Influence of renal nerve activity on arteriolar resistance, ultrafiltration dynamics and fluid reabsorption

Sir George Johnson FRCP (1818-96), high blood pressure and the continuing altercation about its origins

NEW FINDINGS

What is the topic of this review? The review takes a historical approach to examining where in the body it might be possible to identify the most common cause, or causes, of long-term hypertension. It gathers evidence from histology, human and animal physiology, and computational modelling. The burden of decades of controversy is noted. What advances does it highlight? The review highlights the distinctive pathology of the afferent renal circulation and what its consequences are for the widespread view that essential hypertension is caused by elevated peripheral vascular resistance.

ABSTRACT

The widely promulgated notion that long-term elevation in mean arterial blood pressure (MAP) can be caused by raised peripheral vascular resistance remains a subject of vigorous debate. According to the 1967 mathematical model of Guyton and Coleman, such a causal relationship is impossible, kidney function being the determining factor. We explore this altercation starting with Sir George Johnson's 19th-century renal vascular histological observations in patients with Bright's disease. We note the striking physiological measurements in hypertensives by Gómez and Bolomey in the 1950s, moving on to the mathematical modelling of the circulation from the 1960s up to the ∼100-parameter computer models of the present day. Confusion has been generated by the fact that peripheral resistance is raised in hypertension in close proportion to MAP whilst cardiac output often stays normal, an apparent autoregulation, the mechanism of which is poorly understood. All models allowing for the circulation to be an open system show that isolated changes in peripheral resistance cannot lead to long-term hypertension, but models fail so frequently to account for results from experiments such as salt loading that their credibility with regard to this key finding is compromised. Laboratory animal models of adrenergic renal actions resonate with a contemporary emphasis on the sympathetic nerve supply to the kidney as contributing to the characteristically markedly elevated renal afferent resistance that appears to be the most common cause of hypertension. Remarkably, there remains no account of the way in which the fixed structural changes in vessels observed by Johnson relate to this sympathetic overactivity, which can itself be modified by drugs in the medium term. In this account, we seek to locate the crime scene and identify a smoking gun.

Electrical stimulation of renal nerves for modulating urine glucose excretion in rats

BACKGROUND

The role of the kidney in glucose homeostasis has gained global interest. Kidneys are innervated by renal nerves, and renal denervation animal models have shown improved glucose regulation. We hypothesized that stimulation of renal nerves at kilohertz frequencies, which can block propagation of action potentials, would increase urine glucose excretion. Conversely, we hypothesized that low frequency stimulation, which has been shown to increase renal nerve activity, would decrease urine glucose excretion.

METHODS

We performed non-survival experiments on male rats under thiobutabarbital anesthesia. A cuff electrode was placed around the left renal artery, encircling the renal nerves. Ureters were cannulated bilaterally to obtain urine samples from each kidney independently for comparison. Renal nerves were stimulated at kilohertz frequencies (1-50 kHz) or low frequencies (2-5 Hz), with intravenous administration of a glucose bolus shortly into the 25-40-min stimulation period. Urine samples were collected at 5-10-min intervals, and colorimetric assays were used to quantify glucose excretion and concentration between stimulated and non-stimulated kidneys. A Kruskal-Wallis test was performed across all stimulation frequencies (α = 0.05), followed by a post-hoc Wilcoxon rank sum test with Bonferroni correction (α = 0.005).

RESULTS

For kilohertz frequency trials, the stimulated kidney yielded a higher average total urine glucose excretion at 33 kHz (+ 24.5%; n = 9) than 1 kHz (- 5.9%; n = 6) and 50 kHz (+ 2.3%; n = 14). In low frequency stimulation trials, 5 Hz stimulation led to a lower average total urine glucose excretion (- 40.4%; n = 6) than 2 Hz (- 27.2%; n = 5). The average total urine glucose excretion between 33 kHz and 5 Hz was statistically significant (p < 0.005). Similar outcomes were observed for urine flow rate, which may suggest an associated response. No trends or statistical significance were observed for urine glucose concentrations.

CONCLUSION

To our knowledge, this is the first study to investigate electrical stimulation of renal nerves to modulate urine glucose excretion. Our experimental results show that stimulation of renal nerves may modulate urine glucose excretion, however, this response may be associated with urine flow rate. Future work is needed to examine the underlying mechanisms and identify approaches for enhancing regulation of glucose excretion.

The macro- and microcirculation of the kidney

Acute kidney injury (AKI) remains one of the main causes of morbidity and mortality in the intensive care medicine today. Its pathophysiology and progress to chronic kidney disease is still under investigation. In addition, the lack of techniques to adequately monitor renal function and microcirculation at the bedside makes its therapeutic resolution challenging. In this article, we review current concepts related to renal hemodynamics compromise as being the event underlying AKI. In doing so, we discuss the physiology of the renal circulation and the effects of alterations in systemic hemodynamics that lead to renal injury specifically in the context of reperfusion injury and sepsis. The ultimate key culprit of AKI leading to failure is the dysfunction of the renal microcirculation. The cellular and subcellular components of the renal microcirculation are discussed and how their injury contributes to AKI is described.

