Hypertension due to renal artery stenosis results from the kidney response to hypoperfusion. Cells of the juxtaglomerular complex secrete renin that converts angiotensinogen to angiotensin I (AT I), which in turn is converted to angiotensin II (AT II) by angiotensin-converting enzyme. This peptide increases aldosterone secretion and water retention, and hence leads to arterial hypertension [1,2]. Renin-AT II-induced expansion of the extracellular volume enhances atrial natriuretic peptide (ANP) release from myocardial cells . AT II and ANP have antagonistic effects on the cardiovascular system and both are involved in the pathophysiology of hypertension and cardiac hypertrophy [4,5].
Recently, it has become evident that adipose tissue is a rich source of metabolic substrates, including free fatty acids (FFAs) and angiotensinogen . In addition, adipose tissue is the target of AT II and ANP [6,7]. It has been suggested that activation of the renin-AT II system leads to concomitant changes in lipid metabolism, including increased adipose lipoprotein lipase (LPL) activity, a decisive factor of FFA metabolism, and FFA mobilization . It has been suggested that ANP increases triglyceride degradation in adipose tissue and subsequently enhances FFA release from the adipocytes. The ANP-mediated lipid-mobilizing result seems to be independent to sympathetic nervous system activation, and it possibly reflects a direct effect of the ANP on adipose tissue metabolic function .
Ketamine is a common anaesthetic agent, which demonstrates effects throughout the central nervous system. Ketamine increases blood pressure (BP), heart rate (HR) and cardiac output due to central sympathetic stimulation and inhibition of norepinephrine re-uptake. For this reason, ketamine is commonly administrated in acute hypovolaemic shock, and it may be useful for patients with severe cardiac failure [10,11].
Based on the above indications, it would be of great interest to explore the effect of ketamine on ANP, AT II and FFA concentrations, both in healthy rats as well as in those with renal artery stenosis, in which the ANP and the renin-AT II systems are highly stimulated.
Forty male Wistar rats (180-240 g) were used for this study. The rats were handled in accordance with the guide for the Use of Laboratory Animals, published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1985). There were four experimental groups with ten rats each:
- Group A: Untreated, control group.
- Group B: Ketamine-control group. The animals received no operation on day 1, then ketamine intraperitoneally at a dose of 100 mg kg−1, 20 min before sacrifice [12,13].
- Group C: 2-kidney 1-clip group. The left renal artery of the animals was exposed and a silver clip was applied to it . The surgical procedure was performed on day 1 under anaesthesia induced by ketamine 100 mg kg−1.
- Group D: Ketamine-2-kidney 1-clip group. The animals were submitted to renal artery clipping on day 1 as in Group C, then received ketamine as in Group B 20 min before sacrifice.
The experiment lasted 21 days and the animals were sacrificed on the last day by decapitation. Blood was collected, divided in two tubes with or without ethylene diamine tetracetic acid (EDTA), separated into plasma and serum by centrifugation at 4°C and stored at −80°C. After blood collection, the perirenal adipose tissue was removed, washed thoroughly in water, weighed and stored at −80°C. In addition, the heart was excised and the left ventricle was isolated, weighted and stored at −80°C.
AT II concentrations were determined with a commercially available radioimmunoassay (range 2-500 pg mL−1, cut off value 0-7 pg mL−1, intra-inter assay precision 8.3% and 11%, respectively - Bohmann Diagnostic, Switzerland). LPL activity and AT II content were measured in adipose tissue samples as described previously [14,15]. ANP concentrations in serum were measured by radioimmunoassay (Peninsula Labs, Belmont, USA) with the method of Nagase and colleagues . The quantitative determination of serum FFA concentrations was performed spectrophotometrically at 550 nm using a commercially available FFA assay (Wako-Chemie GMBH, Neuss, Germany; precision 1.1% coefficient of variation for mean concentration 0.99 mmol L−1, linear range from 0 to 2 mmol L−1). The weight of the left ventricle relative to total body weight (left ventricular weight/body weight) was used as a cardiac hypertrophy index .
Statistical analysis was performed by one-way analysis of variance (ANOVA) with Bonferroni's or Dunnett's test between groups. The level of significance was set at 5% (P < 0.05) (SPSS for Windows version 10.0).
