Influence of the renin-angiotensin system on the autoregulation of renal blood flow and glomerular filtration rate in conscious dogs

Role of radionuclide imaging for diagnosis of device and prosthetic valve infections

Cardiovascular implantable electronic device (CIED) infection and prosthetic valve endocarditis (PVE) remain a diagnostic challenge. Cardiac imaging plays an important role in the diagnosis and management of patients with CIED infection or PVE. Over the past few years, cardiac radionuclide imaging has gained a key role in the diagnosis of these patients, and in assessing the need for surgery, mainly in the most difficult cases. Both ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) and radiolabelled white blood cell single-photon emission computed tomography/computed tomography (WBC SPECT/CT) have been studied in these situations. In their 2015 guidelines for the management of infective endocarditis, the European Society of Cardiology incorporated cardiac nuclear imaging as part of their diagnostic algorithm for PVE, but not CIED infection since the data were judged insufficient at the moment. This article reviews the actual knowledge and recent studies on the use of ¹⁸F-FDG PET/CT and WBC SPECT/CT in the context of CIED infection and PVE, and describes the technical aspects of cardiac radionuclide imaging. It also discusses their accepted and potential indications for the diagnosis and management of CIED infection and PVE, the limitations of these tests, and potential areas of future research.

The effects of 18-h fasting with low-carbohydrate diet preparation on suppressed physiological myocardial (18)F-fluorodeoxyglucose (FDG) uptake and possible minimal effects of unfractionated heparin use in patients with suspected cardiac involvement sarcoidosis

BACKGROUND

(18)F-fluorodeoxyglucose (FDG) PET plays an important role in the detection of cardiac involvement sarcoidosis (CS). However, diffuse left ventricle (LV) wall uptake sometimes makes it difficult to distinguish between positive uptake and physiological uptake. The aims of this study were to evaluate the effects of 18-h fasting with low-carbohydrate diet (LCD) vs a minimum of 6-h fasting preparations on diffuse LV FDG uptake and free fatty acid (FFA) levels in patients with suspected CS.

METHODS

Eighty-two patients with suspected CS were divided into 2 preparation protocols: one with a minimum 6-h fast without LCD preparation (group A, n = 58) and the other with a minimum 18-h fast with LCD preparation (group B, n = 24). All patients also received intravenous unfractionated heparin (UFH; 50 IU/kg) before the injection of FDG.

RESULTS

Group A showed a higher percentage of diffuse LV uptake than did group B (27.6 vs 0.0%, P = .0041). Group B showed higher FFA levels (1159.1  ±  393.0, 650.5  ±  310.9 μEq/L, P < .0001) than did group A. Patients with diffuse LV uptake (n = 16) showed lower FFA levels than did other patients (n = 66) (432.1  ±  296.1, 888.4  ±  381.4 μEq/L, P < .0001). UFH administration significantly increased FFAs in both groups, even in the patients with diffuse LV FDG uptake.

CONCLUSIONS

The 18-h fast with LCD preparation significantly reduced diffuse LV uptake and increased FFA levels. In particular, the FFA level was significantly lower in patients with LV diffuse uptake than in patients without LV diffuse uptake. Acutely increasing plasma FFA through the use of UFH may not have a significant role in reducing physiological LV FDG uptake.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Long fasting is effective in inhibiting physiological myocardial 18F-FDG uptake and for evaluating active lesions of cardiac sarcoidosis

BACKGROUND

F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is a promising modality for detecting active lesions of cardiac sarcoidosis (CS). However, determining whether 18F-FDG uptake in the myocardium is physiological is challenging due to metabolic shift in myocardial cells. Although methods for inhibiting physiological myocardial 18F-FDG uptake have been proposed, no standard methods exist. This study therefore aimed to compare the effect of an 18-h fast (long fasting (LF)) with heparin loading plus a 12-h fast (HEP) before 18F-FDG PET scan.

METHODS

We analyzed the effects of LF and HEP on the inhibition of physiological myocardial 18F-FDG uptake in healthy subjects (18 in HEP and 19 in LF) and in patients with known or suspected CS (96 in HEP and 69 in LF). In CS, the lower uptake of 18F-FDG in the myocardium was evaluated. A visual four-point scale was used to assess myocardial 18F-FDG uptake in comparison with hepatic uptake (1 lower, 2 similar, 3 somewhat higher, 4 noticeably higher).

RESULTS

Myocardial 18F-FDG uptake was 1.68 ± 1.06 in LF and 3.17 ± 1.16 in HEP in healthy subjects (p < 0.0001), whereas it was 1.48 ± 0.99 in LF and 2.48 ± 1.33 in HEP in CS patients (p < 0.0001). Logistic regression and regression trees revealed the LF was the most effective in inhibiting myocardial 18F-FDG uptake. In addition, serum free fatty acid levels on intravenous 18F-FDG injection were a possible biomarker.

