(Hypertension. 1996;28:1047-1054.)
© 1996 American Heart Association, Inc.
Articles |
Hypertension in Obese Zucker Rats
Role of Angiotensin II and Adrenergic Activity
the Department of Physiology and Biophysics, University of Mississippi Medical Center, Jackson.
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Abstract |
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We designed our studies to determine whether blood pressure is elevated in obese Zucker rats compared with lean control rats and to test the importance of the renin-angiotensin and adrenergic nervous systems in long-term blood pressure control in this genetic model of obesity. We monitored mean arterial pressure 24 hours per day using computerized methods in 13- to 14-week-old lean and obese Zucker rats maintained on a fixed, normal sodium intake (3.3 mmol/d). Mean arterial pressure (average of 5 days) was higher in obese (100±1 mm Hg) than in lean (86±1) rats. Although control plasma renin activity was lower in obese than in lean rats (3.66±0.15 versus 5.48±0.11 ng angiotensin I/mL per hour), blood pressure sensitivity to exogenous angiotensin II was greater in obese than in lean rats. Blockade of endogenous angiotensin II receptors with losartan (10 mg/kg per day) for 7 days also caused a greater decrease in blood pressure in obese (36±2 mm Hg, n=6) than in lean (25±1, n=5) rats. However, combined
- and ß-adrenergic blockade with terazosin (10 mg/kg per day) and propranolol (10 mg/kg per day), respectively, for 8 days caused only modest decreases in blood pressure in obese (9±3 mm Hg, n=8) and lean (4±2, n=6) rats, despite effective - and ß-adrenergic blockade. These results suggest that increased arterial pressure in obese Zucker rats depends in part on angiotensin II. However, additional mechanisms may also contribute to increased blood pressure in obese Zucker rats.
Key Words: obesity • blood pressure • renin • sympathetic nervous system • kidney • sodium
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Introduction |
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Although obesity is believed to be a major cause of human essential hypertension, the mechanisms responsible for weight-related increases in BP are still obscure. Recent studies in the obese dog model,1 2 3 first described by Wood and Cash,4 have helped to elucidate some of the renal and cardiovascular changes associated with dietary-induced obesity. However, characterization of another animal model of obesity-associated hypertension may provide information about whether the fundamental mechanisms involved in elevating BP are species dependent. In addition, recent work has demonstrated the potential importance of genes in determining variations in human obesity,5 6 7 and the extent to which hypertension and obesity aggregate in family members suggests that they share common antecedents, both genetic and environmental.
The Zucker strain of obese rat shares many similarities with obese humans who have insulin-resistant, type II diabetes as well as a strong genetic component in the transmission of obesity.8 9 10 11 Therefore, this animal model may provide additional insight into the etiology of obesity hypertension. However, reports have conflicted over whether the obese Zucker rat is hypertensive compared with lean rats.12 13 14 15 16 These variable findings could result from different measurement techniques, from differences in the age and/or sex of the rats studied, or from other factors, such as the level of sodium intake.
One of our aims in this study was to test the hypothesis that obese Zucker rats are hypertensive compared with their lean genetic controls. To accomplish this, we measured BP continuously 24 hours per day in conscious, chronically instrumented, age-matched lean and obese Zucker rats, with sodium chloride intake carefully controlled. In addition, because evidence suggests that both the RAS3 17 18 and the sympathetic nervous system19 20 21 may be involved in the pathogenesis of obesity-associated hypertension, we also evaluated the contribution of Ang II and adrenergic activity to BP regulation in obese compared with lean Zucker rats.
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Methods |
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Animals
The experimental protocols were reviewed and approved by the University of Mississippi Medical Center Institutional Animal Care and Use Committee and were carried out according to the Guidelines for the Care and Use of Laboratory Animals of the National Institutes of Health and the guidelines of the Animal Welfare Act. Studies were conducted in male lean and obese Zucker rats (Harlan Sprague Dawley, Inc, Indianapolis, Ind), approximately 13 to 14 weeks of age and weighing 298±3 and 539±5 g, respectively.
