Innervation of the renal proximal convoluted tubule of the rat

Possible organ-protective effects of renal denervation: insights from basic studies

Inappropriate sympathetic nervous activation is the body's response to biological stress and is thought to be involved in the development of various lifestyle-related diseases through an elevation in blood pressure. Experimental studies have shown that surgical renal denervation decreases blood pressure in hypertensive animals. Recently, minimally invasive catheter-based renal denervation has been clinically developed, which results in a reduction in blood pressure in patients with resistant hypertension. Accumulating evidence in basic studies has shown that renal denervation exerts beneficial effects on cardiovascular disease and chronic kidney disease. Interestingly, recent studies have also indicated that renal denervation improves glucose tolerance and inflammatory changes. In this review article, we summarize the evidence from animal studies to provide comprehensive insight into the organ-protective effects of renal denervation beyond changes in blood pressure.

Neuroimmune system-mediated renal protection mechanisms

The autonomic nervous system plays an important role in maintaining homeostasis in organisms. Recent studies have shown that it also controls inflammation by directly altering the function of the immune system. The cholinergic anti-inflammatory pathway (CAP) is one of the neural circuits operating through the vagus nerve. Acetylcholine released from the terminal of the vagus nerve, which is a parasympathetic nerve, acts on the α7 nicotinic acetylcholine receptor of macrophages and reduces inflammation in the body. Previous animal studies demonstrated that vagus nerve stimulation reduced renal ischemia-reperfusion injury. Furthermore, restraint stress and pulsed ultrasound had similar protective effects against kidney injury, which were mainly thought to be mediated by the CAP. Using optogenetics, which can stimulate specific nerves, it was also revealed that activation of the CAP by restraint stress was mediated by C1 neurons in the medulla oblongata. Nevertheless, there still remain many unclear points regarding the role of the nervous and immune systems in controlling renal diseases, and further research is needed.

Role of renal sensory nerves in physiological and pathophysiological conditions

Whether activation of afferent renal nerves contributes to the regulation of arterial pressure and sodium balance has been long overlooked. In normotensive rats, activating renal mechanosensory nerves decrease efferent renal sympathetic nerve activity (ERSNA) and increase urinary sodium excretion, an inhibitory renorenal reflex. There is an interaction between efferent and afferent renal nerves, whereby increases in ERSNA increase afferent renal nerve activity (ARNA), leading to decreases in ERSNA by activation of the renorenal reflexes to maintain low ERSNA to minimize sodium retention. High-sodium diet enhances the responsiveness of the renal sensory nerves, while low dietary sodium reduces the responsiveness of the renal sensory nerves, thus producing physiologically appropriate responses to maintain sodium balance. Increased renal ANG II reduces the responsiveness of the renal sensory nerves in physiological and pathophysiological conditions, including hypertension, congestive heart failure, and ischemia-induced acute renal failure. Impairment of inhibitory renorenal reflexes in these pathological states would contribute to the hypertension and sodium retention. When the inhibitory renorenal reflexes are suppressed, excitatory reflexes may prevail. Renal denervation reduces arterial pressure in experimental hypertension and in treatment-resistant hypertensive patients. The fall in arterial pressure is associated with a fall in muscle sympathetic nerve activity, suggesting that increased ARNA contributes to increased arterial pressure in these patients. Although removal of both renal sympathetic and afferent renal sensory nerves most likely contributes to the arterial pressure reduction initially, additional mechanisms may be involved in long-term arterial pressure reduction since sympathetic and sensory nerves reinnervate renal tissue in a similar time-dependent fashion following renal denervation.

Effects of renal denervation on the NKCC2 cotransporter in the thick ascending limb of the loop of Henle in rats with congestive heart failure

AIM

Congestive heart failure (CHF) is associated with increased renal sympathetic nerve activity and renal sodium retention. Rats with CHF display increased expression of the Na-K-2Cl cotransporter (NKCC2) in the renal medullary thick ascending limb of the loop of Henle (mTAL), and arginine vasopressin (AVP)-stimulated cAMP formation in mTAL segments is increased in rats with CHF. The aim of the study was to investigate the role of RSNA on cAMP formation and NKCC2 expression in mTAL in rats with CHF.

