal vasodilation or sodium excretion. High albumin concentration may increase the sensitivity of the kidney to acute stimulation by a mechanism involving calcium.

Glomerular filtration during furosemide diuresis in the dog

Simultaneous clearance and micropuncture experiments were performed in pentobarbital-anesthetized dogs to determine the effect of furosemide (F; 5 mg/kg) on some of the determinants of GFR during replacement of urine losses. Glomerular capillary pressure (PG) was estimated from stop flow pressure (SFP) plus systemic colloid osmotic pressure (pi alpha). Because renal vasodilation during F administration occurs more often when blood pressure is elevated, two groups of dogs were studied. At endogenous renal perfusion pressure (RPP) of 130 mm Hg, one group responded to F with a 28% increase in renal blood flow (RBF). PG rose (a rise of 18 mm Hg) in proportion to the rise in proximal tubule pressure (PT) (a rise of 20 mm Hg). Thus, the difference in pressures (PG - PT) was unchanged, as was GFR. The second group had RPP lowered by renal artery constriction to a point near the lower limit of autoregulation (104 mm Hg). These dogs responded to F with no increase in RBF; PG was lower and remained constant during F, PT, however, increased (a rise of 10 mm Hg). The difference in pressures (delta P) decreased by 30%, and GFR decreased by 40%. Single nephron glomerular filtration rate (SNGFR) also decreased, and estimated Kf, the ultrafiltration coefficient, actually rose slightly. The major reason for the fall in GFR and SNGFR was due to a decrease in delta P rather than a decrease in Kf. The decrease in delta P can be attributed to failure of the renal vasculature to dilate because PG and RBF remained constant. It is likely that these events will be observed less often at hypertensive BP than at normal BP where renal vascular resistence is already close to a minimum value.

Intrarenal venous and cortical catheter pressures in the dog kidney

To examine the validity of intrarenal venous pressure (IRVP) as a measure of peritubular capillary pressure when obtained with a method applicable in man, IRVP was measured with a 0.9 mm o.d. catheter introduced retrograde into interlobar veins of anesthetized dogs and was compared with a modified needle pressure (cortical catheter pressure = RCCP) measured simultaneously in the same kidneys. In twelve dogs with a mean experimental kidney control sodium excretion of 91 +/- 15 (SEM) micronmol/min IRVP averaged 16.0 +/- 1.1 mmHg and was significantly lower than the average RCCP of 22.6 +/- 1.1 mmHg (P less than 0.001). These pressures compare well with the reported micropuncture pressures in the peritubular capillaries and proximal tubules, respectively, at comparable levels of sodium excretion. IRVP fell significantly during reduction of renal perfusion pressure within the range of autoregulation of renal blood flow and increased during elevation of renal pelvic pressure (PP). At at PP of 60 mmHg, when urine flow had stopped, the PP-IRVP gradient was 22.7 +/- 3.1 and increased to 36.7 +/- 3.8 (P less than 0.001) at a PP of 80. Acute renal vein constriction always increased IRVP before renal vein pressure reached the preceeding control level of IRVP. Increased urine flow during saline volume expansion and furosemid infusion was associated with increased IRVP. The results when compared with micropuncture data indicate that IRVP is a satisfactory expression of peritubular capillary pressure in the experimental conditions included in the study.

Influence of replacement of chloride by sulphate upon urine excretion and glomerular filtration rate in blood perfused isolated dog kidneys

Tubular reabsorption was inhibited in isolated dog kidneys by the progressive substitution of plasma chloride by sulphate. In the absence of antidiuretic hormone activity, urine output remained unchanged owing to an equivalent decrease in glomerular filtration rate. This equilibrium was demonstrated under conditions of "saline natriuresis" and was not disturbed by furosemide. Although the impairment of glomerular filtration rate was accompanied by a decrease of total renal blood flow, the equilibrium was not disrupted by angiotensin antagonism. Sodium excretion was enhanced by low plasma chloride concentrations in the absence, but not in the presence of furosemide. The results are not compatible with a specific role of osmolality, sodium or chloride concentrations in the tubular fluid in the adjustment of glomerular filtration. Simultaneous changes in blood flow and tubular flow resistances might explain the results. It is suggested that, in contrast to the mechanism of tubulo-glomerular feedback found in individual nephrons of hydropenic animals, this intrarenal mechanism might serve to protect the organism against sodium loss under conditions of high intake.