Chronic Instrumentation
All rats underwent surgery for implantation of chronic indwelling arterial and venous catheters. With rats under gas anesthesia (isoflurane) and the use of aseptic conditions, a laparotomy was performed, and a nonocclusive catheter was placed in the abdominal aorta, caudal to both kidneys. A femoral vein catheter was also implanted via a separate incision, and the tip was advanced into the vena cava. Both catheters were routed subcutaneously to the scapular region and exteriorized through a stainless steel button implanted subcutaneously. All incisions were infiltrated with penicillin G and 0.25% bupivacaine HCl solution (Sensorcaine-MPF). A stainless steel spring was used to protect the catheters and tether the rat to the infusion swivel.
Rats were allowed to recover in a warmed cage 1 to 2 hours after surgery. Thereafter, rats were placed in individual metabolic cages in a quiet, air-conditioned room with a 12-hour light/dark cycle. The catheters were connected to a dual-channel infusion swivel (Instech Laboratories, Inc) mounted above the cage. The arterial catheter was filled with heparin solution (1000 USP/mL) and connected via the swivel to a pressure transducer (Cobe) mounted on the cage exterior at the level of the rat.
BP Measurements
MAP and HR were monitored continuously 24 hours per day. Pulsatile arterial pressure signals were amplified, sent to an analog-to-digital convertor, and analyzed by computer with customized software. The analog signal was sampled at 500 Hz per channel for 4 seconds each minute 24 hours per day. A 22-hour period, which excluded 2 hours for daily cage and room maintenance and catheter flushing, was used for calculation of daily MAP and HR.
Sodium Intake
Total sodium intake was fixed at a normal level of 3.3 mmol/d in all experimental protocols. This was achieved by giving a continuous intravenous infusion of 20.5 mL sterile 0.9% saline per day (containing 26 µg mezlocillin and 26 000 U penicillin G). All solutions were infused with a syringe pump (Harvard Apparatus) through a filter (22 µm, Cathivex, Millipore) into the venous line. The sodium infused plus a low sodium rat chow (0.006 mmol sodium per gram, Teklad) constituted all the sodium the rats received. The infusion was started immediately after the rat was placed in the metabolic cage, and 5 to 7 days were allowed for acclimation before control measurements were recorded.
Renal Hemodynamics
GFR and effective renal plasma flow were measured with the use of a 4-hour fasted plasma sample after a 48-hour infusion of 125I-iothalamate (0.01 µCi/kg per minute) and 131I-iodohippurate (0.02 µCi/kg per minute), respectively. Steady state is achieved with this 48-hour infusion protocol; therefore, a sample of the infusate was counted and the infusion rate of isotope substituted for the urinary excretion rate of isotope for the calculation of clearance.22 RBF was calculated from renal plasma flow and hematocrit.
Hormone Assays
Blood was collected in chilled sodium EDTA tubes. Blood samples were then centrifuged at 4°C for 30 minutes at 4000 rpm. Plasma was aliquoted and immediately frozen at -20°C until assayed. PRA was measured by the radioimmunoassay method of Haber et al23 with 125I-labeled Ang I (New England Nuclear) and antibody (Chemicon). Plasma insulin concentration was measured by a radioimmunoassay kit (Cambridge Medical Diagnostic Products). The samples were assayed against rat insulin standards (Linco rat standards, lot No. 1026). Plasma glucose was measured by the hexokinase method (Sigma Chemical Co) with the use of a modified 96-well microtiter plate assay. Urine and plasma electrolytes were analyzed by flame photometry (model IL 944, Instrumentation Laboratories). Hematocrit was measured with the use of microcapillary tubes, and plasma protein concentration was measured by refractometry.
Experimental Protocols
Protocol 1: Renal and Cardiovascular Hemodynamics
After confirming that the rats were in sodium balance and had stable BP and HR, we made control measurements for 5 days to determine average MAP and HR in 31 lean and 30 obese Zucker rats on a normal sodium intake (3.3 mmol/d). On the 3rd day, the rats were fasted for 4 hours, and a 1.8-mL arterial blood sample was collected for measurement of plasma insulin, PRA, glucose, hematocrit, plasma proteins, and 125I and 131I activities. In addition to being used for comparison of baseline hemodynamics and renal function between lean and obese rats, this 5-day period also served as the control period for protocols 2 and 3.