METHODS

Congestive heart failure was induced in male Wistar rats by ligation of the left anterior descending coronary artery. Bilateral surgical renal denervation (DNX) was performed 3 weeks later. Two weeks after DNX, mTAL segments were isolated and stimulated with AVP.

RESULTS

Congestive heart failure rats displayed increased mTAL NKCC2 expression (2.5 ± 0.5 vs. 1 ± 0.2 in Sham rats), which was abolished by DNX. Bilateral denervation decreased basal cAMP levels in unstimulated tubules from CHF rats (CHF: 12.56 ± 7.73 fmol μg(-1) protein vs. DNX-CHF: 7.94 ± 4.33; P < 0.05), as well as from Sham rats (SHAM: 4.70 ± 1.38 vs. DNX-SHAM: 2.36 ± 1.52; P < 0.05). mTAL segments from DNX-CHF and DNX-Sham rats showed decreased AVP (10(-6) M)-stimulated cAMP formation, compared with CHF (CHF: 11.92 ± 4.89 fmol μg(-1) protein vs. DNX-CHF: 4.68 ± 2.47; P < 0.05) and Sham (SHAM: 10.78 ± 5.59 vs. DNX-SHAM: 4.89 ± 2.62; P < 0.05).

CONCLUSION

These results indicate that the renal sympathetic nerves have an effect on NKCC2 expression in the mTAL and might have an effect on cAMP formation in the TAL in CHF.

Changes of renal AQP2, ENaC, and NHE3 in experimentally induced heart failure: response to angiotensin II AT1 receptor blockade

Heart failure (HF) was induced by ligation of the left anterior descending artery (LAD). Left ventricular end-diastolic pressure (LVEDP) >25 mmHg (at day 23 after LAD ligation) was the inclusion criterion. The rats were divided into three groups: sham-operated (Sham, n = 23, LVEDP: 5.6 +/- 0.6 mmHg), HF (n = 14, LVEDP: 29.4 +/- 1.4 mmHg), and candesartan (1 mg.kg(-1).day(-1) sc)-treated HF (HF + Can, n = 9, LVEDP: 29.2 +/- 1.2 mmHg). After 7 days (i.e., 29 days after LAD ligation) semiquantitative immunoblotting revealed increased abundance of inner medulla aquaporin-2 (AQP2) and AQP2 phosphorylated at Ser(256) (p-AQP2) in HF. There was also markedly enhanced apical targeting of AQP2 and p-AQP2 in inner medullary collecting duct (IMCD) in HF compared with Sham rats, shown by immunocytochemistry. Candesartan treatment significantly reversed the increases in both AQP2 and p-AQP2 expression and targeting. In contrast, there were only modest changes in other collecting duct segments. Semiquantitative immunoblots revealed increased expression of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) in kidneys from HF compared with Sham rats: both effects were reversed or prevented by candesartan treatment. The protein abundance of alpha-epithelial sodium channel (alpha-ENaC) was increased while beta-ENaC and gamma-ENaC expression was decreased in the cortex and outer stripe of the outer medulla in HF compared with Sham rats, which was partially reversed by candesartan treatment. These findings strongly support an important role of angiotensin II in the pathophysiology of renal water and sodium retention associated with HF.

Losartan decreases vasopressin-mediated cAMP accumulation in the thick ascending limb of the loop of Henle in rats with congestive heart failure

INTRODUCTION

Vasopressin (AVP) stimulates sodium reabsorption and Na,K,2Cl-cotransporter (NKCC2) protein level in the thick ascending limb (TAL) of Henle's loop in rats. Rats with congestive heart failure (CHF) have increased protein level of NKCC2, which can be normalized by angiotensin II receptor type-1 (AT(1)) blockade with losartan.

AIM

In this study, we investigated whether CHF rats displayed changes in AVP stimulated cAMP formation in the TAL and examined the role of AT(1) receptor blockade on this system.

METHOD

CHF was induced by ligation of the left anterior descending coronary artery (LAD). SHAM-operated rats were used as controls. Half of the rats were treated with losartan (10 mg kg day(-1) i.p.).

