Glomerular surface area is normalized in mice born with a nephron deficit: no role for AT1receptors

We examined whether deficits in glomerular capillary surface area associated with a congenital nephron deficit could be corrected by glomerular hypertrophy. Using unbiased stereological techniques, we examined the time course and mode of glomerular hypertrophy in mice lacking one allele for glial cell line-derived neurotrophic factor (GDNF). These GDNF heterozygous (Het) mice are born with ∼30% less nephrons than wild-type (WT) littermates. An additional group of GDNF Het mice received the angiotensin type 1 (AT1)-receptor antagonist candesartan (Cand; 10 mg·kg−1·day−1) from 5 wk of age to determine the role of AT1receptors in the compensatory hypertrophy. At 10 wk of age, the total volume of renal corpuscles, glomerular capillary surface area, and length of glomerular capillaries in the kidneys of GDNF Het mice were all markedly (∼45%) less than that of WT mice ( P < 0.001). However, by 30 wk, and persisting at 60 wk of age, GDNF Het and WT mice did not significantly differ in any of these parameters. Furthermore, conscious 24-h mean arterial pressure (MAP) did not differ between GDNF Het and WT mice at any time point. MAP of GDNF Het-Cand mice was 20–30 mmHg less than that of GDNF Het-vehicle mice at all three ages, but Cand treatment did not significantly alter glomerular capillary dimensions. In conclusion, we have demonstrated that the deficit in glomerular capillary surface area associated with a congenital nephron deficit can be corrected for in adulthood by an increase in the total length of glomerular capillaries. This process does not require AT1receptor activation.

Total peripheral resistance responsiveness during the development of secondary renal hypertension in dogs

OBJECTIVE

To determine whether the responses of total peripheral resistance and arterial pressure to vasoconstrictor agents are amplified as renovascular hypertension develops in dogs.

METHODS

After baseline measurements, the effects of renal artery stenosis (1K, 1C hypertension) were studied in groups of untreated and enalapril-treated dogs early (1-3 weeks) and later (4-6 weeks) as the hypertension developed. Both resting and open-loop haemodynamic measurements were made and the effects of acute intravenous infusions of vasopressin (0.25, 0.5 and 1.25 ng/kg per min) and phenylephrine (0.125, 0.25 and 0.50 microg/kg per min on arterial pressure, cardiac output and calculated total peripheral resistance responses were measured.

RESULTS

Renal artery stenosis induced an increase in arterial blood pressure in both groups of dogs, with similar changes in haemodynamics also observed in open-loop conditions. The slopes of arterial pressure and peripheral resistance responses to vasopressin and phenylephrine were not significantly changed in early or late hypertension, in either the untreated or enalapril-treated groups.

CONCLUSIONS

Hypertension from renal artery stenosis in dogs was due to nonautonomic, nonangiotensin II mechanisms. There was no evidence of vascular amplification of the effects of vasoconstrictor agents, indicating that this did not play a role in the development of hypertension.

Optimizing microarray in experimental hypertension

BACKGROUND

Genetic noise between outbred animals can potentially be a major confounder in the use of microarray technology for gene expression profiling. The study of paired organs from the same animal offers an alternative approach (e.g., for studies of the kidney in experimental hypertension). The present study was undertaken to determine the level of genetic noise between outbred adult Sprague-Dawley (SD) rats, and to determine the effects of unilateral nephrectomy on changes in gene expression as a basis for the design of microarray studies in experimental hypertension.

METHODS

Male SD rats (approximately 130 g) were acclimatized before measurement of tail-cuff systolic blood pressure (SBP) for 6 control days and 4 days of saline treatment. Left kidney nephrectomy was performed, and the tissue snap-frozen in liquid nitrogen for subsequent RNA extraction. Two weeks later, SBP was measured over 4 control and 8 saline treatment days, and the remaining right kidney removed and frozen. Total RNA purification, preparation of cRNA, hybridization, and scanning of the Rat U34A Affymetrix arrays were performed, and data analyzed using MAS5 software Affymetrix Suite (v5), Bioconductor, as well as statistical methods motivated by relevant simulations.