CONCLUSIONS

LF is effective in inhibiting myocardial 18F-FDG uptake, and consequently, it could be useful for evaluating active lesions of CS in 18F-FDG PET images.

¹⁸F-Fluoro-2-deoxyglucose positron emission tomography in cardiac sarcoidosis

Cardiac sarcoidosis (CS) is a rare and potentially life-threatening disease that causes conduction disturbance, systolic dysfunction, and most notably sudden cardiac death. Accurate diagnosis of CS is thus mandatory; however, a reliable approach that enables diagnosis of CS with high sensitivity and specificity has yet to be established. Recent studies have demonstrated the promising potential of (18)F-fluoro-2-deoxyglucose positron emission tomography ((18)F-FDG PET) in the diagnosis and assessment of CS. Indeed, (18)F-FDG PET provides a wide variety of advantages over previous imaging modalities; however, there are pitfalls and limitations that should be recognized. In this review article, (1) the rationale for (18)F-FDG PET application in CS, (2) suitable pretest preparations, and (3) evaluation protocols for the (18)F-FDG PET images obtained will be addressed. In particular, sufficient suppression of physiological (18)F-FDG uptake in the heart is essential for accurate assessment of CS. Also, (4) recent studies addressing the diagnostic role of (18)F-FDG PET and (5) the clinically important differences between (18)F-FDG PET and other imaging technologies will be reviewed. For example, active sarcoid lesions and their response to steroid treatment will be better detected by (18)F-FDG PET, whereas fibrotic lesions might be shown more clearly by magnetic resonance imaging or other nuclear myocardial perfusion imaging. In the last decade, (18)F-FDG PET has substantially enhanced detection of CS; however, CS would be better evaluated by a combination of multiple modalities. In the future, advances in (18)F-FDG PET and other emerging imaging modalities are expected to enable better management of patients with sarcoidosis.

Losartan chemistry and its effects via AT1 mechanisms in the kidney

Besides the importance of the renin-angiotensin system (RAS) in the circulation and other organs, the local RAS in the kidney has attracted a great attention in research in last decades. The renal RAS plays an important role in the body fluid homeostasis and long-term cardiovascular regulation. All major components and key enzymes for the establishment of a local RAS as well as two important angiotensin II (Ang II) receptor subtypes, AT1 and AT2 receptors, have been confirmed in the kidney. In additional to renal contribution to the systemic RAS, the intrarenal RAS plays a critical role in the regulation of renal function as well as in the development of kidney disease. Notably, kidney AT1 receptors locating at different cells and compartments inside the kidney are important for normal renal physiological functions and abnormal pathophysiological processes. This mini-review focuses on: 1) the local renal RAS and its receptors, particularly the AT1 receptor and its mechanisms in physiological and pathophysiological processes; and 2) the chemistry of the selective AT1 receptor blocker, losartan, and the potential mechanisms for its actions in the renal RAS-mediated disease.

Myocardial imaging with 18F-fluoro-2-deoxyglucose positron emission tomography and magnetic resonance imaging in sarcoidosis

PURPOSE

Despite accumulating reports on the clinical value of (18)F-fluoro-2-deoxyglucose positron emission tomography (18F-FDG PET) and magnetic resonance imaging (MRI) in the assessment of cardiac sarcoidosis, no studies have systematically compared the images of these modalities.

METHODS

Twenty-one consecutive patients with suspected cardiac sarcoidosis underwent cardiac examinations that included 18F-FDG PET and MRI. The association of 18F-FDG PET and MRI findings with blood sampling data such as serum angiotensin converting enzyme levels was also evaluated.

RESULTS

Eight of 21 patients were diagnosed as having cardiac sarcoidosis according to the Japanese Ministry of Health and Welfare Guidelines for Diagnosing Cardiac Sarcoidosis. Sensitivity and specificity for diagnosing cardiac sarcoidosis were 87.5 and 38.5%, respectively, for 18F-FDG PET, and 75 and 76.9%, respectively, for MRI. When the 18F-FDG PET and MRI images were compared, 16 of 21 patients showed positive findings in one (n = 8) or both (n = 8) of the two modalities. In eight patients with positive findings on both images, the distribution of the findings differed among all eight cases. The presence of positive findings on 18F-FDG PET was associated with elevated serum angiotensin-converting enzyme levels; this association was not demonstrated on MRI.

CONCLUSIONS

Both 18F-FDG PET and MRI provided high sensitivity for diagnosing cardiac sarcoidosis in patients with suspected cardiac involvement, but the specificity of (18)F-FDG PET was not as high as previously reported. The different distributions of the findings in the two modalities suggest the potential of 18F-FDG PET and MRI in detecting different pathological processes in the heart.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Effects of carprofen on renal function during medetomidine-propofol-isoflurane anesthesia in dogs

OBJECTIVE

To investigate effects of carprofen on indices of renal function and results of serum bio-chemical analyses and effects on cardiovascular variables during medetomidine-propofol-isoflurane anesthesia in dogs.

ANIMALS

8 healthy male Beagles.