RESULTS

CHF rats were characterized by increased left ventricular end diastolic pressure. Measurement of cAMP in isolated outer medullary TAL showed that both basal and AVP (10(-6) m) stimulated cAMP levels were significantly increased in CHF rats (25.52 +/- 4.49 pmol cAMP microg(-1) protein, P < 0.05) compared to Sham rats (8.13 +/- 1.14 pmol cAMP microg(-1) protein), P < 0.05). Losartan significantly reduced the basal level of cAMP in CHF rats (CHF: 12.56 +/- 1.93 fmol microg(-1) protein vs. Los-CHF: 7.49 +/- 1.08, P < 0.05), but not in Sham rats (SHAM: 4.66 +/- 0.59 vs. Los-SHAM: 4.75 +/- 0.71). AVP-mediated cAMP accumulation was absent in both groups treated with losartan (Los-SHAM: 4.75 +/- 0.71 and Los-CHF: 7.49 +/- 1.08).

CONCLUSION

The results indicate that the increased NKCC2 protein level in the mTAL from CHF rats is associated with increased cAMP accumulation in this segment. Furthermore, the finding that AT(1) receptor blockade prevents AVP-mediated cAMP accumulation in both SHAM and CHF rats suggests an interaction between angiotensin II and AVP in regulation of mTAL Na reabsorption.

Function and regulation of epithelial sodium transporters in the kidney of a salt-sensitive hypertensive rat model

OBJECTIVE

To determine the function and regulation of thiazide-sensitive NaCl co-transporters (NCC), NaK2Cl co-transporters (NKCC2), and epithelial sodium channels (ENaC) in the kidneys of a salt-sensitive hypertensive model.

DESIGN AND METHODS

Neonatal Wistar rats were treated with capsaicin or vehicle. Seven-week-old male rats were treated for 2 weeks with: vehicle plus a normal (Con-NS) or high (Con-HS) sodium diet, and capsaicin pretreatment plus a normal (Cap-NS) or high (Cap-HS) sodium diet. Mean arterial pressure (MAP), renal excretory function, and protein expression determined by western blot were performed.

RESULTS

MAP was increased in Cap-HS compared with other groups. Trichlormethiazide increased urine sodium excretion (UNaV) and urine flow rate (UFR) and decreased MAP in Cap-HS rats only. Furosemide increased UNaV and UFR in Cap-NS, Con-HS and Cap-HS, and decreased MAP in Cap-HS rats only. Amiloride had no effect on UNaV, UFR and MAP in any group. Renal NCC contents were increased in Cap-HS compared with Con-NS, Con-HS and Cap-NS rats, and NKCC2 expression was increased in Cap-NS, Con-HS and Cap-HS compared with Con-NS rats. No change was found in ENaC alpha subunit expression. The capsaicin-induced release of calcitonin gene-related peptide from renal tissues was decreased in Cap-HS and Cap-NS compared with Con-HS and Con-NS rats.

CONCLUSION

NCC and possibly NKCC2, but not ENaC, were functionally upregulated in the kidneys of rats subjected to sensory nerve degeneration plus high salt intake, suggesting that sensory neurotransmitters may regulate the expression of the former but not the latter, which may underlie the development of salt-sensitive hypertension in this model.

Ultrastructural anatomy of the renal nerves in rats

The innervation within mammalian kidneys (intrinsic innervation) has been extensively described in the literature, particularly for rats. In contrast, there is still a lack of detailed description of the morphology of the extrinsic renal nerves leading to the kidney. The aim of the present study was to describe, in detail, the morphology of the renal nerves in rats. Left renal nerves were evaluated in 6 normal adult Wistar rats. After nerve recordings, in order to ascertain that the nerves studied were the extrinsic renal nerves, rats were killed and the nerves prepared for transmission electron microscopy. Morphometry was carried out with the aid of computer software. The total numbers of myelinated and unmyelinated fibers were 22+/-6 and 1246+/-110, respectively, with a ratio of unmyelinated/myelinated fiber of 109+/-26. The diameters of myelinated fibers showed an unimodal distribution with a peak at 3.0 microm but more than 17% of the fibers showed diameters larger than 5 microm. Unmyelinated fiber distribution was unimodal, with peak between 0.5 and 0.7 microm. The present study adds new information on the morphology of renal nerves in rats and provides morphological basis for further studies involving the structural basis of altered renal responses in conditions such as hypertension, ageing, diabetes and peripheral neuropathies.