Neural regulation of renal blood flow: a re-examination

1. The importance of renal sympathetic nerve activity (RSNA) in the regulation of renal function is well established. However, it is less clear how the renal vasculature responds to the different mean levels and patterns of RSNA. While many studies have indicated that small to moderate changes in RSNA preferentially regulate renin secretion or sodium excretion and only large changes in RSNA regulate renal blood flow (RBF), other experimental evidence suggests that small changes in RSNA can influence RBF 2. When RSNA has been directly measured in conjunction with RBF, it appears that a range of afferent stimuli can induce reflex changes in RBF. However, many studies in a variety of species have measured RBF only during stimuli designed to reflexly increase or decrease sympathetic activity, but have not recorded RSNA. While this approach can be informative, it is not definitive because the ability of the vasculature to respond to RSNA may, in part, reflect the resting level of RSNA and, therefore, the vasoconstrictive state of the vasculature under the control conditions. 3. Further understanding of the control of RBF by RSNA has come from studies that have analysed the underlying rhythms in sympathetic nerve activity and their effect on the cardiovascular system. These studies show that the frequency-response characteristic of the renal vasculature is such that higher frequency oscillations in RSNA (above 0.6 Hz) contribute to setting the mean level of RBF. In comparison, lower frequency oscillations in RSNA can induce cyclic vasoconstriction and dilation in the renal vasculature, thus inducing oscillations in RBF. 4. In summary, the present review discusses the neural control of RBF, summarizing evidence in support of the hypothesis that RBF is under the influence of RSNA across the full range of RSNA.

Effects of low dose sympathetic inhibition on glomerulosclerosis and albuminuria in subtotally nephrectomized rats

ABSTRACT.: A potential role of the sympathetic nervous system in progression of renal failure has received little attention. This study examined whether nonhypotensive doses of moxonidine, an agent that reduces sympathetic activity, affects glomerulosclerosis, urine albumin excretion, and indices of renal handling of norepinephrine (NE) in subtotally nephrectomized (SNX) rats. Sprague Dawley rats were SNX or sham-operated (control). SNX rats were either left untreated or treated with moxonidine in a dose (1.5 mg/kg body wt per d) that did not modify telemetrically monitored 24-h BP. Glomerular and renal morphology were evaluated by quantitative histology, immunohistochemistry, and in situ hybridization. Urine albumin excretion rate was analyzed by enzyme-linked immunosorbent assay, and kidney angiotensin II and NE content were measured using HPLC, (3)H-NE uptake, and release. Body and kidney weight and BP were not significantly different between SNX with or without moxonidine. The glomerulosclerosis index was significantly lower in moxonidine-treated (0.88 +/- 0.09) compared with untreated (1.55 +/- 0.28) SNX rats, as was the index of vascular damage (0.32 +/- 0.14 versus 0.67 +/- 0.16). The number of proliferating cell nuclear antigen-positive glomerular and tubular cells per area was significantly higher in untreated SNX rats than in controls and moxonidine-treated SNX rats. The same was true for urine albumin excretion rate. Renal angiotensin II tissue concentration was not affected by moxonidine. In untreated SNX rats, renal nerve stimulation and exogenous NE induced an increase in isolated kidney perfusion pressure (102 +/- 21 versus 63 +/- 8 mmHg). Renal endogenous NE content was significantly lower in SNX rats than in controls (86 +/- 14 versus 140 +/- 17 pg/mg wet weight). Cortical uptake of [(3)H]-NE was not different, but cortical NE release was significantly higher in SNX rats than in controls. Reduced function of presynaptic inhibitory alpha-adreno-receptors is unlikely because an alpha(2)-adrenoceptor antagonist increased NE release. At subantihypertensive doses, moxonidine ameliorates renal structural and functional damage in SNX animals, possibly through central inhibition of efferent sympathetic nerve traffic. In kidneys of SNX rats, indirect evidence was found for increased activity of a reduced number of nerve fibers.

