Renal hemodynamics in conscious rats: effects of anesthesia, surgery, and recovery

The effects of recruitment of renal functional reserve on renal cortical and medullary oxygenation in non-anesthetized sheep

AIM

Recruitment of renal functional reserve (RFR) with amino acid loading increases renal blood flow and glomerular filtration rate. However, its effects on renal cortical and medullary oxygenation have not been determined. Accordingly, we tested the effects of recruitment of RFR on renal cortical and medullary oxygenation in non-anesthetized sheep.

METHODS

Under general anesthesia, we instrumented 10 sheep to enable subsequent continuous measurements of systemic and renal hemodynamics, renal oxygen delivery and consumption, and cortical and medullary tissue oxygen tension (PO2 ). We then measured the effects of recruitment of RFR with an intravenous infusion of 500 ml of a clinically used amino acid solution (10% Synthamin® 17) in the non-anesthetized state.

RESULTS

Compared with baseline, Synthamin® 17 infusion significantly increased renal oxygen delivery mean ± SD maximum increase: (from 0.79 ± 0.17 to 1.06 ± 0.16 ml/kg/min, p < 0.001), renal oxygen consumption (from 0.08 ± 0.01 to 0.15 ± 0.02 ml/kg/min, p < 0.001), and glomerular filtration rate (+45.2 ± 2.7%, p < 0.001). Renal cortical tissue PO2 increased by a maximum of 26.4 ± 1.1% (p = 0.001) and medullary tissue PO2 increased by a maximum of 23.9 ± 2.8% (p = 0. 001).

CONCLUSIONS

In non-anesthetized healthy sheep, recruitment of RFR improved renal cortical and medullary oxygenation. These observations might have implications for the use of recruitment of RFR for diagnostic and therapeutic purposes.

Bilateral renal denervation prevents the development of hypertension during diet-induced obesity in male rats

NEW FINDINGS

What is the central question of this study? What is the role of the renal nerves in the development of obesity, hyperlipidaemia and hypertension during the long-term feeding of a moderately high-fat diet in male obesity-prone rats? What is the main finding and its importance? The renal nerves play a prominent mediatory role, without influencing the establishment of visceral adiposity and atherogenic hyperlipidaemia, in the induction and progression of pressure natriuresis impairment and hypertension during the developmental period of diet-induced obesity.

ABSTRACT

Feeding a moderately high-fat (MHF) diet in male Sprague-Dawley rats induces obesity, pressure natriuresis impairment and hypertension. This study investigated the role of the renal nerves in the impaired pressure natriuresis and hypertension caused by feeding a MHF diet. After collecting baseline data on day 0, 12 rats remained on a low-fat diet (LF group) while the others were switched onto a MHF diet and diverged into obesity-resistant (OR) or obesity-prone (OP). After 4 weeks, half of the OR and OP rats underwent bilateral renal denervation (BRD) to generate four groups: OR, OR/BRD, OP and OP/BRD (n = 12). During 10 weeks, body weight, obesity index, systolic pressure and renal excretory function were measured regularly. After 10 weeks, renal excretory responses to acute salt loading and renal autoregulation were evaluated. The OP and OP/BRD groups had greater increases of body weight and obesity index during the dietary period compared to the other groups, and by week 10 their body weight (425.1 ± 7.2 and 411.9 ± 5.1 g) became considerably larger than that of the LF group (358.5 ± 6.2 g). Renal sodium excretion was reduced by ∼20% at week 4 in the OP and OP/BRD groups, while only the OP group had lower sodium excretion at weeks 6-8 and higher systolic pressure over weeks 5-10 than the other groups and its week 10 systolic pressure reached 138.1 ± 6.7 versus 123.6 ± 2.7 mmHg of the LF group. The OP group showed delayed renal excretory responses to salt loading with rightward and downward shifts in renal autoregulatory curves. Therefore, the renal nerves exert a main mediatory role in the development of pressure natriuresis impairment and hypertension as obesity is established due to the long-term consumption of the MHF diet in male OP rats.

Robust arterial spin labeling MRI measurement of pharmacologically induced perfusion change in rat kidneys

Kidney diseases such as acute kidney injury, diabetic nephropathy and chronic kidney disease (CKD) are related to dysfunctions of the microvasculature in the kidney causing a decrease in renal blood perfusion (RBP). Pharmacological intervention to improve the function of the microvasculature is a viable strategy for the potential treatment of these diseases. The measurement of RBP is a reliable biomarker to evaluate the efficacy of pharmacological agents' actions on the microvasculature, and measurement of RBP responses to different pharmacological agents can also help elucidate the mechanism of hemodynamic regulation in the kidney. Magnetic resonance imaging (MRI) with flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) has been used to measure RBP in humans and animals. However, artifacts caused by respiratory and peristaltic motions limit the potential of FAIR ASL in drug discovery and kidney research. In this study, the combined anesthesia protocol of inactin with a low dose of isoflurane was used to fully suppress peristalsis in rats, which were ventilated with an MRI-synchronized ventilator. FAIR ASL data were acquired in eight axial slices using a single-shot, gradient-echo, echo-planar imaging (EPI) sequence. The artifacts in the FAIR ASL RBP measurement due to respiratory and peristaltic motions were substantially eliminated. The RBP responses to fenoldopam and L-NAME were measured, and the increase and decrease in RBP caused by fenoldopam and L-NAME, respectively, were robustly observed. To further validate FAIR ASL, the renal blood flow (RBF) responses to the same agents were measured by an invasive perivascular flow probe method. The pharmacological agent-induced responses in RBP and RBF are similar. This indicates that FAIR ASL has the sensitivity to measure pharmacologically induced changes in RBP. FAIR ASL with multislice EPI can be a valuable tool for supporting drug discovery, and for elucidating the mechanism of hemodynamic regulation in kidneys.