Comparison of effects of prostaglandins E2 and I2 on rat renal vascular resistance

Effects of PGE2 and PGI2 on renal vascular resistance (RVR) were compared in anesthetized rats. Renal blood flow and systemic blood pressure were measured before and during infusion of PGE2 (2--2 microgram/min) or PGI2 (1--5 microgram/min) into the aorta just proximal to the renal arteries. Both prostaglandins significantly decreased blood pressure and renal blood flow, but effects on RVR were dissimilar. At low doses, PGI2 reduced RVR in 8 of 10 rats; PGE2 increased it in 5 of 7. At higher doses, PGE2 increased RVR in all rats; during PGI2 infusion RVR did not significantly exceed control at any dose. We conclude that, in the intact rat, PGE2 increases RVR either directly or through potentiation of other constrictor stimuli, while PGI2 tends to reduce RVR and diminish the renal response to other constrictor stimuli. PGI2 is the only prostaglandin found to decrease RVR in the rat.

Prostaglandin-mediated hyperemia and renin-mediated hypertension during acute ureteral obstruction

Acute elevation of ureteral pressure to 100 mm Hg in anesthetized dogs (n=7) resulted in an increase (P less than 0.005) in systemic blood pressure form 151 +/- 7 to 163 +/- 7 mm Hg, a transient (approximately 15 min) increase (P less than 0.05) in renal blood flow from 413 +/- 27 to 465 +/- 27 ml/min and a rise (P less than 0.05) in plasma renin activity from 6.0 +/- 1.6 to 10.3 +/- 2.1 ng/ml/hr. Pretreatment with a competitive inhibitor of angiotensin II, i.e. sar1gly8AII, abolished the hypertensive response to acute ureteral obstruction, and pretreatment with 2 mg/kg of either indomethacin (n=6) or meclofenamate (n=3), 15 min before obstruction, prevented the hyperemic response. These results suggest that acute ureteral obstruction leads to hypertension via activation of the renin-angiotensin system and hyperemia via a prostaglandin-initiated mechanism.

Basal vascular tone in the kidney. Evaluation from the static pressure-flow relationship under normal autoregulation and at maximal dilation in the dog

In conscious dogs static pressure-flow relationships (I-P curves) were obtained both for the normal autoregulating kidney vasculature and under conditions of maximal vasodilation induced by infusion of papaverine or acetylcholine into the renal artery. Concentration-response curves for the substances infused showed the typical S-shape. Control I-P curves exhibited an autoregulatory plateau; under maximal vasodilation the I-P curves were pressure-passive up to a perfusion pressure of 50 mm Hg, following a power function (1 = a-Pn) with an exponent greater than 1. Under the influence of acetylcholine, renal blood flow was significantly higher than under control conditions even at a perfusion pressure of 20 mm Hg. This indicates that there is an appreciable vascular tone even at a low pressure. The I-P curves under acetylcholine showed a break at about 50 mm Hg, above which the I-P curves were straight lines.

Determination of glomerular intracapillary and transcapillary pressure gradients from sieving data. I. A mathematical model

Determination of glomerular intracapillary and transcapillary pressure gradients from sieving data. A biomathematical model is described to calculate the intracapillary and transcapillary glomerular pressure gradients from the sieving coefficients (phi: fractional clearances/GFR) of macromolecules such as polyvinylpyrrolidone (PVP). Two differential equations have been developed. The first one calculates local values for GFR in terms of local values for PGC (intracapillary hydrostatic pressure) and pi (oncotic pressure). The second equation calculates the clearance of PVP equimolecular fractions, the sieving equations previously described (24) being used to derive the concentrations of PVP in the filtrate (c2). Two variants of the second equation have been considered, assuming the filtrate in contact with the membrane either "well stirred" or "unstirred" (constant c2 and local c2 gradient models respectively). Computer simulations have been used to illustrate how the sieving curve is modified when the five parameters on which depends the shape of the curve are changed one by one. The sieving curve relates phi to a(s) (hydrodynamically equivalent molecular radius). The determining parameters are: GFP, the mean effective glomerular filtration pressure, epsilon, the slope of the intracapillary pressure, FF, the filtration fraction, Cp0, the protein concentration in arterial plasma and r, the pore radius which is the only structural parameter involved when one assumes the glomerular membrane crossed by cylindrical pores of uniform size and length. The shape of the sieving curve is modified significantly enough by changing GFP, FF and r within reasonable limits, to make it possible to derive GFP and r from experimental sieving data for macromolecules such as PVP or dextrans.