Effects of antihypertensive drugs on autoregulation of RBF and glomerular capillary pressure in SHR

The relationship between systemic blood pressure and glomerular capillary pressure (Pgc) in spontaneously hypertensive rats (SHR) during treatment with antihypertensive drugs is still unclear. The effects of an angiotensin-converting enzyme inhibitor (enalapril), two calcium channel antagonists (nifedipine and verapamil), and an alpha1-receptor blocker (doxazosin) on renal blood flow (RBF) autoregulation, Pgc, and renal segmental resistances were therefore studied in SHR. Recordings of RBF autoregulation were done before and 30 min after intravenous infusion of the different drugs, and Pgc was thereafter measured with the stop-flow technique. When the mean arterial pressure (MAP) was reduced to approximately 120 mmHg by infusions of doxazosin or enalapril, the lower pressure limit of RBF autoregulation was reduced significantly. Nifedipine or verapamil abolished RBF autoregulation. Doxazosin did not change Pgc (43.6 +/- 1.4 vs. 46.7 +/- 1.5 mmHg in controls, P > 0.5), enalapril lowered (41.3 +/- 0.8 mmHg, P < 0.01), and the calcium channel antagonists increased Pgc [53.7 +/- 1.4 mmHg (nifedipine) and 54.8 +/- 1.2 mmHg (verapamil), P < 0.01]. When MAP was reduced to approximately 85 mmHg by drugs, Pgc was reduced to 43.3 +/- 1.7 mmHg after nifedipine (P > 0.2 vs. control), whereas Pgc after enalapril was 38.5 +/- 0.5 mmHg (P < 0.05 vs. control). Enalapril reduced Pgc mainly by reducing efferent resistance. During treatment with calcium channel antagonists, Pgc became strictly dependent on MAP. Monotherapy with nifedipine may increase Pgc and by this mechanism accelerate glomerulosclerosis if a strict blood pressure control is not obtained.

Resetting of renal blood autoregulation during acute blood pressure reduction in hypertensive rats

Decrease in systemic blood pressure, duration of pressure decrease, and change in the activity of the renin or the sympathetic nervous system may represent mechanisms involved in resetting the renal blood flow (RBF) autoregulation found in hypertensive rats. Autoregulation of RBF, plasma renin concentration (PRC), and the time needed for resetting to take place were studied in the nonclipped kidney before and after removal of the clipped kidney of two- kidney, one-clip (2K1C) hypertensive rats and before and after mechanical reduction of the renal arterial pressure (RAP) for 10 min in the spontaneously hypertensive rat (SHR) and in the nonclipped kidney of 2K1C hypertensive rats with and without renal denervation. Mean arterial pressure (MAP) fell from 147 to 107 mmHg 30 min after removal of the clipped kidney, and the lower pressure limit of RBF autoregulation decreased from 113 to 90 mmHg (P < 0.01); PRC fell. Mechanical reductions of RAP from 161 to 120 mmHg in the nonclipped kidney for 10 min did not change RBF, but at 120 mmHg, the lower pressure limit of RBF autoregulation was reduced from 115 mmHg before pressure reduction to 96 mmHg afterwards (P < 0.02). In SHR, similar pressure reduction for 10 min decreased the lower pressure limit of RBF autoregulation from 106 to 86 mmHg (P < 0.01). PRC was unchanged in both models, and denervation did not change RBF autoregulation. When RAP was reduced below the lower pressure limit of RBF autoregulation, RBF decreased approximately 20%; the lower pressure limit of RBF autoregulation remained unchanged. In normotensive Wistar-Kyoto rats, pressure reduction did not change the range of RBF autoregulation. These results indicate that acute normalization of the pressure range of RBF autoregulation in hypertensive rats is dependent on the degree of pressure reduction of RAP, whereas renal innervation and PRC do not play a major role. We propose that the mechanism of resetting is due to afterstretch of noncontractile elements of the vessel wall or is caused by pure myogenic mechanisms. An effect of intrarenal angiotensin cannot be excluded.

Influence of the sympathetic nervous system on renal function during hypothermia

Hypothermia increases preglomerular vasoconstriction leading to decreases in renal blood flow (RBF) and glomerular filtration rate (GFR). Since plasma catecholamine concentrations are increased during hypothermia, the present study was performed to determine the role of the renal sympathetic nervous system in the cold-induced renal vasoconstriction. In Inactin anaesthetized rats, hypothermia at 28 degrees C decreased GFR by 50% but failed to alter efferent renal sympathetic nerve activity (ERSNA). Since hypothermia causes shivering which could have influenced the ERSNA recording, Inactin anaesthetized rats were treated with pethidine or rats were anaesthetized with pentobarbital sodium or Saffan to eliminate cold-induced shivering. In these non-shivering rats, hypothermia produced a reversible decrease in ERSNA in association with a fall in GFR that was of a similar magnitude as in shivering rats. Further studies in Inactin anaesthetized rats showed that the fall in GFR was unaltered by renal denervation, bilateral adrenalectomy or intrarenal administration of the alpha 1-adrenoceptor antagonist prazosin. We conclude that cold-induced renal vasoconstriction is not due to an increase in ERSNA or adrenaline/noradrenaline-mediated activation of renal alpha 1-adrenoceptors.