Impact of anesthesia, sex, and circadian cycle on renal afferent nerve sensitivity

Elevated renal afferent nerve (ARNA) activity or dysfunctional reno-renal reflexes via altered ARNA sensitivity contribute to hypertension and chronic kidney disease. These nerves contain mechano- and chemosensitive fibers that respond to ischemia, changes in intrarenal pressures, and chemokines. Most studies have utilized various anesthetized preparations and exclusively male animals to characterize ARNA responses. Therefore, this study assessed the impact of anesthesia, sex, and circadian period on ARNA responses and sensitivity. Multifiber ARNA recordings were performed in male and female Sprague-Dawley rats (250-400 g) and compared across decerebrate versus Inactin, isoflurane, and urethane anesthesia groups. Intrarenal artery infusion of capsaicin (0.1-50.0 μM, 0.05 mL) produced concentration-dependent increases in ARNA; however, the ARNA sensitivity was significantly greater in decerebrate versus Inactin, isoflurane, and urethane groups. Increases in renal pelvic pressure (0-30 mmHg, 30 s) produced pressure-dependent increases in ARNA; however, ARNA sensitivity was again greater in decerebrate and Inactin groups versus isoflurane and urethane. Acute renal artery occlusion (30 s) increased ARNA, but responses did not differ across groups. Analysis of ARNA responses to increased pelvic pressure between male and female rats revealed significant sex differences only in isoflurane and urethane groups. ARNA responses to intrarenal capsaicin infusion were significantly blunted at nighttime versus daytime; however, ARNA responses to increased pelvic pressure or renal artery occlusion were not different between daytime and nighttime. These results demonstrate that ARNA sensitivity is greatest in decerebrate and Inactin-anesthetized groups but was not consistently influenced by sex.NEW & NOTEWORTHY We determined the impact of anesthesia, sex, and circadian cycle on renal afferent nerve (ARNA) sensitivity to chemical and mechanical stimuli. ARNA sensitivity to renal capsaicin infusion was greatest in decerebrate > Inactin > urethane or isoflurane groups. Elevated renal pelvic pressure significantly increased ARNA; decerebrate and Inactin groups exhibited the greatest ARNA sensitivity. Sex differences in renal afferent responses were not consistently observed. Circadian cycle altered chemosensory but not mechanosensory responses.

Pathophysiology of unilateral ischemia-reperfusion injury: importance of renal counterbalance and implications for the AKI-CKD transition

Unilateral ischemia-reperfusion (UIR) injury leads to progressive renal atrophy and tubulointerstitial fibrosis (TIF) and is commonly used to investigate the pathogenesis of the acute kidney injury-chronic kidney disease transition. Although it is well known that contralateral nephrectomy (CNX), even 2 wk post-UIR injury, can improve recovery, the physiological mechanisms and tubular signaling pathways mediating such improved recovery remain poorly defined. Here, we examined the renal hemodynamic and tubular signaling pathways associated with UIR injury and its reversal by CNX. Male Sprague-Dawley rats underwent left UIR or sham UIR and 2 wk later CNX or sham CNX. Blood pressure, left renal blood flow (RBF), and total glomerular filtration rate were assessed in conscious rats for 3 days before and over 2 wk after CNX or sham CNX. In the presence of a contralateral uninjured kidney, left RBF was lower (P < 0.05) from 2 to 4 wk following UIR (3.6 ± 0.3 mL/min) versus sham UIR (9.6 ± 0.3 mL/min). Without CNX, extensive renal atrophy, TIF, and tubule dedifferentiation, but minimal pimonidazole and hypoxia-inducible factor-1α positivity in tubules, were present at 4 wk post-UIR injury. Conversely, CNX led (P < 0.05) to sustained increases in left RBF (6.2 ± 0.6 mL/min) that preceded the increases in glomerular filtration rate. The CNX-induced improvement in renal function was associated with renal hypertrophy, more redifferentiated tubules, less TIF, and robust pimonidazole and hypoxia-inducible factor-1α staining in UIR injured kidneys. Thus, contrary to expectations, indexes of hypoxia are not observed with the extensive TIF at 4 wk post-UIR injury in the absence of CNX but are rather associated with the improved recovery of renal function and structure following CNX.

Renal oxygenation during the early stages of adenine-induced chronic kidney disease

To assess whether renal hypoxia is an early event in adenine-induced chronic kidney disease, adenine (100 mg) or its vehicle was administered to male Sprague-Dawley rats by daily oral gavage for 7 days. Kidney oxygenation was assessed by 1) blood oximetry and Clark electrode in thiobutabarbital-anesthetized rats, 2) radiotelemetry in unanesthetized rats, and 3) expression of hypoxia-inducible factor (HIF)-1α and HIF-2α protein. After 7 days of treatment, under anesthesia, renal O2 delivery was 51% less, whereas renal O2 consumption was 65% less, in adenine-treated rats than in vehicle-treated rats. Tissue Po2 measured by Clark electrode was similar in the renal cortex but 44% less in the medulla of adenine-treated rats than in that of vehicle-treated rats. In contrast, in unanesthetized rats, both cortical and medullary tissue Po2 measured by radiotelemetry remained stable across 7 days of adenine treatment. Notably, anesthesia and laparotomy led to greater reductions in medullary tissue Po2 measured by radiotelemetry in rats treated with adenine (37%) than in vehicle-treated rats (16%), possibly explaining differences between our observations with Clark electrodes and radiotelemetry. Renal expression of HIF-1α was less after 7 days of adenine treatment than after vehicle treatment, whereas expression of HIF-2α did not differ significantly between the two groups. Renal dysfunction was evident after 7 days of adenine treatment, with glomerular filtration rate 65% less and serum creatinine concentration 183% greater in adenine-treated rats than in vehicle-treated rats. Renal cortical tissue hypoxia may not precede renal dysfunction in adenine-induced chronic kidney disease and so may not be an early pathological feature in this model.