RESULTS

Gene expression profiles in the left control kidney were extremely consistent across animals. The expression profiles of pairs of kidneys from the same animal were, however, more similar than those of kidneys from different animals. Nephrectomy had little effect on the gene expression profiles in the time frame examined.

CONCLUSION

Despite the outbred nature of the rats used in this study, they are useful for gene expression profiling comparisons. The use of paired organs from an individual animal ensures even further genetic identity, allowing determination of genes modified by the treatment of interest.

Paradoxical structural effects in the unilaterally denervated spontaneously hypertensive rat kidney

OBJECTIVE

To determine the effects of chronic denervation on renal vascular structure and function in young adult spontaneously hypertensive rats (SHRs).

DESIGN

Unilateral renal denervation (SHRUDx) or sham-operation (SHRS) was performed in SHRs at 6 weeks of age. At 10 weeks, rats were allocated to one of three procedures designed to examine renal vascular structure and function. A further group underwent bilateral renal denervation.

METHODS

In SHRUDx or SHRS groups, either the kidneys were perfusion-fixed for stereological estimates of artery wall and lumen dimensions or for vascular casting to determine arteriole lumen diameters, or the rats were anaesthetized for estimation of glomerular capillary pressure.

RESULTS

Chronic unilateral renal denervation had no significant effect on the development of hypertension between 6 and 10 weeks of age, as previously reported, but resulted in luminal narrowing of the interlobular artery (denervated group 52 +/- 2 mum, sham-operated group 64 +/- 1 mum; P < 0.01 for interaction between strain and treatment), without alterations in interlobular or arcuate artery wall dimensions. There were no significant effects on either afferent or efferent arteriole lumen diameters. Estimated glomerular capillary pressure was significantly lower in the denervated kidneys of SHRUDx (47 +/- 1 mmHg) compared with kidneys of the SHRS (50 +/- 1 mmHg; P < 0.04). Mean arterial pressure was approximately 12 mmHg lower in the bilaterally denervated SHRs than in the sham-operated SHRs.

CONCLUSIONS

Although bilateral denervation attenuated the development of hypertension in SHRs, unilateral denervation did not, indicating that one neurally intact kidney was sufficient to drive the normal development of SHR hypertension, but only with apparent prohypertensive compensatory changes in the denervated kidney.

Effect of endothelin-1 on regional kidney blood flow and renal arteriole calibre in rabbits

1. Medullary blood flow (MBF) is important in the long-term control of arterial pressure. However, it is unclear which vascular elements regulate MBF. 2. Exogenous endothelin (ET)-1 decreases cortical more than medullary blood flow. We hypothesized that ET-1 would therefore constrict afferent (AA) and efferent arterioles (EA) of juxtamedullary glomeruli less than those of cortical glomeruli. 3. Mean arterial pressure, renal blood flow and cortical (CBF) and medullary (MBF) blood flow, via laser-Doppler flowmetry, were measured before and after intrarenal ET-1 (2 ng/kg per min; n = 6) or vehicle (n = 6) in anaesthetized rabbits. Kidneys were perfusion fixed, vascular casts formed, lumen diameters measured via scanning electron microscopy and relative resistance calculated. 4. Mean arterial pressure was not significantly affected by ET-1 infusion. Cortical glomerular arteriole lumen diameters were significantly reduced in the ET-1-infused group (AA approximately 30%, EA approximately 18%; PA < 0.01), compatible with the decrease in CBF (42 +/- 3%; PGT < 0.01). Juxtamedullary arteriole lumen diameters were also significantly reduced in the ET-1-infused group (AA approximately 34%, EA approximately 21%; PA < 0.01); however, MBF did not decrease. 5. In conclusion, our data suggest that juxtamedullary arterioles are not of primary importance in the regulation of MBF because, despite reductions in juxtamedullary arteriole diameters in response to ET-1, MBF was not decreased.