PROCEDURES

A randomized crossover study was conducted with treatments including saline (0.9% NaCl) solution (0.08 mL/kg) and carprofen (4 mg/kg) administered IV. Saline solution or carprofen was administered 30 minutes before induction of anesthesia and immediately before administration of medetomidine (20 microg/kg, IM). Anesthesia was induced with propofol and maintained with inspired isoflurane in oxygen. Blood gas concentrations and ventilation were measured. Cardiovascular variables were continuously monitored via pulse contour cardiac output (CO) measurement. Renal function was assessed via glomerular filtration rate (GFR), renal blood flow (RBF), scintigraphy, serum biochemical analyses, urinalysis, and continuous CO measurements. Hematologic analysis was performed.

RESULTS

Values did not differ significantly between the carprofen and saline solution groups. For both treatments, sedation and anesthesia caused changes in results of serum biochemical and hematologic analyses; a transient, significant increase in urine alkaline phosphatase activity; and blood flow diversion to the kidneys. The GFR increased significantly in both groups despite decreased CO, mean arterial pressure, and absolute RBF variables during anesthesia.

CONCLUSIONS AND CLINICAL RELEVANCE

Carprofen administered IV before anesthesia did not cause detectable, significant adverse effects on renal function during medetomidine-propofol-isoflurane anesthesia in healthy Beagles.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Influence of short-term versus prolonged cardiopulmonary receptor stimulation on renal and preganglionic adrenal sympathetic nerve activity in rats

Renal and preganglionic adrenal sympathetic nerve activities (RSNA, ASNA) are regulated differentially. Various cardiopulmonary receptor (CPR) stimulation procedures were performed to distinguish short-term and prolonged as well as mechanical and chemical stimulatory effects on RSNA and ASNA. In anesthetized male Sprague-Dawley rats blood pressure, heart rate, left ventricular end-diastolic pressure (LVEDP), RSNA and ASNA were recorded. CPRs were stimulated as follows: Short-term mechanical: LVEDP changes (+/-4, +/-6, +/-8 mmHg) via aortic and caval vein occlusion; Short-term chemical: phenylbiguanide (PBG-bolus, 0.1, 1, 10 microg IV); Prolonged mechanical (15 min): volume expansion (0.9% NaCl, 5% body weight) and hemorrhage, to modulate LVEDP; Prolonged chemical: PBG infusion (32 microg/min IV, for 15 min); Stimulations were done with 1) all afferents intact, 2) bilateral cervical vagotomy (VX), 3) VX + SAD (sino-aortic denervation; short-term protocols and hemorrhage).1) Short-term mechanical stimuli decreased RSNA (-52 +/- 12%) and ASNA (-37 +/- 13%). 2) PBG-bolus decreased RSNA (-54 +/- 12%) but increased ASNA (+40 +/- 13%). 3) Volume expansion decreased RSNA (-55 +/- 7%), ASNA was unaffected. 4) PBG infusion persistently decreased RSNA (-60 +/- 6%) but just shortly increased ASNA (+120 +/- 15%); VX abolished all responses. 5) Hypotensive hemorrhage decreased RSNA (-39 +/- 9%) but increased ASNA (+42 +/- 9%). VX abolished RSNA response; ASNA response only disappeared with VX + SAD.Short-term mechanical CPR stimulation uniformly decreased sympathetic activities, whereas chemical stimulation had opposing effects on renal and adrenal sympathetic responses. All prolonged stimuli decreased RSNA, whereas ASNA was virtually unaffected: Sympathetic out.ow is differentially controlled not only with regard to target organs or afferent receptors but also stimulus time pattern.

Effect of tepoxalin on renal function in healthy dogs receiving an angiotensin-converting enzyme inhibitor

The objective of this study was to investigate renal function in clinically normal dogs receiving tepoxalin, a nonsteroidal inflammatory drug, either in association with or without an angiotensin-converting enzyme inhibitor (ACEI). Ten adult female Beagle dogs were used in the three phases of the study. The dogs were administered the drugs once daily for 7 days (experiment 1: placebo/tepoxalin/tepoxalin and benazepril; experiment 2: enalapril/tepoxalin and enalapril) or for 28 days (experiment 3: tepoxalin and benazepril together). Renal function was assessed by measurement of glomerular filtration rate (GFR) by renal scintigraphy [(renal uptake of 99mTc-diethylenetriaminepentacetic acid (DTPA)] and plasma clearance of 99mTc-DTPA. Compared with the placebo group, renal uptake and plasma clearance of 99mTc-DTPA were not significantly modified after a 7-day period of treatment with tepoxalin or enalapril alone, tepoxalin and benazepril or tepoxalin and enalapril together. No significant change was obtained in GFR after a 28-day period of dosing with tepoxalin and benazepril together. Therefore, it was concluded that tepoxalin did not alter renal function in healthy Beagle dogs receiving ACEI.