The effect of inactin on kidney mitochondrial function and production of reactive oxygen species

Inactin is a long lasting anesthetic agent commonly used in rat studies, but is also shown to exert physiological effects such as reducing renal blood flow, glomerular filtration rate and depressing tubular transport capacity. The effect of inactin on isolated kidney mitochondria is unknown and may be important when studying related topics in anaesthetized animals. The aim of this study was to determine whether inactin exerts effects on mitochondrial function and production of reactive oxygen species. Kidney mitochondrial function and production of reactive oxygen after acutely (5 min) or longer (1.5 hour) anesthetizing rats with inactin was evaluated using high-resolution respirometry. The results demonstrate that inactin significantly improves respiratory control ratio, inhibits complex I in the mitochondrial respiratory chain, reduce both unregulated proton leak and time dependently reduce the regulated proton leak via uncoupling protein-2 and adenine nucleotide translocase. Inactin also contributes to increased mitochondrial hydrogen peroxide production. In conclusion, inactin exerts persistent effects on mitochondrial function and these profound effects on mitochondrial function should to be considered when studying mitochondria isolated from animals anesthesized with inactin.

Angiotensin II-induced hypertension in rats is only transiently accompanied by lower renal oxygenation

Activation of the renin-angiotensin system may initiate chronic kidney disease. We hypothesised that renal hypoxia is a consequence of hemodynamic changes induced by angiotensin II and occurs prior to development of severe renal damage. Male Sprague-Dawley rats were infused continuously with angiotensin II (350 ng/kg/min) for 8 days. Mean arterial pressure (n = 5), cortical (n = 6) and medullary (n = 7) oxygenation (pO₂) were continuously recorded by telemetry and renal tissue injury was scored. Angiotensin II increased arterial pressure gradually to 150 ± 18 mmHg. This was associated with transient reduction of oxygen levels in renal cortex (by 18 ± 2%) and medulla (by 17 ± 6%) at 10 ± 2 and 6 ± 1 hours, respectively after starting infusion. Thereafter oxygen levels normalised to pre-infusion levels and were maintained during the remainder of the infusion period. In rats receiving angiotensin II, adding losartan to drinking water (300 mg/L) only induced transient increase in renal oxygenation, despite normalisation of arterial pressure. In rats, renal hypoxia is only a transient phenomenon during initiation of angiotensin II-induced hypertension.

Mechanisms of sodium balance: total body sodium, surrogate variables, and renal sodium excretion

The classical concepts of human sodium balance include 1) a total pool of Na+ of ≈4,200 mmol (total body sodium, TBS) distributed primarily in the extracellular fluid (ECV) and bone, 2) intake variations of 0.03 to ≈6 mmol·kg body mass−1·day−1, 3) asymptotic transitions between steady states with a halftime (T½) of 21 h, 4) changes in TBS driven by sodium intake measuring ≈1.3 day [ΔTBS/Δ(Na+ intake/day)], 5) adjustment of Na+ excretion to match any diet thus providing metabolic steady state, and 6) regulation of TBS via controlled excretion (90–95% renal) mediated by surrogate variables. The present focus areas include 1) uneven, nonosmotic distribution of increments in TBS primarily in “skin,” 2) long-term instability of TBS during constant Na+ intake, and 3) physiological regulation of renal Na+ excretion primarily by neurohumoral mechanisms dependent on ECV rather than arterial pressure. Under physiological conditions 1) the nonosmotic distribution of Na+ seems conceptually important, but quantitatively ill defined; 2) long-term variations in TBS represent significant deviations from steady state, but the importance is undetermined; and 3) the neurohumoral mechanisms of sodium homeostasis competing with pressure natriuresis are essential for systematic analysis of short-term and long-term regulation of TBS. Sodium homeostasis and blood pressure regulation are intimately related. Real progress is slow and will accelerate only through recognition of the present level of ignorance. Nonosmotic distribution of sodium, pressure natriuresis, and volume-mediated regulation of renal sodium excretion are essential intertwined concepts in need of clear definitions, conscious models, and future attention.

An anatomically unbiased approach for analysis of renal BOLD magnetic resonance images

Oxygenation defects may contribute to renal disease progression, but the chronology of events is difficult to define in vivo without recourse to invasive methodologies. Blood oxygen level-dependent magnetic resonance imaging (BOLD MRI) provides an attractive alternative, but the R2* signal is physiologically complex. Postacquisition data analysis often relies on manual selection of region(s) of interest. This approach excludes from analysis significant quantities of biological information and is subject to selection bias. We present a semiautomated, anatomically unbiased approach to compartmentalize voxels into two quantitatively related clusters. In control F344 rats, low R2* clustering was located predominantly within the cortex and higher R2* clustering within the medulla (70.96 ± 1.48 vs. 79.00 ± 1.50; 3 scans per rat; n = 6; P < 0.01) consistent anatomically with a cortico-medullary oxygen gradient. An intravenous bolus of acetylcholine caused a transient reduction of the R2* signal in both clustered segments ( P < 0.01). This was nitric oxide dependent and temporally distinct from the hemodynamic effects of acetylcholine. Rats were then chronically infused with angiotensin II (60 ng/min) and rescanned 3 days later. Clustering demonstrated a disruption of the cortico-medullary gradient, producing less distinctly segmented mean R2* clusters (71.30 ± 2.00 vs. 72.48 ± 1.27; n = 6; NS). The acetylcholine-induced attenuation of the R2* signal was abolished by chronic angiotensin II infusion, consistent with reduced nitric oxide bioavailability. This global map of oxygenation, defined by clustering individual voxels on the basis of quantitative nearness, might be more robust in defining deficits in renal oxygenation than the absolute magnitude of R2* in small, manually selected regions of interest defined exclusively by anatomical nearness.