Predominant postglomerular vascular resistance response to reflex renal sympathetic nerve activation during ANG II clamp in rabbits

We have shown previously that a moderate reflex increase in renal sympathetic nerve activity (RSNA) elevated glomerular capillary pressure, whereas a more severe increase in RSNA decreased glomerular capillary pressure. This suggested that the nerves innervating the glomerular afferent and efferent arterioles could be selectively activated, allowing differential control of glomerular capillary pressure. A caveat to this conclusion was that intrarenal actions of neurally stimulated ANG II might have contributed to the increase in postglomerular resistance. This has now been investigated. Anesthetized rabbits were prepared for renal micropuncture and RSNA recording. One group (ANG II clamp) received an infusion of an angiotensin-converting enzyme inhibitor (enalaprilat, 2 mg/kg bolus plus 2 mg·kg−1·h−1) plus ANG II (∼20 ng·kg−1·min−1), the other vehicle. Measurements were made before (room air) and during 14% O2. Renal blood flow decreased less during ANG II clamp compared with vehicle [9 ± 1% vs. 20 ± 4%, interaction term (PGT) < 0.05], despite a similar increase in RSNA in response to 14% O2in the two groups. Arterial pressure and glomerular filtration rate were unaffected by 14% O2in both groups. Glomerular capillary pressure increased from 33 ± 1 to 37 ± 1 mmHg during ANG II clamp and from 33 ± 2 to 35 ± 1 mmHg in the vehicle group before and during 14% O2, respectively (PGT< 0.05). During ANG II clamp, postglomerular vascular resistance was still increased in response to RSNA during 14% O2, demonstrating that the action of the renal nerves on the postglomerular vasculature was independent of the renin-angiotensin system. This further supports our hypothesis that increases in RSNA can selectively control pre- and postglomerular vascular resistance and therefore glomerular ultrafiltration.

DIFFERENTIAL NEURAL CONTROL OF GLOMERULAR ULTRAFILTRATION

The renal nerves constrict the renal vasculature, causing decreases in renal blood flow (RBF) and glomerular filtration rate (GFR). Whether renal haemodynamics are influenced by changes in renal nerve activity within the physiological range is a matter of debate. We have identified two morphologically distinct populations of nerves within the kidney, which are differentially distributed to the renal afferent and efferent arterioles. Type I nerves almost exclusively innervate the afferent arteriole whereas type II nerves are distributed equally on the afferent and efferent arterioles. We have also demonstrated that type II nerves are immunoreactive for neuropeptide Y, whereas type I nerves are not. This led us to hypothesize that, in the kidney, distinct populations of nerves innervate specific effector tissues and that these nerves may be selectively activated, setting the basis for the differential neural control of GFR. In physiological studies, we demonstrated that differential changes in glomerular capillary pressure occurred in response to graded reflex activation of the renal nerves, compatible with our hypothesis. Thus, sympathetic outflow may be capable of selectively increasing or decreasing glomerular capillary pressure and, hence, GFR by differentially activating separate populations of renal nerves. This has important implications for our understanding of the neural control of body fluid balance in health and disease.

Preglomerular and postglomerular resistance responses to different levels of sympathetic activation by hypoxia

This study investigated the effects of graded reflex increases in renal sympathetic nerve activity (RSNA) on renal preglomerular and postglomerular vascular resistances. With the use of hypoxia to reflexly elicit increases in RSNA without affecting mean arterial pressure, renal function and stop-flow pressures were measured in three groups of rabbits before and after exposure to room air and moderate (14% O2) or severe (10% O2) hypoxia. Moderate and severe hypoxia increased RSNA, primarily by increasing the amplitude of the sympathetic bursts rather than their frequency. RSNA amplitude increased by 20 +/- 6% (P < 0.05) and 60 +/- 16% (P < 0.05), respectively. Moderate hypoxia decreased estimated renal blood flow (ERBF; 26 +/- 7%; P = 0.07), whereas estimated glomerular capillary pressure (32 +/- 1 versus 34 +/- 1 mmHg; P < 0.05) and filtration fraction (FF; P < 0.01) increased. In response to moderate hypoxia, calculated preglomerular (approximately 20%) and postglomerular (approximately 70%) resistance both increased, but only the increase in postglomerular resistance was significant (P < 0.05). In contrast, severe hypoxia decreased ERBF (56 +/- 8%; P < 0.01), GFR (55 +/- 9%; P < 0.001), and glomerular capillary pressure (32 +/- 1 versus 29 +/- 1 mmHg; P < 0.001), with no change in FF, reflecting similar preglomerular (approximately 240%; P < 0.05) and postglomerular ( approximately 250%; P < 0.05) contributions to the vasoconstriction and a decrease in calculated K(f) (P < 0.05). These results provide evidence that reflexly induced increases in RSNA amplitude may differentially control preglomerular and postglomerular vascular resistances.