Contribution of renal nerves to renal blood flow variability during hemorrhage

We have examined the role of the renal sympathetic nerves in the renal blood flow (RBF) response to hemorrhage in seven conscious rabbits. Hemorrhage was produced by blood withdrawal at 1.35 ml.min(-1).kg-1 for 20 min while RBF and renal sympathetic nerve activity (RSNA) were simultaneously measured. Hemorrhage was associated with a gradual increase in RSNA and decrease in RBF from the 4th min. In seven denervated animals, the resting RBF before hemorrhage was significantly greater (48 +/- 1 vs. 31 +/- 1 ml/min intact), and the decrease in RBF did not occur until arterial pressure also began to fall (8th min); however, the overall percentage change in RBF by 20 min of blood withdrawal was similar. Spectral analysis was used to identify the nature of oscillations in each variable. Before hemorrhage, a rhythm at approximately 0.3 Hz was observed in RSNA, although not in RBF, whose spectrogram was composed mostly of lower-frequency (< 0.25 Hz) components. The denervated group of rabbits had similar frequency spectrums for RBF before hemorrhage. RSNA played a role in dampening the effect of oscillations in arterial pressure on RBF as the transfer gain between mean arterial pressure (MAP) and RBF for frequencies > 0.25 Hz was significantly less in intact than denervated rabbits (0.83 +/- 0.12 vs. 1.19 +/- 0.10 ml.min(-1).mmHg-1). Furthermore, the coherence between MAP and RBF was also significantly higher in denervated rabbits, suggesting tighter coupling between the two variables in the absence of RSNA. Before the onset of significant decreases in arterial pressure (up to 10 min), there was an increase in the strength of oscillations centered around 0.3 Hz in RSNA. These wer accompanied by increases in the spectral power of RBF at the same frequency. Arterial pressure fell in both groups of animals, the dominant rhythm to emerge in RBF was centered between 0.15 and 0.20 Hz and was present in intact and denervated rabbits. It is speculated that this myogenic in origin. We conclude that RSNA can induce oscillations in RBF at 0.3 Hz, plays a significant role in altering the effect of oscillations in arterial pressure on RBF, and mediates a proportion of renal vasoconstriction during hemorrhage in conscious rabbits.

Transient renal impairment in acute intracerebral haemorrhage

Transient impairment of renal function was found in 30 of 78 patients with acute intracerebral haemorrhage (ICH). Patients with a history of renal disease, dehydration, nephrotoxic drugs or septicaemia were excluded. In the 1st week, the mean serum creatinine level was 3.4 (range 1.7-7.7) mg/dl, which returned to normal in 2-4 weeks. Employing multivariate stepdown logistic regression analysis, Glasgow Coma Scale score and pulse pressure were found to be significantly related to renal impairment manifesting with a raised serum creatinine level, whereas pupillary asymmetry was of borderline significance. An acute rise in intracranial pressure following ICH may result in sympathetic overactivity, which may account for the renal impairment observed in our patients.

Juxtamedullary afferent and efferent arterioles constrict to renal nerve stimulation

Sympathetic neural control of afferent and efferent arterioles of inner cortical (juxtamedullary) glomeruli has not been established, in part, because of difficulty accessing these vessels, normally located deep below the kidney surface. In this study we utilized the rat hydronephrotic kidney model to visualize the renal microcirculation and to quantitate the responses of juxtamedullary arterioles to brief (30 sec) renal nerve stimulated (RNS). Juxtamedullary afferent and efferent arterioles constricted in a frequency-dependent fashion to RNS, achieving a maximal constriction of 35% to 8 Hz stimulation. In these same kidneys, outer cortical afferent arterioles also constricted to RNS but outer cortical efferent arterioles did not. Microinjection of norepinephrine (NE) around single outer cortical efferent arterioles (to avoid the constriction of preglomerular vessels) constricted the efferent arterioles. However, the afferent arterioles of the same glomeruli were considerably more responsive to microinjected NE. Thus, the lack of constriction of outer cortical efferent arterioles to RNS may relate, in part, to their low sensitivity to NE, the primary neurotransmitter. These direct observations indicate that the juxtamedullary efferent arterioles are responsive to renal nerve stimulation whereas the outer cortical efferent vessels are not. These results, which should be cautiously extrapolated to normal filtering kidneys, indicate that glomerular hemodynamic changes evoked by the sympathetic nervous system are different for outer cortical and inner cortical glomeruli.