Lipoprotein lipase in hemodialysis patients: indications that low molecular weight heparin depletes functional stores, despite low plasma levels of the enzyme

BACKGROUND

Lipoprotein lipase (LPL) has a central role in the catabolism of triglyceride-rich lipoproteins. The enzyme is anchored to the vascular endothelium through interaction with heparan sulphate proteoglycans and is displaced from this interaction by heparin. When heparin is infused, there is a peak of LPL activity accompanied by a reduction in triglycerides (TG) during the first hour, followed by a decrease in LPL activity to a stable plateau during the remaining session while TG increase towards and beyond baseline. This suggests that tissue stores of LPL become depleted. It has been argued that low molecular weight (LMW) heparins cause less disturbance of the LPL system than conventional heparin does.

METHODS

We have followed LPL activity and TG during a dialysis-session with a LMW heparin (dalteparin) using the same patients and regime as in a previous study with conventional heparin, i.e. a primed infusion.

RESULTS

The shape of the curve for LPL activity resembled that during the earlier dialyses with conventional heparin, but the values were lower during dialysis with dalteparin. The area under the curve for LPL activity during the peak period (0-180 minutes) was only 27% and for the plateau period (180-240 minutes) it was only 36% of that observed with conventional heparin (p < 0.01). These remarkably low plasma LPL activities prompted us to re-analyze LPL activity and to measure LPL mass in frozen samples from our earlier studies. There was excellent correlation between the new and old values which rules out the possibility of assay variations as a confounding factor. TG increased from 2.14 mmol/L before, to 2.59 mmol/L after the dialysis (p < 0.01). From 30 minutes on, the TG values were significantly higher after dalteparin compared to conventional heparin (p < 0.05).

CONCLUSION

These results indicate that LMW heparins disturb the LPL system as much or more than conventional heparin does.

Lower plasma levels of lipoprotein lipase after infusion of low molecular weight heparin than after administration of conventional heparin indicate more rapid catabolism of the enzyme

The functional pool of lipoprotein lipase (LPL) is anchored to heparan sulfate at the vascular endothelium. Injection of heparin releases the enzyme into the circulating blood. Animal experiments have shown that the enzyme is then extracted and degraded by the liver. Low molecular weight (LMW) heparin preparations are widely used in the clinic and are supposed to release less LPL. In this study, we infused a LMW heparin into healthy volunteers for 8 hours. The peak of LPL activity was only about 30% and the subsequent plateau of LPL activity only about 40% compared with those seen with conventional heparin. When a bolus of heparin was given after 4 hours' infusion of LMW or conventional heparin, only relatively small, and similar, amounts of LPL entered plasma. This suggests that the difference between LMW and conventional heparin lay in the ability to retain LPL in the circulating blood, not in the ability to release the lipase. Triglycerides (TGs) decreased when the heparin infusion was started, as expected from the high circulating LPL activities. After 1 to 2 hours, TG levels increased again, and after 8 hours they were about twice as high as before the heparin infusion. This indicates that the amount of LPL available for lipoprotein metabolism had become critically low in relation to TG transport rates. This study indicates that LMW heparin compared with conventional heparin causes as much or more depletion of LPL and subsequent impairment of TG clearing.

Volume-selective 1H MR spectroscopy for in vivo detection of valproate in patients with epilepsy

We performed volume-selective 1H MR spectroscopy (1H-MRS) on 12 patients on valproate monotherapy to detect valproate in vivo in the brain. We also acquired reference valproate spectra in vitro in subphysiological 15 g/l albumin solution in saline, in which valproate showed two resonance peaks at 0.7 and 1.2 ppm and a minimum detection threshold of 240 mg/l. In vivo 1H-MRS spectra in brain showed peaks between 0.6 and 1.6 ppm. Simultaneous serum valproate concentrations did not correlate with these integrated MRS peaks. On follow-up, changes in these signals also did not correlate with increasing serum valproate levels. The inconsistency of in vivo 1H-MRS signals at varying serum levels and the high detection levels in vitro suggest that valproate signals are missed in vivo because valproate is metabolised or strongly bound, presumably to brain macromolecules.

Effects of carprofen on renal function and results of serum biochemical and hematologic analyses in anesthetized dogs that had low blood pressure during anesthesia

OBJECTIVE

To investigate effects of IV administered carprofen on indices of renal function and results of serum biochemical and hematologic analyses in dogs anesthetized with acepromazine-thiopentone-isoflurane that had low blood pressure during anesthesia.

ANIMALS

6 healthy Beagles.

PROCEDURE

A randomized crossover study was conducted, using the following treatments: saline (0.9% NaCl solution)-saline, saline-carprofen, and carprofen-saline. Saline (0.08 ml/kg) and carprofen (4 mg/kg) were administered IV. The first treatment was administered 30 minutes before induction of anesthesia and immediately before administration of acepromazine (0.1 mg/kg, IM). Anesthesia was induced with thiopentone (25 mg/ml, IV) and maintained with inspired isoflurane (2% in oxygen). The second treatment was administered 30 minutes after onset of inhalation anesthesia. Blood gases, circulation, and ventilation were monitored. Renal function was assessed by glomerular filtration rate (GFR), using scintigraphy, serum biochemical analyses, and urinalysis. Hematologic analysis was performed. Statistical analysis was conducted, using ANOVA or Friedman ANOVA.