Chronic angiotensin converting enzyme inhibition enhances renal vascular responsiveness to acetylcholine in anaesthetized rabbits

OBJECTIVE

To determine whether 6 weeks continuous treatment with an angiotensin converting enzyme (ACE) inhibitor reduced renal vascular responsiveness in vivo, since this treatment results in extensive phenotypic conversion of afferent arteriolar cells from contractile to endocrine-like, renin secretory cells.

METHODS

Enalapril (10 microg/kg per h s.c.) was delivered continuously for 6 weeks. In anaesthetized rabbits (treated or sham), arterial blood pressure and renal blood flow were measured and renal responsiveness tested by constructing dose-response curves to bolus doses of phenylephrine, angiotensin II and acetylcholine delivered directly into the renal artery.

RESULTS

ACE inhibition resulted in a significant shift to the left in the renal vascular conductance responses to acetylcholine (P < 0.005) and angiotensin II (P < 0.05), indicating enhanced, not reduced, responsiveness to these agents. There were no significant effects of chronic ACE inhibition on the conductance responses to phenylephrine.

CONCLUSIONS

Contrary to our hypothesis, 6 weeks ACE inhibition did not reduce renal vascular responsiveness to three vasoactive agents, suggesting that the phenotypic changes observed in the afferent arterioles and to a lesser extent the interlobular arteries, were either insignificant or compensated for by other changes in renal circulatory control.

Effects of naloxone on the haemodynamic and renal functional responses to plasma volume expansion in conscious rabbits

We tested whether the opioid antagonist naloxone affects responses to plasma volume expansion (PVE) in conscious rabbits. Under basal conditions, naloxone (6 mg x kg-(1) plus 0.3 mg x kg(-1) x min(-1) i.v.) had no observable effect, except to slightly reduce heart rate. During vehicle treatment, PVE (Haemaccel; 1 ml x kg(-1) min(-1) for 30 min plus 0.2 ml x kg(-1) x min(-1) for 60 min i.v.) reduced haematocrit by 7.1+/-0.8% (from 34.8+/-1.1%), and increased central venous pressure by 3.0+/-0.9 mmHg (from -2.8+/-1.5 mmHg), cardiac output by 42+/-9 ml min(-1) x kg(-1) (from 152+/-17 ml x min(-1) x kg(-1)), systemic vascular conductance by 0.49+/-0.11 ml x min(-1) x mmHg(-1) kg(-1) (from 1.58+/-0.23 ml x min(-1) x mmHg(-1) x kg(-1)), urine flow by 0.13+/-0.04 ml x kg(-)x min(-1) (from 0.12+/-0.02 ml kg(-1) x min(-1)) and sodium excretion by 21+/-5 micromol x kg(-1) min(-1) (from 5+/-2 micromol x kg(-1) x min(-1)). During naloxone treatment, the PVE-induced changes in haematocrit and central venous pressure were similar to those during vehicle treatment, but the increases in cardiac output (24+/-7 ml kg(-1) min(-1)), systemic vascular conductance (0.25+/-0.05 ml min(-1) x kg(-1) x mmHg(-1)), urine flow (0.09+/-0.03 ml x kg(-1) min(-1)) and sodium excretion (11+/-4 micromol x kg(-1) x min(-1)) were 31-49% less. These observations indicate that endogenous opioids mediate some of the circulatory and renal excretory responses to PVE in conscious rabbits.