Influence of endogenous angiotensin on the renovascular response to norepinephrine

The purpose of this study was to elucidate the role of endogenous angiotensin II in mediating the renovascular effects of renal adrenergic stimulation. Six conscious dogs instrumented for monitoring of renal blood flow were subjected to step increases every 10 minutes in the rate of norepinephrine infusion into the renal artery. Under control conditions, infusion of norepinephrine (10-40 ng/min per milliliter per minute of control renal blood flow) increased plasma renin activity and decreased renal blood flow progressively by approximately 10-75%. When increments in angiotensin II during norepinephrine infusion were abolished by fixing plasma levels of angiotensin II at either normal or high concentrations by chronic infusion of captopril plus angiotensin II, renal blood flow responses to adrenergic stimulation were greatly attenuated at rates of norepinephrine infusion that decreased renal blood flow up to approximately 40% under control conditions. Thus, acutely generated angiotensin II appeared to contribute to the renovascular effects of norepinephrine. However, when endogenous levels of angiotensin II were suppressed to low levels by chronic infusion of captopril alone, norepinephrine induced severe renal ischemia at much lower rates of infusion than occurred when the renin-angiotensin system was intact. Since this enhanced sensitivity to norepinephrine did not occur during chronic captopril infusion when angiotensin II was given simultaneously at rates that restored mean arterial pressure to normotensive levels or higher, low arterial pressure during chronic captopril administration may predispose the kidneys to excessive renal vasoconstriction during renal adrenergic stimulation.

The renal response to electrical stimulation of renal efferent sympathetic nerves in the anaesthetized greyhound

1. The effect of direct electrical stimulation of the renal efferent nerves upon renal haemodynamics and function was studied in greyhounds anaesthetized with chloralose and artificially ventilated. The left kidney was neurally and vascularly isolated, and perfused with blood from one of the femoral arteries at a constant pressure of 99 +/- 1 mmHg. Renal blood flow was measured with a cannulating electromagnetic flow probe placed in the perfusion circuit, glomerular filtration rate by creatinine clearance, urinary sodium excretion by flame photometry and solute excretion by osmometry. Beta-Adrenergic receptor activation was blocked by the infusion of dl-propranolol (17 micrograms kg-1 min-1). The peripheral ends of the ligated renal nerves were stimulated at 0.5, 1.0, 1.5 and 2.0 Hz. 2. At 0.5 Hz frequency only osmolar excretion was significantly reduced (10.3 +/- 3.2%, P less than 0.05, n = 6). Reductions in sodium excretion (53.6 +/- 8.5%, P less than 0.01, n = 6) and water excretion (26.9 +/- 8.0%, P less than 0.05, n = 6) and further reductions of osmolar excretion (20.7 +/- 3.7%, P less than 0.01, n = 6) were observed at 1.0 Hz; however, these were observed in the absence of significant changes in renal blood flow and glomerular filtration rate. Significant reductions were observed in glomerular filtration rate at 1.5 Hz (16.3 +/- 4.1%, P less than 0.02, n = 5) and in renal blood flow at 2.0 Hz (13.1 +/- 4.0%, P less than 0.05, n = 5). Further reductions in urine flow and sodium excretion were also observed at these higher frequencies. 3. These results clearly show that significant changes in renal tubular function can occur in the absence of changes in renal blood flow and glomerular filtration rate when the renal nerves are stimulated electrically from a zero baseline activity up to a frequency of 1.5 Hz. Higher frequencies caused significant changes in both renal haemodynamics and function.

Continuous measurement of renal cortical blood flow and renal arterial blood flow during stimulation of the renal nerve

Stimulation of the renal sympathetic nerve in young pigs with biphasic pulses of current (3 mA, 800 microseconds per phase, 5, 10 and 50 Hz) produced decreases in arterial and cortical blood flow in the kidney, with the greatest decreases occurring at the highest stimulus frequencies. The decrease in cortical flow lagged that in arterial flow by 1.53-1.99 s; the delay increased with decreasing frequency but was unaffected by captopril, an angiotensin-converting enzyme-blocking agent. This result was consistent with the hypothesis that stimulation of the sympathetic nerve causes constriction first in the afferent arteriole and then in the efferent arteriole. Systemic arterial pressure increased during stimulation of the nerve; the increase was greater in intact nerves than in nerves that had been crushed proximal to the point of the stimulus, indicating that pigs do have renal afferent nerves. Pressure increased after the stimulus ended, but the increase abated or changed to a decrease after administration of captopril. The changes in flow were unaffected by administration of captopril, but were markedly reduced by the blocking agent labetalol (renal arterial flow, 77 +/- 14 per cent; cortical flow, 70 +/- 12 per cent). Thus, the observed changes in flow resulted from direct stimulation of the sympathetic nerves and not from stimulation of the renin-angiotensin system, which affects the pressure response after the stimulus.

Proximal tubular stop flow pressure: an index of glomerular capillary pressure?