Protocol 2: Role of Ang II in BP Regulation
Experiments were conducted in chronically instrumented, conscious lean (n=5) and obese (n=6) Zucker rats maintained on a normal sodium intake of 3.3 mmol/d. After 5 days of control measurements, rats were given an intravenous infusion of losartan (EI DuPont de Nemours & Co), a selective angiotensin type 1 receptor antagonist, at a rate of 10 mg/kg per day, for 8 days. This intravenous dose of losartan has been shown to effectively block Ang II receptors within 3 to 4 hours of infusion in rats.24 After losartan administration, a 9-day recovery period was allowed. MAP and HR were monitored continuously as described above. Renal clearances and blood sampling for measurement of plasma insulin, PRA, plasma glucose, hematocrit, plasma proteins, and 125I and 131I activities were made during the 3rd day of the control period, 4th day of the experimental period, and 7th day of the recovery period.
We assessed the effectiveness of Ang II receptor blockade in lean and obese Zucker rats by comparing MAP responses to a single intra-arterial bolus injection of Ang II (0.4 µg per rat) before, during, and after losartan administration. Because bolus doses of vasoactive drugs given on the basis of body weight may result in higher blood concentrations in obese than in lean animals, the same total amount of Ang II was given to lean and obese rats.
Protocol 3: Role of the Adrenergic System in BP Regulation
Experiments were conducted in chronically instrumented, conscious lean (n=6) and obese (n=8) Zucker rats maintained on a normal sodium intake of 3.3 mmol/d. Total volume infusion was 43 mL/d in this group of rats. In addition to the 20.5 mL of saline given daily, 22.5 mL of distilled water was added per day to increase the solubility of terazosin. After 5 days of control measurements, rats were given an intravenous infusion of the 1-adrenergic blocker terazosin (Abbott Laboratories) and the ß-adrenergic blocker propranolol (Sigma) at a rate of 10 mg/kg per day each for a period of 8 days. Previous studies25 have shown that these doses effectively block 1- and ß-adrenergic receptors. A recovery period of 8 days was allowed. MAP and HR were monitored continuously as described above. Renal clearances and blood sampling for measurement of plasma insulin, PRA, plasma glucose, hematocrit, plasma proteins, and 125I and 131I activities were made during the 3rd day of the control period, 7th day of the experimental period, and 7th day of the recovery period.
We assessed the effectiveness of -adrenergic blockade by comparing MAP responses to a single intra-arterial bolus injection of phenylephrine (4 µg per rat) before, during, and after - and ß-adrenergic blockade. Similarly, we assessed the effectiveness of ß-blockade by comparing MAP responses to a single intra-arterial bolus injection of isoproterenol (0.35 µg per rat) before, during, and after - and ß-adrenergic blockade.
Statistical Analyses
Results are expressed as mean±SE. Between- and within-group effects were evaluated by two-factor ANOVA with repeated measures on one factor (time) and with Dunnett's t test when appropriate.26 Statistical significance was established at a value of P<.05.
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Results |
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Protocol 1: Renal and Cardiovascular Hemodynamics
The average of 5 days of continuous BP monitoring showed that obese Zucker rats were hypertensive compared with their lean controls (Table 1
). For the 5-day control period, MAP in 30 lean rats averaged 86±1 mm Hg compared with 100±1 in 31 obese rats (P<.05). HR was significantly lower in obese (365±4 beats per minute) than in lean rats (392±4, P<.05).
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The average daily food intake in lean rats was 16±1 g/d compared with 26±1 in obese rats. Despite differences in food intake, sodium intake was not different between lean and obese rats because the rats were fed a sodium-deficient chow and sodium intake was controlled by infusion of isotonic saline (Table 1
). Sodium excretion was not different in lean and obese rats, but urinary potassium excretion was significantly lower in lean (3.15±0.10 mmol/d) than in obese (5.40±0.16) rats. The only source of potassium intake in these rats was the rat chow, and food intake was nearly twofold higher in obese rats. Therefore, the higher potassium intake in obese rats most likely explains the higher urinary potassium excretion.