Localization of tubular uptake segment of filtered Cystatin C and Aprotinin in the rat kidney

AIM

The renal tubular uptake of 125I-Aprotinin (*Ap) is on average located more superficially than its filtration site, causing transfer of some of *Ap filtered in deep to more superficial cortical zones. 125I-Cystatin C (*Cy) showed less uptake in deep cortical zones than Ap, suggesting a longer and/or a more superficial tubular uptake site. To test that hypothesis and to quantify the outward transfer of the filtered polypeptides, we estimated the tubular uptake pattern of the tracers in perfusion fixed rat kidneys after intravenous injection of *Cy and *Ap.

METHODS

Autoradiographs were made from 10 mum thick slices of Microfil nephron casts from outer (OC), middle (MC) and inner (IC) cortical zones to quantify cortical border-crossing *Ap transfer. Single nephron glomerular filtration rate (snGFR) was estimated as the zonal uptake of *Ap corrected for *Ap transfer, divided by its time-integrated plasma concentration and the zonal number of glomeruli.

RESULTS

*Ap and *Cy uptake fell exponentially along the proximal convoluted tubule (PCT), indicating an uptake proportional to luminal concentration. Uptake in IC exceeded that in MC and OC nephrons. The per cent PCT length with *Cy uptake (67.2 +/- 1.6) exceeded that of *Ap (54.6 +/- 1.8). The zonal border-crossing PCT length (29-34% of total PCT) from deep to more superficial cortical zones transferred 4-6% more *Cy than *Ap.

CONCLUSION

Greater tubular uptake length of *Cy than of *Ap causes more cortical border-crossing of *Cy. The zonal snGFR estimated from Aprotinin uptake corrected for border-crossing agreed well with that obtained with the Hanssen ferrocyanide technique.

Aprotinin uptake in the proximal tubules in the rat kidney I. Length of proximal tubular uptake segment

Aprotinin (Ap), a basic polypeptide with a molecular weight of 6500, is filtered at the glomerular membrane without steric restriction and is completely absorbed by the proximal tubule cells. Here Ap is broken down to amino acids, but no breakdown products enter the peritubular circulation during the first 20 min following an intravenous injection. These properties have recently been exploited for measurement of local glomerular filtration rate, based on the assumption that the proximal tubular uptake site is located at the level of the filtering glomerulus. To evaluate that assumption we have now made serial autoradiographs of the rat kidney 20 min after intravenous injection of 2-750 microg of 125I-Aprotinin. With all doses the percent 125I-containing proximal tubular transections were about 50 in the outer and middle cortex and 35 in the inner third. We interpret these numbers to mean that all filtered Ap is taken up in the first two thirds of the proximal convoluted tubular length and does not reach the pars recta. Since the proximal tubule on average is located more superficial than its glomerulus, measurement of local Ap uptake will tend to overestimate glomerular filtration rate in outer layers of the cortex. Quantitative estimate of this "displacement" will be presented in a companion article.

Effects of renal denervation on tubular sodium handling in rats with CBL-induced liver cirrhosis

This study was designed to examine the effect of bilateral renal denervation (DNX) on thick ascending limb of Henle's loop (TAL) function in rats with liver cirrhosis induced by common bile duct ligation (CBL). The CBL rats had, as previously shown, sodium retention associated with hypertrophy of the inner stripe of the outer medulla (ISOM) and increased natriuretic effect of furosemide in vivo, and semiquantitative immunoblotting showed increased expression of the furosemide-sensitive Na-K-2Cl cotransporter type 2 (NKCC2) in ISOM from CBL rats. DNX significantly attenuated the sodium retention in the CBL rats, which was associated with normalization of the natriuretic effect of furosemide, as well as a significant reduction in the expression of NKCC2 in the ISOM. However, the marked hypertrophy of the ISOM found in CBL rats was not reversed by DNX. Together, these data indicate that increased renal sympathetic nerve activity known to be present in CBL rats plays a significant role in the formation of sodium retention by stimulating sodium reabsorption in the TAL via increased renal abundance of NKCC2.