Central to the assumption that glomerular capillary pressure (Pgc) can be equated with the sum of arterial oncotic pressure (pi art) and the pressure in a blocked proximal tubule ("stop flow" pressure, Psf) is that filtration ceases in the blocked nephron. Should filtration not cease, but continue at a rate equal to tubular reabsorption between the block and the glomerulus, Psf, for a given Pgc, will depend on the distance between block and glomerulus. This would have serious consequences for the interpretation of Psf, particularly in respect to its frequent use in analysis of the tubuloglomerular feedback (TGF) mechanism. Experiments were performed in anaesthetized Wistar rats to examine whether a length dependency of Psf exists and, if so, to what extent this relationship alters during maximal TGF stimulation by loop of Henle perfusion. A length dependency of Psf existed both in the absence and presence of loop flow. The regression coefficients were significantly different from 0 and from each other. Pgc cannot thus be equated with the sum of Psf and pi art. The length dependent error in Psf makes it unsuitable for the quantitative analysis of TGF and glomerular haemodynamics.

Functional effects on glomerular hemodynamics of short-term chronic cyclosporine in male rats

We evaluated the effects of chronic cyclosporine (CsA) administration on the determinants of nephron filtration rate (SNGFR) using micropuncture techniques (mp) in male Munich-Wistar rats. Animals received CsA (30 mg/kg SQ) in olive oil daily for 8 d before mp. Controls (PFC) were pair fed. SNGFR, glomerular capillary hydrostatic pressure gradient (delta P), nephron plasma flow (SNPF), plasma protein oncotic pressure (pi A), and glomerular ultrafiltration coefficient (LpA) were quantitated in each experiment. CsA was associated with a lower SNGFR due to decreases in SNPF and a major reduction in delta P but no decrease in LpA. Plasma volume expansion (PVE) caused SNGFR, delta P, and SNPF to increase in both CsA and PFC without eliminating the differences between CsA and PFC. CsA/PVE rats responded normally to angiotensin II (AII) infusion indicating that the low delta P associated with CsA is not due to unresponsiveness to AII. Prior renal denervation caused SNGFR and SNPF to increase in CsA-treated animals but failed to alter the reduction in glomerular capillary pressure after CsA or to eliminate the glomerular hemodynamic differences between treated animals and pair-fed controls. This constellation of glomerular hemodynamic abnormalities suggests that the renal effect of short-term chronic CsA administration is mediated primarily by a reduction in the afferent effective filtration pressure resulting from an imbalance between pre- and postglomerular vascular resistances.

Increase in proximal tubular fluid reabsorption by renal nerve stimulation. A split oil droplet study

The influence of renal sympathetic nerve stimulation on fluid reabsorption in the proximal tubules was studied in anaesthetized male Sprague-Dawley rats. Direct stimulation with a frequency of 2 Hz was applied, and a modification of the split oil droplet technique was used. The fluid reabsorption was determined as the half-time (t1/2) of the shrinking droplet. In the control situation t1/2 was 30.7 +/- 3.4 s. On stimulation at 2 Hz, t1/2 decreased in all nine rats studied by an average of 25 +/- 5%, to 22.4 +/- 2.6 s (P less than 0.01). The decrease in t1/2 indicates an increased rate of proximal tubular fluid reabsorption. The results support the concept that the anatomically established adrenergic innervation of renal proximal tubules participates in the direct regulation of tubular fluid reabsorption, a role which might be important in the control of the extracellular volume.

Influence of renal denervation on vascular responsiveness of isolated rat intrarenal arteries

Microsurgical renal denervation of the rat has been reported to increase blood loss and bleeding time after a standardized kidney resection. To investigate the vascular effects of denervation, isolated intrarenal arteries were studied using sensitive 'isometric' recording equipment. Four pieces of evidence were obtained to indicate an effective functional denervation I week after renal nerve transection: (i) Phentolamine reduced the K+-induced contraction in controls but not in denervated arteries. (ii) The K+-induced contraction was significantly smaller in denervated than in control arteries. (iii) Noradrenaline (NA) was a significantly more potent vasoconstrictor (4 x) in denervated than in control arteries. (iv) Cocaine increased the NA sensitivity in control arteries (3 x), whereas it failed to do so in denervated vessels. Vasopressin, 5-hydroxytryptamine (5-HT), NA (in the presence of cocaine), prostaglandin F2 alpha (PGF2 alpha) and dopamine (DA) produced concentration-dependent contractions in the mentioned order of potency. Denervated arteries were found to be about two to three times more sensitive to the vasoconstrictors than control arteries. Angiotensin I and II had no contractile effect in any of the vessel segments examined. Indomethacin-pretreated arteries also failed to respond to angiotensin II. Neuropeptide Y produced only weak contractions and failed to influence the NA concentration-response relationship in either control or denervated arteries. In conclusion, renal denervation caused a general supersensitivity of the vascular smooth muscle cells to both circulating and non-circulating vasoconstrictors. Our results cannot explain the increased blood loss and bleeding time seen after denervation, but rather support the view that the enhanced bleeding was caused by an interrupted vasoconstrictor influence of the sympathetic nerves.