GFR values per gram of kidney weight were not statistically different between lean and obese rats (Table 1). However, because total kidney weight was more than 40% greater in obese (3.190±0.053 g) than in lean (2.251±0.040) rats, absolute GFR was approximately 50% higher in obese (2.54±0.14 mL/min) than in lean (1.62±0.11) Zucker rats. Absolute sodium reabsorption was also significantly elevated by approximately 52% in obese (0.37±0.03 mmol/min) compared with lean (0.24±0.02) rats. Although a tendency for higher RBF values per gram of kidney weight was observed in obese rats, this difference was not statistically significant. However, absolute RBF was significantly higher in obese (6.62±0.37 mL/min) than in lean (3.97±0.28) rats.
Fasting plasma insulin levels were almost sevenfold higher in obese than in lean rats, and obese rats also had a significantly higher plasma glucose concentration. PRA was significantly lower in obese than in lean rats (3.66±0.15 versus 5.48±0.11 ng Ang I/mL per hour, Table 1).
Protocol 2: Role of the RAS in BP Control
Losartan administration for 7 days significantly decreased BP in lean and obese rats (Fig 1). ANOVA revealed that BP fell more in obese than in lean rats, reaching statistical significance on day 2 of losartan infusion. On the 7th day of losartan, MAP decreased by 37±1 mm Hg in obese rats, compared with 25±1 in lean rats. After losartan was stopped, MAP returned to levels not different from control by day 7 in lean and obese rats. HR was lower in obese than in lean rats during control conditions (350±3 versus 378±3 beats per minute), and this difference was maintained during and after losartan administration (Table 2).
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Before losartan infusion, the systemic pressor response to 0.4 µg Ang II was higher in obese than in lean rats, as evidenced by a greater area under the BP curve (ie, the increment in MAP times minutes after Ang II: 205±5 versus 147±16 mm Hg·min) (Table 3). The changes in peak MAP from baseline values, however, were not different between lean and obese rats. After 7 days of losartan, the pressor response to the same dose of Ang II was markedly reduced in lean and obese rats compared with their respective baseline values, indicating the effectiveness of Ang II receptor blockade. After losartan administration, the pressor response to Ang II in lean and obese rats tended to return to control values (Table 3).
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Seven days of intravenous losartan administration had no significant effect on food intake, sodium and potassium intakes, or GFR in either lean or obese rats compared with their respective control values (Tables 2 and 4). However, losartan administration significantly increased RBF in both lean and obese rats from control values. After losartan was stopped, RBF returned to levels not different from control in obese and lean rats after 7 days of recovery. Urinary sodium excretion was not different between lean and obese rats before, during, and after losartan administration; however, obese rats excreted twice as much urinary potassium as lean Zucker rats during control and during losartan administration (Table 4).
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Losartan had no significant effect on either fasting plasma insulin or glucose levels in lean or obese rats compared with their respective control values (Table 4). PRA was significantly lower in obese than in lean rats during the control period, and losartan administration significantly increased PRA in lean and obese rats to 15.00±1.21 and 9.04±0.40 ng Ang I/mL per hour, respectively. After 7 days of recovery from losartan, PRA was still significantly elevated in both lean and obese rats compared with control values (Table 4).
Protocol 3: Role of the Sympathetic Nervous System in BP Control
Terazosin and propranolol administration caused a rapid fall in BP in lean and obese Zucker rats. MAP for the 8-day period of adrenergic blockade averaged 77±2 mm Hg in lean rats and 85±2 in obese rats (Table 5). As shown in Fig 2, however, MAP in lean rats gradually returned toward control during the 8-day period of adrenergic blockade, and by day 8, the fall in MAP in lean rats was not statistically different from control values (4±2 mm Hg). In contrast, MAP in obese rats remained significantly lower than control values throughout the 8-day period of adrenergic blockade, and by day 8, the fall in MAP in obese rats was statistically different from control values (9±3 mm Hg, P<.05). After adrenergic blockade was stopped, MAP increased rapidly in both groups, returned to control on the 2nd day of recovery, and actually rose above control values in lean and obese rats; however, only days 3 to 5 of recovery in obese rats were statistically different from control. HR was lower in obese than in lean rats during control conditions (371±10 versus 405±7 beats per minute) and decreased significantly from control values in both groups during adrenergic blockade (328±5 and 368±9, respectively) (Table 5).