RESULTS

Values did not differ significantly among the 3 treatments. For all treatments, sedation and anesthesia caused changes in results of serum biochemical and hematologic analyses, a decrease in mean arterial blood pressure to 65 mm Hg, an increase of 115 pmol/L in angiotensin II concentration, and an increase of 100 seconds in time required to reach maximum activity counts during scintigraphy.

CONCLUSIONS AND CLINICAL RELEVANCE

Carprofen administered IV before or during anesthesia did not cause detectable significant adverse effects on renal function or results of serum biochemical and hematologic analyses in healthy Beagles with low blood pressure during anesthesia.

Role of angiotensin II in dynamic renal blood flow autoregulation of the conscious dog

The influence of angiotensin II (ANGII) on the dynamic characteristics of renal blood flow (RBF) was studied in conscious dogs by testing the response to a step increase in renal artery pressure (RAP) after a 60 s period of pressure reduction (to 50 mmHg) and by calculating the transfer function between physiological fluctuations in RAP and RBF. During the RAP reduction, renal vascular resistance (RVR) decreased and upon rapid restoration of RAP, RVR returned to baseline with a characteristic time course: within the first 10 s, RVR rose rapidly by 40 % of the initial change (first response, myogenic response). A second rise began after 20-30 s and reached baseline after an overshoot at 40 s (second response, tubuloglomerular feedback (TGF)). Between both responses, RVR rose very slowly (plateau). The transfer function had a low gain below 0.01 Hz (high autoregulatory efficiency) and two corner frequencies at 0.026 Hz (TGF) and at 0.12 Hz (myogenic response). Inhibition of angiotensin converting enzyme (ACE) lowered baseline RVR, but not the minimum RVR at the end of the RAP reduction (autoregulation-independent RVR). Both the first and second response were reduced, but the normalised level of the plateau (balance between myogenic response, TGF and possible slower mechanisms) and the transfer gain below 0.01 Hz were not affected. Infusion of ANGII after ramipril raised baseline RVR above the control condition. The first and second response and the transfer gain at both corner frequencies were slightly augmented, but the normalised level of the plateau was not affected. It is concluded that alterations of plasma ANGII within a physiological range do not modulate the relative contribution of the myogenic response to the overall short-term autoregulation of RBF. Consequently, it appears that ANGII augments not only TGF, but also the myogenic response.

Effects of Ca(2+) channel activity on renal hemodynamics during acute attenuation of NO synthesis in the rat

In cultured vascular muscle cells, nitric oxide (NO) has been shown to inhibit voltage-dependent Ca(2+) channels, which are involved in renal blood flow (RBF) autoregulation. Therefore, our purpose was to specify in vivo the effects of this interaction on RBF autoregulation. To do so, hemodynamics were investigated in anesthetized rats during Ca(2+) channel blockade before or after acute NO synthesis inhibition. Rats were treated intravenously with vehicle (n = 10), 0.3 mg/kg body wt N(G)-nitro-L-arginine-methyl ester (L-NAME; n = 7), 4.5 microg. kg body wt(-1). min(-1) nifedipine (n = 8) alone, or with nifedipine infused before (n = 8), after (n = 8), or coadministered with L-NAME (n = 10). Baseline renal vascular resistance (RVR) averaged 14.0 +/- 1.2 resistance units and did not change after vehicle. RVR increased or decreased significantly by 27 and 29% after L-NAME or nifedipine, respectively. Nifedipine reversed, but did not prevent, RVR increase after or coadministered with L-NAME. RBF autoregulation was maintained after L-NAME, but the autoregulatory pressure limit (P(A)) was significantly lowered by 15 mmHg. Nifedipine pretreatment or coadministration with L-NAME limited P(A) resetting or suppressed autoregulation at higher doses. Results were similar with verapamil. Intrarenal blockade of Ca(2+)-activated K(+) channels also prevented autoregulatory resetting by L-NAME (n = 8). These findings suggest NO inhibits voltage-dependent Ca(2+) channels and thereby modulates RBF autoregulatory efficiency.