The role of the kidney in essential hypertension

1. Many forms of human and experimental hypertension begin with compromised renal function. Essential hypertension may be another such case. 2. The kidneys of subjects with essential hypertension excrete normal amounts of salt and water at higher-than-normal renal perfusing pressures. Other overt signs of renal dysfunction are few; renal disease is excluded by definition. However, renal blood flow and glomerular filtration rate are usually less than normal in essential hypertension. 3. Renal afferent resistance can be calculated from arterial pressure, renal blood flow, and an estimate of glomerular capillary pressure. These calculations indicate that afferent resistance is increased to two or more times normal in essential hypertension. 4. It is not clear whether afferent constriction causes hypertension or results from it. The ability of high pressure to produce vascular damage points to the latter. But, most essential hypertensives show low-to-normal plasma renin levels and a marked afferent dilation after saline loading. These observations do not suggest nephrosclerosis: they are consistent with a causal role for afferent constriction. 5. We can speculate that, in essential hypertension, there is a defect in one of the mechanisms that sets afferent resistance. Afferent constriction could result from extrinsic influences (neural or humoral) or something totally within the kidney, such as abnormal handling of information from the macula densa. 6. The effect of afferent constriction on salt-and-water excretion would theoretically be offset by elevated arterial pressure so that the actual salt-and-water excretion would be normal, but only so long as the arterial pressure remained elevated.

Effects of norepinephrine and angiotensin II on the determinants of glomerular ultrafiltration and proximal tubule fluid reabsorption in the rat

In 26 Wistar rats with surface glomeruli, the determinants of glomerular ultrafiltration and peritubular capillary uptake of proximal reabsorbate were studied before and during intravenous infusions of norepinephrine or angiotensin II. Regardless of whether renal perfusion pressure (AP) was permitted to increase, both hormones produced elevations in single nephron filtration fraction due to declines in glomerular plasma flow with little change in nephron glomerular filtration rate. The resulting large increases in the efferent arteriolar oncotic pressure, piE, were accompanied by equivalent increases in the mean glomerular transcapillary hydraulic pressure difference, deltaP. Equality of piE and deltaP, denoting filtration pressure equilibrium, obtained before and during infusion of either hormone. Por both hormones, when elevations in AP were allowed, marked and roughly proportional increases in the resistance to blood flow through single afferent and efferent arterioles occurred, whereas when increases in AP were prevented by partial aortic constriction increases in resistance were confined primarily to the efferent arteriole. Tespite the marked increases in piE, absolute rates of proximal tubule fluid reabsorption, on the average, were unchanged by these hormones due to the opposing effects of marked decreases in efferent arteriolar plasma flow rate and, to a lesser extent, increases in peritubular capillary hydraulic pressure.

Autoregulation of renal circulation in mannitol-loaded dogs

Renal haemodynamics and autoregulatory behaviour of the renal vessels have been investigated in mannitol-loaded dogs both for free-flow and for stop-flow conditions. Arterial pressure has been decreased by aortic clamping. For free-flow conditions: (i) relative constancy of RBF and GFR are well preserved in the entire autoregulatory pressure range, i.e., over 80 mm Hg arterial pressure; (ii) decreased GFR is due to enchanced intratubular pressure; (iii) autoregulation is connected with a sharp rise in afferent resistance, accompained by a passive decrease in efferent resistance. For stop-flow conditions: (i) enchancement of total vascular resistance is due to an increase in the passive resistance of the postglomerular vessels; (ii) afferent resistance drops to minimal values as casused by the relaxation of the corresponding arterioles; (iii) autoregualation is abolished: pressure-flow relations are linear over the entire arterial pressure range examined.

Quantitative aspects of autoregulation in the canine kidney

Autoregulatory changes in series connected vascular resistances were studied in the normal canine kidney. Two sets of observations were made: a) ureter pressure was increased by ureteral obstruction and b) arterial pressure was decreased by aortic clamping. It was assumed that in both experimental panels the decrement in intrarenal resistance is solely due to dilatation of the preglomerular vessels. Proximal tubular pressure is thought to equal deep venous (wedged) pressure under free flow and ureteral pressure under stop flow conditions. Calculation of the glomerular filtration coefficient yielded the same value in both sets of observations: k similar to 2 ml/min/100 g per 1 mm Hg effective filtration pressure. Glomerular capillary pressure rises steeply and roughly parallels arterial pressue below the autoregulatory range (about 40-80 mm Hg), whereas in the autoregulatory range (about 90-150 mm Hg) its value is stabilized at 74 plus or minus 1 (S.E.)mm Hg. Afferent resistance is negligible in the hypotensive range and increases progressively in the normotensive range. Changes in efferent and venular resistance are insignificant in the whole pressure range considered, i.e. 40 to 150 mm Hg.