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During control conditions, the peak MAP response to 4 µg phenylephrine was significantly greater in obese compared with lean rats (43±5 versus 28±4 mm Hg, P<.05). However, the area under the BP curve after phenylephrine was similar in lean and obese rats. A similar peak decrease in MAP in response to isoproterenol was observed in lean and obese rats, as well as a similar area under the BP curve in response to isoproterenol. After 8 days of adrenergic blockade, the pressor response to phenylephrine and the depressor response to isoproterenol were markedly diminished in lean and obese rats, providing evidence for effective blockade of - and ß-adrenergic receptors. After 8 days of recovery, the BP responses to the adrenergic agonists returned to control levels in lean and obese rats (Table 6).
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Eight days of intravenous administration of terazosin and propranolol had no significant effects on food intake, sodium and potassium intakes, or GFR in either lean or obese rats compared with their respective control values (Tables 5 and 7). Adrenergic blockade significantly increased RBF only in obese rats. Urinary sodium excretion was not different between lean and obese rats before, during, and after adrenergic blockade; however, obese rats excreted twice as much potassium as lean Zucker rats. Adrenergic blockade had no significant effect on fasting plasma insulin or glucose levels in lean or obese Zucker rats. PRA was lower in obese than in lean rats and was not changed significantly by adrenergic blockade (Table 7).
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Discussion |
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The most important findings of these studies are (1) the obese Zucker rat was hypertensive compared with its lean genetic control; (2) elevated BP in obese compared with lean Zucker rats depended in part on Ang II; and (3) adrenergic activity appears to play a modest role in raising arterial pressure in obese Zucker rats.
Hemodynamics in Obese Zucker Rats
Previous studies have reported conflicting results about whether obese Zucker rats are hypertensive compared with their lean controls. Several studies, using indirect, tail-cuff measurements or direct, acute measurements of intra-arterial pressure, have reported that the BP of the obese Zucker rat is not elevated.12 13 27 28 In contrast, other reports, using direct intra-arterial pressure measurement over several hours in conscious or anesthetized rats, have indicated that arterial pressure is significantly higher in obese than in lean Zucker rats.14 15 16 29 30
Some of the differences in previous studies may be related to the methods of BP measurement or to differences in sodium intake. In some cases, sodium intake was not carefully controlled. Acute measurements of BP, particularly with the tail-cuff technique, are susceptible to errors induced by handling stress, for example, that could render the measured value unrepresentative of the 24-hour average BP.
To determine whether BP is elevated in obese Zucker rats, we measured MAP continuously, 24 hours per day, in rats left undisturbed in their metabolic cages. We also carefully controlled sodium intake so that both lean and obese rats received essentially the same amount of sodium despite large differences in food intake. Our results indicate that MAP in obese Zucker rats is approximately 14 mm Hg higher than in lean control rats. This difference in MAP agrees with previous reports of moderate hypertension in obese Zucker rats, showing elevations of 14 to 22 mm Hg from baseline MAP in obese compared with lean Zucker rats.14 15 16 29 30
HR, monitored 24 hours per day, was significantly lower in obese than in lean rats, a finding consistent with previous reports in which HR was monitored for only a short time.28 30 However, the finding of lower HRs in obese Zucker rats differs from previous studies in obese dogs and humans in which HR was increased rather than decreased compared with lean controls.1 2 3 31 Decreased HR in obese Zucker rats could signify decreased activity of the cardiac sympathetic nerves, increased parasympathetic tone, or a decrease in the intrinsic rate of the heart. Currently, the mechanisms responsible for lower HR in obese than in lean Zucker rats are not clear. However, Barringer and Bunag28 reported that baroreflex HR responses, whether manifested as bradycardia or tachycardia, were markedly attenuated in obese compared with lean Zucker rats. These investigators also suggested that efferent sympathetic and parasympathetic mediations of HR reflexes were unevenly blunted in obese compared with lean Zucker rats, with more parasympathetic than sympathetic dysfunction.