Chronic renal blood flow measurement in dogs by transit-time ultrasound flowmetry

To test the validity of transit-time ultrasound flowmetry for chronic measurement of renal blood flow in dogs, we compared this method with the renal clearance of para-aminohippuric acid (CPAH) (corrected for hematocrit), and with direct volumetric measurements. When flow-probes were implanted without silastic sheeting to stabilize the implant, there was significant disparity between the (within-dog) mean levels of renal blood flow estimated by flow-probe and CPAH. In contrast, when the flow-probe implants were stabilized with silicone sheeting, there was close agreement in each dog between the flow rates measured by the two methods. When flow-probes were calibrated volumetrically in situ, there was a close linear relationship between flow derived from the flow-probe and that measured volumetrically (r = 0.98 +/- 0.02). We conclude that valid, chronic measurement of renal blood flow in dogs can be achieved using transit-time ultrasound flowmetry, provided the implant is stabilized with silicone sheeting.

Renal effects of rilmenidine in volume-loaded anaesthetized dogs

1. In anaesthetized, fluid expanded rats rilmenidine has diuretic and natriuretic effects. There is strong evidence that the natriuresis is mediated by putative imidazoline receptors. In contrast, in conscious euvolaemic dogs rilmenidine has a diuretic effect that is entirely attributable to activation of alpha 2-adrenoceptors, but no natriuretic effect. To determine whether the effects of rilmenidine are truly species dependent, or merely dependent upon the influences of anaesthesia and volume status, we tested the effects of rilmenidine in pentobarbitone anaesthetized, volume-loaded dogs. 2. The effects of rilmenidine in anaesthetized, volume-loaded dogs were similar to those found in conscious euvolaemic dogs. Compared with vehicle treatment, levels of glomerular filtration rate, urine flow and haematocrit were increased following rilmenidine treatment. No effect of rilmenidine on sodium excretion was observed. 3. We conclude that the renal responses to rilmenidine in dogs are largely unaffected by anaesthesia and plasma volume status. In particular, the natriuretic effect seen in rats was not observed. We conclude that putative imidazoline receptors do not have a major influence on sodium excretion in dogs.

Functional response to graded increases in renal nerve activity during hypoxia in conscious rabbits

Changes in renal sympathetic nerve activity (SNA) are postulated to influence renal function in selective ways, such that different levels of activation produce particular renal responses, initially in renin release, then sodium excretion, with changes in renal hemodynamics occurring only with much greater stimulus intensities. The aim of this study was to determine the renal hemodynamic and excretory responses to graded physiological increases in renal SNA induced by breathing different hypoxic gas mixtures. Experiments were performed in seven conscious rabbits subjected to four gas mixtures (14% O2, 10% O2, 10% O2 + 3% CO2, and 10% O2 + 5% CO2) and instrumented for recording of renal nerve activity. After a 30-min control period, rabbits were subjected to one of the four gas mixtures for 30 min, and then room air was resumed for a further 30 min. The four gas mixtures increased renal SNA by 14, 38, 49, and 165% respectively, but arterial pressure (thus renal perfusion pressure) was not altered by any of the gas mixtures. The greatest level of sympathetic activation produced significant falls in glomerular filtration rate (GFR), renal blood flow, sodium and fluid excretion, and significant increases in plasma renin activity. These returned to levels not significantly different from control conditions in the 30-min period after the gas mixture. When the changes to the various gas mixtures were analyzed within each rabbit, a significant linear relationship was found with all variables to the increase in SNA. Renal denervation in a separate group of seven rabbits completely abolished all of the above responses to the different gas mixtures. Thus graded activation of renal nerves induced by changes in inspired gas mixtures resulted in graded decreases in renal blood flow, GFR, and sodium excretion and graded increases in renin activity, with the changes occurring across a similar range of nerve activities; there was no evidence for a selective change in any renal variable.