Renal and extrarenal handling of a new imaging compound (99m-Tc-MAG-3) in the rat

We investigated the renal clearance, the extrarenal clearance, the biodistribution, the plasma extraction ratio and the imaging quality on a scintillation camera of a new substance: 99mTc-mercaptoacetyltriglycine (MAG-3) in rats. Simultaneously 125I-hippurate (OIH) and 51Cr-EDTA were used as reference substances. In scintillation camera studies, 123I-hippurate served as reference. MAG-3 was prepared by kit labelling without further purification, as it would be used in clinical studies. However in three separate rats, HPLC purified MAG-3 was used. High renal clearance and extraction ratio was found but these were less than that for OIH (1.95 ml/min per 100 g BW vs 2.76 ml/min per 100 g BW and 64% vs 85% respectively). The extrarenal clearance was higher for MAG-3 than for OIH (0.20 ml/min per 100 g BW vs 0.04 ml/min per 100 g BW), presumably because of bile excretion. HPLC purification increased the renal excretion of MAG-3: the clearance was 2.53 ml/min per 100 g BW, the extraction fraction 75%, but the extrarenal clearance was unchanged. The imaging quality was comparable to that of 123I-hippurate in early pictures, but extrarenal activity, presumably representing bile, was observed in late pictures. We conclude that MAG-3 has some potential for the replacement of OIH but clinical studies are necessary to test whether the same conditions and limitations exist in humans.

Glomerulotubular balance during renal sympathetic stimulation

In volume-expanded dogs receiving ethacrynic acid, a linear relationship, glomerulotubular balance (GTB), applies between the remaining sodium reabsorption and the glomerular filtration rate (GFR) during mechanical aortic constriction. To examine whether GTB applies during sympathetic stimulation, the GFR was progressively reduced by 70-75% in anaesthetized dogs by renal nerve stimulation, intrarenal norepinephrine infusion or by selective stimulation of alpha-adrenoceptors by intrarenal methoxamine infusion. Linear relationships (GTB) were obtained (r greater than 0.9). Reabsorption was not different during the various kinds of sympathetic stimulation, but less than during aortic constriction; the largest difference in NaCl reabsorption at comparable GFR amounted to 10-15% and was obtained 30-40% below control GFR, whereas inhibition of NaHCO3 reabsorption was uncertain. To inhibit NaHCO3 reabsorption and associated NaCl reabsorption in the proximal tubules, acetazolamide (30 mg kg-1) was administered instead of ethacrynic acid. No difference in reabsorption was observed at comparable GFR during norepinephrine infusion and mechanical aortic constriction. Hence, GTB applies during sympathetic stimulation. Compared with data obtained during aortic constriction, alpha-adrenergic stimulation reduces proximal reabsorption of NaCl and, possibly, NaHCO3 and exerts no effect on distal transcellular NaCl reabsorption.

A microsurgical method for experimental kidney denervation

A successful microsurgical method for denervation of the kidney in the rat is presented. Unilateral kidney denervation was performed in eight animals. Ten sham-operated animals were used as controls. Under ether anaesthesia, the kidney vessels on one side were exposed. The peritoneum above the vessels was incised and the area was stained with 1% toluidine blue solution. The nerves were identified. With microsurgical technique, the nerves were carefully separated from the vessels and resected. Tissue specimens from both kidneys in all animals were removed and analysed for norepinephrine content. There was no normal difference between the kidneys in the sham-operated rats. About 1 week after denervation, the norepinephrine concentration in the denervated kidney showed a 95% reduction, indicating a complete denervation. The described microsurgical procedure was a rapid, simple, and reproducible method for kidney denervation in the rat.

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

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

Hemodynamic characterization of hypertension induced by chronic intrarenal or intravenous infusion of norepinephrine in conscious rats

The present study was designed to determine the hemodynamic changes underlying the hypertension induced by chronic intrarenal infusion of norepinephrine (NE) in conscious rats. NE was infused for a 5-day period intrarenally with osmotic minipumps via a chronic catheter in the right suprarenal artery at rates of 4 and 36 micrograms . kg-1 . hr-1 or intravenously at a rate of 36 micrograms . kg-1 . hr-1. Control rats received a 1 microliter . hr-1 intrarenal infusion of pyrogen-free 0.9% NaCl. In separate experiments, short-term effects were measured continuously during a 22- to 24-hour intrarenal infusion of 4 and 36 micrograms NE . kg-1 . hr-1 or intravenous infusion of 36 micrograms NE . kg-1 . hr-1. Intrarenal infusion of NE produced a more pronounced long-term hypertensive effect than infusion of the same dose intravenously. This hypertension was characterized by a rapid and sustained increase in total peripheral resistance index (TPRI). Despite of the initial renal vasoconstriction, specifically produced during the first 24 hours of intrarenal NE application, cardiac index (CI) in parallel to stroke volume index (SVI) decreased significantly during intrarenal as well as during intravenous NE infusion. Furthermore, no signs of sodium retention were observed. Both rates of intrarenal NE infusion have been shown previously to produce a significant long-term increase in plasma potassium concentration, and the present study indicates that this is presumably the result of decreased urinary potassium output. It is concluded that chronic hypertension produced by intrarenal or intravenous infusion is not volume-dependent. The relatively greater increase in TPRI during intrarenal NE infusion is attributed to vascular wall receptor sensitization by increased plasma potassium levels resulting from effects of intrarenally present NE on tubular cation exchange mechanisms.