The impact of four different classes of anesthetics on the mechanisms of blood pressure regulation in normotensive and spontaneously hypertensive rats

Most anesthetics induce characteristic hemodynamic changes leading to blood pressure (BP) reduction but the role of renin-angiotensin system (RAS), sympathetic nervous system (SNS) and nitric oxide (NO) synthesis in this BP reduction is unknown. We therefore studied the influence of four widely used anesthetics - pentobarbital (P), isoflurane (ISO), ketamine-xylazine (KX) and chloralose-urethane (CU) - on the participation of these vasoactive systems in BP maintenance. BP effects elicited by the acute sequential blockade of RAS (captopril), SNS (pentolinium) and NO synthase (L-NAME) were compared in conscious and anesthetized Wistar or spontaneously hypertensive rats (SHR). Except for pentobarbital all studied anesthetics evidenced by diminished BP responses to pentolinium. The absolute pentolinium-induced BP changes were always greater in SHR than Wistar rats. KX anesthesia eliminated BP response to pentolinium and considerably enhanced BP response to NO synthase inhibition in SHR. In both rat strains the anesthesia with ISO or CU augmented BP response to captopril, decreased BP response to pentolinium and attenuated BP response to NO synthase inhibition. In conclusion, pentobarbital anesthesia had a modest influence on BP level and its maintenance by the above vasoactive systems. Isoflurane and chloralose-urethane anesthesia may be used in cardiovascular experiments if substantial BP decrease due to altered contribution of RAS, SNS and NO to BP regulation does not interfere with the respective research aim. Major BP reduction (namely in SHR) due to a complete SNS absence is a major drawback of ketamine-xylazine anesthesia.

Blood pressure-renal blood flow relationships in conscious angiotensin II- and phenylephrine-infused rats

Chronic ANG II infusion in rodents is widely used as an experimental model of hypertension, yet very limited data are available describing the resulting blood pressure-renal blood flow (BP-RBF) relationships in conscious rats. Accordingly, male Sprague-Dawley rats (n = 19) were instrumented for chronic measurements of BP (radiotelemetry) and RBF (Transonic Systems, Ithaca, NY). One week later, two or three separate 2-h recordings of BP and RBF were obtained in conscious rats at 24-h intervals, in addition to separate 24-h BP recordings. Rats were then administered either ANG II (n = 11, 125 ng·kg(-1)·min(-1)) or phenylephrine (PE; n = 8, 50 mg·kg(-1)·day(-1)) as a control, ANG II-independent, pressor agent. Three days later the BP-RBF and 24-h BP recordings were repeated over several days. Despite similar increases in BP, PE led to significantly greater BP lability at the heart beat and very low frequency bandwidths. Conversely, ANG II, but not PE, caused significant renal vasoconstriction (a 62% increase in renal vascular resistance and a 21% decrease in RBF) and increased variability in BP-RBF relationships. Transfer function analysis of BP (input) and RBF (output) were consistent with a significant potentiation of the renal myogenic mechanism during ANG II administration, likely contributing, in part, to the exaggerated reductions in RBF during periods of BP elevations. We conclude that relatively equipressor doses of ANG II and PE lead to greatly different ambient BP profiles and effects on the renal vasculature when assessed in conscious rats. These data may have important implications regarding the pathogenesis of hypertension-induced injury in these models of hypertension.

Sex differences in renal medullary endothelin receptor function in angiotensin II hypertensive rats

We hypothesized that angiotensin (Ang) II hypertensive rats have impaired natriuresis after renal medullary endothelin (ET) B receptor stimulation that would be more evident in male versus female rats. Acute intramedullary infusion of the ET(B) agonist sarafotoxin 6c in normotensive male rats increased sodium excretion from 0.51±0.11 μmol/min during baseline to 1.64±0.19 μmol/min (P<0.05) after S6c. After 2 weeks of Ang II infusion (260 ng/kg per minute SC), male rats had an attenuated natriuretic response to S6c of 0.62±0.16 μmol/min during baseline versus 0.95±0.07 μmol/min after S6c. In contrast, ET(B)-dependent natriuresis was similar in female hypertensive rats (0.48±0.07 versus 1.5±0.18 μmol/min; P<0.05) compared with normotensive controls (1.05±0.07 versus 2.14±0.24 μmol/min; P<0.05). Because ET(A) receptors also mediate natriuresis in normotensive female rats, we examined ET(A) receptor function in female Ang II hypertensive rats. Intramedullary infusion of ET-1 increased sodium excretion in both hypertensive and normotensive female rats, which was partially blocked by the ET(A) antagonist BQ-123. Maximum ET(B) receptor binding in inner medullary membrane preparations was comparable between vehicle and Ang II hypertensive females; however, maximum ET(B) binding was significantly lower in male hypertensive rats (1952±251 versus 985±176 fmol/mg; P<0.05). These results indicate that renal ET(B) function is impaired in male Ang II hypertension attributed, at least in part, to a reduced number of ET(B) binding sites. Furthermore, renal ET receptor function is preserved in female rats during chronic Ang II infusion, suggesting that renal ET receptor function could serve to limit hypertension in females compared with males.