Stimulation of the renin-angiotensin system by endothelin subtype A receptor blockade in conscious dogs

Previous studies in dogs have shown additive or even synergistic effects of combined angiotensin-converting enzyme inhibition and either nonselective endothelin subtype A/B (ETA/B) or selective endothelin subtype A (ETA) receptor blockade on renal vascular resistance and mean arterial blood pressure. A possible mechanism underlying this interaction may be a stimulation of the renin-angiotensin system during endothelin (ET) receptor blockade. We therefore investigated whether plasma renin activity and renin release are regulated by the ETA receptor. Experiments were made in conscious, chronically instrumented dogs receiving either saline or the selective ETA receptor antagonist LU 135252 (10 mg/kg IV). Eighty to 100 minutes after the administration of LU 135252 (n=5), heart rate (99+/-7 versus 81+/-6 bpm; P<0.05) and renal blood flow (327+/-40 versus 278+/-36 mL/min; P<0.05) were increased significantly, whereas mean arterial blood pressure tended to be lower (93+/-5 versus 105+/-7 mm Hg). These changes were associated with a 2-fold increase in plasma renin activity (0.74+/-0.12 versus 0.37+/-0.10 ng angiotensin I per milliliter per hour; P<0.05). Measurements of renin release at various renal perfusion pressures (n=5) with the use of a vascular occluder implanted around the left renal artery revealed that ETA receptor blockade did not alter renin release at resting renal perfusion pressure (78+/-25 versus 71+/-39 U/min) but strongly enhanced the sensitivity of pressure-dependent renin release <80 mm Hg approximately 2.2-fold. In conclusion, selective ETA receptor blockade is associated with a stimulation of the circulating renin-angiotensin system, which results from both a sensitization of pressure-dependent renin release and a larger proportion of blood pressure values falling into the low pressure range, where renin release is stimulated. These find-ings strengthen the view that ET and the renin-angiotensin system closely interact to regulate vascular resistance and provide a physiological basis for synergistic hypotensive effects of a combined blockade of both pressor systems.

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

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

Differential effects of sulindac on renal hemodynamics and function in the rat

Renal hemodynamics were studied using an electromagnetic perivascular flow sensor in anesthetized rats injected i.v. with vehicle, 5 or 10 mg/kg body weight (b.w.) sulindac. No hemodynamic changes occurred with vehicle (n = 6), but mean arterial pressure was significantly decreased (by 15 mmHg) with sulindac (n = 12). In the 5 mg/kg b.w. sulindac group (n = 7), renal blood flow progressively and significantly increased from 7.88 +/- 0.36 to 8.98 +/- 0.58 ml/min, except during concomitant intrarenal infusion of 3 mg/kg b.w. per h proadifen (n = 7). The pressure limits for efficient and no renal blood flow autoregulation remained unchanged (approx. 100 and 80 mmHg, respectively). In the 10 mg/kg b.w. sulindac group (n = 5), renal blood flow did not change but autoregulatory pressure limits were lowered by 10 mmHg 2 h after treatment (P < 0.025). Also, Na+ retention was marked. Prostanoid excretion in urine was significantly reduced with either dose but basal plasma renin activity was not (about 8 ng/ml per h; n = 15). When plasma renin activity was enhanced after a reduction in renal perfusion pressure (n = 21), it was decreased from 11.5 +/- 1.2 to 7.4 +/- 0.2 ng/ml per h only by 10 mg/kg b.w. sulindac (P < 0.05; n = 6). In conclusion, differential effects of sulindac on renal hemodynamics, Na+ excretion and plasma renin activity were demonstrated. Renal hemodynamic changes could be related in part to the cytochrome P-450 arachidonic acid pathway.

Cardiac baroreflex properties in myocardial infarcted rats

Recent studies demonstrated that chronic but not acute myocardial infarction impairs the cardiopulmonary reflex. The aim of the present study was to evaluate the baroreflex in awake rats bearing short-term (1 day) or long-term (30 days) myocardial infarction. Left ventricular infarction was produced by ligation of the anterior descending branch of the left coronary artery. In order to examine the baroreceptor reflex function by means of sigmoidal curvefitting analysis in conscious rats, reflex heart rate responses were elicited by alternate intravenous injections of phenylephrine (change, +5 to +40 mmHg) and sodium nitroprusside (change, -5 to -40 mmHg). Infarcted rats showed either hypotension plus tachycardia (1 day) or bradycardia (30 days) in resting conditions. The baroreceptor reflex gain (sensitivity) was significantly increased in 30 days (5.20 +/- 0.33 bpm/mmHg, p < 0.01) but not in 1 day (3.78 +/- 0.20 bpm/mmHg) infarcted rats when compared to sham rats (3.83 +/- 0.16 bpm/mmHg). Transmural antero-medio-lateral infarcted areas spanned over nearly 37% (1 day group) and 35% (30 days group) of the left ventricular circumference. Myocardial hypertrophy was showed in right ventricle (39%, p < 0.01) as well as in right (35%, P < 0.05) and left atria (127%, p < 0.001) in the 30 days but not in the 1 day infarcted group. The enhancement of baroreflex correlated significantly with the extent of myocardial necrosis in the 30 days infarcted group. We conclude that baroreflex control of heart rate is well preserved in short- but exaggerated in long-term myocardial infarction. The enhancement of the baroreflex gain could reflect a compensatory mechanism to the impairment of the cardiopulmonary reflex following chronic myocardial infarction and thus contributing to sustain the arterial pressure and heart rate in low levels.