Effect of renal nerve stimulation on the activity of the tubuloglomerular feedback mechanism

The influence of renal nerve stimulation on the tubuloglomerular feedback mechanism was studied on anaesthetized rats. The analyses was made by comparing the single-glomerular filtration rate (SNGFR) measured from late proximal tubules with SNGFR measured from distal tubules in the same nephron. In the former situation the flow to the macula densa cells is interrupted and in the latter the macula densa is influenced by the flow passing by. In the control situation the SNGFR measured proximally was 47.7 +/- 2.2 nl . min-1 (mean +/- SE) and 40.1 +/- 1.8 measured distally indicating an activated tubuloglomerular feedback. During renal nerve stimulation (2-3 Hz), the SNGFR fell to 38.5 +/- 2.3 and 33.5 4/- 1.7 when measured proximally and distally, respectively. The results indicate that the tubuloglomerular feedback mechanism is unaffected by renal nerve stimulation.

The cortical and medullary blood flow at different levels of renal nerve activity

The effect of (1) renal denervation and (2) stimulation of the renal nerve on the regional renal blood flow were determined by the Rb uptake method. Under control conditions the total renal blood flow was 3.64 +/- 0.09 ml X min-1 X g-1 tissue increasing significantly (p less than 0.02) to 4.39 +/- 0.28 ml X min-1 X g-1 after denervation. Upon stimulation of the peripheral portions of the sectioned renal nerves the blood flow decreased almost linearly with the frequency of stimulation reaching 0.99 +/- 0.24 ml X min-1 X g-1 at 10 Hz. Utilizing the relation between blood flow and stimulation frequency the control blood flow correspond to a spontaneous activity of 1.5 Hz. As expected the cortical blood flow responded in the same way as for the total renal blood flow. In the renal medulla denervation gave a much more pronounced response where e.g. the inner medullary flow increased from 0.88 +/- 0.09 to 1.30 +/- 0.16 ml X min-1 X g-1, i.e. a 50% increase (p less than 0.05). Stimulation with 2 Hz produced a steep fall in the blood flow, whereafter it decreased linearly with the stimulation frequency reaching 0.11 ml X min-1 X g-1 at 10 Hz stimulation. This demonstrates again that the renal medulla is sensitive to renal nerve activity primarily in the low level range. It should be remarked, however, that the 86-Rb uptake method reflects the effective blood flow, which might differ from the blood flow in absolute terms.(ABSTRACT TRUNCATED AT 250 WORDS)

Compensatory hypertrophy of single nephrons following ischemic injury in the rat

In Sprague-Dawley rats, the increase in single nephron glomerular filtration rate (SNGFR) following uninephrectomy is due to an increase in glomerular plasma flow (GPF) along with an increase in the glomerular capillary ultrafiltration coefficient (Kf). Hypertrophy of individual nephrons also occurs when renal mass is reduced by disease or ischemic injury. To characterize the factors determining SNGFR in the minority of nephrons that recover from a severe ischemic insult, rats were studied 4 weeks after 1 hour of complete unilateral renal artery occlusion and the results compared to those obtained in normal rats. Indirect determination of the dynamics of glomerular ultrafiltration along with microangiographic studies indicated that despite a reduction in total renal blood flow in the postischemic kidney, a minority of nephrons were hyperperfused. The increase of GPF in these nephrons along with a significant increase in Kf was responsible for the observed increase in SNGFR.

Effect of adrenergic blockade with bretylium and phentolamine on glomerular filtration rate in the rainbow trout, Salmo gairdneri Rich., adapting to saline water

In resting conscious trout, Salmo gairdneri, a gradual increase in external salinity (brackish water) resulted in an immediate decline of glomerular filtration rate and urine flow. These renal responses were partly blocked by the administration of bretylium and phentolamine. The presence of adrenergic nerve endings in the trout kidney and these pharmacological results together provide evidence of neural control of renal haemodynamics. The action of vasoactive substances and innervation is discussed briefly in relation to their importance for normal renal function.