Qualitative and quantitative morphology of renal nerves in C57BL/6J mice

The detailed morphology of the renal nerves in mice has not been reported previously. The aims of this study were to describe the general morphology of the extrinsic renal nerve in C57BL/6 mice, and determine its morphometric parameters. The major renal nerve innervating the left kidney was isolated in five mice. Thin sections of the nerve segments were then examined by transmission electron microscopy. The renal nerve averaged 35.4 +/- 3.6 (S.E.M.) microm in diameter and 741 +/- 104 microm in area. The renal nerve contained an average of 830 +/- 169 unmyelinated fibers and only 4.6 +/- 1.7 myelinated fibers. The axon diameter of myelinated and unmyelinated fibers averaged 2.2 +/- 0.3 microm and 0.76 +/- 0.02 microm, respectively. The diameter of the unmyelinated fibers ranged from 0.3 to 2.0 microm, and the distribution histogram was unimodal. The majority of fibers (85%) had diameters of 0.6-1.0 microm. These results are similar to those obtained for renal nerves of rats with respect to the predominance of unmyelinated fibers. However, the diameter of unmyelinated fibers is larger in rats and the distribution histogram of rat unmyelinated fibers is bimodal, in contrast to the unimodal distribution in mice. The morphological description of the renal nerves in mice provides baseline data for further investigations of the structural basis of altered autonomic reflexes. The results will be useful in analyses of genes that influence the development and structure of sympathetic and sensory innervation of the kidney in genetically manipulated mice.

Alpha-2 and beta-adrenergic receptors mediate NE's biphasic effects on rat thick ascending limb chloride flux

The sympathetic neurotransmitter norepinephrine (NE) influences renal sodium excretion via activation of adrenergic receptors. The thick ascending limb (THAL) possesses both alpha-2 and beta-adrenergic receptors. However, the role(s) different adrenergic receptors play in how isolated THALs respond to NE are unclear. We tested the hypothesis that both alpha-2 and beta-adrenergic receptors are responsive to NE in the isolated THAL, with alpha-2 receptors inhibiting and beta-receptors stimulating chloride flux (J(Cl)). THALs from male Sprague-Dawley rats were perfused in vitro, and the effects of 1) incremental NE, 2) the alpha-2 agonist clonidine, and 3) the beta-agonist isoproterenol on J(Cl) were measured. Low concentrations (0.1 nM) of NE decreased J(Cl) from a rate of 114.2 +/- 8.1 to 93.5 +/- 14.6 pmol. mm(-1). min(-1) (P < 0.05), with the nadir occurring at 1 nM (67.7 +/- 8.8 pmol. mm(-1). min(-1); P < 0.05). In contrast, greater concentrations of NE significantly increased J(Cl) from the nadir to a maximal rate of 131.0 +/- 28.5 pmol. mm(-1). min(-1) at 10 microM (P < 0.05). To evaluate the adrenergic receptors mediating these responses, the THAL J(Cl) response to NE was measured in the presence of selective antagonists of beta- and alpha-2 receptors. A concentration of NE (1 microM), which alone tended to increase J(Cl), decreased THAL J(Cl) (from 148.9 +/- 16.4 to 76.2 +/- 13.6 pmol. mm(-1). min(-1); P < 0.01) in the presence of the beta-antagonist propranolol. In contrast, a concentration of NE (0.1 microM), which alone tended to decrease J(Cl), increased THAL J(Cl) (from 85.5 +/- 20.1 to 111.8 +/- 20.1 pmol. mm(-1). min(-1); P < 0.05) in the presence of the alpha-2 antagonist rauwolscine. To further clarify the role of different adrenergic receptors, selective adrenergic agonists were used. The alpha-2 agonist clonidine decreased J(Cl) from 102.4 +/- 9.9 to 54.0 +/- 15.7 pmol. mm(-1). min(-1), a reduction of 49.1 +/- 11.0% (P < 0.02). In contrast, the beta-agonist isoproterenol stimulated J(Cl) from 95.3 +/- 11.6 to 144.1 +/- 15.0 pmol. mm(-1). min(-1), an increase of 56 +/- 14% (P < 0.01). We conclude that 1) the sympathetic neurotransmitter NE exerts concentration-dependent effects on J(Cl) in the isolated rat THAL, 2) selective alpha-2 receptor activation inhibits THAL J(Cl), and 3) selective beta-receptor activation stimulates THAL J(Cl). These data indicate the response elicited by the isolated rat THAL to NE is dependent on the neurotransmitter concentration, such that application of NE in vitro biphasically modulates J(Cl) via differential activation of alpha-2 and beta-adrenergic receptors in a concentration-dependent manner.