Autonomic Cardiovascular Responses to Heme Oxygenase Inhibition in Conscious Rats

Carbon monoxide (CO) is produced in the course of heme degradation from biliverdin by heme oxygenase (HO) in various tissues, including the central nervous system. Recent studies suggest the inhibition of HO activity increases arterial pressure mediated by the autonomic nervous system. The present study was designed to investigate the autonomic regulation of cardiovascular responses to inhibition of endogenous CO production by the HO inhibitor Zinc deuteroporphyrin 2, 4-bis glycol (ZnDPBG) by using direct sympathetic nerve recordings in conscious, chronically instrumented rats. ZnDPBG induced increases in mean arterial pressure (MAP) (P<0.05) and renal sympathetic nerve activity (RSNA) (P<0.05) but no significant change in heart rate (P>0.05) in intact rats. In atropine-treated rats, ZnDPBG also induced increases in MAP (P<0.05) and RSNA (P<0.05) but no change in heart rate (P>0.05). In sinoaortic denervated rats, ZnDPBG induced increases in MAP (P<0.05), heart rate (P<0.05), and RSNA (P<0.05). ZnDPBG shifted the baroreflex curve for RSNA upward and to the right, which was characterized by increases in the maximum and minimum response and midpoint pressure without altering the maximum gain. These results indicate that inhibition of HO activity within the central nervous system causes sympathoexcitation, resulting in an increase in arterial pressure. We conclude that the CO/HO system plays an important role in cardiovascular regulation by modulating sympathetic tone.

Biphasic effects of ANP infusion in conscious, euvolumic rats: roles of AQP2 and ENaC trafficking

Atrial natriuretic peptide (ANP) acutely promotes water and sodium excretion, whereas subchronic effects involve water retention. Renal hemodynamics, water and sodium excretion, and aquaporin-2 (AQP2) and epithelial Na channel (ENaC) subcellular trafficking were determined in response to continuous ANP infusion in conscious rats, where body sodium and fluid balance was constantly maintained. ANP (0.5 μg·kg−1·min−1) evoked a transient (peak at 10 min) fivefold diuresis followed by reduced urine production to control levels (30- to 90-min period). The fractional distal water excretion was significantly increased initially and then decreased in response to ANP. There was no change in the subcellular localization of AQP2 and AQP2 phosphorylated in PKA consensus site S256 (p-AQP2) 10 min after ANP infusion. In contrast, after 90 min a marked increase in apical labeling of AQP2 and p-AQP2 was observed in the inner and outer medullary collecting ducts but not in cortical collecting ducts. In support of this, ANP induced plasma membrane targeting of AQP2 in transiently AQP2-transfected cells. ANP infusion evoked an instant increase in renal sodium excretion, which persisted for 90 min. Ten minutes of ANP infusion induced no changes in the subcellular localization of ENaC subunits, whereas a marked increase in apical targeting of α- and γ-subunits was observed after 90 min. In conclusion, 1) ANP infusion induced a sustained natriuresis and transient diuresis; 2) there were no changes in the subcellular localization of AQP2 and ENaC subunits after 10 min of ANP infusion; and 3) there was a marked increase in apical targeting of AQP2, p-AQP2, and α- and γ-ENaC after 90 min of ANP infusion. The increased targeting of ENaC and AQP2 likely represents direct or compensatory effects to increase sodium and water reabsorption and to prevent volume depletion in response to prolonged ANP infusion.

Age-related reduction in estrogen receptor-mediated mechanisms of vascular relaxation in female spontaneously hypertensive rats

Hypertension increases with aging, and changes in vascular estrogen receptors (ERs) may play a role in age-related hypertension in women. We tested whether age-related increases in blood pressure in female spontaneously hypertensive rats (SHRs) are associated with reduction in amount and/or vascular relaxation effects of estrogen and ER. Arterial pressure and plasma estradiol were measured in adult (12 weeks) and aging (16 months) female SHRs, and thoracic aorta was isolated for measurement of active stress, 45Ca2+ influx, and ERs. Arterial pressure was greater and plasma estradiol was less in aging females than in adult females. In aorta of adult females, Western blots revealed alpha- and beta-ERs that were slightly reduced in aging rats. In endothelium-intact vascular strips, phenylephrine (Phe; 10(-5) mol/L) caused greater active stress in aging rats (9.3+/-0.2) than in adult rats (6.2+/-0.3x10(4) N/m2). 17beta-estradiol (E2) caused relaxation of Phe contraction and stimulation of vascular nitrite/nitrate production, which was reduced in aging rats. In endothelium-denuded strips, E2 still caused relaxation of Phe contraction, which was smaller in aging rats than adult rats. KCl (51 mmol/L), which stimulates Ca2+ influx, produced greater active stress in aging rats (9.1+/-0.3) than in adult rats (5.9+/-0.2x10(4) N/m2). E2 caused relaxation of KCl contraction and inhibition of Phe- and KCl-induced 45Ca2+ influx, which were reduced in aging rats. Thus, aging in female SHR is associated with reduction in ER-mediated NO production from endothelial cells and decrease in inhibitory effects of estrogen on Ca2+ entry mechanisms of smooth muscle contraction. The age-related decrease in ER-mediated vascular relaxation may explain the increased vascular contraction and arterial pressure associated with aging in females.

Chronic NOS inhibition reverses systemic vasodilation and glomerular hyperfiltration in pregnancy