The cardiac hypertrophy index and serum FFA concentrations were significantly higher in the 2-kidney 1-clip rats than in the normal ones (Table 1).
In the 2-kidney 1-clip rats, adipose tissue LPL activity was significantly decreased compared to the control groups (Groups A and B), and ketamine administration did not affect LPL activity either in normal or in banded animals (Table 2).
In renal-banded rats (Groups C and D), AT II concentrations were significantly higher than in the normal controls. AT II concentrations both in serum and in adipose tissue were not affected by ketamine administration in any of the groups (Table 2).
ANP levels were significantly increased in the renal-banded rats (Group C: 69 ± 5.4 pg mL−1 and Group D: 71 ± 5.1 pg mL−1) compared to the normal ones (Groups A and B). Ketamine administration profoundly increased ANP concentrations in normal animals (Group B: 44 ± 1.7 pg mL−1) but not in the control group (Group A: 35 ± 1.5 pg mL−1). However, in the banded rats (Group D), ketamine treatment did not induce any significant alteration in serum ANP levels compared to the renal-banded controls (Group C) (Fig. 1).
AT II is thought to be a key regulator of the cardiovascular system, and to play a fundamental role in the development of hypertension and of left ventricular hypertrophy [1,2]. In addition, AT II may exert physiological actions in other tissues, including the peripheral and central nervous systems. In the central nervous system, AT II increases sympathetic nervous system activity , while in the periphery its action includes stimulation of catecholamine secretion and vasoconstriction . It should also be kept in mind that AT II and catecholamines may induce cardiac ANP secretion [20,21].
AT II can also induce significant changes in serum lipid parameters by decreasing adipose LPL activity [14,22]. It inhibits adipocyte proliferation through an AT II receptor-mediated mechanism. Blockade of the AT II receptors, in contrast, stimulates adipogenesis, FFA storage and reduces FFA plasma concentrations . Other studies have also suggested that ANP-related lipid-mobilizing pathways may contribute to adipose tissue lipolysis and FFA liberation into the blood stream [9,23]. The results of the present study suggest that the high serum AT II concentrations lead to increased left ventricular weight, increased ANP secretion and to reduced adipose tissue LPL activity in the renal-banded animals.
Earlier experimental studies have suggested that ketamine interferes with lipid metabolism by increasing FFA concentrations in serum, possibly as a result of sympathetic nervous system activation. Furthermore, in doses over 100 mg kg−1 ketamine seems to down-regulate adipose LPL activity, and additionally to enhance FFA oxidation by increasing medium-chain acyl-CoA content in muscles . To the best of our knowledge, no studies have been so far reported, in which the simultaneous effects of ketamine on endocrine (AT II, ANP) and lipid parameters in a model of renal artery stenosis have been investigated.
In the present study, ketamine treatment had no effect on serum AT II and adipose tissue LPL activity in either normal or renal-banded rats. However, it augments circulating concentrations of both ANP and FFA in normal animals. It seems that ketamine exerts a direct or indirect effect, possibly through sympathetic activation, on ANP secretion. Moreover, the increased FFA levels may be due to ketamine-mediated sympathetic stimulation or to ANP-induced lipolysis  and not to serum insulin fluctuations, since previous studies have shown that ketamine administration does not alter insulin concentrations in serum .
In contrast to the normal animals, the stimulating effect of ketamine on ANP and FFA secretion is not detectable in renal-banded rats. A possible explanation is that in the 2-kidney 1-clip animals, the renin-AT II and ANP systems are so highly activated that a peak plateau of ANP and FFA secretion may have been reached, and thus ketamine could have no further effect on serum ANP and FFA levels.
In summary, the interesting finding of this data is that ketamine induces noteworthy changes in several endocrine and metabolic parameters, such as serum ANP and FFA concentrations. Importantly enough, the ketamine-induced mobilization of both FFA and ANP may interfere with electrolyte-water and oxidative metabolism regulation. However, as the renin-AT II and ANP systems are maximally activated during renal artery stenosis, ketamine does not seem to exert an additive response to serum ANP and FFA levels, and therefore its baseline contribution to these factors is abolished.
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