Immunofluorescent imaging of beta 1- and beta 2-adrenergic receptors in rat kidney

BACKGROUND

beta-Adrenergic receptors (beta-ARs) are known to participate in the regulation of glomerular filtration, NaCl reabsorption, acid-base balance, and renin secretion; however, the precise histologic localization of beta-AR at putative signaling sites involved in these processes remains an open issue.

METHODS

We used a set of subtype-specific rabbit antibodies to visualize beta(1)- and beta(2)-AR in rat kidney by immunohistochemistry and specified cells and segments of the nephron thought to be regulated by catecholamines. In addition, the relative proportion of beta-AR subtypes in cortical and medullary portions of rat kidney was determined by Western blotting and by competing [(125)I]-cyanopindolol binding with the beta(1)- or beta(2)-selective antagonists bisoprolol or ICI 118,551, respectively.

RESULTS

Immunoreactivity for beta(1)-AR was found in mesangial cells, juxtaglomerular granular cells, the macula densa epithelium, proximal and distal tubular segments, and acid-secreting type A intercalated cells of the cortical and medullary collecting ducts. Immunoreactivity for beta(2)-AR was predominantly localized in the apical and subapical compartment of proximal and, to a lesser extent, distal tubular epithelia (suggesting interactions with luminal fluid catecholamines). Both subtypes were dense in the membranes of smooth muscle cells from renal arteries. Concordant data were obtained by radioligand binding and immunoblotting of membranes prepared from cortical and medullary portions of the kidney.

CONCLUSION

Our data provide an immunohistochemical basis for the cellular targets of beta-adrenergic regulation of renal function. Moreover, they could help to devise therapeutic strategies directed at renal beta-ARs.

Defective fluid and HCO(3)(-) absorption in proximal tubule of neuronal nitric oxide synthase-knockout mice

Using renal clearance techniques and in situ microperfusion of proximal tubules, we examined the effects of N(G)-monomethyl-L-arginine methyl ester (L-NAME) on fluid and HCO(3)(-) transport in wild-type mice and also investigated proximal tubule transport in neuronal nitric oxide synthase (nNOS)-knockout mice. In wild-type mice, administration of L-NAME (3 mg/kg bolus iv) significantly increased mean blood pressure, urine volume, and urinary Na(+) excretion. L-NAME, given by intravenous bolus and added to the luminal perfusion solution, decreased absorption of fluid (60%) and HCO(3)(-) (49%) in the proximal tubule. In nNOS-knockout mice, the urinary excretion of HCO(3)(-) was significantly higher than in the wild-type mice (3.12 +/- 0.52 vs. 1. 40 +/- 0.33 mM) and the rates of HCO(3)(-) and fluid absorption were 62 and 72% lower, respectively. Both arterial blood HCO(3)(-) concentration (20.7 vs. 25.7 mM) and blood pH (7.27 vs. 7.34) were lower, indicating a significant metabolic acidosis in nNOS-knockout mice. Blood pressure was lower in nNOS-knockout mice (76.2 +/- 4.6 mmHg) than in wild-type control animals (102.9 +/- 8.4 mmHg); however, it increased in response to L-NAME (125.5 +/- 5.07 mmHg). Plasma Na(+) and K(+) were not significantly different from control values. Our data show that a large component of HCO(3)(-) and fluid absorption in the proximal tubule is controlled by nNOS. Mice without this isozyme are defective in absorption of fluid and HCO(3)(-) in the proximal tubule and develop metabolic acidosis, suggesting that nNOS plays an important role in the regulation of acid-base balance.