Inhibition of lipoprotein lipase induced cholesterol ester accumulation in human hepatoma HepG2 cells

It has been suggested previously that lipoprotein lipase may act as a ligand to enhance binding and uptake of lipoprotein particles. In the present study we have examined the capacity of bovine milk lipoprotein lipase to induce intracellular accumulation of triglyceride and cholesterol ester by VLDL (Sr 60-400) isolated from Type IV hypertriglyceridemic subject (HTg-VLDL) in HepG2 cells, independent of its lipolytic activity. We have also attempted to elucidate the cellular receptor mechanisms responsible for these effects. HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester were dependent on the presence of an active lipase. Bovine milk lipoprotein lipase (LPL) increases triglyceride mass by 301% +/- 28% (P < 0.0005) and cholesterol ester mass by 176% +/- 12% (P < 0.0005). These HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester did not occur when heat-inactivated lipase was used. Rhizopus lipase could replace LPL and cause equivalent increases in intracellular triglyceride and cholesterol ester (472% +/- 61%(P < 0.005) and 202% +/- 25% (P < 0.025) respectively vs. control). HTg-VLDL treated with LPL and reisolated also caused equivalent increases (274% +/- 18%(P < 0.01) and 177% +/- 12% (P < 0.005) for triglyceride and cholesterol ester). LDL also caused increases in intracellular cholesterol ester (189% +/- 20%(P < 0.005)), although three times more LDL cholesterol had to be added to achieve the same effect. These LDL-induced increases were effectively blocked by monoclonal antibodies directed against the B,E receptor binding domains of apo B (-97% +/- 13% (P < 0.0005) with anti-apo B 5E11 and -68% +/- 13% (P < 0.05) for anti-apo B B1B3) or by anti-B,E receptor antibodies (-77% +/- 7% (P < 0.01) antibody C7). These same antibodies had little effect on the HTg-VLDL+LPL-induced increases in cholesterol ester (+21%, +15% and -22% for 5E11, B1B3 and C7, respectively). Monoclonal anti-apo E antibodies also had no effect on LDL-mediated increases in intracellular cholesterol ester, but had a small and significant effect on VLDL-mediated increases in cholesterol ester. However, heparin, which interferes with cell surface proteoglycan interaction, was very effective at blocking HTg-VLDL-mediated increases in cholesterol ester in the presence of LPL (-86% +/- 8% P < 0.0005). Heparin was also effective in the presence of Rhizopus lipase (-79%) or lipolyzed re-isolated HTg-VLDL (-95%). These results suggest that lipoprotein lipase may enhance the uptake process beyond its role in lipolytic remodelling but does not appear to be an absolute requirement. In contrast, heparin had no effect on LDL-mediated cholesterol ester accumulation. Lactoferrin, which inhibits interaction with the low density lipoprotein receptor-related protein (LRP), was also very effective at inhibiting HTg-VLDL increases in intracellular cholesterol ester (-95% +/- 6%, P < 0.01). However, there was no effect of either heparin or lactoferrin on HTg-VLDL-mediated triglyceride accumulation. Thus cell surface heparin sulphate may facilitate intracellular lipid acquisition by providing a stabilizing bridge with the lipoproteins and enhance uptake through receptor-mediated processes such as LRP.

Influence of endogenous angiotensin on the renovascular response to norepinephrine

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

Blood pressure control in arterial- and cardiopulmonary receptor denervated dogs

The mechanisms influencing arterial blood pressure and heart rate were studied in conscious foxhounds after chronic sino-aortic and cardiopulmonary denervation (N = 6). In previous investigations it was shown, that this denervation produces hypertension and tachycardia, which is confirmed by the present study: Mean arterial blood pressure increased from 101 +/- 3 to 123 +/- 6 mmHg (P less than 0.05), and heart rate rose from 85 +/- 6 to 124 +/- 5 beats min-1 (P less than 0.001). The variability of mean arterial blood pressure, but not that of heart rate increased (from 6 +/- 1 to 22 +/- 2 mmHg (P less than 0.001). The administration of the alpha-adrenergic blocker prazosin reduced both mean arterial blood pressure (-33 +/- 8 mmHg, P less than 0.01) and its variability (-12 +/- 1 mmHg, P less than 0.01), thus suggesting an alpha-adrenergic mediated hypertension. beta-blockade by propranolol blunted the heart rate increase (-24 +/- 5 beats min-1, P less than 0.05). Although plasma renin activity increased in the denervated dogs, converting enzyme inhibition had little effect on mean arterial blood pressure and heart rate. In conclusion, chronic sino-aortic and cardiopulmonary denervation enhances the alpha and beta-adrenergic component of cardiovascular control in a different fashion. While the alpha-adrenergic component induces fluctuations around an elevated arterial blood pressure level, the beta-adrenergic tone to the heart increases without any significant increase in variability.