Kidney Function in Obese Zucker Rats
Reports have conflicted about renal function in obese Zucker rats. By 12 to 13 weeks of age, whole-kidney GFR has been reported to be either normal29 or elevated13 in anesthetized obese Zucker rats compared with lean littermates. In the present study, a higher MAP in obese Zucker rats was required to achieve sodium balance, indicating impaired pressure natriuresis, similar to the impairment of pressure natriuresis that has been reported for obese dogs32 and abdominally obese adult humans.33 Theoretically, impaired renal excretory capability could be caused by decreased GFR or by increased tubular reabsorption. Our findings that GFR and RBF in conscious obese Zucker rats are increased by approximately 50% and that sodium reabsorption is also markedly elevated compared with lean Zucker rats suggest that altered pressure natriuresis in obese Zucker rats may be due to increased tubular reabsorption rather than decreased GFR.
The mechanisms responsible for altered renal function and hypertension in obese Zucker rats are not clear. Thus, another aim of this study was to test the possible contribution of the RAS and adrenergic system to altered renal function and hypertension in obese Zucker rats.
Role of the Adrenergic System in Obese Zucker Rats
One mechanism that could contribute to elevated tubular reabsorption, altered pressure natriuresis, and hypertension in obesity is increased sympathetic activity. Increased caloric intake has been suggested to activate the sympathetic nervous system, and caloric restriction may suppress sympathetic activity in experimental animals and humans, as assessed by various indirect methods, such as norepinephrine turnover in peripheral tissues.20 Landsberg21 suggested that hyperinsulinemia in obesity may stimulate the ventromedial hypothalamus, resulting in increased sympathetic outflow, renal sodium retention, and elevated arterial pressure.
Activation of the sympathetic nervous system appears to be important in obesity-induced hypertension in the dog. Seven days of - and ß-adrenergic receptor blockade markedly reduced BP in obese hypertensive dogs.34 In addition, Kassab et al35 reported that renal denervation markedly attenuated the rise in BP associated with the induction of obesity. However, reports have conflicted about the level of sympathetic activity in obese Zucker rats. Evaluation of sympathetic function by measurements of norepinephrine turnover in individual organs as an index of sympathetic activity suggested a decrease in sympathetic activity in obese Zucker rats that was organ specific.36 For example, norepinephrine turnover was decreased in the pancreas and in interscapular brown adipose tissue but not in the heart or white adipose pads of obese Zucker rats.36 However, previous reports also suggest that obese Zucker rats exhibit enhanced pressor sensitivity to norepinephrine.37
To evaluate the contribution of the adrenergic nervous system to the elevated BP observed in obese compared with lean Zucker rats, we assessed the effects of chronic - and ß-adrenergic receptor blockade with terazosin and propranolol. Eight days of - and ß-adrenergic receptor blockade modestly reduced BP in obese and lean Zucker rats; however, BP in lean rats returned to levels not different from control. Thus, these results support the possibility that adrenergic activity may contribute, albeit modestly, to the elevated BP in obese Zucker rats.
Adrenergic blockade for 8 days did not change PRA in lean or obese Zucker rats, suggesting that the contribution of ß-adrenergic receptor stimulation to renin release was not greater in obese than in lean rats. These observations also suggest that other factors besides differences in adrenergic activity may contribute to the difference in PRA observed between lean and obese Zucker rats.
Role of the RAS in Obese Zucker Rats
In this study, PRA was significantly lower in obese than in lean Zucker rats during control conditions of normal sodium intake. Studies by Harker et al38 have also demonstrated that PRA and total renin content in obese Zucker rats are decreased compared with values in lean littermates. However, the observation that PRA is reduced in obese Zucker rats does not necessarily imply that the RAS is unimportant in mediating increased BP. Several studies have suggested that in vivo vascular responsiveness to Ang II is increased in obese Zucker rats,37 39 leading to the hypothesis that obese Zucker rats have increased pressor responsiveness to Ang II.
To quantify the role of the RAS in this model of hypertension, we compared the cardiovascular and renal effects of chronic Ang II receptor blockade with losartan in lean and obese Zucker rats. Although losartan reduced arterial pressure in lean and obese Zucker rats, the fall in BP was significantly greater in obese rats; after 7 days of losartan, MAP in obese rats fell by 37 mm Hg, compared with a decrease of 25 mm Hg in lean Zucker rats. This observation suggests that elevated BP in obese Zucker rats depends in part on Ang II.