The chronic role of nitric oxide (NO), independent of prostaglandin synthesis, in the primary peripheral vasodilation, increased glomerular filtration rate (GFR), and renal plasma flow (RPF) in normal pregnancy remains to be defined. The purpose of the present study was to chronically inhibit NOS to return systemic vascular resistance (SVR), cardiac output (CO), GFR, and RPF to nonpregnant values. Pregnant rats received the nitric oxide synthase (NOS) inhibitor, nitro-L-arginine methyl ester (L-NAME), orally from gestational days 7 through 14. Results were compared with nonpregnant and untreated pregnant rats. At 14 days gestation, CO significantly increased in pregnant vs. nonpregnant rats (187 +/- 17 vs. 125 +/- 10 ml/min, P < 0.05) as SVR decreased (0.64 +/- 0.08 vs. 1.08 +/- 0.08 mmHg. ml(-1). min, P < 0.05) and mean arterial pressure was unchanged (117 +/- 5 vs. 125 +/- 2 mmHg, not significant). Pregnant rats also demonstrated increased GFR (3,015 +/- 33 vs. 2,165 +/- 136 microl/min, P < 0.01) and RPF (7,869 +/- 967 vs. 5,507 +/- 290 microl/min, P < 0.05) vs. nonpregnant rats. L-NAME-treated pregnant rats had values for CO (118 +/- 7 ml/min), SVR (1.09 +/- 0.07 mmHg. ml(-1). min), GFR (2,264 +/- 150 microl/min), and RPF (5,777 +/- 498 microl/min), which were no different than nonpregnant animals. In summary, similar to human pregnancy, primary peripheral vasodilation occurs early in rat pregnancy. Furthermore, the hyperdynamic circulation and glomerular hyperfiltration of normal rat midterm pregnancy can be chronically reversed by NOS inhibition. These findings suggest a role for endothelial damage and decreased NO in the pathogenesis of preeclampsia.

Chloralose/ketamine anaesthesia preserves a form of postprandial sodium chloride balance in Wistar rats

Studies on the mechanisms underlying Na balance in anaesthetized rats are complicated by the fact that the most frequently used barbiturate anaesthetics attenuate or abolish this phenomenon. In the present study we show that a combination of nonbarbiturate anaesthetics: chloralose (140 mg/kg i.v.) and ketamine (30 mg/kg i.m. ), preserve the ability of rats to excrete intragastrically applied NaCl loads dose dependently. Thus rats anaesthetized with this regime excreted 86-102% of in- tragastrically applied NaCl whereas rats anaesthetized with thiobutabarbitone sodium (Inactin) excreted only 20-28%. We conclude that chloralose/ketamine anaesthesia is suitable for studies on Na balance mechanisms.

Renal effects of acute amino acid infusion in hypertension induced by chronic nitric oxide blockade

L-Arginine is the physiological substrate of nitric oxide, a vasodilator that controls blood pressure and renal hemodynamics in the basal state. In the present studies, we produced chronic nitric oxide blockade by oral administration of the L-arginine analogue NG-nitro-L-arginine methyl ester, which produced sustained hypertension and increased renal vascular resistance in conscious rats. Acute excess L-arginine had little effect on blood pressure but completely normalized renal vascular resistance and increased renal plasma flow in chronically nitric oxide-blocked hypertensive rats. In contrast to L-arginine, D-arginine had no renal hemodynamic effects in either normal or chronically nitric oxide-blocked rats. Acutely administered glycine was ineffective in vasodilating the chronically nitric oxide-blocked rat kidney, in a dose that produced renal vasodilation in normal rats. These findings indicate the following: (1) Hypertension induced by chronic nitric oxide blockade due to substituted L-arginine analogue cannot be acutely reversed with excess L-arginine, suggesting that the maintenance of the hypertension is not solely caused by competitive inhibition of nitric oxide production; (2) in contrast, the kidney remains responsive to L-arginine whereas the renal vasodilator response to glycine is abolished in this model of hypertension.

Spontaneous changes in arterial blood pressure and renal interstitial hydrostatic pressure in conscious rats

1. Previous work has demonstrated a positive relationship between experimentally induced changes in arterial pressure (AP) and renal interstitial hydrostatic pressure (RIHP). The purpose of the present study was to test the hypothesis that RIHP is positively correlated with the normal changes in AP that occur spontaneously in conscious rats. 2. Rats were chronically instrumented for the recording of AP (via an aortic catheter) and RIHP. RIHP was measured by implanting a Millar microtransducer, whose tip had been encapsulated in a 35 microns pore polyethylene matrix (5 mm long, 2 mm o.d.), approximately 5 mm below the renal cortical surface. 3. A total of 56 h of simultaneous analog recording of AP and RIHP was obtained from ten rats. Each 1 h segment was digitized and evaluated at frequencies of 1, 0.1, 0.02 and 0.01 Hz. 4. In forty-nine out of fifty-six of these 1 h recordings taken at 1 Hz, there were significant positive linear correlations between AP and RIHP (mean r = 0.32) with a mean slope of 0.11 mmHg RIHP/1 mmHg AP. Low-pass filtering to 0.01 Hz significantly increased the r value to 0.48. 5. These results demonstrate that spontaneous changes in AP and RIHP are positively correlated. The spontaneous coupling of AP and RIHP may be of importance in the regulation of salt and water excretion by the pressure diuresis mechanism.

Effect of hyperfiltration on long-term follow-up of glomerular filtration rate in male Wistar rats

It has been suggested that a prolonged course of hyperfiltration could lead to progressive deterioration of renal function. In order to test this hypothesis, the following protocol was applied to 60 male Wistar rats. At 12 weeks of life, the rats were submitted to a surgical procedure: sham operation (25 rats), unilateral nephrectomy (25 rats) or 3/4 nephrectomy (10 rats). The three groups were again divided into two subgroups: one with high-protein intake (36%) and one with a low-protein intake (12%). In order to avoid any additional traumatic procedure which could shorten the animal's life, the glomerular filtration rate (GFR) was measured without blood sampling, using a previously validated technique based on an image recorded by a gamma camera between the 9th and the 10th min after intravenous injection of 99m technetium diethylenetriaminepentaacetate (DPTA). The sum of both kidneys and bladder activity was expressed as a percentage of the injected dose. The test was performed before surgery and every month thereafter. Six weeks after surgery, the highest filtration rate was found in the rats with "two kidneys/high-protein diet", followed by the "two kidneys/low-protein diet", the "one kidney/high-protein diet", the "one kidney/low-protein diet" and the "1/2 kidney". The overall GFR in the one kidney/high-protein diet rat and in the 1/2 kidney rat was respectively 80% and 55% of the pre-operative values. Until 109 weeks of age, the survival rate was comparable in the five groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of permanent cannulation of the jugular vein on pharmacokinetics of amoxycillin and antipyrine in the rat