Preservation of renal function by angiotensin during chronic adrenergic stimulation

The purpose of the present study was to determine the role of angiotensin II (Ang II) in mediating renal responses to chronic intrarenal norepinephrine infusion. Norepinephrine was continuously infused for 5 days into the renal artery of unilaterally nephrectomized dogs at progressively higher daily infusion rates: 0.05, 0.10, 0.20, 0.30, and 0.40 micrograms/kg/min. In three additional groups of dogs, norepinephrine infusion was repeated during chronic intravenous captopril administration to fix plasma Ang II concentration at 1) low levels (no Ang II infused), 2) high levels in the renal circulation (Ang II infused intrarenally at a rate of 1 ng/kg/min), and 3) high levels in the systemic circulation (Ang II infused intravenously at a rate of 5 ng/kg/min). In the control group of animals with intact renin-angiotensin systems, there were progressive increments in mean arterial pressure (from 96 +/- 4 to 141 +/- 6 mm Hg) and plasma renin activity (from 0.4 +/- 0.1 to 10.9 +/- 4.5 ng angiotensin I/ml/hr) and concomitant reductions in glomerular filtration rate and renal plasma flow to approximately 40% of control during the 5-day norepinephrine infusion period. In marked contrast, when captopril was infused chronically without Ang II, mean arterial pressure was 20-25 mm Hg less than that under control conditions, and the renal hemodynamic effects of norepinephrine were greatly exaggerated; by day 3 of norepinephrine infusion, both glomerular filtration rate (16 +/- 2% of control) and renal plasma flow (12 +/- 4% of control) were considerably lower than values in control animals (86 +/- 4% and 80 +/- 8% of control, respectively). Similarly, when a high level of Ang II was localized in the renal circulation during captopril administration, mean arterial pressure was depressed, and again there were pronounced renal responses to norepinephrine. Conversely, when Ang II was infused intravenously during captopril administration, mean arterial pressure was not reduced, and the glomerular filtration rate and renal plasma flow responses to norepinephrine were similar to those that occurred under control conditions. These findings indicate that the renin-angiotensin system prevents exaggerated renal vascular responses to chronic norepinephrine stimulation by preserving renal perfusion pressure.

Renal effects of angiotensin-converting enzyme inhibition in congestive heart failure

Some studies report that inhibition of angiotensin-converting enzyme (ACE) improves renal function in patients with congestive heart failure, whereas others report that renal deterioration is a frequent complication of treatment with ACE inhibitors. This article explores the mechanisms by which antagonism of the renin-angiotensin system improves kidney function in some patients while causing harm in others. ACE inhibition may alter renal blood flow, glomerular perfusion pressure, basement membrane activity and renal tubular function both directly and indirectly. In most patients, renal function is maintained as other neurohormonal mechanisms compensate for the negative effects and permit the positive effects (such as improved renal flow) to predominate. However, when physiologic characteristics or iatrogenic interventions (such as volume reduction or prostaglandin inhibition) limit the effectiveness of neurohormonal compensation to maintain renal autoregulation, clinically important deterioration in renal function may occur. An understanding of the renal effects of ACE inhibitors permits their safe and effective use in most patients with congestive heart failure.

Resetting of renal autoregulation in conscious dogs: angiotensin II and alpha1-adrenoceptors

It has recently been shown, that common carotid occlusion (CCO) impairs autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR). This study was designed to investigate the mechanisms by which a moderate sympathetic stimulus influences RBF and GFR autoregulation. CCO provided a moderate sympathetic stimulus, and impaired autoregulation by increasing the lower autoregulatory limit of RBF and GFR by 21-30 mmHg. Basal RBF and GFR were not affected. A low-dose intrarenal infusion of the alpha 1-adrenoceptor agonist methoxamine (which did not change total RBF or GFR) induced a similar shift as CCO (n = 5, RBF: +31 +/- 11 mmHg, P less than 0.05; GFR: +24 +/- 4 mmHg, P less than 0.01). In another group it was shown, that a combination of CCO with an intrarenal angiotensin II (A II) blockade (saralasin) did not significantly alter the response to CCO (n = 7). These data suggest an alpha 1-adrenergic pathway for the sympathetic resetting of autoregulation. An augmented A II formation does not play a major role in mediating this effect.

Sympathetic modulation of renal hemodynamics, renin release and sodium excretion

In anesthetized animals it has been shown previously, that the influence of electrical stimulation of efferent renal nerves on renal function with increasing stimulation frequencies can be graded; renin release is affected at low, sodium excretion at intermediate and vascular resistance at high stimulation frequencies. Experiments in conscious dogs are reviewed, which present evidence for a similar functional dissociation under physiological conditions. Moderate activations of the renal sympathetic nerves, which do not change renal blood flow 1) decrease sodium excretion independent of changes in angiotensin II, 2) interact with the pressure-dependent mechanism of renin release by resetting its threshold pressure and 3) modulate autoregulation by increasing the lower limits of glomerular filtration rate and renal blood flow-autoregulation. These findings may contribute to our understanding of the role of the renal nerves in the pathophysiology of congestive heart failure and hypertension.