There are several possible explanations for this increased dependence of arterial pressure on Ang II in obese Zucker rats. First, circulating Ang II levels may be increased in obese compared with lean rats. This seems unlikely because PRA was actually lower in obese than in lean rats. The second possibility is that there may be activation of a local tissue RAS in obese rats that is not reflected by PRA. However, we did not design the present study to test this possibility, and further studies are needed to determine whether activation of the RAS occurs in specific tissues of obese Zucker rats. Our previous studies in obese dogs suggest that weight gain activated the RAS,3 and similar observations have also been made in obese humans.17 18 However, in all of these studies, stimulation of the RAS was accompanied by increased PRA, in contrast to the reduction in PRA observed in obese Zucker rats. Thus, if local tissue Ang II formation is increased in obese Zucker rats, independent of PRA, this would be in contrast to obese dogs and humans.
The third possible explanation for the increased dependence of BP on Ang II is that obese Zucker rats may have increased sensitivity to the pressor actions of Ang II. This possibility fits with our observation that the BP responses to injections of exogenous Ang II were enhanced in obese compared with lean rats; in obese rats, there was a greater area under the rise in BP curve, suggesting enhanced vascular responsiveness to Ang II. This greater BP response to Ang II in obese rats occurred despite the fact that the same total amount of Ang II was injected into both groups of rats, even though the obese rats were much larger than the lean rats. Therefore, our protocol may have underestimated the enhanced pressor sensitivity to Ang II in obese rats.
Because obese Zucker rats display baroreflex impairment,28 the greater rise in BP during a bolus injection of Ang II could be in part due to inadequate baroreflexes. However, previous studies have shown that obese Zucker rats exhibit exaggerated pressor responses to Ang II even after ganglionic blockade, suggesting hyperresponsiveness of the vasculature to Ang II.37 Studies by Zemel et al39 have also shown that obese Zucker rats exhibit a greater in vitro vascular reactivity to Ang II. These observations fit with the possibility that the higher BP observed in obese Zucker rats compared with lean controls may result in part from enhanced sensitivity to Ang II.
Although losartan caused greater reductions in BP in obese than in lean Zucker rats, Ang II receptor blockade did not cause greater increases in RBF or PRA in obese rats. Thus, the greater BP effects of Ang II blockade in obese rats cannot be attributed to a greater sensitivity of the renal vasculature to the effect of Ang II. It is possible that the renal tubules in obese rats may be more sensitive to Ang II. If this were the case, blockade of Ang II would shift pressure natriuresis toward lower BP in obese compared with lean rats, thereby leading to a greater BP reduction. However, we did not design the present study to specifically test the intrarenal mechanisms by which chronic blockade of Ang II receptors alters pressure natriuresis in obese compared with lean Zucker rats. Further studies are needed to explore these possibilities.
In summary, the results of this study indicate that obese Zucker rats have higher MAP than lean Zucker rats. The increased BP in obese rats appears to depend in part on the RAS because chronic Ang II blockade with losartan caused a greater reduction in BP in obese than in lean rats. Because PRA was reduced in obese Zucker rats and because obese rats exhibited a greater pressor response to exogenous Ang II compared with lean control rats, our results suggest that obesity may be associated with increased sensitivity of the chronic BP effects of Ang II. The precise mechanisms for this effect, however, are still unclear. In addition, increased adrenergic activity may play a modest role in raising BP in obese Zucker rats, because combined - and ß-adrenergic blockade caused a slightly greater reduction in BP in obese than in lean Zucker rats.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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The authors' research was supported by grants HL-51971, HL-39399, and HL-23502 from the National Institutes of Health. The losartan used in these studies was kindly supplied by EI DuPont de Nemours & Co, Inc, Wilmington, Del. M. Alonso-Galicia was supported by a graduate fellowship from GLAXO Research Institutes.
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Footnotes |
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Reprint requests to John E. Hall, PhD, Department of Physiology and Biophysics, University of Mississippi Medical Center, 2500 N State St, Jackson, MS 39216.
Received November 2, 1995; first decision November 30, 1995; accepted July 5, 1996.
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