The effect of chronic cannulation of the rat jugular vein on the pharmacokinetics of amoxycillin and antipyrine administered by the i.v. and oral routes has been evaluated. Animals that received the i.v. dose of amoxycillin on the eighth day after jugular vein cannulation showed decreased clearance (4.0 +/- 0.3 ml/min) and steady-state volume of distribution (105 +/- 8 ml) compared to animals that received the i.v. dose on the fourth day (5.5 +/- 1.1 ml/min and 155 +/- 17 ml, respectively). Rats first dosed by the i.v. route showed an oral bioavailability of 54 +/- 12%, whereas for those first dosed by the oral route the calculated bioavailability was 31 +/- 6%. Antipyrine was administered to rats by the i.v. and oral routes on the first and fourth days after jugular vein cannulation. Animals intravenously dosed on the fourth day showed a decreased clearance (1.9 +/- 0.3 ml/min) compared to rats intravenously dosed on the 1st day (2.7 +/- 0.6 ml/min). Antipyrine bioavailability was larger in animals first dosed by the i.v. route than in animals first dosed by the oral route (173 +/- 43 and 74 +/- 15%, respectively). These results argue against the use of crossover studies in rats with permanently implanted cannulas since kinetic changes induced by cannulation can be larger than previously proposed.

Estimation of bioavailability on a single occasion after semisimultaneous drug administration

A method for determination of absorption rate and bioavailability was developed to reduce the influence of intraindividual variability and applied to the absolute intraperitoneal availability of lithium in the rat. In this method the test and the reference doses are given with a few hours' interval, and the resulting concentration-time data are analyzed by nonlinear regression. The bioavailability estimation by this approach was compared to that of the traditional method, with test and reference doses given on separate days. The mean estimates of availability were 1.035 +/- 0.109 (N = 7) and 0.984 +/- 0.052 (N = 11) for the traditional and the alternative method, respectively. Thus, the precision was better in the latter. Major influences of dose- or time-dependent kinetics of lithium on the availability estimates were excluded by the design used. The estimation of bioavailability was robust with respect to the choice of absorption and disposition model and the duration of sampling. The plasma clearance of lithium was 169 +/- 15 ml/hr/kg, with a terminal half-life of 5.0 +/- 0.5 hr (N = 5).

Effect of sino-aortic denervation in comparison to cardiopulmonary deafferentiation on long-term blood pressure in conscious dogs

The isolated and combined influence of cardiopulmonary and sinoaortic denervation on long-term blood pressure (MAP), heart rate (HR), plasma renin activity (PRA) and plasma volume (PV) was studied in 11 conscious, chronically instrumented foxhounds receiving a normal sodium diet. MAP, HR, PV and PRA remained unchanged in the 5 dogs after bilateral thoracic vagal stripping, which eliminates the cardiopulmonary afferents. After sino-aortic denervation in another 5 dogs there was equally little change when compared to the control group. Only total baroreceptor and cardiopulmonary denervation (7 dogs) revealed significantly higher levels of MAP (119.6 +/- 4.6 vs. 100.4 +/- 1.5, P less than 0.01), HR (118.2 +/- 3.7; vs. 84.1 +/- 3.5; P less than 0.0001), and PRA (3.6 +/- 0.9 vs. 0.9 +/- 0.2; P less than 0.05). In conclusion, the function of either arterial baroreceptors or cardiopulmonary receptors is sufficient for normal circulatory control. When both groups of receptor afferents are interrupted, MAP, HR, and PRA rise to significantly higher levels. Thus, both systems interact in a sense of a non-additive attenuation on "cardiovascular centres". This may clarify previous disputes concerning neurogenic hypertension, and supplies information for the role of the renin-angiotensin system in blood pressure control.

Sodium and water reabsorption in the proximal and distal nephron in conscious pregnant rats and third trimester women

1. Sodium and water reabsorption were determined in virgin controls and rats at 9 and 20 days of pregnancy during a water diuresis induced and maintained by infusion of 2.5% dextrose and in normally hydrated women during the third trimester and again 2-3 months after delivery. 2. Reabsorption from the proximal tubules and distal nephron segments was assessed using the clearance of lithium as a marker of the sodium and water delivery from the proximal tubules to the loops of Henle. 3. Glomerular filtration rate and whole-kidney reabsorption of sodium and water were significantly increased at both stages of pregnancy in rats and late in pregnancy in women when compared to the respective non-pregnant control. Fractional reabsorption was unaltered in pregnant rats but increased in pregnant women. 4. Proximal tubule sodium and water reabsorption were increased in pregnant rats and women; fractional reabsorption was unaltered. 5. Absolute reabsorption of sodium and water in distal nephron segments were increased during pregnancy in both rats and women. Although increases in fractional reabsorption were detected in pregnant women, this was not the case at either stage of gestation in rats.