Repeatable measurement of local and zonal GFR in the rat kidney with aprotinin

The basic polypeptide aprotinin (Ap), mol. wt 6513, is freely filtered in glomeruli and completely reabsorbed by the proximal tubules. Cellular processing is slow with return to plasma of breakdown products beginning after 20-30 min. When corrected for Gibbs-Donnan distribution of Ap between glomerular filtrate and plasma (i.e. 0.65 at a plasma protein concentration of 50 mg ml-1), the renal clearance of [125I]Ap, estimated as the ratio of kidney uptake and integrated non-protein bound plasma 125I concentration, equals that of [51Cr]EDTA (urine + kidney content). Zonal GFR per gram tissue was obtained from uptake in three to six samples from outer and inner cortex (OC, IC) and the cortico-medullary border zone, 5-30 min after i.v. injection in rats. Control GFR in OC was 2.05 (SD 0.39) ml g-1 min-1 and the IC/OC ratio 0.66 (SD 0.14). Repeated local clearances (CI and CII) were obtained by injecting a second tracer (i.e. [131I]Ap) 15 min after the first injection (i.e. [125I]Ap), which by then had a low plasma concentration. The kidneys were removed at 30 min, frozen and dissected. During control conditions CII/CI averaged 1.06 in OC and IC, and the coefficient of variation (CV) between CII/CI ratios of individual tissue samples was 2% in both zones. Lowering left renal arterial pressure before the second injection reduced GFR proportionally in both zones (34 and 37%) with a CV of intersample CII/CI ratios of 5%. We conclude that the method allows precise and repeatable measurements of local and zonal GFR.

Sympathetic and sensory innervation of the urinary tract in young adult and aged rats: a semi-quantitative histochemical and immunohistochemical study

The sympathetic innervation of the urinary tract of young adult (4 months) and aged (24+ months) rats has been examined by glyoxylic acid-induced fluorescence for the detection of noradrenaline and by immunofluorescence using antisera against tyrosine hydroxylase (TH) and neuropeptide Y (NPY). Immunostaining for calcitonin gene-related peptide (CGRP), known to be present in pelvic sensory nerves, was also performed. Semi-quantitative estimations of nerve densities were made of noradrenergic and peptidergic fibres innervating the smooth musculature of the ureter, bladder and urethra, and of the urinary tract vasculature. In the aged rats the overall patterns of innervation remained unchanged. However, with the exception of the vesical vasculature, the density of noradrenergic innervation decreased as did the intensity of histofluorescence. A similar pattern of results was observed by TH and NPY immunofluorescence. The results present evidence for a diminution in the sympathetic control of the urinary tract in aged rats. The pattern and density of CGRP-immunoreactive nerves was unchanged in the aged animals suggesting that pelvic visceral sensory innervation is more resistant to the effects of advancing age.

A study of catecholamine concentrations in selected renal segments of cats of different ages

Renal concentrations of the catecholamine neurotransmitters norepinephrine (NE) and dopamine (DA) in 19 cats were measured by reverse-phase high pressure liquid chromatography (HPLC) with electrochemical detection. Animals of five different age groups (2-4, 6-8, 10-12 and 16-20 weeks and adult animals one year and older) were anesthetized and the kidneys were excised, sectioned into cortical, outer medullary and inner medullary segments, and processed for HPLC. There were no statistical differences in cortical NE concentrations between the 2-4 week and adult age groups, suggesting that peak noradrenergic nerve terminal density in the cortex is achieved at or before 2-4 weeks of age. NE concentration in the outer medulla and DA concentration in the cortex and outer medulla increased from the 2-4 to the 10-12 week age group but did not change significantly in the older animals. There were significant decreases in inner medullary NE and DA concentrations from the 10-12-week to the 16-20 week age group. The decrease in concentration of inner medullary catecholamines in the early postnatal period is consistent with the reported total loss of tyrosine hydroxylase- and dopamine beta hydroxylase-immunoreactive inner medullary nerve terminals at approximately 12 weeks of age.