Autoregulation of renal blood flow, glomerular filtration rate and renin release in conscious dogs

The relationship between renal artery pressure (RAP), renal blood flow (RBF), glomerular filtration rate (GFR) and the renal venous-arterial plasma renin activity difference (PRAD) was studied in 22 chronically instrumented, conscious foxhounds with a daily sodium intake of 6.6 mmol/kg. RAP was reduced in steps and maintained constant for 5 min using an inflatable renal artery cuff and a pressure control system. Between 160 and 81 mm Hg we observed a concomitant autoregulation of GFR and RBF with a high precision. The "break off points" for GRF- and RBF-autoregulation were sharp and were significantly different from each other (GFR: 80.5 +/- 3.5 mm Hg; RBF: 65.6 +/- 1.3 mm Hg; P less than 0.01). In the subautoregulatory range GFR and RBF decreased in a linerar fashion and ceased at 40 and 19 mm Hg, respectively. Between 160 mm Hg and 95 mm Hg (threshold pressure for renin release) PRAD remained unchanged; below threshold pressure PRAD increased steeply (average slope: 0.34 ng AI.ml-1.h-1.mm Hg-1) indicating that resting renin release may be doubled by a fall of RAP by only 3 mm Hg. At the "break-off point" of RBF-autoregulation (66 mm Hg) renin release was 10-fold higher than the resting level. It is concluded that under physiological conditions (normal sodium diet) GFR and RBF are perfectly autoregulated over a wide pressure range. Renin release remains suppressed until RAP falls below a well defined threshold pressure slightly below the animal's resting systemic pressure. RBF is maintained at significantly lower pressures than GFR, indicating that autoregulation of RBF also involves postglomerular vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of atrial natriuretic factor on blood pressure, heart rate, and renal functions in conscious, spontaneously hypertensive rats

Atrial natriuretic factors, polypeptides released by atrial myocytes, may play a role in the control of blood pressure and the regulation of renal salt and water excretion. Our studies were designed to assess the role of a synthetic peptide, atriopeptin II, on blood pressure and heart rate, renal hemodynamics, and salt and water excretion in conscious, spontaneously hypertensive rats and in normotensive Wistar-Kyoto rats. Changes in mean arterial pressure and heart rate were recorded following intravenous bolus injections (0.1, 1.0, 10, 100 micrograms/kg) of atriopeptin II in 5 spontaneously hypertensive and 5 Wistar-Kyoto rats. In a second group of rats the peptide was infused for 90 minutes in two different doses: low dose, 1 microgram/kg + 2 micrograms/kg/hr; and high dose, 10 micrograms/kg + 20 micrograms/kg/hr. Bolus injections of atriopeptin II resulted in dose-dependent decreases in mean arterial pressure in the hypertensive, but not in the normotensive, rats; heart rates remained unchanged. Blood pressure decreased gradually during the sustained infusion of both doses of atriopeptin II, with the spontaneously hypertensive strain showing increased sensitivity compared to the Wistar-Kyoto strain. Heart rate decreased in both strains during infusion of the high dose; the decrease was significant only in the hypertensive rats. The low dose of atriopeptin II increased the clearance of free water in both strains of rats; sodium excretion was increased only in the hypertensive rats. The high-dose atriopeptin II was associated with transient natriuresis, unaltered glomerular filtration rate, and decreased effective renal blood flow in both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against the hypothesis that prostaglandins are the vasodepressor agents of pregnancy. Serial studies in chronically instrumented, conscious rats

Renal hemodynamics increase dramatically during pregnancy, and pressor responsiveness to exogenous administration of vasoconstrictors is attenuated. We investigated whether or not vasodilatory prostaglandins mediate these phenomena. Trained, chronically instrumented, conscious pregnant rats were used. Control values of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated at midgestation (P less than 0.01 and P = 0.05 from prepregnant means, respectively), and effective renal vascular resistance was decreased (P = 0.05). Indomethacin (4.5-6.5 mg/kg body weight [BW]) failed to decrease renal hemodynamics at this stage of pregnancy; in fact, it raised GFR somewhat further (P less than 0.05). Systemic pressor responsiveness to bolus administration of norepinephrine and angiotensin II (AII) was significantly attenuated by at least gestational day 20. Neither indomethacin (7 mg/kg BW) or meclofenamate (6 mg/kg BW) affected the refractory response. The renal vasculature was also relatively unresponsive to an intravenous infusion of AII (5 ng X kg-1 X min-1) during late gestation (day 19); in particular, the fall in ERPF in response to AII (16 +/- 3%) was markedly less than that observed in the prepregnant condition (34 +/- 3%; P less than 0.05). Indomethacin (6 mg/kg BW) failed to restore this blunted response, and further attenuation was evident, despite the presence of the inhibitor (gestational day 21). We conclude that vasodilatory prostaglandins do not appear to mediate the rise in renal hemodynamics, and the attenuation of the systemic and renal pressor responsiveness observed during pregnancy, insofar as these phenomena were unaffected by acute cyclooxygenase inhibition in unstressed, conscious rats.

Renal hemodynamics during pregnancy in chronically catheterized, conscious rats

Two types of experiments were performed, cross-sectional and longitudinal. In the cross-sectional studies, rats were mated, later prepared surgically, and then 5 or more days after surgery, each examined twice during days 11 to 15 or days 18 to 20 of gestation. Nonpregnant rats matched for age and prepregnant weight served as controls. In the longitudinal studies, rats were catheterized and, starting 6 days later, examined twice; then the same rats were mated and each was studied on days 5, 8, 12, 16, and 20 of gestation, as well as on day 5 postpartum. In the cross-sectional studies, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated by approximately 26% and 20%, respectively, above nonpregnant controls at 11 to 15 days of gestation (GFR, 2739 +/- 94 vs. 2181 +/- 134 microliters . min-1, P less than 0.005; ERPF, 9367 +/- 295 vs. 7785 +/- 422 microliters . min-1, P less than 0.01). By 18 to 20 days of gestation, GFR and ERPF had returned to levels that were not significantly different from nonpregnant values. The longitudinal studies confirmed these findings in every respect and further revealed that GFR and ERPF were elevated above nonpregnant values as early as day 5 of gestation (P less than 0.005). Thereafter, they rose to peak values, at 12 and 16 days of gestation, of 3122 +/- 144 and 10,584 +/- 541 microliters . min-1, and then returned to nonpregnant levels by day 20 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)